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Strike Early with PER1 fa Isew0r k syst e ms PER1 offers a complete range of products suitable for all forms of slab construction: SKYDECK - The fastest system for flat slabs 0

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,

..

,

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I

...

Guide to Flat Slab Formwork and Falsework

concrete structures group CONSTRUCT is an association of member companies dedicated to the task of improving the construction efficiency of in-situ concrete frames and associated structures. The membership focuses on Specialist Concrete Frame Contractors, supported by other associated market leaders especially in the formwork, reinforcement and concrete industries. The aims and objectives of CONSTRUCT are to: promote excellence in the design and construction of reinforced concrete frames and structures identify and address issues which impede best practice and buildability actively work with clients, designers and others in the project team to maximise benefits in speed and economy during the design and construction of in-situ concrete frames and structures promote innovation in the use of concrete in reinforced concrete frames and structures co-ordinate the views of CONSTRUCT members and to act in their interest promote and encourage training and education initiatives

For further information contact: Colin Cleverly, Executive Secretary, CONSTRUCT, Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS, UK Tel: 01344 725744, Fax: 01344 772426, E-mail: [email protected],www.construct.org.uk


Eur I n g P. F. Pallett BSc CEng FICE FCS

Ternporary Works ConsuItant

This Guide is the result of a collaborative Partners in Technology programme between CONSTRUCT (Concrete Structures Group), the University of Birmingham, the University of Leeds and the Building Research Establishment. The research study of the construction of the seven-storey in situ European Concrete Building Project has informed this Guide.

Published by The Concrete Society on behalf of CONSTRUCT

concrete s ~ ~ u c t ~ r e s

Keywords Formwork, Falsework, Striking, Backpropping, Building, Best practice, Concrete structures, Floors, Flat slabs, Loading, Safety, Concept selection, Strength assessment Reader interest

Classification

Building clients, developers, permanent works and temporary works design engineers, frame constructors’ technical and supervisory staff, planning supervisors, proprietary equipment suppliers and main contractors involved in building.

Availability

Content Status User

Unrestricted Best practice guidance Committee guided Clients, developers, engineers and building constructors


ISBN 0 946691 90 8 Order Reference CS 140

0CONSTRUCT Concrete Structures Group 2003 Published by The Concrete Society on behalf of CONSTRUCT, Concrete Structures Group The Concrete Society, Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS, UK Tel: +44 (0) 1344 466007, Email: [email protected], www.concrete,org.uk Further copies may be obtained from The Concrete Bookshop. Email: [email protected] www.concretebookshop.com

All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without the prior permission of the copyright owner. Enquiries should be addressed to The Concrete Society. Although The Concrete Society (limited by guarantee) does its best to ensure that any advice, recommendations or information it may give either in this publication or elsewhere is accurate, .no liability or responsibility of any kind (including liability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by the Society, its servants or agents.

Summary and Foreword

Summary Formwork and falsework play a significant part in the overall benefits achievable in the construction of in situ concrete buildings. Process control and an improved understanding of temporary works by designers, constructors and suppliers have been identified from research as crucial to successful design, leading to safe completion of flat slab concrete buildings. Many of the principles apply to other forms of construction. Detailed information and guidance for all members of the construction team is given in this Guide to enable improved designs and construction techniques to be adopted, thus promoting more efficient and economic construction of commercial concrete buildings. The Guide has been structured to aid readability, and follows the stages of construction. New principles for the early striking of flat slabs in buildings are introduced, together with recommended methods of assessing concrete strength. The relative arrangement of falsework and backpropping has a significant effect on the force transmitted through the supporting slab into the backpropping. Four methods of assessing backpropping loads are presented in order to help designers and constructors to plan efficient construction schemes. One of these is in the form of a spreadsheet, which is included on a CD ROM. Theoretically, construction loads can exceed the design service load on newly cast slabs, and the Guide makes proposals so that permanent works designers can investigate and take appropriate action. This Guide should lead to the quicker, safer construction of buildings and provide longer term economic benefits to the industry, to constructors and ultimately to clients. It should be of interest to all those in the construction team.

Foreword This Guide is a consensus view developed during the construction of the seven-storey in situ concrete building for the European Concrete Building Project by the multi-disciplinary team of consultants, contractors, researchers and specialist suppliers involved with the project. Construction took place between January and May 1998 inside Hangar No. 2 at the Building Research Establishment’s Laboratory, Cardington, Bedfordshire. The project was part of a major collaborative programme between BRE, CONSTRUCT, BCA, Reinforced Concrete Council and others, with considerable assistance from industry. The author of this Guide was employed by CONSTRUCT (the Concrete Structures Group) and the University of Birmingham. The work reported herein was carried out under a Contract jointly funded by CONSTRUCT and the Secretary of State for the Environment, Transport and the Regions (now the Secretary of State for Trade and Industry) placed in March 1998. Any views expressed are not necessarily those of the Secretary of State for Trade and Industry.

vii

ing Group The Guide was prepared under the guidance of a steering group: Eur. Ing. P. F. Pallett (Chairman)

Temporary Works Consultant

Professor A. W. Beeby P. Campbell Professor L. A. Clark C. T. Cleverly (Secretary) J. E. C. Franklin C. Goodchild C. F. Heathcote Rev. M. James S. Lillie J. J. A. Maw Dr R. Moss Eur. Ing. T. Pope D. Smith D. A. B. Thomas T. S. Viney R. T. Ward

University of Leeds CONSTRUCT University of Birmingham CONSTRUCT Kwikform UK Ltd Reinforced Concrete Council PERI Ltd Malcolm James Consultancy Byrne Brothers Ltd J.J.A. Maw Consultants Building Research Establishment PERI Ltd Ischebeck Titan Ltd Health and Safety Executive, Technology Division Hanson Birchwood Consultant

Corresponding Members: J. N. Clarke V. R. Dunham S. Ferguson D. S. Lazarus Professor A. E. Long Eur. Ing. Dr W. F. Price Dr M. N. Soutsos C. J. Wilshere OBE

viii

The Concrete Society Peter Brett Associates Syntech Consulting Engineers, Perth, Australia Ove Arup & Partners The Queen's University of Belfast' Sandberg (now Lafarge Cement UK) University of Liverpool Consultant

ontent Summary Foreword Steering group List of figures List of tables List of equations Abbreviations Definitions

vii vii ... v111 xii xii 1 . .

Xlll

...

Xlll

xiv

1

Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

2

Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

Pre-contract stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General - building design considerations Loading considerations - permanent structure Slab thickness Column and wall kickers Staircases Trimming of holes Effect of concrete type on temporary works Temporary works concept selection Types of temporary works for in situ flat slab construction 3.9.1 Individual props and beams, with proprietary panels or plywood as formwork 3.9.2 Steel skeletal systems, with beams and plywood as formwork 3.9.3 Table systems 3.9.4 Flying form systems 3.9.5 Precast concrete planks (participating) 3.9.6 Permanent concrete planks (non-participating) 3.10 External protection and enclosure considerations

6

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

4 4.1 4.2

4.3 4.4

4.5 4.6 4.7 4.8

Preconstruction planning stage. . . . . . . . . . . . . . . . . . . . . . . . . Management of the temporary works The temporary works design brief 4.2.1 Individual props and beams, with proprietary panels or plywood as formwork 4.2.2 Steel skeletal systems, with beams and plywood as formwork 4.2.2 Table systems 4.2.3 Flying form systems 4.2.4 Precast concrete planks (participating) 4.2.5 Permanent concrete planks (non-participating) Loading during construction Procurement of temporary works 4.4.1 General 4.4.2 Working platforms and edge protection 4.4.3 Edge distance of standards 4.4.4 Column forms - lifting, access and stability 4.4.5 Level adjustment of head jacks, base jacks and props 4.4.6 Length of formwork beams 4.4.7 Implications of changes Permit to load and permit to strike Sequence of work Striking - planning considerations Method statements

6 7 9 9 11 12 12 13 14

18

21 22 24

28

30

36 37 39 39

ix

Contents

5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

Construction stage . . . . . . . . . . . . . . . . . . . . . . . .

6 6.1 6.2 6.3 6.4 6.5

Background to striking and backpropping . . . . . . . . . General Philosophy for loading and striking flat slabs Deflection of slabs struck at early age Assessment of concrete strength at early age Backpropping 6.5.1 General 6.5.2 Backpropping assumptions 6.5.3 Loads in backprops 6.5.4 Sequence and installation procedure for backpropping Loading a slab to above the design service load 6.6.1 Introduction 6.6.2 Proposals 6.6.3 Effects of proposals 6.6.4 Alternative measures

6.6

.

.

.

..

.

. 41 42 42 43 44 45 46 47 49

General Material handling Loading-out platforms Temporary works to columns Temporary works to walls and lift shafts Temporary works to slabs Temporary works to precast slabs Cranage Implications of changes

50

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.

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51 52 54 56 57 60

72

7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

application . . . . . . . . . . General Method to establish required concrete strength No backpropping required One level of backpropping Two levels of backpropping Sequence of striking soffit formwork Sequence of installing and removal of backpropping Accidental overloading of slabs

-

. . . . . . . 77

8

References . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

09

.

93

Striking and backpropping

77 81 82 82 83 85 86 88

Annexes Annex A AI A2 A3 A4 A5

- The Cardington Project

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. . .

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. . . .

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. .

. .

.

.

93 94 94 96 96

Construction Equipment for Floors 1 and 2 - Props and timber beams Equipment for Floors 3 and 4 - Ischebeck Titan Ltd Equipment for Floors 5 and 6 - Kwikstage Shoring 55 Floor 7 - Birchwood Omnia Ltd - Precast plank and Densit

Annex B

- Temporary works design checklist .

.

. .

. . . . .

. . . .

. . . . .

Annex C - Comparison of load transferred . . . . . . . . . . . . . . . . . . .

,

97 100

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Annex D Worked example of striking and backpropping to second floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 D1 D2 D3 D4

X

General Loadings To establish the criteria for striking Further considerations - strength assessment

101 101 102 103

Contents

D5 D6 D7 D8 D9 D10 D11 D12 D 13 D14

103 104 104 104 105 106 106 107

Second floor - Backpropping considerations for falsework erection Second floor - Backpropping considerations for casting the third floor Method One - Backprop loads by use of percentages of load Method Three - Backprops loads by use of further simplified formulae Method Four - Backprop loads from spreadsheet Discussion Second floor - Pre-load of backprops and allowance for cracking Second floor - Discussion on loading to above the service load Second floor - Backpropping considerations for accidental overloading Fourth floor - Distributed load from falsework

108 108

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Annex E Theory of loading a slab to above the design service load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . El E2 E3

E4 E5

. . . . . . 109 109

Introduction Theory: base criteria Loads E3.1 Load factors: independent loads E3.2 Load duration E3.3 Load allowances E3.4 Temporary load patterns E3.5 Loads: self-weight of formwork and falsework E3.6 Pre-loads in back props E3.7 Load combination factors Deflection: Serviceability Limit State Collapse: Ultimate Limit State E5.1 Bending E5.2 Shear E5.3 Bond E5.4 Load factors: temporary nature of load E5.5 Material factors of safety

109 110

111

113

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Annex F Backpropping spreadsheet. . . . . . . . . . . . . . . . . . . . . . . . 118 118 F 1 Conditions of use F2 F3 F4

Annex G G1 G2 G3

118

Spreadsheet liability Introduction Operation of the Excel spreadsheet Worked examples

- Allowance for cracking in spreadsheet. .

119 119 130

. .

. .

.

. .

.

The distribution coefficient Ratio of curvature Effective deflection factor (FJ

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CD ROM containing Backpropping spreadsheet (CPR0P.XLS)

. . . . .

. 140 140 140 140

. . . . 142 inside back cover

Xi

FiQures and tables

List 0%figuues Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 2 1 Figure 22 Figure 23 Figure 24 Figure 25 Figure 26 Figure 27 Figure A 1 Figure A2 Figure A3 Figure D1 Figure D2 Figure El Figure E2 Figure F1

Floor four of the ECBP under construction. (page 1) Typical temporary works management flow diagram. (page S) Typical kickers to perimeter columns. (page 10) Isometric view of possible staircase construction. (page 12) Typical individual props and proprietary beams. (page 14) Typical arrangement with steel skeletal system. (page 15) Typical aluminium frame table with decking. (page 16) Typical aluminium table with precast plank system. (page 18) Typical section at edge of slab. (page 32) Typical loading-out platforms in use. (page 43) Plan of column form at corner and edge of building. (page 45) Typical plan arrangements of formwork to a lift shaft. (page 45) Typical precast block for wall kicker. (page 46) Effect on deflected shape of different backpropping positions. (page 65) Diagrammatic representation of backpropping of suspended slabs. (page 68) Extent of theoretical overloading of flat slabs due to concreting operations above. (page 75) Proposal 2a: fJ’, required to satisfy Equation 2 at SLS. (page 75) Proposal 2a: wln,,, should be less than Pbto satisfy Equation 13 at SLS. (page 75) Proposal 2b: nln,,, should be less than 1.O to satisfy Equation 14. (page 76) Flowchart for striking flat slabs in buildings. (page 79) Flowchart for backpropping flat slabs in buildings. (page 80) Graph of ratio of loading to concrete strength factor. (page 81) No backpropping. (page 82) Backpropping one level. (page 83) Backpropping two levels. (page 84) Sequence of striking flat slabs. (page 85) Typical backpropping layout planning chart from ECBP. (page 87) View of completed building. (page 93) Typical cross section of ECBP. (page 95) Plan of building. (page 95) Situation after striking second floor. (page 102) Backpropping one-level. (page 106) Typical ultimate bending moment capacity graph for a C28/35 concrete. (page 114) Typical ultimate shear capacity of a C28/35 concrete. (page 114) Diagrammatic representation of backpropping. (page 119)

Table 1 Table 2 Table 3 Table 4 Table C1 Table El Table E2 Table E3 Table F 1

Summary of concrete strength assessment methods for early striking. (page 60) Effect of falsework and backpropping locations. (page 64) Method One - percentage of load distribution from temporary works loads. (page 68) Checklist for striking and backpropping procedures. (page 78) Percentage of load distribution from temporary works loads. (page 100) Loads and combination factors at serviceability limit state (SLS). (page 112) Loads and combination factors at ultimate limit state (ULS). (page 112) Values of material partial safety factor ym at ultimate limit state (ULS). (page 116) Schedule of sample A4 outputs from spreadsheet. (page 129)

xii

Equations and abbreviations

Sd eqUathl§ Note: Page numbers refer to the first page on which an equation appears. Some equations are repeated at appropriate points in the text.

Equation I Equation 2 Equation 3 Equation 4 Equation 5 Equation 6 Equation 7 Equation 8 Equation 9 Equation 10 Equation 11 Equation 12 Equation 13 Equation 14 Equation 15 Equation 16 Equation 17 Equation 18 Equation 19 Equation 20 Equation 2 1 Equation 22 Equation 23 Equation 24 Equation 25 Equation 26 Equation 27 Equation 28 Equations 29-33

Loading factor criterion (page 54) Cracking factor criterion (page 54) Effective deflection factor criterion (page 54) Loading factor defined (page 55) Cracking factor defined (page 55) Striking criteria for crack width control (page 55) Concrete strength factor (page 56) Minimum characteristic concrete strength (page 56) Change in prop length (page 63) Added load versus spare capacity (Method One) (page 65) Load in backprops for one level of backprops (Method Two) (page 68) Load in first layer backprops for two levels of backprops (Method Two) (page 69) Load in lower layer backprops for two levels of backprops (Method Two) (page 69) Load in backprops for one level of backprops (Method Three) (page 69) Load in first layer backprops for two levels of backprops (Method Three) (page 69) Load in lower layer backprops for two levels of backprops (Method Three) (page 69) Ratio of unfactored construction load to ultimate design load (page 73) Ratio of ultimate construction load to ultimate design load (page 74) Minimum required characteristic concrete strength at time of striking (page 81) Combination factors for ultimate loads at rare load case (page 110) Alternative combination factors for ultimate loads at rare load case (page 110) Combination factors for ultimate loads at frequent load case (page 110) Alternative combination factors for ultimate loads at frequent load case (page 110) Partial safety factor for temporary nature of load (page 115) Material partial safety factor (page 116) Effective deflection coefficient (puge 126) Effective modulus of elasticity for varying slab thicknesses (page 128) Distribution coefficient (page 140) Calculation of effective deflection coefficient (page 141)

Abbreviations BCA BRE BS CXdYY CDM C(HSW) ClRlA CONSTRUCT DETR ECBP HSE HSW LOLER NSCS PIT PWD SLS TMC TW TWC TWD ULS

British Cement Association Building Research Establishment British Standard grade of concrete (cylinder strength/ cube strength) Construction (Design and Management) Regulations 1994 Construction (Health, Safety and Welfare) Regulations 1996 Construction Industry Research and Information Association Concrete Structures Group Department of the Environment, Transport and the Regions (now DTi) European Concrete Building Project Health and Safety Executive Health and Safety at Work etc. Act 1974 Lifting Operations and Lifting Equipment Regulations 1998 National Structural Concrete Specification for Building Structures Partners in Technology (Scheme of the DETR, now DTi) permanent works designer serviceability limit state temperature-matched curing temporary works temporary works co-ordinator temporary works designer ultimate limit state

xiii

Definitions

Definitions Backpropping: Propping installed at levels below the slab that supports the falsework in order to distribute the load on the uppermost slab to suitable supports, such as lower slabs or to the foundations. Backprops can be required at more than one level. (In some parts of the world, backpropping is called ‘re-shoring’.) Constructor: Any organisation carrying out construction operations on a site. Falsework: Any temporary structure used to support the permanent structure until it can support itself. Flat slab: A floor slab with or without drops and supported, generally without beams, by columns preferably without column heads. It may be solid or may have recesses formed on the sofit so that the soffit comprises a series of ribs in one or two directions (i.e. a waffle or coffered slab). Formwork: A structure, usually temporary, for containing poured concrete, moulding it to the required dimensions and supporting it until it is able to support itself. It consists primarily of face contact material and the bearers directly supporting the face contact material. Post-propping: Temporary propping installed after placing permanent formwork prior to steel fixing and concreting, where the permanent formwork will carry its own weight and the construction loads, but the load from the in situ concrete is carried by the post-propping. Repropping: The system used during construction in which the supports to a recently cast slab are removed and replaced in a planned sequence. Supporting slab: The suspended floor slab immediately below the floor slab under construction. The sofit formwork and falsework for the next slab above will be erected on this slab. Temporary works: A structure used in the construction of the permanent structure. It is usually removed on completion. Temporary works co-ordinator: The person appointed to have overall responsibility for the technical and procedural aspects of the design, procurement, erection and use of the formwork, falsework and scaffolding.

Flat slabs have no supporting beams or drop heads

This Guide covers the procedural and technical aspects of the formwork, falsework and striking operations involved in flat slab in situ concrete construction in the UK. Flat slabs are considered as slabs with a flush soffit that may incorporate beam strips within their depth and are one- or two-way spanning. They are without supporting beams or drop heads. The Guide is, however, applicable to flat slabs with downstand beams along their edges, and many of the principles discussed will be applicable to other types of construction.

Figure 1: Floor four of the ECBP under construction.

1

Scope

The Guide will principally be of interest to those in, the building industry involved in the construction of in situ reinforced concrete slabs of all sizes for single and multi-storey construction. By following the construction process, from concept by the client, through to construction, it will assist all members of the construction team. Guidance is given on the application of the Construction, Design and Management (CDM) Regulations, design (including consideration of loading slabs above their design load), general imposed loads and construction details.

Interest to those in the building industry constructing in situ RC and precast slabs for single and multi-storey buildings

Topics covered include temporary works management, approvals, edge protection, working platforms, temporary cladding and enclosures. Improved understanding of the process will reduce construction time and lead to safer, more economic use of in situ and precast slabs in building construction. An important feature of the Guide is an improved method and understanding of backpropping and striking. The formwork construction methods described are ‘state-of-the-art’ for single and multistorey construction, but, in some cases, particularly in high-rise construction, the methods used may be inefficient andor uneconomic but be viable in the light of other considerations.

Methods stated may be inefficient and uneconomic

The findings in this Guide are based on the study of the seven-storey in situ concrete frame building built as part of the European Concrete Building Project (ECBP) at the BRE Laboratory, Cardington, Bedfordshire between January and May 1998. (Refs 1 and 2 ) More details of the construction of the ECBP are given in Annex A.

ECBP: see Annex A

2

2 Introduction

Recommendations - Introduction 1.

Construction safety and economy begins a t a building‘s concept and design stages, not on site.

2.

Designers, particularly Permanent Works Designers, should be made aware that appropriate design decisions can optimise the temporary works. The direct economic benefit for clients is that less expensive buildings can be built more quickly, efficiently and safely.

3.

Temporary Works Co-ordinators should be appointed by the constructor, and should be available a t an early stage of the construction process to provide guidance to other members of the construction team.

This Guide should lead to the quicker, safer construction of buildings and provide longer term economic benefits to the industry, to constructors and ultimately to clients. It should be of interest to all those in the construction team. Guide structured into Sections by likely reader

The Guide has been structured to aid readability, and follows the stages of construction. For example, clients will wish to concentrate on reading Chapters I to 4, but may not necessarily read other sections in depth. Some important topics are repeated at different points, but with the emphasis altered to suit the anticipated reader.

Safety and economy begin at concept and design stages

Construction safety and economy begin at a building’s concept and design stages, not on site; the safe and effective use of site manpower is influenced not only by operative’s competence and supervision, but also by the quality and adequacy of the designs prepared - for both the permanent structure, and the temporary works. The research at Cardington identified that “it is within the process not within the hardware (product) that opportunities for high performance will lie”. Much of the advice in this Guide is aimed at improving the process and identifying areas where change will generate benefits.

PWDs influence on temporary works

Encouraging designers, particularly Permanent Works Designers (PWDs) and their representatives, to make appropriate design decisions that optimise the temporary works has direct economic advantages for the client in producing less expensive buildings that can be built more quickly, efficiently and safely. This approach promotes the recommendations of the Egan report (Ref. 3) for giving clients better value and good engineering. This document is not intended to impose restrictions on Permanent Works Designers, but temporary works have a significant influence on the final structure and its buildability. In particular, PWDs are responsible for decisions that will affect the safety of constructors and users. The constructor, of course, has a significant part to play in temporary works. A major recommendation of this Guide, in order to improve the process engineering, is to promote the responsibilities that the constructor should take in managing the temporary works. The importance of the role of the Temporary Works Co-ordinator (TWC) in managing the assembly, construction, striking, backpropping, and all the technical aspects of the temporary works cannot be overstated. Ideally the TWC should be appointed by the constructor, and should be available at an early stage of the design process to provide guidance to other members of the construction team. One of the original concepts of the ECBP at Cardington, shown in Figure 1, under the main proposal to the Partners in Technology scheme, was to assess the performance of the temporary works (‘Task 2’). The output from the research, although not as originally envisaged, provided

3

Introduction

meaningful information of use to the industry - in the circumstances it concentrated the researchers on the smaller details so that future fast-track construction processes can be improved in design, in procurement, and particularly in management procedures. The research identified the important contribution that both the Permanent Works Designer and the client make to the safety and economics of the temporary works. The CDM Regulations (Ref. 4.) impose duties on all designers. Consideration of the temporary works and safety of all persons are important issues for all members of the construction team.

Clients' influence on temporary works

The importance of the Permanent Works Designer and Temporary Works Co-ordinator having an understanding of the backpropping and striking requirements for flat slab construction is highlighted in this Guide. It is likely that recent flat slab buildings which have low ratios of live load to dead load have been stressed during construction above their service load. How this can be avoided in practice is a major aspect of this document and is discussed in detail in Section 6.6 and Annex E.

PWD and TWC to understand backpropping and striking of flat slabs

Striking and backpropping are the subject of a separate BRE Report under Task 4 (Ref. 2). This Guide discusses the criteria in detail in Chapter 6, and includes in Section 7.2 a new proposed method of assessing the strength of a concrete slab during any construction operation. To assist readers, two flowcharts, Figures 20 and 21, are included. The complexity of backpropping, with the various options necessary for safe control and loading of flat slabs during construction, is detailed in Chapter 6, with four methods for calculating the backpropping loads. Annex D includes a worked example.

Consideration of loading slabs during construction

The experiences gained from the ECBP indicate that there are advantages in considering complete temporary enclosures for buildings under construction, although these may impose unacceptable limitations on the concept of the temporary works.

Advantages in enclosure of buildings

This Guide is not a design manual, but is intended to be complementary to the Concrete Society's Formwork - a guide to goodpractice (Ref. 5) and to BS 5975 Code ofpracticefor falsework (Ref. 6). The ECBP was built using the first edition of the National Concrete Frame Specification, which was revised in May 2000 with the title National structural concrete specijication for building construction (Ref. 7).

Complementary publications: Formwork Guide to good practice, Code of practice on falsework, NSCS

Most of the comments in this Guide relate to experiences from Cardington, and examples of the positive and negative experiences are used to illustrate and inform the industry. To give the broader view, comments from the Steering Group have been incorporated. It is accepted that there are risks in projecting results from only a few studies to be accepted as general practice. The ECBP structure was at or beyond the edge of current practice and therefore the structural effects observed are greater than those encountered in normal practice. For example, the proposed method of striking flat slabs from Task 4 has been confidently adopted for similar structures within defined limits of type of structure, slab thickness, etc. The arrangement of this Guide follows the stages in the construction process. Some items and recommendations that are relevant at several stages are repeated and cross referenced. Recommendations are summarised at the start of each chapter. The involvement and responsibilities of the client, PWD and TWC throughout the construction process are summarised here:

Chapter 3 - Pre-contract stage - client and/or PWD involvement The client and the PWD are involved in considering the basic layout and possible construction methods, including initial layouts, loadings, slab sizes, column layouts, and safety issues by the designer.

Chapter 4 - Preconstruction planning stage - client, PWD and TWC involvement The contract has been awarded and detailed planning and procurement starts. At this stage the TWC and the PWD should establish the detailed concept, procurement, early striking criteria, method of assessing concrete strengths, risk assessments, detailed method statements, etc.

Chapter 5 - Construction stage - TWC and PWD involvement The TWC should be co-ordinating the suppliers, erectors and users of the temporary works. Detailed procedures need to be established and monitored. Safety concerns are raised and actions implemented; this may include backpropping procedures.

4

Risks in projecting results from ECBP to industry

I ntraduction

I

Chapters 6 and 7 - Backpropping and striking the slabs - TWC and PWD involvement

~

Once a slab is cast, procedures for striking the formwork and falsework need careful consideration. These may include requirements for backpropping. New methods of determining strength in situ will save time without compromising safety or serviceability.

5

tract st

Recommendations 1.

Permanent Works Designers (PWDs) should consider the implications of changes in the design of a structure on the economics of construction.

2.

Consideration should be given to the complete temporary enclosure of the building during construction to improve productivity.

3.

Unless specified otherwise, the self-weight of reinforced concrete in flat slab construction should be calculated using a specific weight of 24 kN/m3.

4.

Issues relating to the site boundary need to be addressed at tender stage. For multi-storey construction, protection (edge fans or nets) around the entire building will be required.

5.

Adequate edge protection should be provided for all operations, including precast floor construction.

6. Reinforcement should be detailed to suit the method by which the structure is to be built: the constructor should be involved in the detailing.

7.

Detailing of column reinforcement is important, particularly when reinforcement couplers are to be used in kickers.

0.

Column kickers should be at least 100 mm high.

9. Kickerless construction is not recommended for perimeter columns. 10. Downstand beams should be avoided: for economic slab formwork, beam strips within the slab depth are preferred. 11. Where upstand beams are used, such as to trim large openings, reinforcement should be scheduled separately, not with the main slab. This facilitates precasting the beams on site.

12. Stair flights can affect the temporary works and their design should be carefully considered.

13. Precast stair flights should be carefully detailed and safety aspects of installation considered by both the PWD and the TWC. 14. Precast stair flights should be planned for early erection during construction, to facilitate access for operatives and staff to the working areas.

It is not intended that the PWD and the client should be limited by the constraints of the temporary works, but the following design considerations for the permanent building are given in good faith, based on ECBP experience. The comments are for consideration by the PWD and the client at the early, and often critical, concept stage. Where design and build or management contracting is envisaged for a project, the constructor should be selected early to give technical temporary works advice to the PWD. The expertise of the experienced frame contractor will help the PWD to make correct decisions.

6

3.2 Loading considerations

When designing large-span flat slabs with few columns, procedures during construction to avoid overstressing the slabs are very important. This is discussed in detail in Section 6.6. Constructor responsible for construction

Designers to take account of any temporary works

While the PWD’s responsibilities may have been fulfilled during design in considering buildability and carrying out the appropriate hazard identification and risk assessment, responsibility for construction remains with the constructor. Responsibility for permanent and temporary works design and for constructing the building safely is clearly a requirement under the CDM Regulations (Ref. 4). Clause 28 of the Approved Code of Practice to the Regulations states: “Designers need to understand how the structure can be constructed, cleaned and maintained safely. This involves ensuring that the overall design takes full account of any temporary works, for example falsework, which may be needed, no matter who is to develop these works.” A typical flow chart of temporary works procedures is shown in Figure 2. It is not exhaustive, as site and contractual conditions will vary.

Pre-tender Health and Safety Plan to be available at tender stage

The PWD has a responsibility to ensure that the building can be constructed safely. Unusual or critical aspects of the design, e.g. backpropping or striking procedures, should be identified in the pre-tender Health and Safety Plan. This document should be made available at tender stage and will enable tenderers to make an informed choice of construction technique. It is an important duty of the Planning Supervisor to ensure the plan is complete, and includes items such as provision of edge protection and other forms of protection. Although the PWD usually has little control over how the building is actually built, and cannot be expected to be an expert in backpropping and striking, the PWD has a responsibility to communicate information such as the rationale for the design and descriptions of the works, and to ensure the overall safety of the construction. Communication on such matters as backpropping and striking at both tender and construction stages is therefore important. The technical aspects of striking and compliance with the principles of striking are discussed further in Chapter 6. Ideally, provisions for safety at the early design stage should not just be for a safe working environment, but should improve productivity on the project, and therefore be self-financing. This Guide recommends that consideration be given to temporary enclosure of the structure and that more research into the subject is carried out (see Section 3.10).

TR52

Consider the column/precast plank interface

Consideration of the surface finish to the soffit of the flat slab is outside the scope of this document. Guidance on plain formed concrete finishes is given in Concrete Society Technical Report 52 (Ref. 8). It is recommended that PWDs, when assessing precast alternatives to in situ construction, should give more consideration at design stage to the column/plank interface and discuss the implications with the relevant parties at contract stage. Early discussions with TWDs who have experience in such matters, and who have design solutions, will provide further benefits to the client. Permanent works designers should be more aware of the implications of changes in design of the structure on the economics of Construction. For example, the introduction of a column near the centre of one of the 7.5 m bays in the ECBP to support the staircase had the immediate effect of making any table or flying form system inoperable. On a PWD re-design, the central column was removed! Another example of the effect of the PWD on the selection of temporary works system was the removal from the design of any cross-walls, and the insertion of temporary steel crossbracing. This bracing was not fixed to the level immediately below the slab being cast, which made removing the falsework easier.

Detail reinforcement to site procedures

Ideally, reinforcement should be detailed to suit the method of construction, and thus the constructor should be involved in the detailing, as recommended in the CONSTRUCT Guide to contractor detailing (Ref. 9). This had beneficial results at the ECBP where reinforcement for four of the seven floors was contractor-detailed. Design and detailing of reinforcement can affect the temporary works, and so the following sections include comments relating to reinforcement.

3.2 Loading considerations Commercial buildings, imposed service load 2.5 kN/mZ

- permanent structure

The ECBP was designed for a characteristic imposed load of 2.5 kN/mZfor general office loading in accordance with BS 6399: Part 1 (Ref. IO), which is considerably less than the higher loads commonly required by letting agents “to cover all future eventualities”. This had direct effects on the structure as it allowed thinner slabs and a lighter overall structure, and gave economies in both structure and foundation design.

7

3.2 Loading considerations

I

.

Client

PWD

TWD

I

!

TWC

I

Site

!

Supplier

I

I --------, ---------L-------I

comments,

I

I

I

I I

j(

I

I

I

I

Prepare Health and Safety Plan. Design brief, contract specification, and construction drawinqs

I

;

-- I ,

;

assessment

I

Comment on method of conCrete strength assessment, striking and backpropping

--; I

I

;-

! I

.I I

;

Establish responsibilities Appoint TWC and TWD

I

Award contract and appoint constructor

Commenton supporting slab strength, and backpropping

;

-

)/

!

(See proforma Annex B)

--

drawings etc.

drawing

I

Issue preliminary drawings andlor TW sketches

-

I

I

_-

1-

I

I

I

I

I

I

I

;

I

9. . I

c). I

. I

Permit to load system approvals. Backpropping as required

I

--

I I' I

Figure 2: Typical temporary works management flow diagram. Note: The diagram does not illustrate the roles of the various parties, e.g. planning supervisor, under health and safety legislation.

8

Advisory

3.4 Column and wall kickers

0.25 x 24 kN/m3

Normal construction: use concrete density of 24 kN/m3

At the ECBP the loadings for the 250 mm-thick solid reinforced concrete slab were considered as follows: Self-weight concrete slab 6.00 kN/m2 (calculated on the basis of density of 24 kN/m3) 1.OO kN/m2 partitions raised floor 0.5 0 kN/m2 ceiling services 0.50 kN/m2 Total self-weight load of structure Characteristic imposed load

8.00 kN/m2 2.50 kN/m2

Hence total PWD unfactored design load

10.50 kN/m2

DD ENV 1991-2-1 (Ref. 11) recommends a value for the density of concrete of 24 kN/m3 plus 1 kN/m3 for reinforced and prestressed concrete, implying a total of 25 kN/m3 for concrete. A footnote states that the “density of concrete may be in the range 20-28 depending on local materials.” This Guide recommends the use of 24 kN/m3 for normal weight reinforced concrete in flat slab construction’. Information about the PWD’s assumptions about self-weight and characteristic imposed loads used in the design, together with data on concrete characteristic strengths, will be required at the preconstruction stage by the TWD and the TWC. See the temporary works design checklist in Annex B.

3.3 Slab thickness Selection of slab thickness by the PWD depends on many factors, including planned current use of the building, likely future use, overall height constraints and proposed plan layout of columns. Use Economic concrete frame elements for initial span/depth ratio against load

The extremely useful book published by the Reinforced Concrete Council, Economic concrete frame elements (Ref. 12), gives many permutations of slab design with charts of span/depth ratios for typical characteristic loads. For the solid flat slab at the ECBP, with its imposed design load of 2.5 kN/m2, superimposed dead load from partitions etc. of 2.0 kN/m2, and 7.5 m column grid, the book indicates a solid slab thickness of about 275 mm. Thus the ECBP design at 250 mm thick was 10% thinner and would be expected to be more critical in flexure, particularly during early striking. A full discussion of the merits of the structural design of the ECBP is outside the scope of this Guide, but thinner slabs enable reductions in overall weight and hence foundations, reductions in height which in turn reduce cladding area and hence costs. In addition, lighter slabs result in lighter, less expensive formwork solutions.

Implications of intentional camber

At an early stage in the design, the PWD should consider whether the slab to be cast should incorporate a camber and decide if any intentional precamber should be left in the slab. The implications of this for the formwork need to be communicated to the constructor, and should appear in the temporary works design brief. Obviously many other factors will affect the deflected shape of the slabs as cast, such as order of casting, amount of reinforcement, state of finishes and partitions, etc., at the time. The determination of values for precambering is outside the scope of this Guide but inclusion of precambering requirements may affect the selection of the temporary works concept.


Bc9 Guide Kickerless construction

Pressure to maximise commercial building space, together with clients’ frequent requirements for large open areas, generally based on a 1.5 m partition module, nearly always dictate that perimeter columns and walls are flush with, or very close to, the outside edge of the slab. Up to the present, columns and walls have often been specified as ‘kickerless construction’ (see BCA Guide, Ref. 13)

I Bulk weight density is the overall weight per unit volume of a material, including a normal distribution of voids and pores, and in SI units is quoted in kg/m3. In everyday usage this term is frequently abbreviated to ‘density’ (which is strictly mass per unit volume). This Guide follows the recommendation in the ENV to use units of kN/m3 for density.

9


and the reinforcement detailed accordingly. For perimeter columns at the ECBP this was seen to have serious safety implications during construction and kickerless construction is not recommended for perimeter columns. The kicker provides a location for the perimeter column forms to prevent them being dislodged. See Figure 3 below and Figure 11 in Section 5.4.

Kickers for perimeter columns

perimeter columns.

Where a kicker is required, a minimum height of 100 mm is recommended2 (see NSCS, Ref. 7) with well-compacted concrete of the same strength as the adjoining concrete.

Minimum kicker lOOmm

Casting separate kickers to columns did not significantly delay construction as they were cast at the same time as the slab. Separate casting of kickers requires adequate supervision to ensure that the quality of a small volume of concrete is consistent with the structure.

Separate kickers not found to delay construction

Where high-strength concrete is specified for columns, and the slab is of lower strength, casting separate kickers can have construction implications. This is due to the difficulty in procuring and placing small quantities of high-strength concrete just for the kickers; it is generally not structurally acceptable for the kicker to be weaker than the main column. The TWC should reconsider the order of casting the columns and cast some kickers at the same time; preparing and casting kickers for perimeter columns while at the same time casting a few internal columns as ‘kickerless’ would allow a reasonable concrete delivery volume of the higher strength concrete.

To aid fast-track construction the use of drop heads andor column heads should be avoided, but if they have to be incorporated, round columns should have round heads and square columns should have square heads.

Avoid drop heads and column heads

The choice of surface finish to columns can have implications on the selection and economics of the formwork. For example, on a contract detailed with a few round columns of one size, cardboard one-use formers may be expected to be more economic than purpose-made circular forms in steel or GRP. If column heads are detailed, fixing reinforcement to the column becomes particularly difficult and a much more expensive column formwork arrangement is needed; thus the PWD can influence the cost to the client by careful selection of buildable details. Perimeter wall formwork without kickers is not likely to be a safety problem on the edge of the building. The TWD will have considered how to support the external formwork face, either by an external independent scaffold, or by a bracket attached to the previously cast wall below, to reduce the risks of instability of the external form face. Where kickerless construction is used on internal or perimeter wall formwork, the use of precast concrete blocks, as shown in Figure 13 in Section 5.5, has benefits in speed of Construction and in ensuring that the walls in the building are built directly over each other, a particular advantage for thin walls. See also Ref. 13. The PWD needs to be aware of the likely construction details when detailing the reinforcement to walls and columns, particularly when using reinforcement couplers. An obvious solution is to 2 For water-resistant construction, it is recommended that kickers are 150 mm high and cast monolithically with the slab (Ref. 7).

10

When using couplers consider position of lap

3.5Staircases

position the bar laps and any reinforcement couplers on the assumption that kickers will be used. As recommended earlier, the constructor should be involved in detailing the reinforcement. The method of fitting the vertical reinforcement bars should be considered, particularly as they have to be moved into a vertical position, held in place and then inserted and screwed into the coupler - this operation can be particularly difficult on perimeter columns and walls when no formwork is in position.

3.5 Staircases Economy begins at design

Construction economy begins at the design stage, not on site - this is particularly true for staircases, which are necessary in all buildings. The nature of stair flights, with varying riser and tread details, always makes them a strong candidate for precasting; precasting on-site can be done if only a few are needed. The optimum size of the precast flight depends on crane capacity, and ease of road transportation, among other factors. It is obvious that, once construction gets underway, feeding precast stair units down from the top of the completed frame can be difficult and time-consuming, so stair flights should be installed early during floor construction. This may not eliminate all ladder access, which may be needed for access to the very top level, but will allow easy access for operatives and staff to the working areas.

Stair flight design important for TW at early.stage of contract

The design of the stair flight can affect the temporary works. It is convenient to cast stairs in straight flights, possibly including landings at one or both ends. On a tall storey a half landing is invariably required. Initially, the ECBP was designed with an in situ stub column in the middle of the stair bay - this would have significantly affected the selection of formwork, and made the use of any flying or table form system impossible in those areas.

ECBP: innovative staircase

The final design incorporated an innovative staircase - two opposing straight flights connected with an intermediate half landing. The half landing was split into two, with half of the landing on each flight and a central joint with intersecting reinforcement. After placing the flights by crane onto temporary supports, the ‘joint’ was filled with Densit3, and the supports to the ‘Z’ stair struck out after a few days. Although the twin half landings glued with Densit were a novel idea, they did not permit the staircases to be used easily for access because of the amount of falsework backpropping down several levels of staircase! As construction moved upwards, so did the precast stair fixing, and so the backpropping interfering with the access also moved upwards.

Consider safety aspects

The PWD should highlight the safety aspects of precast staircase installation; in some cases the PWD might detail the connections, but the detailed method statement for installation should be prepared by the TWC and agreed with the PWD. Failure to allow for lateral movement while placing precast stairs has caused collapses, and attention to detail, particularly at connections, is vital.

Consider lateral forces when placing stair units at connections Backpropping of stair flights

The staircase design may have assumed temporary support and the PWD should communicate to the TWC the backpropping requirements for such work. Ideally, individual precast stair units should be designed to be supported on the adjacent floors and not on lower half landings. The TWC should establish a method statement with detailed procedures and agree it with the PWD before construction. Another solution is to design the precast staircase as an independent tower supported off a central square integral column, see Figure 4. The precast unit would be a section of square hollow column with a stair flight andor part landing attached on two sides. The next twisted half, complete with its central square half-floor-height column, would then seat on top of the lower column unit which has already been placed. The stair flight would then become substantially self-supporting and not require any further falsework for support. Additional reinforcement would be placed into the hollow square column, and concreted, possibly stitched into the floor slabs for stability during casting of the next set of columns andor slab.

‘Densit’, also known as ‘compact reinforced composite’ or ‘CRC Jointcast’, is a proprietary product that is mixed on site and used to ‘glue’ precast concrete components together. It is a very quick setting product made from a blend of Portland cement, microsilica, superplasticiser, fine aggregate (sand), water and 6% steel fibres. At ECBP it was batched in 37.5 litre quantities and gave compressive strengths of around I50 MPa.

3

11

3.5Staircases

Figure 4: Isometric view of possible staircase construction.

This solution, although it might increase the stairwell dimensions and reduce the client’s net lettable floor area, would shorten erection times, reduce maintenance at structural joints, and could be designed to give almost uninterrupted access to the construction using the stair flight. With careful scheduling of delivery, such stair units could be used with flying form and table systems if each unit was installed after the tables had been flown out.

3.6 Trimming of holes Commercial buildings often require large openings and holes for services such as air-conditioning ducts and lifts. The edge trimming of large openings needs detailed consideration by the PWD, particularly as thinner slabs become more common. Downstand beams are to be avoided for economic flat slab formwork; beam strips within the slab depth are preferred. These beam strips can be difficult for the PWD to accommodate within thin concrete slabs; structural steel sections may be required. At the ECBP, the PWD trimmed the large staircase opening with an upstand beam on the edge of the building, and it was expected that the constructor would precast this beam. The proprietary formwork suppliers had all assumed that a precast upstand beam would be used. The steel reinforcement was scheduled so the beam could be precast, with the reinforcement scheduled with the main slab. Restrictions on storage on site meant that reinforcement was procured by floors only as required, as is common on sites. With the exception of the roof construction, the steel reinforcement for these beams never arrived on site early enough for them to be precast! They were cast in situ, effectively as part of the main slab pour.

Avoid downstand beams Preference for beam strips

Consider separate scheduling for upstand beams

If items are expected to be precast, the PWD should consider scheduling them separately. This could also be advantageous with contractor detailing of reinforcement (see Ref. 9).

3.7 Effect of concrete type on temporary works Specifiers should be aware of the implications on surface finishes of using high-strength concrete. It can be difficult to vibrate, and obtaining a satisfactory surface against normal plywood on the ECBP was exacerbated by the inclusion of polypropylene fibres4 in the high-strength concrete on the first three floors. This was significantly harder to vibrate, and these columns have larger areas of blowholes.

4

Polypropylene fibres were added as part of the fire resistance research programme for the ECBP.

12

Surface finishes and high-strength concrete

3.8 Temporary works concept selection

The flowing concrete used at ECBP was a hazard

A flowing C30I37F concrete was specified for the fourth floor. As supplied, it was so heavily retarded that it became a hazard because the grout ran down the openings and between the plywood joints onto the lower floors. It covered the falsework support equipment at head level, jamming the jacks, which required extra fettling before reuse. Even three floors further down the building, the grout had still run down the openings and spread over the floors, making walking very difficult for several days. Fortunately no one was hurt, although several people slipped and were still slipping two days later when erecting column forms.

Flowing C30/37F difficult to cure and/or Protect

The flowing C30/37F concrete supplied was still so fluid on the late afternoon of placing that no curing or slab protection could be applied to the top surface. With a normal concrete, the first bay placed would have stiffened by the time the pour ended, allowing curing and protection to commence. Such extremely fluid concrete appears to be suitable on sites with all-weather protection, and in dry, temperate (not freezing) conditions where early curing and protection of the top surface is not critical. At the ECBP it was also very difficult to make satisfactory cubes and demould them for temperature-matched cube storage. These points should be taken into account when designing with a flowing concrete mix.

3.8 Temporary works concept selection There are many reasons for selecting a particular temporary works concept; the rationale for selecting one system in preference to another is a complex interaction between costs, building shape and the technology used. A useful guide to the selection of temporary works is BCA’s Advances in concrete construction technology (Ref. 14) which, although published in 1989, remains a useful review of fast-track techniques. The number of floors in the building, and the size, complexity and location of the site all affect the final concept chosen.

Temporary works account ’ of the cost for UP to 350o of concrete structures Direct influence of PWD on efficieno/ and economy

The industry has, in the past, considered ‘temporary works’ as entirely the constructor’s problem temporary supports and scaffolding are not Bill of Quantity items, so they are rarely measured. The only temporary works item measured is the formwork used for forming the surface face of the finished concrete. A comment often heard from PWDs was “Whether a few props are used or a sophisticated proprietary system is used, it has nothing to do with the PWD”. Fortunately, this attitude is changing and by considering the overall construction process in detail, savings are being generated. Clients and PWDs are not always aware that the temporary works (materials, labour and plant) account for as much as 35% of the cost of a concrete building frame (Ref. 2), split between materials and labour/plant. Typically, costs of labour and plant are three-quarters of the total cost of the temporary works. Encouraging PWDs to make design decisions that optimise the temporary works therefore has direct economic advantages to the client in that less expensive buildings can be built more quickly, efficiently and safely. All these ideas promote the Egan Report’s (Ref. 3) requirements of giving clients better value and good engineering. Since the late 1980s aluminium frame support systems have been increasingly used in the UK building industry. These systems are expensive to buy, but are regarded by many formwork subcontractors as capital investment and the ‘work-horse’ of the industry. In contrast, systems from continental Europe of simple adjustable props with beams on the top have started to be marketed in the UK. On economic grounds, not all sites can justify the selection of aluminium systems for slab support, and traditional steel skeletal systems still have a place in the industry.

ECBP Task 2 not involved in measuring production rates Two uses of equipment per system

In attempting to record the manhours involved in different falsework and formwork systems it is significant that several researchers on the ECBP project independently came to the same conclusion that it was very difficult to provide meaningful research on labour utilisation because of other factors. For example, there was no incentive to increase work rate, except to keep to the pre-set programme, thus “working hard” was seen to be similar to “standard rate” and provided the area was completed in time for the programme there was little discernible difference. It was not the intention of the Task 2 programme to identify production rates and tabulate the results, but the recording of the likely concept selection parameters was included in the Task objectives: These are outlined in Section 3.9. Each floor of the ECBP was to be cast in one pour, which meant that the formwork systems had to cover a complete floor of 1350 m2. Because they were generally used on two floors, only two uses per system were achieved. This provided a small learning curve for the three systems. The three generic systems used at the ECBP Cardington are discussed in detail in Annex A.

13

3.9 Types of temporaw works

Six main systems are used for flat slab construction:

1.

Individual props and beams

2.

Steel skeletal systems

3.

Table systems

4.

Flying form systems

5.

Precast concrete planks (participating)

6.

Precast concrete planks (non-participating).

To help clients, PWDs and constructors select suitable systems for individual contracts, the advantages and disadvantages of each type are listed below. Nearly all the falsework systems described can be built to permit a camber in the permanent work, although this may be easier to accommodate in the simpler arrangements. The lists are not exhaustive and individual site needs should be considered.

3.9.1 Individual props and beams, with proprietary panels or plywood as formwork Individual props and beams suit smaller contracts and, although labour-intensive, can be erected simply. They are particularly good for low height soffits, up to about 3 m, and where items have to be man-handled into position. As there are no units to 'make-up', they also suit one- and possibly two-storey structures with little repetition. A typical arrangement of the basic components, shown without any lateral stability, is in Figure 5.

"

Simple, good for low height one- and possibly two-storey

U

Figure 5: Typical individual props and proprietary beams. Note: Lateral stability not shown.

This system is ideal on slab areas with walls at opposite ends. This allows the props and beams to be stabilised in at least one direction from the walls of the existing structure.

Need stabilising from existing structure(s)

Use on structures with downstand beams is difficult, particularly in providing stability and continuity for strength. It can be difficult to provide external working platforms with downstand beams and an external scaffold system may be required. When proprietary panels are used as the soffit formwork, panel marks will always show on the soffit of the structure. It is unrealistic to expect the panels to be new, unless this is specified. Minor damage or blemishes on the panels will show on the resulting soffit. This may be acceptable, for example, if a false ceiling is to be fitted.

Panel marks will show

3.9.2 Steel skeletal systems, with beams and plywood as formwork The steel systems are generally based on scaffold systems, and operatives are usually familiar with their use and erection. These systems have more components than most other systems, but have the

14

Stable and rigid systems, during and after erection

3.9 Types of temporary works

Working platforms, access toeboards and guardrails not shown for clarity

Stop end at construction joint shown struck with bars for next slab projecting

I

Face material (shown as plywood sheets 1.22 x 2.44 m)

I

Falsework supports with diagonal bracing shown dotted

Figure 6: Typical arrangement with steel skeletal system. Note: Working platforms and intermediate members are not shown.

benefit of being extremely rigid both during and after erection. The vertical members are generally of 48.3 mm outside diameter and suit standard scaffold fittings. A typical arrangement on a building slab is shown in isometric view in Figure 6. For clarity, the working platforms have not been shown. Suit complex shapes, heights, bay size changes etc

The skeletal systems are much more adaptable than the other concepts, because of the variety of vertical and horizontal components available ‘off the shelf‘. They are therefore ideally suited for complex shapes, and where storey heights and bay sizes vary. They generally have jacking, bracing, platforms, decking, etc., as standard. The head assemblies suit both aluminium and timber bearers.

Generally designed as freestanding falsework

Skeletal systems are usually designed by the TWD as freestanding falsework structures with the lateral forces transmitted through the system’s bracing into suitable restraints, generally by friction on the supporting floor or slab. Because they are unlikely to corrode they are suitable for use in chemical works, heavy industrial plants, etc., and can also be used in construction in marine environments.

Cranage not essential

Cranage is generally not needed for erecting and dismantling so they are suitable for use in exposed or very high locations in the UK, where cranes would not be allowed to work for significant parts of the year due to high winds, rain etc. The greater weight of the systems makes them more stable.

Advantage - robust Disadvantage - labour intensive

Although they have the disadvantage that they are labour intensive to erect and dismantle, they are very robust, and because so few decisions are needed to put them together as intended, less skill is needed.

3.9.3 Table systems Handled as tables, say, 12 m long max.

Table systems usually comprise large-diameter aluminium props (typical diameters are 100 to 150 mm), with long threaded lengths, and connected together with frames and cross-bracing. The stiffness of the head of the assembly is derived from long lengths of aluminium primary beams. These tend to limit the lengths of table handled to 12 m with standard components, but longer lengths are possible. A typical example of a table, complete with its plywood decking, is shown in Figure 7.

Ideal for flat slabs

Most UK frame contractors use table systems for flat slab construction, generally with aluminium beams in both directions at the top.

15


I

a

*

Figure 7 : Typical aluminium frame table with decking.

Tables have the benefit that, once made up, they enable rapid construction, and the benefits increase with repetitive use. They can be used with cross-wall construction or columns, but are most economic when there is access to opposite faces of the building for direct removal of the tables. The operation of flying out the table form from a floor slab may cause it to pass outside the site, and building boundaries, i.e. over adjacent property. The over-flying of other properties is an essential planning consideration. They suit flat slab and repetitive construction. Table systems become economic at over eight uses, but because many frame contractors own aluminium systems, they will use the system with fewer repeats as conventional falsework. These systems can be used successfully for ‘strip and re-erect’ for fewer uses. Cycle times as short as four days have been achieved with careful planning.

Crane access to two opposing sides best

Consider over-flying of adjacent properties Economy - over eight uses

Four-day cycle possible

Suppliers generally assume that aluminium table systems are restrained at head level by the permanent works, and that no horizontal loads are applied to the falsework tables. The TWD should clarify with the PWD that the permanent works can safely transmit such loads. An example of the applied horizontal load is the minimum lateral stability force of 2%% of the applied vertical loads from Clause 6.4.4.1 of BS 5975 (Ref. 6).

Horizontal loads taken by the permanent works, not the table

Tables need space to be flown out of the building (to one or both sides), with allowance made for clearance between the building and adjacent structures or objects and to allow for cantilever access platforms. A minimum end allowance of 500 mm is recommended from the face of the building. To the sides of each table a minimum clearance to columns and walls of 40 mm per side should be allowed, so some infill support is necessary at arrises when used with cross-wall construction.

Clearance: end 500mm side 40mm

Methods of handling tables are discussed in Section 4.2.3. Where tables are handled with fourlegged slings, as the table starts to move outside the edge of the building, the first two slings can be attached. At this stage the inner two crane positions are ‘hidden’ under the cast slab. Depending on the procedure used, the table may need to be ‘rested’, i.e. with part projecting out of the building, and possibly imparting an upward load on the edge of the recently cast slab. The table has to be manoeuvred further out of the building before the rear slings can be fitted. Usually these are fitted through simple trap doors left in the sofit form. These will show as squares about 300 x 300 mm on the sofit. Such trap doors are not needed when tables are flown using a purpose made C-hook that fits underneath the sofit formwork. The design of buildings with downstand beams on the front perimeter needs careful consideration. This is because the support system for the edge forms and platforms uses the bearers projecting past the building edge. A downstand beam prevents these beams projecting, and makes the arrangement discontinuous. Altering the edge make-up of the tables affects the crane handling properties. Care may be needed when certain aluminium alloys are used for long periods near seawater.

16

Trap doors in soffit form

Care with edge beams


3.9.4 Flying form systems Flying forms are large areas of formwork, generally larger than the table systems described above. Unlike tables, which are made up from vertical members like props, flying forms comprise two long deep steel or aluminium trusses. These create a much stiffer and stronger unit and are used to ‘fly’ the units out of the building, preferably from one side. Seated on top are conventional aluminium secondary bearers and either plywood sheeting or trough/waffle formers.

Clearance: end 500 mm side 40 mm

Flying form systems ideally suit flat slab and repetitive construction. They are most economic at over ten uses. There must be enough room to fly the entire unit beyond the building line and then back in again, with allowance made for clearance from the building to adjacent structures and objects. A minimum end allowance of 500 mm from the face of the building is recommended. Hence flying forms ideally need clearances from 12 to 25 m from the building line. To the sides of each flying form a minimum clearance to columns and walls of 40 mm per side should be allowed to permit safe movement when ‘flying’ and operating the system. When such systems are used with cross-wall construction some areas of make-up will be required between the edge of the table sheeting and the top of the cross-wall, and some infill support is necessary at arrises.

Preferably no internal beams

The design of buildings with downstand beams needs careful consideration. Design of flying form trusses means that downstand beams up to, say, 500 mm deep can often be accommodated with these systems on the perimeter of the building. Specialist flying form systems are available for internal drop beams.

Four-day cycle possible

The expected cycle time with flying forms is four days, floor to floor, though shorter times have been achieved in the Far East with careful planning and design of the forms.

3.9.5 Precast concrete planks (participating)

Generally one-way spanning

Precast concrete planks are usually 50 to 75 mm-thick and are used as permanent soffit formwork. The planks are reinforced and the bottom reinforcement becomes the permanent reinforcement to the slab. The in situ concrete topping to the planks is designed to act compositely with the planks. Traditionally designed for one-way spanning applications, variations have been manufactured for two-way spanning slabs. For two-way spanning planks, reinforcement must be provided in two directions in the bottom mat, i.e. within the plank; this was one of the innovations at ECBP. The side ‘lap’ of the planks to provide reinforcement continuity requires the bars from adjacent precast planks to be intermeshed and the gap to be filled with a ‘structural concrete stitch’ to provide bond. A proprietary product (‘Densit’) was used at the ECBP to provide this. See also Section 5.7.

Suited to congested sites

See Sections 4.2 and 5.7

The planks are manufactured in widths to suit transportation, usually up to a maximum of 2.4 m, and in lengths to suit the building. Such systems are particularly suited to congested sites as the planks can be delivered on a ‘just-in-time’ basis to avoid on-site storage. See Figure 8. The planks generally have projecting lattice top steelwork. This increases their ability to span in the temporary condition between the falsework supports, before the in situ structural topping concrete is placed. The planks are usually designed to be self-supporting to span about 2 m between the falsework. This is discussed further at Sections 4.2 (e) and 5.7. The floor planks used in buildings will span one way up to 10 m.

CIRIA Report Permanent formwork in construction

Falsework may be needed PWD to consider column details

A logical development for precast panels as permanent formwork is the use of fibre cement products, possibly to produce a fully participating structural form which would improve both the strength and durability of the concrete. For information on the commercial considerations of permanent formwork, see the CIRINConcrete Society Publication C558 Permanent formwork in construction (Ref. 15).

When using precast planks, falsework may be required to support the planks during assembly and during the placing of the topping concrete, see Section 5.7. Some propping may be needed on spans greater than 3 m. The PWD should consider the detailing at the columns and the construction tolerances at an early stage of design.

17


4L

3.9.6

Figure 8: Typical aluminium table with precast plank system. Note: TWD must consider stability of such assemblies.

Permanent concrete planks (non-participating)

Concrete planks only 30 mm thick have been used as permanent formwork as face contact materials to replace conventional materials, such as plywood. The thin planks will not span over long distances without unacceptable deflections, and reinforcement lattice girders are incorporated to allow them to be placed over falsework supports at about 2 m centres. Unlike the planks discussed at Section 3.9.5, these thin planks are not considered to contribute to the strength of the slab, and require normal bottom mat steel reinforcement to be fixed. Similarly, handling is limited by vehicle width to about 2.4 m and 10 m in length. The thin planks are easier to modify at columns and other features. Such systems are particularly suited to congested sites as the planks can be delivered on a ‘just-intime’ basis to avoid on-site storage. The planks generally have projecting lattice top steelwork to facilitate their temporary spanning condition on falsework, before the in situ structural topping concrete is placed. The planks are designed to be self-supporting to span about 2 m between the falsework, see Section 4.2.5.

Falsework supports at say 2 m centres

These planks suit two-way spanning slabs, as the main reinforcement is not part of the plank. Care is needed to ensure that the bottom steel transverse to the planks can be fitted through the lattice of the planks.

Fitting steel through lattice

The thin precast concrete floor planks used in buildings will accommodate spans up to 10 m. Other materials are available as permanent formwork; for more information on the applications and commercial considerations of permanent formwork, see the CIRINConcrete Society Publication 558 Permanentformwork in construction (Ref. 15).

Suit up to 10 m

CIRIA Report Permanent formwork in construction

3.I0 External protection and encioswre considerations The safety of all persons, including operatives and the public, is a primary requirement in all construction operations. Although construction at the ECBP was intended to be carried out without external scaffolding it was observed that, even with experienced operatives working in controlled conditions and without any weather constraints (it was indoors!), objects fell from the edge of the building. Primary responsibility for edge protection is with the constructor, although ideally the constructor should discuss this with the PWD and client to reduce risk at an early stage in the contract, as the initial concept can affect the protection requirements. The Construction (Health, Safety and

18

Edge protection

3.10 External protection and enclosure considerations

Welfare) C(HSW) Regulations (Ref. 16) specifically refer to the safety of “any person”. External protection for all persons should have been considered in the design risk assessment. A useful guide is the HSE book Protecting the public -your next move (Ref. 17). Safety nets as leading edge protection

Fit anchorages early in contract

External protection vs fans Client‘s duty

Health issues of complete enclosure vs economy, speed, improved quality

Research needs Edge protection using precast units difficult

Edge protection at leading edges can be provided with safety nets. In order to reduce the cost of safety nets, suitable provision should be made for their rigging, such as providing anchorage points in the columns and walls. This need not be an onerous extra cost to the structure, because, under CDM requirements, provision of suitable anchorages for long-term maintenance of the structure is often required. Planning for the location and fitting of the anchorages early in the contract can have significant benefits to overall safety and productivity. The location and the temporary works concept for a structure can affect the type of protection that is possible. For example, a full external sheeted scaffolding is not possible with a flying form or table form system as it would interfere with the form removal, so a protection fan, suitably designed and of sufficient strength, would be the preferred solution. Timing of the installation of the fan can be critical. The client has the duty to ensure that adequate resources have been allocated for safety, and for any necessary permits, for example, for debris fans oversailing the pavement. This will involve obtaining approval in advance. Although the Planning Supervisor can advise, if asked, the duty remains with the client. Where external protection is to be used, consideration should be given to the complete temporary enclosure of the building during construction. This will improve workmanship and quality of work by making most operations independent of the weather. Improved working conditions on construction sites through such means have benefits on safety, efficiency, recruitment and operator satisfaction, so moves to improve working environments can only be for the good. There are however health issues to be addressed when considering enclosures, such as dust, fumes, heat, solvents and ventilation. Tightly woven containment nets have been shown to be effective in trapping concrete dust. Elaborate enclosures have been developed in Germany and Japan, some with facilities for travelling overhead cranes so that they can use more sophisticated construction methods and achieve high quality control. Although popular in colder parts of Europe, such enclosure systems tend only to be considered in the UK for renovation work. Total enclosure may require large-span temporary roof systems and new products have been developed specifically for this type of construction work with clear spans up to 44 m being achievable with ‘standard’ proprietary equipment. Particular attention should be paid to safe lifting in erection and dismantling. The steering group for this Guide believe that there are significant benefits in using temporary cladding and enclosures, as well as the obvious one of being able to continue working safely and efficiently in bad weather, but that more research on the subject is required. At the ECBP an alternative construction method, using precast units for the roof, had been expected to overcome many of the problems of edge protection. However, edge protection on two sides of the falsework was difficult to provide. This was because of the orientation of the primary beams on the falsework which only provided cantilevered support to an external platform on two sides of the building - providing access on the other two sides was difficult.

The Task 2 research report states “Placing planks at the leading edge is hazardous and risks can only be partly reduced through the use of safety harnesses.” This is discussed further in Section 4.2.5. Provision of edge protection to precast flooring differs from that on in situ slab construction and should be considered at an early stage in the temporary works planning. Likely solutions are: provision of an external scaffold suitably sheeted at all levels of construction; incorporation of cantilevered edge protection into the supporting falsework framework for all exposed edges; or attachment of protection fans or nets to a lower level. The C(HSW) Regulations (Ref. 16) clearly require the hierarchy of safety for providing protection from falls to be considered. All construction should be safe, and although shorter construction times are demanded, with less dependence on temporary works, and possibly eliminating external scaffolding, the chosen system still requires a safe method of work. Minimum Class 6 medium duty fan 1.0 kN/mZ

For reinforced concrete slab construction a fan or net should be considered with a maximum loading equivalent of 1.O kN/mZ.Such a fan is classified as Class B medium duty to BS 5973 (Ref. 18).

19

3.10 Drternal protection and enclosure considerations

Development of simple edge protection systems with guardrails and toeboards for placing on the cast slab, with or without incorporation of a protection fan, are recommended. Some proprietary systems are already available, but PWDs and clients should think about the provision of edge protection during construction. PWDs have responsibilities under the CDM Regulations to ensure that their designs can be built safely. The incorporation of standard fixing details into slabs designed to transmit safely the loads from edge protection may be a suitable approach, similar to the way that anchorage points are fitted to structures for connecting safety harnesses.

20

PWDs and clients to consider anchorage points

4 Preconstruction planning stage Recommendations - Preconstruction planning stage Management and procedures 1.

2.

3.

Temporary Works Co-ordinators (TWC) should be appointed for all building construction involving formwork and falsework. The TWC should be from the organisation building the concrete frame, and must have suitable technical knowledge and appropriate training to understand the implications of any changes to the TW schemes, and the effects on the structure of any changes in the construction sequence, such as backpropping and striking. A procedure for striking and removing falsework and backpropping should be agreed and followed. The subject should be on the agenda of every construction progress site meeting. Temporary works training at both technical and supervisory levels is needed in the industry.

4.

Procedures for documents such as the Permit to load the falsework, and Permit to strike the formwork/falsework must be in written format: verbal procedures are not adequate. 5. The TW design brief should specify the performance of the falsework. 6. The TW design brief should specifically request enough copies of drawings for the parties involved in all aspects of the temporary works. Equipment procurement 7.

8.

9.

10. 11.

Working platforms and edge protection should be shown in all temporary works schemes. Even if they are not part of the supply and so ‘by others”, they should have been considered and shown. Calculations for temporary works should be available, prepared either by the TWD or by the proprietary supplier, and should form part of the suppliers‘ contract to supply equipment. On all areas of manually erected formwork, where the primary and/or secondary beams are to be struck out, handled and re-erected by hand, the length of beams used in the design should be carefully considered by the TWD to facilitate handling. When using flying tables, TWDs and TWCs should consider edge tables carefully, as their correct design can significantly affect the process and productivity. For safety and ease of adjustment during erection and subsequent striking, the level of the handle on telescopic props should be considered.

Column equipment procurement

12. Suitable working platForms and edge protection should be included, preferably on all sides, together with safe means of providing stability. 13. All column formwork assemblies should incorporate crane lifting points. The weight of the form should be known. Continues overlea$..

21

4.1 Management of the ternporav works

Column equipment procurement (continued) 14. Fixings cast into slabs for stabilising column forms should be designed to reduce tripping hazards. 15. Column forms should incorporate access by a vertical ladder, preferably with hoops for extra safety, and with a suitable trap door detail for access onto the working platform. Precast concrete 16. The interface of responsibilities when supplying precast planks should be established very early during procurement. Precast planks will normally require falsework. The position of support (or props) required should be shown on the precast panel layout drawing, together with any requirements for post-propping. 17. Tolerances between in situ concrete and precast units should be agreed be tween the PWD, TWD and precast supplier before construction.

4.1 MaonagemenU off Uhe 8emporaG-y works The appointment of an individual, the Falsework Co-ordinator, is recommended in the code of practice on falsework, BS 5975 (Ref. 6), and was one of the principal recommendations in the 1976 Bragg Report (Ref. 19). The Co-ordinator has specific roles, not only during the planning and design stages, but throughout the construction. It is recommended that a Temporary Works Co-ordinator (TWC) is appointed early in the planning process to co-ordinate and manage the design, procurement and operations of the temporary works. There has been a trend for organisations and individuals to define the boundaries of their responsibilities and to refuse to undertake roles in areas they believe to be outside these boundaries. One consequence is that collective responsibility for health and safety issues may remain ‘in the back of the mind’. However, when a main constructor (usually the principal contractor) employs a frame contractor as a subcontractor, overall responsibility for safety remains with the main constructor, although other contractual responsibilities may be passed on to the subcontractor. The frame contractor will be under the management of the main contractor and will have to liaise with the latter’s technical departments, and possibly with the appointed TWC.

Nominate a Temporary Works Co-ordinator at very early stage

Main constructor has overall responsibility for safety

Management of temporary works is sometimes considered to be about minimising formwork costs, reducing the number of components, etc. However, it is often more efficient and economical for the project as a whole if construction time is reduced by increasing the quantity of formwork equipment, plant and labour. An example from ECBP is discussed in Section 4.6 where increasing the number of column forms would have resulted in a shorter cycle.

Reducing the quantity of formwork can be counter-productive

It is recommended that, for the construction of the concrete frame, frame constructors appoint an individual to handle their temporary works co-ordination who should work closely with the main contractor and its TWC. It is recommended that the frame contractor’s co-ordinator is directly employed by that organisation.

Frame constructor to appoint own TWC

The appointment of the TWC under design and build contracts appears to be straightforward, and is normally made early in the planning stage. However, in design and build joint ventures the frame contractor will probably appoint its own co-ordinator. It is recommended that all co-ordinators are involved as early as possible with the PWD to assist in design decisions, as there are significant advantages to the PWD in early liaison. It is important to note that, under the CDM Regulations, any temporary works co-ordinator is considered to be a “designer”. Under the law a designer is required to have sufficient competence and resources for health and safety to undertake the role satisfactorily. Since frame constructors have their own responsibilities for safety during the erection of the building frame, it is crucially important that their own co-ordinator is in a position to control the appropriate activities on the site.

TWC are ‘designers‘ (CDM Reg 13)

As already discussed, this Guide recommends more efficient management of temporary works and that each constructor should take more responsibility, i.e. take on the role of the temporary works co-ordinator, whether or not the main contractor has made their own arrangements.

Frame constructor’s TWC to be in a position to control activities

22

4.1 Manaqement of the temporary works

TWC to be trained

Analysis of the management of the temporary works at the ECBP demonstrated the importance of the requirement in the CDM Regulations that the TWC has suitable technical knowledge and appropriate training. This ensures that the TWC understands the implications of any changes proposed by site staff or by proprietary equipment suppliers. Such proposed changes are discussed below, in Section 4.4.7 and also in Chapter 5. Managing health and safety matters successfully on a project involves many activities - the appointment of a principal contractor, risk assessments, method statements, a Health and Safety Plan for the pre-tender and construction phase, and so on. The temporary works should play an important role in the documents and be considered at an early stage. A typical flow chart for the management of temporary works is shown in Figure 2. (Important documents for reference are Refs 16, 17 and 34.)

Suppliers acting as designers have responsibilities

Calculations should form part of the final contract, but not necessarily be submitted at tender stage

Standard solutions

‘Custom and practice’ must be inherently safe

Reduction in skill level and TW knowledge on site requires greater emphasis on training and competence of TWCs

The supplier and designer of a precast permanent formwork system that requires falsework support by others have an obligation to ensure that it can be safely erected and that the constructor, spec-ifically the TWC, has enough information to co-ordinate the technical aspects of the safe use of the system. The interface of responsibility should be established between the precast plank supplier, the PWD and the TWD at a very early stage of procurement, and proper control exercised. Suppliers often act as ‘designers’, whether planned or not, and need to be aware of their responsibilities (see also Section 5.7). The method of internal support must take into account the location of the reinforcement, allowances for supports at joints, tolerances, etc. At the ECBP no calculations were submitted to verify the adequacy of the falsework. None of the proprietary system suppliers were asked for calculations to verify the systems adopted and presumably assumed that custom and practice would be acceptable, supported by tables of ‘standard solutions’, based originally on their standard calculations. It has not been ascertained whether they undertook separate calculations. It may be that the TWC did not request calculations to verify the structural adequacy and safety of the system because of the unusual nature of the contract. It is recommended that calculations should be submitted, which should form part of the proprietary equipment suppliers’ contract to supply equipment. There are valid commercial reasons why it should not be a requirement to submit calculations at tender stages. Designers have a duty under the CDM Regulations (Ref. 4) to take account of hazards and risks, and constructors have a responsibility under the C(HSW) Regulations (Ref. 16) to ensure that the structure is stable and safe - a structure in this context specifically includes the temporary works. Other checks may also be required, such as under the organisation’s internal quality assurance procedures, and some major clients require contractually independent checks on temporary works. The merit of calculations is that in all cases they provide an ‘auditable trail’ in the event of any incident: ‘standard calculations’ for ‘standard solutions’ should be available to verify the adequacy of the ‘standard’ temporary works. What are the legal implications of not carrying out any design verification or calculations? Obviously, as the height of the falsework and the mass of the slab to be supported increase, the magnitude of loading and imperfections increases. This is accepted in the draft EuroNorm prEN12812 (Ref. 20) for falsework where falsework is accepted for simple structures to be erected to low heights as ‘standard’. Thus custom and accepted good practice, provided they are proven, are acceptable. However, the first floor at the ECBP was over 4 m from floor to soffit - hardly standard floor height! Any ‘custom and practice’ solution must be inherently safe before it is accepted - otherwise such practice should never be allowed or condoned. On a conventional building contract, the main constructor ensures that temporary works advice and suitable technical information are presented correctly. Fewer building contractors operate temporary works design offices and the skill base among operatives is also reducing, with fewer apprenticeships and less training. It is already difficult to find experienced staff who can understand and prepare meaningful temporary works calculations. The main source of information for most builders will be suppliers and specialist consultants. A few large companies, generally involved in civil engineering, still operate temporary works design offices, but for most builders the temporary works skill base is from the suppliers, who design, verify and then supply their own schemes.

This is to avoid calculations produced by one company being used as the basis of supply of a similar system by others, at lower commercial rates! 5

23

4.1 Management of the temporary works

So as frame constructors take on more responsibility for temporary works, they also have to become more technically aware. This means that the level of training of TWCs should be increased so they can understand and check the quality and competence of supplier’s designs - after all, as designers, they have significant responsibilities under the CDM Regulations. The current transition from permissible stress to limit state design methods for temporary works may initially cause some confusion. The proprietary suppliers are already aware of the new design methods and await publication of the EuroNorms such as prEN12812 draft (Ref. 20) in final form. All that they require is to verify their ‘standard solution’ designs for the majority of the standard floor slabs and heights envisaged.

Awareness of permissible stress vs limit state

The reduction in skill level is already apparent. Consider the proposed European performance standard on falsework - had the proprietary suppliers at ECBP submitted second-order limit-state calculations to the frame constructor, in accordance with prEN12812 (Ref. 20), few of the constructor’s staff could have assessed their accuracy! EuroNorms will formalise the specification of temporary works, e.g. falsework and scaffolding, in a similar way to the introduction in the UK of the National structural concrete specification (Ref. 7). At the ECBP, the quality of the proprietary suppliers’ proposal documents varied significantly, from full A0 drawings, through coloured A3 drawings to simple A4 black and white sketches. The coloured drawings could not be photocopied easily, whereas the A4 sketch was regarded as the most informative because it could be copied and issued easily to several operatives at a time. It is recommended that the supplier andor TWD should be instructed in the TW design brief to supply enough copies of appropriately sized working drawings to eliminate the need for on-site copying. The project specification in the National structural concrete specijication (NSCS) (Ref. 7) suggests default values for the number of copies of drawings at tender, acceptance and construction stages.

Performance requirements

Ensure enough copies of drawings are available Suggested default values in NSCS

4.2 The temporary works design brief At the pre-construction planning stage the TWC should prepare the design brief for the temporary works. A sample brief is at Annex B. The brief should include relevant information from the PWD, material to be supplied by the constructor, on-site guidance and concept suggestions and it will state the design loadings to allow the TWD to prepare a realistic design.

Temporary works design brief: see Annex B

This Guide discusses aspects to be considered by the TWD and suppliers when preparing the TW scheme and when procuring equipment. Many of these aspects are obvious, but even under the supposedly controlled environment at the ECBP, items were forgotten, not included, or not known to make a difference when changed! It is worth noting that BS 5975, the code of practice for falsework (Ref. 6 ) , includes procedures for controlling the temporary works and recommends the preparation of design briefs. The generic systems discussed in Section 3.9 for the PWD and client concept are repeated in the following sections, but from the constructor’s viewpoint in planning the temporary works. 4.2.1 Individual props and beams, with proprietary panels or plywood as formwork Stability of individual prop and beam systems requires careful control at all stages of erection. Unless there are adequate walls or columns alternative provision for stability such as bracing should be provided. Suppliers’ procedures should be followed, and detailed method statements established. Several arrangements do not include diagonal components for bracing, on the assumption that lateral stability is provided by the stiffness of the soffit formwork, once it has been erected (see Figure 5). If there are walls on several sides of the slab being constructed, then the formwork can be ‘wedged’ between them, but if there are only a few columns, stability needs to be carefully considered.

Need ‘wedging’ against works if not stable as freestanding structure

A critical stage identified at ECBP was the placing of secondary beams and fitting sheets of plywood (2.44 x 1.22 m) by working overhand. Suitable working platforms must be established during erection and use and for access to the sofit members during removal. The proximity of the building edge is discussed in Section 4.4.3.

24

Access for operatives to carry out striking?

4.2 The temporary works design brief

Prop handle level

The system is ideal for transportation because the props and beams pack flat onto pallets. Adjusting prop heights is usually easy, but on long props, the handle may be difficult to reach and adjust. This system can fit any shape or plan area of building, and is often used by suppliers on complicated schemes for the make-up areas around the edges of other systems and at awkward shapes.

BS EN 1065

The European Standard for adjustable steel props, BS EN 1065 (Ref. 21), has introduced five classes of prop with a total of 32 different props. Although only the nominal characteristic strengths of the props are stated, an annex or UK National Application Document may in the future give guidance on safe working loads for the more common prop sizes.

Proprietary beam prEN 13377

Four types of proprietary timber beam used in soffit formwork are the subject of a proposed European Standard prEN 13377, published for comment in 1998 (Ref. 22). Because the main propping components are so versatile, they can be used on other parts of a contract, possibly by other subcontractors, so local losses of equipment can be a problem with these systems.

4.2.2 Steel skeletal systems, with beams and plywood as formwork Inherently very stable. See Figure 6

These systems are inherently very stable and can withstand significant mistreatment before they may need down-rating. A typical arrangement is shown in Figure 6 in Section 3.9.2. The systems are ideal for transportation because they pack flat onto pallets of standards and beams. They are usually steel, and are easy to adjust in height.

Working platforms easily aCCmmodated

Working platforms can be accommodated with these systems at any position in the soffit area - a particular benefit for taller slab soffits. Further, limiting one of the ledger dimensions to suit the system’s staging gives simple access provision anywhere under the sofit, just by moving the staging. Note that all sides and ends of working platforms need edge protection.

Can be erected by ‘semi-skilled’ operatives

Because of their modular arrangement, and the strength of the joints when they are connected, the basic structure can usually be erected by semi-skilled operatives, unlike table forms which need skilled handling. These systems only require cranage periodically to move pallets of components, so striking and repositioning operations can be carried out to suit progress and are not determined by the availability of crane time - an important benefit. Although a crane can help with moving bundles of beams, etc., it is not essential, and so skeletal systems are not sensitive to wind conditions. Training operatives in crane control is not as critical as with, for example, flying form systems. Manual handling of long beams is covered in Section 4.4.6.

Economics of aluminium

For the small constructor, it is rarely economic to hire an aluminium system for completing a large flat slab system. Because the loads on flat slabs are relatively small, the bearers can span further between supports, making the use of aluminium technically effective. However, when commercial hire rates and the initial cost of aluminium material are taken into account, it is sometimes more economic to purchase conventional timber bearers. Frame constructors often invest in aluminium alloy systems, because it becomes economic to use their own equipment, particularly as bearers on steel skeletal falsework systems.

4.2.3 Table systems Detailed method statements required, trained slingers/ signallers etc

Operating table systems needs careful control. Operating, striking, handling and moving them requires a detailed method statement, and constant on-site supervision. Generally, table operation requires a higher standard of operative, with trained slingers/signallers for crane control, etc.

Table size and movement method

The size of table handled is a function of the building shape, the safe reach of the crane and the method of operation. Tables are moved either with a large purpose-made C-hook or with fourlegged unequal length slings, preferably with compensating clutches. The supplier must provide guidance to the constructor on the methods and equipment needed to operate the system safely.

Consider crane reach when replacing tables on next floor

Large table areas up to 85 mz using standard components are possible but may be limited by crane capacity. Note that the crane has to reach to place the table back onto the next floor, possibly operating on a larger radius than when first connected to the table.

25

4.2 The temporaty works design brief

Allowance should be made in the programme for assembling the tables before first use. If assembly is to be carried out on site, enough working space and cranage must be provided.

Allow in planning for assembly time

Infill panels between tables and columns will be needed and the ease of erecting and striking them must be considered. They have to be removed before the tables can be moved. If the TWD has assumed that the sofit formwork will provide lateral restraint to the table, the infill panels may be required to transmit lateral forces, in tension andor compression, to the permanent works.

Infills may be required to transmit lateral forces

Trolleys for moving tables should be of the fixed type and secure. Some systems involve placing small wheel assemblies under individual legs. If these are not secured to the legs they will come off when the table is moved clear of the slab.

Trolleys to be prevented from falling

One method of moving tables with trolleys uses fixed length slings; the table unit is first struck and then rolled out of the building into space by a third of its length. The front pair of slings is attached. The table is rolled out further on the trolley, leaving the inner wheels on the slab, and gently lowered so that the centre ring of the four slings is in line with the centre of gravity projected normally from the formwork. At this point the system will be ‘rested’ on the slab above. The other slings are attached and the table unit removed completely.

Fixed length slings

A better method uses unequal length chains, connected at one-third and two-thirds of the length of the unit, which is rolled out in stages. These compensating chains allow units to be slung horizontally, making them easier to reposition at the next casting location. Note that the connection of the inner sling positions may, depending on the timing and crane availability, require the table to be ‘rested’ on the upper slab. The method statement should clearly describe the procedure. As discussed in Section 5.8, extracting tables without suitable control and allowing them to tip and ‘rest’ on the slab after the centre of gravity has passed the edge of the building is deprecated.

Unequal slings preferred

Tipping of tables and resting on underside of cast slab not under crane control is deprecated

Flying forms and tables can accommodate downstand beams but it is necessary to check that the entire truss assembly, when struck, can drop down enough to come out from beneath the downstand. Safety is of paramount concern, and although table forms often have working platforms attached at the top level, there will be open edges to guard during and after removal from a floor. Edge protection and other safety measures, such as harnesses, should be considered during striking at the floor level being struck, and possibly at the level above where the forms are being positioned. Protection is particularly important if the slab guardrails have to be removed in order to position the table forms. Edge or fall protection at ground floor level during the initial assembly of the formwork is rarely provided, but should be considered.

Open edges and edge protedion

Clause 75 of the Approved Code of Practice to the lifting regulations (Ref. 23) states: “Where access to a working place on the lifting equipment necessarily results in removal of edge protection and exposure of an unguarded edge, as little edge protection as possible should be removed, and should be replaced as soon as possible”.

Removal of edge protection

This clearly includes the operation of handling the table with operatives both above or below the slab just cast. The accuracy of the initial assembly of the units can have a direct effect on overall production.

Accuracy of initial assembly

4.2.4 Flying form systems

The comments in Section 4.2.3 for tables equally apply to flying forms, but because they are larger, they require more careful handling. Typical areas handled are 100 m2, with lengths up to 25 m. Flying forms and tables spanning the full width of a building have significant advantages because they can be moved faster, eliminate the end-to-end joining necessary with smaller tables, and have fewer uprights, thereby allowing faster erection and striking. The weight of the larger units means that a larger crane, possibly with greater reach, may be necessary to fly them successfully out of buildings. The crane should not have a luffing jib, as the forms should come out smoothly horizontally. Flying forms and tables can accommodate downstand beams but it is necessary to check that the entire truss assembly, when struck, can drop down enough to come out from beneath the downstand.

26

Detailed method statements required, trained slingers or signallers etc

Careful crane selection


Safety is of paramount concern, and although the forms will have working platforms attached at the top level, there will be open edges to guard during and after removal from a floor. Edge protection and other safety measures, such as harnesses, should be considered during striking at the floor level being struck, and at the level above where the forms are being positioned. Protection is especially necessary if the flying forms cantilever over the slab edge. Removal of edge protection: see Clause 75 of ACOP in Section 4.2.3 Supplier’s method statement

Clause 75 of the Approved Code of Practice to the lifting regulations (Ref. 23) refers to temporary removal of edge protection and is reproduced in Section 4.2.3 above. This clearly includes the operation of handling the flying form with operatives above or below the slab just cast. Always refer to the supplier’s method statement for the operation of flying forms.

4.2.5 Precast concrete planks (participating) Early involvement with PWD

With precast plank systems the importance of early involvement between the PWD, supplier and TWC has already been stressed in Section 4.1. Temporary works aspects during construction are discussed in Section 5.7.

Tolerances important . See CIRINCS guide on permanent formwork

Where planks are simply spanning between walls, the constructor should ensure that the tolerances on the wall and the plank manufacture are suitable. The bearing width at the plank seating will be affected by the tolerances. For building work, the normal tolerances are given in the joint CIRINConcrete Society publication Permanent formwork in construction (Ref. 15). The width of seating also depends on the material of the supporting structure. On long spans, say over 3 m, intermediate propping may be required to carry steel fixing loads and loads from the topping concrete.

Intermediate propping?

Lifting frames on long lengths

Handling precast planks requires careful control, and for planks over 6 m long, purpose-made lifting frames will probably be needed. The manufacturer’s advice should be followed.

Stability of falsework when precast planks not seated on permanent work

Where the precast planks are not seated directly onto the permanent works, such as on slabs with columns and on long spans requiring jointing, intermediate falsework supports are required (see also Section 4.1). The falsework arrangement for precast planks is significantly different from those previously discussed in several ways: the planks generally incorporate lattice reinforcement to allow them to span in the temporary condition up to, say, 2 m. The falsework will usually be detailed by the supplier to show the planks supported at about 2 m centres, transverse to the span of the units. Thus the initial erection of the falsework is a skeletal frame arrangement with stiff connecting bearersheams in one direction, and without full transverse stability at the head in the direction at right angles. Further, stability in this direction relies on friction from the precast unit, and is, of course, only effective once the unit has been placed.

Working platforms

When a single direction of supporting beams is used working platforms at the edges of the building must be carefully detailed. Along the length of the main beams they may project outside the building line and create a less-than-ideal platform6. In the other direction, apart from cantilever brackets off the falsework uprights, there is little opportunity to provide support.

Is platform safe before

When detailing these platforms to the building edge the TWD should consider seriously whether they are safe before the precast units are placed, in order to avoid the falsework overbalancing and becoming unstable. Instability is less likely with table systems as the weight of secondary beams and the face contact material to the sofit create restoring moments.

precast units placed?

Orientation markers

The method statements and risk assessments will have identified the hazards. The lifting operation should be controlled (see Refs 23 and 24). Precast plank units can weigh 3.5 tonnes or more and cranage must be suitable. Orientation markers and lifting points should be specified. Consideration should be given to the location of the operatives during the placing operation, particularly when safety harnesses are required. The location and adequacy of the first ‘hook-on’ point and subsequent anchorages should be considered. Precast planks with projecting lattice members provide good ‘hook-on’ points, once they are erected and secure, but they also create tripping hazards in the placing area, as identified in an unpublished HSE safety report on the ECBP. A typical view is shown in Figure 8 in Section 3.9.5.

The platform is formed by secondary beams sitting on top of the main beams with decking added. This means that the top of the working platform is significantly higher than the actual soffit level of the precast planks, making access and fixing of the slab edge forms hazardous.

6

27


As the supporting falsework will be of a skeletal nature, access to the jointing areas will be required, certainly from the top, but possibly also from underneath. Where a gap has to be left between planks, a small section of soffit formwork has to be made up and held in position to support the jointing compound while it hardens. Depending on the width of the gap several support solutions are possible; if small, it may be possible to ‘hang’ it from above, but if there is a large gap, conventional sofit formwork, supported from below on falsework, may be required, with suitable access.

Access to plank joints

There is nothing to prevent the skeletal framework from ‘lozenging’ as the falsework is moved from floor to floor. This is illustrated by imagining the table in Figure 7 (Section 3.9.3) without the decking and secondary members fitted. This is in contrast to a made-up table form with its stiff soffit. The solution is to install plan bracing to the skeletal framework in the table near the head level.

Plan bracing to prevent ’lozenging‘

Where precast planks require post-propping to transfer additional construction loads to the supporting slab, the location of the props should be detailed. Stability of such props before loading is a consideration. Once loaded, the effect on the supporting slab should be carefully considered. In certain situations, the strength of the supporting slab may become the criterion for casting the new slab. This is discussed in more detail in Chapter 6.

Post-propping an1 of supporting

Although the foregoing discussion appears to be critical about the use of precast planks in flat slab construction, their use is to be encouraged; this Guide highlights the need for design, management and control of the processes by the supplier andor manufacturer. Attention to detail at the early stages of a contract will provide benefits when using precast planks.

Attention to deta stage will providt when using f

4.2.6 Precast concrete planks (non-participating) Most of the comments in Section 4.2.5 above on the use of thick planks also apply to thin precast planks used as permanent formwork. Effectively, the thin permanent formwork acts as the face contact material. Generally large areas of formwork planks can be handled and placed very quickly, but care should be taken in high winds.

Large areas p Care in high

Plywood butted together, nailed to bearers below, and fitted in two directions will provide some lateral stiffness to the formwork arrangement. In contrast, thin permanent formwork planks, often with bearers in only one direction, and with no positive connections between the planks and bearers, are unlikely to provide any transverse stability to the falsework. It is usually recommended in such cases that the falsework is designed to be self-stabilising, and not reliant on the planks for stability.

Thin planks U to contribut falsework ste

I

At the ECBP the planks to the roof were 30 mm thick in units up to 10 m long. A special lifting frame had to be designed, tested and used on the site. The LOLER Regulations (Ref. 23) specify the responsibilities and testing requirements for such equipment. The fragility of permanent formwork needs to be considered when detailing the falsework supports. Although plank suppliers often stipulate temporary supports at, say, 2 m centres, the plank designer will have assumed a distributed loading from the topping concrete, plus a transient construction operations load of 1.5 kN/m2 on the system. This loading pattern may not take account of temporary point loads during construction, such as from a bundle of reinforcement being lowered by crane onto the planks. Unlike conventional plywood, which is supported at, say, 406 mm centres’ on the bearers, the thin planks are spanning five times as far, say, 2000 mm, and the effect of a point load is much greater. By careful control, and by placing the load spreaders from any temporary load directly over the falsework bearers, such overloading of thin planks will be minimised.

Fragility of planks

4.3 Loading during construction The PWD should make available to the TWC the loads used in the design of the slab (see Section 3.2). However, during the construction stage other loads will apply, e.g. self-weight of the formwork and falsework, imposed loads from operatives on working areas, storage of materials and plant. Specific provisions should be made when materials or plant are to be used on the structure.

PWD to cornrni design loads to ni

In building construction the self-weight of soffit formwork for flat slabs and its associated falsework may be assumed to be 0.50 kN/mZ.Individual cases may require separate checking - see Section 4.2.3 of the Formwork Guide (Ref. 5).

Self-weight of formwork as: 0.50 kN/m

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Will there be rn or plant on stri

~

~

_

~

_

4.3 Loading during construction

Impact placing precast units

Where precast elements are placed, such as precast planks onto falsework, and impact may occur, Clause 4.4.2 of BS 5975 (Ref. 6) recommends that the self-weight of the elements is increased. The increase depends on their weight and how they are placed.

Construction operations load 1.5 kN/m2 (Service Class 2)

The imposed load on temporary works from operatives and man-handled plant is taken as 1.5 kN/mZ. This allows for the placing of the fresh concrete, and includes hand tools and small mechanical plant used in placing operations, such as vibrator motors. The area considered includes all adjacent walkways around the actual soffit area. (See Clause 4.4.3.1 of BS 5975 (Ref. 6).) This load allowance also applies where permanent formwork is used.

Origin of suppliers‘ 2.0 kN/m2

It is common to see TWD calculations for falsework using a single imposed load on the formwork and falsework of 2.0 kN/mZ, this being the summation of the construction operations load (1.5 kN/mZ)and the temporary works self-weight (0.50 kN/mZ).

Access for access and inspection allow 0.75 kN/m2 (Service Class 1)

Where allowance has to be made for access and inspection purposes, an imposed load of 0.75 kN/m2 is considered adequate. (See BS 5975 and the Formwork Guide.) This is known in EN specifications as Service Class 1 loading and is intended for inspection purposes and operations with light tools only but without material storage. It would be prudent to consider that any concrete slab just cast and able to be walked on could provide access for inspection and light operations, such as placing and removing of backprops, so this loading is considered the minimum that any concrete slab and/or permanent formwork should be designed to carry. The total load on a slab depends on the imposed construction operations loads, the self-weights (of permanent and temporary works) and the stage of construction. These must be known when assessing the early striking times and the back-propping necessary(see Chapters 6 and 7).

Example of calculation of loads The example is for a 250 mm-thick flat slabs at ECBP. All values are in kN/m2 Column 1 Recommended RC concrete density 24 kN/m3

(Service class 1 loading)

Recommended concrete density for falsework design: 25 kN/m3 Recommended concrete density for the supporting slab, striking and backpropping considerations: 24 kN/m3

Self-weight of concrete slab (recommended density 24 kN/m3) (assumed density 25 kN/m3) Self-weight of the temporary works (i.e. formwork and falsework) Superimposed construction load Hence TWD’s design service load Additional imposed construction load Hence TWC’s load during concreting

Column 2

6.00 0.50 0.75 7.25 0.75 __ 8.00 kN/mZ ~

6.25 0.50 0.75 7.50 0.75 ~

(from BS 5975 8)

~

8.25 kN/m2

The TWD, when considering the worst case for the temporary works design, will generally take the larger value of 8.25 kN/m2. This agrees with DD ENV 1991-2-1 Basis of design and actions on structures (Ref. 11) which assumes a density of 24 kN/m3 plus 1 kN/m3 for ‘unhardened concrete’ while it needs supporting during its stiffening phase. In flat slab construction this would not apply to the whole area of the slab concrete at one time. For the critical supporting slab, it is more accurate for the TWD to use the total load, based on the actual density of the concrete, of 7.25 kN/mZ.The additional imposed construction load is from the weight of operatives placing the concrete and is a short-term load. There is a difference of nearly 3% between the loads during concreting calculated using the recommended and assumed density of concrete. This difference can be used advantageously in backpropping calculations. The loads used in backpropping calculations, and whether construction operations loads or formwork self-weight need to be considered, are discussed in more detail in Section 6.5.2.

7

406 mm centres is a module of a 2440 mm length of plywood, providing support at the junction of two sheets.

Clause 4.4.3. I of BS 5975 ‘Working areas’ recommends a total imposed loading for construction operations, such as when concreting a slab, of 1.5 kN/m2. Hence the two values of 0.75 kN/m2.

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4.3 Loading during construction

It was demonstrated at the ECBP that, as the ratio of design service load to self-weight reduces on thinner slabs, the loading and backpropping considerations take on more importance for very early striking of formwork and the history of load on the slabs.

Further consideration of loadings, see Section 6.6

The speed of construction can affect the loads. For example, a large flying form system might be designed by the TWC to a full bay width, so on striking and removing the falseworWformwork the entire slab needs to be self-supporting when struck and could be subjected immediately to the full weight of the formwork system placed on it. In contrast, a smaller skeletal or tower system might allow repropping to be inserted. Thus the loads to be considered are very different. More information and recommendations on backpropping are given in Chapters 6 and 7.

4.4 Procurement of temporaw works 4.4.1 General

The successful constructor will make assumptions at tender stage about the equipment to be used and have obtained prices andor built up unit rates. On a design and build contract, these rates will be based on experience from similar contracts, and the design of the permanent works will be influenced by the equipment available for the temporary works, and will often be related to equipment owned and regularly used within the contracting organisation. On conventional contracts, the temporary works in the PWD’s final design need to be verified and compared to the original tender assumptions. The estimatorhpplier completing and pricing the latest scheme should determine the quantities actually required. The competitive nature of equipment pricing, by definition, often makes comparisons very dificult, with questions such as “Does it allow for the scaffold boards?, What plywood has been assumed?’ To demonstrate the most cost-effective solution the proprietary supplier will be under commercial pressure to specify the minimum equipment to suit the contract and thus show a low equipment cost, with additional items as optional extras - these are often safety items such as the working platforms. If such items are not part of the TW design brief, the supplier may be justified in expecting that they will be supplied from on-site resources, so need not be included in the list of items required - again emphasising the importance of the TW design brief. The quantity of equipment to be procured is directly affected by the programme. It has already been emphasised in Section 4.1 that it is often more efficient and economic, for the project as a whole, to reduce construction time by increasing the quantity of formwork equipment.

Importance of TW design brief see Annex B

Minimum equipment not always most economic

Where a constructor is using their own equipment, there will often be added pressures in procurement to use only items held in stock, to avoid buying in special items or hiring extra equipment. Where practicable, timber and wood-based products should be procured from renewable and sustainable sources.

Timber and wood products from sustainable sources

With equipment that is to be man-handled between floors in multi-storey construction, the use of some extra support equipment should be considered. For example, extra base jacks may facilitate an early start on the subsequent floor: on striking and moving, the formwork and head items come out first, and the bases last, while opposite order is needed by the operatives on the next floor when re-erecting the system! It may sometimes be worthwhile increasing quantities so falsework erection can start on a subsequent area, reducing the waiting time while ,equipment is struck from one area and moved to the next. Of course, this does not apply to sections of falsework that remain assembled and are crane-handled in units, such as tables and flying forms.

Additional items when considering order of use

The condition of components will vary. Items such as adjustable jacks need fettling between uses; if they are not used for a while the threads may seize and be hard to operate. Aluminium sections can be susceptible to damage on site due to misuse and long props can be bent in handling. To allow for such ‘not perfect’ items the prudent site will over-order. This should be considered at the planning stage, because quantities used in the tender price will often be based on the exact take-off of the drawing, and not necessarily be the amount required by the site. Close liaison is needed between constructor and supplier when calling off equipment. For example, base jacks are the first items used in erection, so they should be the first items delivered if delays are to be avoided.

30

Liaise over call-off for delivery

4.4 Procurement of temporary works

TW design brief: see Annex B

All these points can lead to discrepancies and underestimating the quantities of equipment actually needed to operate the system safely. This Guide recommends the preparation of carefully considered design briefs to ensure that the temporary works scheme prepared by proprietary suppliers or constructors’ TWDs is designed to the same documentation. Provided that the temporary works conform to the brief, this will lead to safer operation of the temporary works. A typical temporary works design brief is shown in Annex B. At the ECBP, several aspects of temporary works were identified that require more consideration at the contract planning stage. They relate mainly to the design of the temporary works but have implications for the construction stage. These are discussed in the following paragraphs.

4.4.2 Working platforms and edge protection Show working platforms and suitable edge protection on drawings

Barriers and nature of the work

Impact on exposed platforms Use of safety nets

Working platforms and edge protection should be shown in all temporary works drawings. If these are to be prepared by a supplier the TW brief should indicate that they are required and should be included in the scheme. Particular attention should be given to edge protection to all slabs during erection of edge forms, concreting and removing/dismantling of the forms. It should be noted that safety protection can be achieved by keeping persons away from exposed edges by use of barriers of suitable strength and rigidity. The Construction (Health, Safety and Welfare) Regulations (Ref. 16) are specific about such provisions for all activities; in addition, specific reference is made to the requirement when, due to the nature or short duration of the work, compliance is not reasonably practicable. An example is the flying out of falsework tables from underneath a recently cast slab -this may require the temporary removal of guardrails to the lower slab to permit the table to come out. Generally, at the slab level under construction, platforms will be incorporated in the supplier’s selected system, such as outstand brackets on scaffolds, projecting timber or aluminium primary/ secondary bearers, or from the flying form projecting from the building line. Such platforms can be vulnerable to impact, such as from crane loads, and care should be taken in exposed locations. The emphasis should be on correct use of the crane. Edge protection may be by safety nets (see also Section 3.10). Nets can be rigged internally, generally horizontally to fit either partly or fully under the slab, safeguarding temporary openings. They can also be erected vertically or at any angle. Fitted externally on steel brackets or ‘gallows’ they can eliminate external scaffolds, so the slab edges remain clear to be worked on, with the bottom of the net tied back to the underside or edge of a lower slab. This is shown in BS EN 1263-2 (Ref. 25). Nets are generally more effective when spanning large distances. Principally designed to arrest falls from up to 6 m, they are usually rigged for a 2 m fall. This Guide recommends use of 60 mm mesh nets to BS EN 1263-1 (Ref. 26) for concrete-related activities. Nets have traditionally been used as ‘fall arrest’ items and not as ‘prevention’ and come lower down the safety hierarchy of the C(HSW) Regulations. They provide simple effective prevention as work progresses up a building, freeing operatives from the restraints of harnesses and more elaborate fall arrest systems.

Placing precast planks at leading edges

When using a precast plank system, placing the planks at leading edges becomes more hazardous and risks can only be partly reduced through the use of safety harnesses. Such personal protective equipment should only be used as a last resort, and other methods of edge protection and safety of operatives should be sought. The provision of adequate working platforms in such operations is difficult, and needs to be very carefully considered in the design of the overall system of planks and falsework. Further guidance on edge protection is given in the Precast Flooring Federation Code of practice for safe erection of precast concrete flooring (Ref. 24).

Access underneath formwork

Access is required to the underneath of soffit formwork for sealing plywood joints, levelling the soffit formwork, fixing items such as LOK test inserts, and, of course, for striking out the components. The access needed depends on the system used. Although sites often refer to such platforms as ‘crash decks’, they are really working platforms to prcvent operatives and material falling. Only the steel skeletal system used at ECBP had provision for an all-over working platform.

Base jacks

When using flying forms or tables, with levelling facilities at the bottom only, access via a working platform is not required within the table for its general operation, but a working platform is essential

31


for the safe fixing and striking of the infill strips. The infill strip dimension between flying tables is rarely a frame module and the proprietary working platform supports do not fit, so other arrangements for access are needed.

Working platforms for infill strips

Working platforms should be in the right position and at the right height to enable the work to be carried out effectively. Often the platform is used for several operations - for example, placing precast planks will dictate one level of platform with relevant guardrails, but once placed, work on the topping slab and edge forms to the slab will require a higher guardrail, so the TWD has to consider the subsequent operations. 4.4.3 Edge distance of standards The plan arrangement of the temporary works on multi-storey construction should allow for a defined clear edge distance for falsework standards, ledgers or frames from the slab edge of the structure. The TWD should consider this distance and not place components close to the edge in order to reduce the cantilever arrangement of the soffit formwork above.

TWD to consider items close to edge

The location of the side protection handrails and posts on the edge of the floor supporting the falsework should be considered. In this way the supporting equipment (falsework) can be safely checked and dismantled with the site ‘permanent or temporary guardrails’ fitted. The TWD and the PWD have specific responsibilities to consider the safety aspects of edge protection during this stage of construction, particularly when no external scaffolding is envisaged. A stability check on any falsework with a projecting working platform, as shown at Figure 9, is an essential part of the design.

Consider stability with overhanging platforms

In the absence of specific guidance, the recommended distance from the slab edge to the centreline of the edge standard of the falsework is 400 mm, as shown in Figure 9. Increasing the 400 mm dimension increases the length of any cantilever at higher level.

Recommended edge distance 400 mm

Although the recommendation relates to all systems, at the ECBP the individual prop system was most affected because it was detailed with an edge distance of only 100 mm. The system relied on erecting individual props using a patented tripod requiring a dimension larger than 100 mm which meant that the tripod was omitted when erecting edge props on floor slabs above ground level. When using flying table systems demountable guardrails are needed in order for the tables to be flown out, but such systems still require sufficient working space at the slab edge to allow guardrails to be fitted, even if they are only temporary. Various proprietary temporary guardrail systems are available, and it has already been suggested (Section 3.10) that more permanent provisions might need to be incorporated by the PWD for such handrails. At the ECBP cast-in sockets were fitted to the slab for connecting the permanent scaffold tube and fitting handrail.

Consider risk of overturning

Backpropping to

be considered

32

Figure 9: Typical section at edge of slab.

See also Sections 4.2.2 and 4.2.3 for removable guardrails


When an external independent tied scaffold (Ref. 18) is provided to a building, it may possibly be used as the external edge protection. The design risk assessment will identify whether the external scaffolding can be considered to provide edge protection. 4.4.4 Column forms - lifting, access and stability Procure column forms with access, lifting details, stability and ladder provision

Provision should be made in the procurement of column formwork for adequate working platforms. Full working platforms should be provided with edge protection. Where practicable, working platforms should be fitted to all sides. A safe means of providing stability should always be included when procuring column formwork9. Separate prefabricated tower scaffolds will rarely provide satisfactory access for erecting, concreting and dismantling column forms. However, planning may determine that safe access is required for fixing column reinforcement as well. This may eliminate the need for an integral formwork working platform if the TWD has designed the working platform to suit both operations.

Hoops if above 2.5 m Avoid inclined ladders

It is recommended that, where possible, column forms should incorporate access by a vertical ladder, preferably with hoops for extra safety above 2.5 m, and with a suitable trap door detail for access onto the working platform. Separate inclined ladders are a potential hazard because there is rarely one ladder for each column form, so the ladders have to be constantly moved between column forms, and they are therefore rarely tied at the top. In operation it is necessary to climb to the top of each column several times so permanent ladders have distinct benefits. Particular consideration should be given to the column forms near the edges of the building, especially the corner columns.

Consider tripping hazards at prop feet connection

The use of push-pull props in two directions at right angles will give stability. The use of projecting lengths of bar for connecting the raking prop to the slab is not recommended. It was clearly shown at the ECBP to create a tripping hazard, both when the bars were cast into the slab, and when they were placed into drilled holes in the slab top. Many recoverable fixing details are available, and post-drilled fixings into the slab for connecting the prop base plates should be permitted by the PWD and used on site. Where there is no external scaffolding the method statement should discuss the working platform, guardrails and propping necessary for the corner columns to the building. This may mean out-ofalignment propping to cater for the existing guardrails that will be required to remain in position.

Incorporated lifting points

All column formwork assemblies should incorporate crane lifting points. The weight of the form should be known and its centre of gravity should be determined, generally by on-site trials after assembly and prior to first use. These items should be part of the supplier’s obligations in the design and supply of proprietary system forms. Care is needed in designing connecting devices, both for column-to-prop connections and for joining column sections together. Captive components with no loose parts are preferred. 4.4.5 Level adjustment of head jacks, base jacks and props

All systems require some form of adjustment in order to level in the soffit formwork and subsequently to destress the falsework legs when striking. In building applications, where the top of the floor slab is level, it is expensive to insert adjustment at both the top and bottom of the system. It is also time-consuming in levelling in the system. The selection of head or base jacks depends on how the system is moved. If the formwork is to be handled as large made-up tables, or as flying forms, jacking should be provided at the bottom, but for strip and re-erect systems it should be at the top. On a typical large flying table scheme there will be areas that are intended to be manually struck and moved, such as around staircases, so both types of jacking would be expected in the overall scheme. On the steel skeletal systems, it is often better to use longer top jacks, braced for stability, than to insert levels of ledgers or transoms near the top. Not only does this use less equipment, it also gives a less congested working platform for access for stripping the sofit formwork. Both suppliers of proprietary column formwork at the ECBP stated that they are rarely asked to provide access platforms for column formwork. Responsibility for safety should start with correct procurement.

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Where falsework was used to support precast planks as permanent formwork at the ECBP it was noted that it would have been better if the jacks had been placed at the bottom of the standard with no adjustment at the top. Allowance for bedding the units and protecting the soffit could have been by use of thick non-compressible paper as packing. Wherever long props are used the physical method of adjustment needs considering. At the ECBP the adjusting handle for the propping system on the first two floors was about 2.5 m above the slab. This meant that adjustment had to be carried out off a small platform! Adjustment and striking of edge props while standing on a small platform alongside the building edge put the operative well above the height of the adjacent guardrail, and safety harnesses should have been used. Prop suppliers should give consideration to the level of the handle in use.

Height of prop adjustment device

4.4.6 Length of formwork beams An aspect often overlooked by TWDs and suppliers when drafting TW schemes is the length of the primary and secondary falsework members - these are often detailed in long lengths. This may reduce the supplier’s overall scheme cost as longer lengths reduce the number of joints and discontinuities in the soffit, improve the stiffness of the structure, and can reduce site theft. (It is easier to ‘remove’ a 3.6 m beam than a 7.2 m beam!) Longer beams will often be less expensive to hire or buy. The critical operation in the use of beams is striking them from underneath the slab. Where the system is designed to be handled as a large table form or flying form, the length of unit handled is dictated by the length of the primary members connecting the table legs together, so full-length primary members are beneficial, and the assembly remains connected and is moved out by mechanical means. In contrast, all other applications require the primary and secondary members to be struck out and handled as individual items, so the TWD should consider the safety implications and lengths of beam detailed for separate handling. It is a recommendation of this Guide that, on all areas of manually erected formwork where the primary andor secondary beams are to be struck out, handled and re-erected by hand, the length of beams used in the design be carefully considered by the TWD to facilitate handling. The Manual Handling Operations Regulations (Ref. 27) give clear limits for individual operative capabilities; striking operations involve operatives removing beams from the top of the falsework and man-handling them to the lifting platform, prior to removal for subsequent re-use. Since the operation is generally limited to two operatives, a suggested limit on the weight of beams is between 25 and 38 kg. The location of the beams and their relationship to the ground or supporting slab should also be considered, together with the requirement for adequate working platform(s). A corollary of this limit on length is the need for the TWD to design the supporting falsework specifically to suit these shorter beam lengths and to allow for the possibility of increased leg loads in certain internal standards due to the elastic reactions from continuity. The 10% continuity factor from Clause 6.5.3.1 of BS 5975 (Ref. 6) may not be appropriate, such as for beams on three supports, and a more detailed analysis may be justified. When designing independent tables with short lengths of beam, the TWD should assess the stability of the falsework during concreting. Increasing the edge distance to the standard nearest the edge (see Section 4.4.3) also increases the cantilever of the sofit formwork. The resistance to overturning of the table may need to be checked with the loading from the working platform, the self-weight of the arrangement, and also with the extra imposed concrete load from the longer cantilever. On short lengths of table this may require either tying tables together to prevent overturning, or controlling the order of placing concrete; the latter cannot generally be achieved on site. The lighter aluminium systems have less resistance to overturning than comparable steel systems. The worst case for overturning is with concrete adjacent to the slab edge form and only on the cantilever, and no resistance from any concrete placed inside. This may require the falsework to be tied to the supporting slab to resist the overturning.

Length and weight of manually handled items to suit a two man lift Suggested limit of 38 kg for two men handling one item Adequate working platForms

Beware elastic reactions from shorter length beams

Overturning of tables at cantilever edges using short beams

Sequence of concreting at edges and overturning

4.4.7 Implications of changes It is important that TWCs, suppliers and constructors are made aware of the reasons for selecting specific items of equipment and resist substitutions. Although, commercially, the culture of accepting what is offered as a substitute without any delay is tempting, overall productivity and

34

Technical effect of equipment substitution


speed may be compromised. There may also be technical reasons why certain products cannot be substituted - for example, two 4.8 m beams in place of a single 9.6 m beam might, with extra beams to allow for laps, be a suitable substitution for secondary beams in formwork, provided the plywood joints remain supported; but such a substitution would significantly affect the design of the supporting falsework if considered as a change for primary beams. Although in the ‘real world’ such changes are regrettably commonplace, and accepted as part of the construction process, the implications on construction productivity should be questioned by site managers. If the equipment is only used a couple of times, such reductions in output may be acceptable. However, in many cases the effects on productivity can be large. If the changes in the example above were considered on a commercial site, the correct lengths for optimum efficiency would have been procured, and the TWC would have understood in detail the direct effect on productivity of the changes. Three such changes were highlighted at the ECBP that would have had a direct effect on the commercial viability of the TW schemes, as designed, by significantly changing their productivity. 1. Change in length of flying form increased number of crane lifts per floor by 20%

Change in length offlying form units

The frame contractor requested a reduction in length of the flying form units. This would have resulted in 20% more units being used and handled, and would have reduced production rates by at least 15%. The request for the reduction in length was made because the clearance of the ECBP building in the Cardington Hangar limited the maximum crane handling length of the flying form units. This change was caused by the proprietary supplier who was given the clearance in a detailed Temporary Works Design Brief which conformed at tender stage, but increased the lengths of units in their ‘construction drawings’ for commercial reasons andor ignored the dimension limit. Once on site, and a few days before intended delivery, the constructor contacted the flying form supplier direct and revised drawings were issued with split flying form units. This effectively increased by 20% the planned number of crane lifts. Thus any TWD expecting to use this type of equipment must ensure that it has the necessary dimensional clearance for the flying form, and has obtained any necessary ‘air rights’ if flying in congested city centre sites.

2.

Change in length offlying table units

This change relates to the use of the flying tables made up with aluminium props as legs and braced together with aluminium frames, supporting long lengths of primary aluminium beams. To understand the implications of this change, an understanding of the mode of operation is necessary. The aluminium props were connected together with frames into six- or 12-legged tables, and their heads were joined by long lengths of deep primary aluminium beams. Lighter aluminium bearers were now placed at right angles, at closer centres (about 488 mm) to support the plywood. When handling large tables, it is the length of the primary beams that provides longitudinal strength to the framework, so these lengths dictate the size of units handled, and hence the economy. This is clearly shown in Figure 7. Larger tables reduce the number of units to be handled, reducing both labour and crane costs. The arrangement planned by Task 2 for economic use of the system was detailed on the supplier’s drawing. This proposed the use of only 12 tables for construction of a floor. This gave three tables across the width of the building, using 225 mm deep primary beams fitted as two 9 m long and one 6.4 m long.

Change in primary beam length on table form increased number of crane lifts per floor by 33%

When the constructor requested delivery, he was advised that not all the long lengths of 225 mm deep primary beams were available. Without reference to the management, the supplier was asked by the constructor to redraw the scheme based on the sizes of beam locally available. This revised scheme significantly changed the arrangement, and used 16 tables for construction of a floor. The tables were made up of four tables across the building, using two 8 m long and two 4.2 m long tables. This reduced the number of vertical legs, but, significantly, increased the number of table movements from 12 to 16, i.e. by 33%, increasing both the crane costs and the labour rate when handling the tables. Using more tables also increased the labour time in completing the make-up areas between the tables. This simple change in beam length meant that the table sizes had to be reduced, increasing by 33% the number of tables moved per floor, with a likely corresponding reduction in overall performance.

35


The writer of this Guide is of the opinion that the site management and the TWC were not at the time aware of the planning implications of this change, because a month later he was told by the supplier that, had the implications been communicated to them, efforts would have been made to locate enough 225 mm-deep beams of the required long length from stock! This would have had significant consequences on a commercial site. 3.

Change in steel skeletal grid layout

This change resulted from circumstances outside the project’s control, and was also affected by the change in beam length discussed above. It concerned the steel skeletal system that was supplied and designed around known available (free) equipment. The decision was also taken by the ECBP to use the shorter aluminium beams already on site. A steel skeletal support system is a very rigid birdcage arrangement, and the plan spacing of the legs is normally governed by the safe spanning capacity of the primary and secondary bearers. In building work for thin slabs this would normally be on their largest plan grid size of 2.4 x 2.4 m. Two factors, outside the control of Task 2, affected the layout, firstly the requirement to limit the plan grid size to 1.8 x 1.8 m to suit the items held by the plant hire supplier, and secondly the limitations on layout in having to use the aluminium primary beams already on site in 8 m and 4.2 m lengths. The choice of secondary members would not affect the grid layout. The optimum layout of steel skeletal shoring shown on the original supplier’s drawing was a 1.8 x 2.4 m grid of equipment and would have weighed about 15.8 tonnes. Altering the layout to a 1.8 x 1.8 m grid, but redesigned to suit the aluminium primary beams already on site, would have increased the quantity of equipment to about 17.1 tonnes. As finally erected, the shoring estimated from site sketches and based on the 1.8 x 1.8 m grid of equipment supplied and now using the on-site aluminium 8 m and 4.2 m primary beams weighed about 20.0 tonnes. The direct effect of the two factors was to increase the weight of equipment to be man-handled, erected and dismantled from 15.8 to 20.0 tonnes. This changed the falsework layout in plan from the optimum 1.8 x 2.4 m grid to a 1.8 x 1.8 m grid and positioned the standards to suit the shorter beams. This resulted in an increase of 26% in the weight of equipment to be man-handled, with a likely corresponding decrease in production efficiency.

Change in layout to suit available items: increased weight to be man-handled by 26%

4.5 Permit to load and permit to strike It is recommended that a verbal procedure for ‘permit to load’ and permit to strike’ is not adequate for a large building site, and that a procedure involving written confirmation should be used. The site management should not allow any change in procedure, especially after the detailed preconstruction procedure agreements. A ‘formal permission to load the falsework’ is one of the principal activities of the TWC, as stated in Clause 2.5.2.2(m) of BS 5975 for construction sites, and this should be no different for large building sites involving temporary works.

Use a written confirmation procedure

As this Guide relates to temporary works, the term ‘permit to load’ is used and not ‘permit to concrete’, which implies checks on reinforcement, cover, etc. Obviously reinforcement and cover have a significant effect on the performance of the structure, particularly its load capacity, as wrongly positioned reinforcement may reduce the design capacity, and appropriate checks need to be established.

Importance of checks on reinforcement, cover etc

In temporary works it is approval to load the assembly of falsework and formwork that this Guide requires. This may include the support of steel permanent decking or precast permanent formwork units, before concreting; in such cases, the management may require staged approvals. The procedure for the approval is likely to be shown on two sections of a single form.

Permit to load

Useful guidance on checking falsework and formwork on site is given in a Concrete Society series of checklists. (Refs 28 and 29)

Useful guidance in Concrete Society Checklists

The permit to load system for the falsework at the ECBP was initially operated using a signed form, but subsequently became a verbal matter with the constructor signing the day book (a bound book) instead of a form with the client. The purpose of the written system was to ensure that both research

36

4.6 Sequence of work

and construction teams had taken into account all aspects prior to ‘loading’ or ‘striking’ the temporary works. This was very carefully considered and agreed upon by many parties involved at the precontract stage of the ECBP. Permit to strike

The comments about the ‘permit to load’ relate equally to the ‘permit to strike’, but with the added concerns of backpropping in multi-storey construction. Obviously the permanent structure should be strong enough before the supporting falsework is removed, but, in multi-storey construction the supporting slab is very often also transmitting part of the imposed load into the lower slab(s) through backpropping. These backprops are falsework as they are helping to support the permanent slab until it can carry its own weight. Any ‘permit to strike’ should incorporate any backpropping procedures. Chapter 6 of this Guide gives detailed guidance on striking and backpropping. The importance of the sequence of removal of the falsework and any related backpropping cannot be stressed too strongly.

Permits for permanent formwork

The comments above for permits to load and to strike apply equally to other permanent formwork systems that require propping, such as steel profile decking.

Important to follow agreed striking procedures

A non-adversarial procedure is to be commended, but even on the ‘controlled’ ECBP site with the agreed verbal procedure, the back props between levels five and six were removed prematurely by accident. This altered sequence of removal generated a theoretical unit load of 15.25 kN/m2 on the sixth floor slab (BR 394, Ref. 2), compared to the unfactored design load of 10.50 kN/mZ.The sixth floor had reached its design characteristic concrete strength at the time of the incident. A serious disadvantage of the verbal procedure adopted was its inability to cope with staff changes or absences at critical stages in the project. (The removal of these props took place on a Sunday.) A general reduction in engineering ability and understanding on building sites and the importance of controlling the operatives, particularly if the recommendations on very early striking are to be adopted, makes the need for formal approval to load and strike and to follow agreed procedures even more necessary. The discipline of maintaining a ‘permit to load’ and ‘permit to strike’ systedprocedure, issued and signed by the agreed signatories, has the significant merit of being maintained in a folder or book on site - available to all site staff, even when senior staff are absent. Any agreed procedure and order of striking props should be included in the same place. A check by the supervisor on the Sunday in question would not have found a signed approval form, and removal of backpropping at the lower floor level should not have commenced. The suggestion by site that these approvals on building works be kept to a written record in the site agent’s day book is not considered a satisfactory alternative to a permit form system.

4.6 Sequence of work When planning the order of construction, TWDs and TWCs should consider the implications of the sequence. The shape of the building, number of floors and sequence of construction all affect the programme. Starting column construction in the wrong place affects subsequent operations. Early striking benefits lost if no reuse!

Early striking of slabs, thereby making the formwork available for reuse very quickly, possibly on the morning of the second day after the pour, may not be the most practical solution for a site. If there is no place for storing the forms, then the benefits of early striking are lost. The size and shape of the building under construction have a direct bearing on the planning.

Consider casting columns under the formwork

For example, on buildings with a small plan area, overseas experience has shown that it is generally more economic to start slab formwork on the floor above before forming the columns. This requires enough additional formwork until the struck formwork becomes available. The added difficulty of forming columns under the formwork and cost of additional material are usually more than offset by the time savings on the project as a whole. In contrast, on a large floor area job, the sequencing might allow the soffit formwork to move up a floor, but offset laterally, thus allowing unimpeded column construction on the newly struck slab. At the ECBP, the shape of the building, number of floors and sequence of construction had a direct effect on the safety of operations. Simple items, like the order of casting columns, had a considerable knock-on effect on safety of steel-fixing at the next slab level. The effects were particularly

37

4.6 Sequence of work

noticeable when fast-track flying tables were used, as outlined below in an edited extract from the research report: “For the system of tables used, it is possible to identify an ‘inner’ line of tables and an ‘outer’ line of tables. For ECBP, there were four lines of tables, two inner, two outer. On a different sized building, there would be no more than two outer lines, but could be more or less than two inners (e.g. a three bay building has one inner and two outers; a five bay building has three inners and two outers).

Order of column casting affeds safety

It is clear that the inner tables are easier to move than the outer ones as the erect and dismantle section of the falsework at the end with the staircase opening and upstand beam has to be dismantled before the outer ones can be moved.

For the ECBP project, therefore, it would have been possible to strike the inner lines of tables, and immediately fly them up to the next floor. In this way, 50% of the falsework would be moved straight awaylo. If the middle infill strip is then completed, this results in 50% of the slab being decked out. The steelfixers could then begin their work. However, such a fast-track solution creates a problem: starting decking from the middle of the slab means there are two leading edges which must be protected. In addition, tables clearly cannot be placed whilst the column forms are still in place, so the column-casting sequence may need careful consideration. Such a solution will also have a knock-on effect on other processes, e.g. steel fixing. It was common for the steelfixers to start at one end and work towards the other. If they started in the middle and worked outwards, this might require a different reinforcement design11.”

Further benefits could have been achieved by redesigning the edge tables in the end bays at the ECBP to make them easier for crane handling. Some edge tables could have been almost completely struck by crane. If the sequence had allowed the inner table to be flown first, followed by an edge table, this would have given an edgelend for the steelfixers to start. As there was only one re-use this was not adopted at ECBP. The TWD and the TWC should consider edge tables very carefully, as their correct design can make a significant difference to the process and productivity. The efficiency of the concreting process is an important consideration in design and will dictate the area of slab that can be concreted, thus determining the amount of s o a t formwork required. Above ground level, a pour for the floor slabs of a typical frame can be over 500 m2 in area and up to 30 m in any dimension (ECBP floor slabs were single pours of 675 m2). The BCA guide Concretingfor improved speed and efficiency (Ref. 30) discusses the influence of pour size on the ECBP slab construction by comparing the single pour to two or four pours on separate days.

Typical frame slab area 500 r n 2

Construction joints are acceptable provided they do not compromise the performance of the structure. Even when the use of a construction joint is not intended, the possibility of an unexpected cessation of a pour should be planned for; this may require holding extra equipment, such as expanded metal, on site as a contingency.

Plan for cessation of slab pour

Another lesson from sequencing of the work relates to the number of column forms used. Considering also the comments in Section 3.4 on columns with and without kickers, it would seem that the exact order of column construction takes on greater importance. With the usual contract adopting only one generic system of construction (unlike the ECBP with its four types), the care taken in planning the exact sequence of column casting, related to the programme of fitting the falsework around the columns, will determine the optimum number of column forms and provide a safer, and often faster, construction cycle. The extra cost of another column form would be offset by the advantages in time and safety, as stated in an edited extract from the research report: “Each floor of the building had 20 columns. Seven sets of column forms were supplied (at the request of the contractor, who had concerns about storage). This meant that the 20 columns were poured over three days, the first pour taking place the day after the slab was poured. An innovation on this project was under the Task 4 research to investigate early striking of the slab falsework, with slabs often struck 24 hours after pouring. Part of the striking process involved removing the handrails from the slab falsework and repositioning them at the edge of the slab, making a permanent guardrail around the edge of the newly-cast slab. In addition, striking involved removing the working platform around the edge of the slab falsework. When a slab was struck after 24 hours, it was sometimes the case that the working platform and handrails on the slab falsework alongside the third set of columns were removed and repositioned before the column was struck’*. Hence, this meant no access was possible to the outside edge of the column shutter. This made striking these columns very hazardous. The solution would be to pour the columns in only two stages, hence using ten column form setsl3”. 10

For a five-bay building, the inner tables represent even more of the total slab area (60%).

11

Another example favouring contractor detailing. (Ref. 9.)

38

Constructor to consider order of casting and

i

~

~

~

~

,

form can significantly improve progress

“

”

4.8 Method statements

4.7 Striking

- planning considerations

The criteria for striking slabs recommended in CIRIA Report RI36 (Ref. 31) relates concrete strength at the time of striking to the design 28-day strength of the slab and allows for the actual loading applied at the various stages of construction. Further information on this method for striking slab formwork is outlined in Section 5.3.6 of the Formwork Guide (Ref. 5), which also includes worked examples in its appendices. Serviceability limits give faster striking times

A major recommendation from the research (Ref. 2) on the ECBP was the verification that accepting serviceability as the governing criterion could give faster striking times, yet still ensure a safe structure that was not harmed during construction. Provided the crack extent, crack width and deflection during construction do not exceed those under the design service load conditions, then the structure might reasonably be deemed not to be impaired. This innovative approach was part of the Task 4 PIT project work, and the background to the findings is discussed.in detail in Chapter 6. Chapter 7 gives practical guidance on the application of these innovative methods.

Information to be available to TWC by the PWD

Whichever method is used to decide the basis for striking, the PWD should make available to the TWC the following information for each floor: (a)

Characteristic strength of the concrete in N/mmz (stating whether cube or cylinder strength)

(b)

Design imposed load on the slab in kN/m2 (including all partitions, service loads, etc. and expressed as unfactored loads)

(c)

Assumed self-weight of the slab in kN/mZ (excluding finishes and partitions)

(d)

Assumed self-weight of any finishes, partitions, etc. in kN/m2 to be applied to the slab.

Advance agreement on concrete strength assessment

The method of assessing the concrete strength for striking should be established and agreed between the parties at the pre-construction phase (see also Section 6.4). Both the PWD and TWC need to be confident that the lower bound concrete strength can be accurately determined by the site testing proposed. The BRE-recommended method (Ref. 32) for early-age strength assessment is to use a pull-out test, the LOK test, based on an average of four results. The pull-out test requires the PWD to accept some cast-in inserts and some local damage to the concrete.

Location of concrete critical for striking assessment

The location for the assessment of concrete strength should be carefully considered. The top surface near to the interior columns and at mid-span are often critical areas for strength assessment of slab concrete, and may dictate locations of LOK tests andor supportive concrete sampling for cube tests.

Implications of pull-out testing on finishes

As LOK testing is the preferred method of assessing strength, early consideration by the PWD of the implications of the fixing positions is required. Ideally they should be considered for the top surface of the concrete slab. Where LOK inserts are to be fixed in the sofit, a 400 mm square panel is constructed in the sofit plywood which is removed to allow testing of the insert to take place, before striking the main formworklfalsework. No problems were encountered at the ECBP with the construction of these panels. Clients or specifiers wanting LOK tests as well as specifying finishes requiring purpose-made GRP or steel formwork should be aware of the consequences.

Backpropping and striking on agenda at site meetings

The detailed backpropping and striking procedure was regularly discussed at all ECBP site progress meetings and required concrete strengths at time of striking to be established in advance. In view of its importance, and relevance to the PWD and the TWC, it is recommended that this subject is added to the agenda of every construction progress site meeting.

4.8 Method statements Suppliers to have method statements available

Suppliers, importers and users of systems of formwork and falsework should regularly update their method statements to consider local practices and materials and reflect what is actually happening

12 The first two sets of columns on each floor were generally struck after 12 hours, to allow subsequent sets to be poured 24 hours later. The third set was not always struck at this age as there was no urgent need for the shutters, and, once struck, storage space for the shutters had to be found. 13 In retrospect, the frame constructor agreed that pouring columns in two days would have been better, and would have had the additional effect of reducing each floor cycle by one day.

39

4.8 Method statementr

on site. Method statements are an important way for suppliers to communicate their intended safe operating methods. Although all manufacturers, importers and suppliers of equipment and articles for use at work have a legal requirement under the HSW Act Section 6 (Ref. 35) to provide “adequate information about the use for which it is designed”, the quality and accuracy of issued method statements varies significantly. The method statement should be relevant to UK conditions and equipment. For example, a supplier’s method statement on video that shows continental 500 mm-wide sheets of plywood being laid down overhand on top of beams does not reflect the risks and hazards associated with laying down UK size sheets of plywood the “right way round” which are 2440 mm long. The TWC’s role as co-ordinator takes on even more importance when construction involves more than one equipment supplier, as occurred with both the steel skeletal system and the precast planks. The interface responsibilities to ensure that adequate method statements for the combined systems are produced require good overall knowledge of the construction process; and the most appropriate author of such temporary works method statements would be the TWC.

40

TWC to co-ordinate when several s u p p l i e ~i ~ ~

k

d

5 Construction stage

Recommendations - Construction stage

-

General platforms, fans, etc. 1. A radio for crane control that can be used by the slinger or signaller ‘handsfree’ makes the job easier and improves safety and performance. 2. Consideration should be given to providing temporary debris fans, possibly as netting, at lower levels to catch items on the edge. 3. Adequate loading-out platforms should be incorporated in the construction process. They should be properly designed, and ideally fit flush with the top surface of the slab. 4. Loading-out platforms should be clearly marked with their safe load. 5. Loading-out platforms intended for use with brick pallets should be designed for 20 kN/m2 imposed load and limited to one pallet high. Columns 6. Provision should be made for adequate working platforms to be incorporated in column forms, complete with full platforms and guardrails on all four sides. 7. Methods to provide adequate stability when only half a column form is erected should be developed, with particular reference to corner columns. 8. Columns should be cast about 20 mm into the slab. Table and flying forms 9. Proprietary suppliers should consider incorporating a fixed length plan brace a t or near the head level, particularly when assembling falsework for tables. 10. When using tables that are rolled out from below, only secured trolleys should be used to handle them. 11. When operating flying tables or forms all members of the team should receive basic training as slingers or signallers and be aware of the correct crane signals. 12. Compensating clutches should be used on all table and flying form lifts involving four unequal legged slings. Precast units 13. When using precast planks that require temporary support, the TWC should co-ordinate the falsework, paying particular attention to provisions for adequate working platforms and/or connection points for safety harnesses. The movement of such falsework between floors may need plan bracing. 14. All precast units should be marked with orientation pointers and lifting point positions identified to comply with the supplier’s issued erection drawing(s) and method statement(s).

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5.1 General

5.1 General As already stated in Chapter 2, studying the process of construction is most likely to provide benefits for future contracts. The comments in this section relate to experiences at Cardington, and will be of interest to suppliers, manufacturers and users of temporary works equipment. The experiences gained from the contract indicate that there are advantages in considering complete temporary enclosures to a building under construction, provided the use of fast-track flying table systems is not envisaged. The benefits of all-weather working, external protection to public and outside operatives, plus reduced dependence on edge protection might offset the extra cost of enclosure, but would limit crane handling of equipment. The concept should be to improve the process engineering of the construction and give better guarantees of completion on time. There is no unique way to reduce or eliminate a risk as each site and organisation are different. The risk assessment, a requirement under the Management of Health, Safety and Welfare Regulations (MHSW) (Regulation 3), should have identified the needs; in addition the C(HSW) Regulations (Regulation 6) (Ref. 16) will have considered avoiding falls from heights. The philosophy is to follow the safety hierarchy, with working platforms as the first order of protection, while harnesses and nets are a lower order of protection, and used only if providing a working platform is not practicable. The solution at a particular site also depends on the training and competence of the operators, the quality of supervision, and the type and familiarity of the equipment supplied in response to the designl4. The nature of the industry, with its changing work force, also has an effect on the solution adopted; more experienced operatives require less training.

Consider complete enclosure

More experienced operatives require less training

For example, consider the removal of flying tables from a lower floor; the Regulations make provision for the temporary removal of barriers where it is not reasonably practicable to operate with them in position, but the Regulations still require “suitable and sufficient means to prevent, so far as is reasonably practicable, any person falling.” Thus consideration of the lower order protection, such as nets or harnesses, is appropriate. The use of harnesses in certain temporary works activities has been shown from the ECBP research to hinder performance and compromise safety, so the importance of the risk assessment at an early planning stage of construction is emphasised.

Temporary removal of barriers

The Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) (Ref. 23) were introduced after the ECBP was completed and during the preparation of this Guide, but have not significantly altered the requirement for safe handling of all temporary works, both mechanically and manually.

LOLER

5.2 Material handling The handling of temporary works equipment on site needs careful study; not only because of the method of work adopted, but because of the varying ways in which the constituent parts are handled. Comments have been made in Section 4.4.6 on the lengths of components. Handling starts with procurement, for example, specifying whether stillages, banded or loose, wheeled pallets, etc. are used for initial delivery. From arrival on site, the internal movements need considering, as components or when assembled into larger units for handling, such as flying tables. Under nearly all conditions of construction, once a floor slab has been cast, there will be peripheral items to move up to a higher level. On the flying tables, boxes can be fitted to the standards to move some of the loose parts, while on the skeletal systems items can be collected on pallets, or possibly banded if larger or longer items are to be handled, prior to moving to loading-out platforms. These will be all the manually fitted items, such as the make-up pieces between units, props and beams around openings andor staircases, including temporary guardrails, tube and fittings, etc. The use of manual pallet shifters is recommended. The original planning of the ECBP was to construct the building by passing items manually up through the stairway openings to subsequent levels. Although a reasonable concept, it was found that many of the components were either too long, or too heavy, when palleted together to pass up manually. The storey height of 3.75 m also restricted vertical handling. This meant that a loadingout platform was used at every level, as discussed in the next Section.

14

Once again the TWD and possibly the PWD have an influence on the safety of the system selected.

42

Procurement important

5.3Loading-out platforms

Unsafe lifts from lower floors

Although not observed at the ECBP, an unsafe method is sometimes seen: a crane is connected to a stillage inside the building line, which is under a recently cast slab with projecting working platform. This requires removal of the local guardrail. This practice should be discouraged and will generate an unsafe lift, unless special equipment such as a balanced C-hook is used.

5.3 Loading-out platforms Use loading-out platforms if no external scaffolding

The original concept of the ECBP was to construct the building with limited cranage, but manual handling between floors was found to be impractical and a loading-out platform was used for all the construction methods and systems. Due to the difficulty and cost of loading-out platforms, described in the following paragraphs, the additional cost to the temporary works of ‘designing out’ their requirement should be actively pursued. It should be remembered, however, that such platforms may also serve following trades, making them more cost-efficient and beneficial for the overall project.

Projecting beams to form platforms cause steps

Ideally, the loading-out platform should not simply consist of beams projecting from the building, laid on top of the cast slab and made into platforms. This method produces a step in the platform, because it is not at the same level as the cast slab top surface, and the beams can introduce reverse bending into the newly cast slabs.

Fit top surface flush

It is recommended that loading-out platforms are incorporated which are properly designed, and fit flush with the top surface of the slab they are to service. Fitting platforms to suit the cast top surface of the floor slab will facilitate handling equipment on pallets with small manual pallet shifters; pallets can then be moved from underneath the previously cast slab to the platform, for direct connection and slinging onto the crane. A typical application of several platforms in use on a building is shown in Figure 10, each platform having guardrails, toeboards and notices identifying the safe load on the platform.

to current activity

It is also recommended that consideration is given to having two such platforms to keep pace with the varying levels of construction: one at the level being struck with its platform top flush to the

Figure 10: Typical loading-out platforms in use.

43

5.3 Loading-out platforms

top surface of the lower slab, and one at the higher level flush to the next floor. The top of this second platform should ideally be set to the top of the recent new slab to aid material movement for following trades. In certain circumstances, such as when manually handling beams or face contact materials to the working area of the soffit formwork, a third platform might be justified. This platform is ideally fitted with its top surface not at soffit level, but raised up by the slab thickness. This would avoid repositioning its level after concreting the slab, and might require a small temporary ramp down to the soffit level. There is also a good technical argument for having more than one loading-out platform. The likely applied imposed loading on a platform should be considered. A typical quoted loading for general material storage is between 4.0 and 4.5 kN/m2 (Service Class 5 loading) and would be generated by pallets of equipment weighing up to only 400 kg. The legs of pallets create very high local loads and special platform surfaces are required to cater for the impact and point loads applied. Thus it is not good practice to lift a pallet onto either a decked-out plywood area of soffit formwork, or onto a recently cast slab that has not yet been struck.

General storage loading 4.5 kN/m2 (Service Class 5)

When brickwork is to be handled into the building, pallet loads will increase significantly - for example, the footprint load from a single pallet of bricks can vary from 14 to 17.5 kN/m2 depending on the weight of commonly used bricks. It is recommended that loading-out platforms intended for use by brick pallets should be designed for an imposed load of 20 kN/mZ.

Brickwork loading-out platforms - design for imposed load of 20 kN/m*

Lifting loads from one loading-out platform to another has the benefit of the pallet being landed onto a designated area, with correct decking, guardrails and toeboards. Considering the safe handling and slinging of the loads: the platforms, being external to the structure, will not interfere with general construction, and should give better vision and more lifts for the crane. From a process point of view, the materials delivered to the site for a particular floor will have a common route from delivery to floor location. Loading-out platforms should always be clearly marked with sign boards showing the safe load in kNIm2.

Sign boards showing safe load

Proprietary loading-out platforms are available in the form of system towers, with integrated safety rails and gates, for either front face loading or side access for congested sites. Typical safe loads are 2000 kg per platform on a 2.4 x 2.4 m system module. Loading-out platforms can have implications for both the PWD and the client, in that ground space may have to be allowed for them from the foundation level, see Figure 10. Support does not have to be taken from the ground: it can be taken from the building, and proprietary systems are available for cantilever loading-out platforms, i.e. projecting out from the building line. They generally comprise twin steel beams with a raised platform between, thus making wheeled access onto the platform harder than moving equipment off the platform onto the concrete floor. They have been found to be both expensive to hire and impractical on a busy site, as they often are required to be proof tested under the lifting regulations (Ref. 23) prior to fitting and use at each floor.

Possible proof testing of cantilever platforms

5.4 Temporary works to columns Section 4.4.2 recommends that adequate working platforms are included when procuring column forms. They should be complete with full platforms and guardrails to all sides. The TWC should verify that this is what has been supplied.

Adequate platforms

The stability of columns is normally achieved with push-pull props on two faces at right angles. This is particularly difficult to arrange for rectangular corner columns to buildings, and methods to provide adequate stability when only half a column form is erected should be developed. Figure 11 shows the arrangement at (a) and (c) where only one push-prop can be attached to the first half of the 'L' forms erected, whereas the arrangement at (b) permits both props to be fitted and provides two-way stability to the form for the left hand corner only. Note that the access platform at the top will not be complete until the second half 'L' has been fitted.

Stability - particularly of corner columns

44

5.5 Temporary works to walls and lift shah3

‘F (a) Not safe

(b) Safe

(c) Not safe

Figure 11: Plan of column form at corner and edge of building.

Avoid loose parts

The method of connecting the two halves of the column form together needs to be carefully considered. Ideally there should be no loose parts that could be dislodged during fixing or striking of the forms. The method of lifting the column formwork should be considered - proprietary lifting points should, if possible, be incorporated on the formwork.

Connection to slab?

When considering the fixing details for the propping used to stabilise the column forms, particular care should be taken to reduce tripping hazards, such as by using projecting bars from the top of the slab, as discussed in Section 4.4.4.

Cast columns 20 mm into slab

The practice of casting columns about 20 mm above the final sofit level (Section 5.5.2 of the Formwork Guide (Ref. 5 ) ) is recommended to allow for elastic shortening of the falsework as load is applied.

Importance of curing

The production of durable concrete requires the correct curing of columns to reduce water loss from the surface. Once the forms are struck, the method of curing, and time of application of the curing compound andor protective membrane can affect the development of durable concrete. A delay in commencing curing of more than three hours after striking can be critical. Further guidance on curing is given in Concrete Society Current Practice Sheet 112 (Ref. 33).

5.5 Temporary works to walls and lift shafts The client will often want the layout of the flat slab structure to be as flexible as possible, with open plan areas and as few walls as possible, but the PWD will need to incorporate lateral stability into the structure. Generally this will be achieved by cross-walls, either erected separately or cast in situ as construction progresses. The lengths of walls will be kept to a minimum, and will often be sited around the lift shaft, providing benefits for lateral stability and fire protection. Formwork to walls and lift shafts will follow traditional methods, either as panel formwork with make-ups as necessary, or as traditional soldiers and walings with some complexity at the corners, as shown in Figures 12(a) and 12(b). An innovative idea for the fast construction of lift shafts is to Precast

Make-up panels

(a) Panel forms

(b) Soldiers and walings

Simple formwork

(c) Precast with straights

Figure 12: Typical plan arrangements of formwork to a lifi shafi.

45

5.5 Temporary works to walls and lie shafts

incorporate precast corner units, as shown in Figure 12(c), thus allowing simple straight sections of formwork to be used between the precast units. This speeds up erection and use. The design of double-faced wall formwork is covered in the Formwork Guide (Ref. 5). The use of kickerless construction for walls was discussed in Section 3.4, and one method of providing kickerless construction for walls in multi-storey construction is the use of precast concrete blocks, either cruciform or waisted, as shown in Figure 13. These can be either site- or factory-made to suit a particular building dimension. The blocks, approximately 50 to 75 mm thick, can be made with a through hole in the top, and are suspended off the top tie rod of the formwork. The blocks can be positioned at wall ends or at tie rod positions, depending on the arrangement and length of the formwork. The use of such blocks ensures vertical alignment and accuracy of reinforcement of subsequent walls at a higher level. The system is also adaptable for use on perimeter walls. The practice of casting the walls about 20 mm above the final soffit level (Formwork Guide (Ref. 5)) is recommended to allow for elastic shortening of the falsework as load is applied.

Wall with slab cast

Precast block

Kickerless construction (Section 3.4)

Wall is over the wall below

Cast walls 20 mm into slab

Part of wall formwork

Figure 13: Typical precast block for wall kicker.

5.6 Temporary works to slabs When initially assembling the falsework, particularly when tables are used, consideration should be given by suppliers to incorporating a form of plan brace at or near the head level. This would have two significant benefits, firstly, ensuring that such towers are erected square and, secondly, giving additional stiffness at head level during handling and use.

Plan brace as aid to assembly

Erection and use of falsework near slab edges, not only during erection but also when striking, can be hazardous: it will depend on the proximity of the temporary guardrail, but operatives reaching and handling items close to the top of the falsework are very close to the edge and possibly at risk. Recommendations on edge distance are given in Section 4.4.3.

Edge distance hazards

A critical aspect observed at the ECBP was the use of harnesses and the placing of plywood sheets and beams at the leading edge. This is always a difficult area, but operative safety has to be paramount. The solution is to assess the work and the risks involved carefully by reference to the method statements issued by the suppliers.

Use of harnesses

Provision should be made for temporary debris fans, possibly netting, at lower levels to catch items on the edge (see Section 3.10). Special provisions would be needed at the corners of the building. Even under the research conditions at the ECBP there were at least five incidents involving equipment accidentally falling off the building from the floor being worked on. Fortunately, nobody was injured in any of the incidents, although one of the TW consultants was rather shaken.

Provision for debris fans if no external scaffold

One of the incidents involved the flying aluminium tables, when one of the trolleys used for moving the units became detached and dropped out of the building. It is recommended that only fixed

46

5.7 Temporary works to precast slabs

trolleys be used to handle such tables. They do not need to be permanently attached, but must be captive when the table is moved. The erection of guardrails and toeboards as temporary floor edge protection, just after the slab has been cast, needs careful consideration. Edge protection research

Tolerances of permanent works

Elastic shortening of vertical steel standards

0.5 mm per metre plus 0.5 mm per joint (1.0 mm if timber joint) Typical values for aluminium allow 0.9 mm per metre

Initial set-up level differs with individual props

Restrain soffit formwork against the permanent works

The C(HSW) Regulations (Ref. 16) state the legal requirements, and detailed guidance is given in the HSE book Health and safety in construction (Ref. 34). Ideally edge protection should be fitted before the supporting falsework and its own edge protection are removed. Some sort of fixing into the top or edge of the new slab is required, but it cannot be clamped underneath the soffit, because the formwork is still in place. If a very fast strike is needed to achieve the optimum construction cycle, then these hand-rail fixings may be fitted to the top of the slab while it is still being protected and cured, often with protective sheetingkoverings. Hence a practical detail is needed, and selection of a suitable solution has the distinct safety and speed advantage that the building’s guardrails are fitted once only. Proprietary soffit systems are unlikely to suit exactly the permanent works dimensions, and will also have to allow for the tolerances in casting the columns - see NSCS (Ref. 7). Make-up sections between tables, ends of formwork beams, fitting of areas to the edge of the systems, and around the columns, may all be difficult areas for access. Working platforms need to be considered to all these areas, especially where LOK-test inserts (for concrete strength assessment) are required to be left in the undersides of slabs (see also Section 4.7). It is normal practice to set the soffit formwork to a level slightly higher than the required finished soffit level to allow for elastic shortening of the standards and take-up at the formwork and falsework joints. This is good practice and Clause 6.5.2 of BS 5975 (Ref. 6) gives some guidance on allowances to be considered. For the storey heights used in building these tolerances should be between 3 and 5 mm, dependant on whether steel or aluminium systems are used. It is recommended that, in the absence of detailed calculations, the following values of elastic shortening be used: 0.5 mm per metre (steel) Shortening of vertical standard 0.9 mm per metre (aluminium) 0.5 mm per joint (metal) Take-up at joints in standards 1.O mm per timber joint Take-up at timber bearers During concreting, the falsework supports will shorten and the soffit formwork adjacent to the existing walls and columns will move down accordingly. It is recommended that columns and walls are cast about 20 mm high to allow for this. (See Sections 5.4 and 5.5). At the ECBP, when adjusting the soffit formwork to the correct level, it was found that setting the formwork approximately 5 mm low before the final adjustment suited most of the falsework systems. The exception was the individual prop system, which benefited from setting the formwork a little high. Where the falsework system has been designed as the top restrained by the permanent works, the lateral stability will need to be checked before concreting. The connection of the system to walls or columns needs to be considered in all directions. It is common practice to use the face contact material to transfer these horizontal forces in compression. Casting the walls and columns into the slab assists this restraint as the face material can be butted directly to the permanent works

Set jacks low with long beams prior to final levelling

When using lengths of stiff beam continuous over several standards, the physical act of raising one jack to level will lift the system slightly at other positions - in contrast, if one tried to set the system high, lowering one jack was found to be very difficult.

Cure correctly

Correct curing of the top surface of slabs is required to reduce water loss from the surface. The method of curing, and time of application, should be agreed. Guidance on curing is given in Concrete Society Current Practice Sheet 112 (Ref. 33).

5.7 Temporary works to precast slabs The use of precast planks as permanent formwork may or may not require temporary supports. For example, in housebuilding the planks are often supported directly on the walls, but in large commercial

47

5.7 Temporary works to precast slabs

open-plan flat-slab structures with few columns, such as the ECBP, the systems often require falsework as support to the planks until a structural topping concrete has been placed. Where falsework is involved, the interface of responsibilities should be established at a very early stage of procurement and proper control exercised (see Section 4.1 and Figure 8). At ECBP no formal falsework design was undertaken: although the precast plank supplier did suggest some proposals to BRE, these were not a falsework design. There are fundamental differences in the design of falsework with a sofit form to those with only primary beams and thus without continuity at the head level. The designer of the falsework for a precast system has a key duty as a designer under the CDM Regulations (Ref. 4) to ensure that the assembly will be stable at all stages of construction, i.e. including the temporary stage when supported on falsework. The supplier of the precast system also has a duty under the HSW Act (Ref. 35) to provide sufficient information to the contractor, and specifically the TWC, to co-ordinate the technical aspects relating to the safe use of their product. The TWD should address the detailed concerns at an early stage of the falsework system design, such as: stability of temporary works systems without continuity at the top, particularly where restraint to the falsework is expected from the permanent works, through the precast planks

I f falsework required establish interface early

Fundamental difference in falsework design: a) with soffit form b) with planks discontinuous

TWD’s concerns: stability, access, platforms

access to infill strips between planks working platforms for placing units and subsequent slab operations. Placing precast planks at leading edges can become a hazardous operation and risks can only be partly reduced through the use of fall arrest safety harnesses. The problem arises because the supporting system has no deck; the precast planks are acting as permanent formwork and are intended to eliminate the necessity for any temporary formwork. A solution may be to provide a complete working platform, which could be expensive. However, it may be that only a part working platform is required - with appropriate edge protection where no platform is provided. What is clear is that before construction is started the risk assessment process should lead to the preparation of detailed method statements including sequences of erection, essential for the safe operation of temporary works.

Risk assessment and method statements at leading edges

It is recommended that all precast units be marked with orientation pointers and lifting point positions identified to comply with the supplier’s issued erection drawing and method statements. This is particularly necessary where panels may appear similar, but actually have different reinforcement, and wherever panels are designed to be adjacent, such as on two-way spanning units. At the ECBP this would have considerably reduced erection times.

Orientation pointers

At the ECBP there were problems at the joints between panels with projecting reinforcement, and with the tolerances at the joints between the columns and the precast planks. They are discussed in the following paragraphs, and arose from a lack of co-ordination between the PWD, the precast plank supplier and the constructor. These details should have been discussed and resolved between the PWD, the supplier and the TWD during the preconstruction stage (see Chapter 4). #en precast units are required with projecting reinforcement, clashing of reinforcement from adjacent panels must be avoided. The innovative use of precast panels at the ECBP with two-way spanning panels ‘stitched’ together was a unique opportunity to study the implications and learn lessons from the supply and use of such arrangements. If ‘North-South’ marks are included on the units they can be individually matched to avoid reinforcement clashes.

Avoid clashing reinforcement from adjacent panels

It is recommended that PWDs should give more consideration at the design stage to the column/ plank interface and should discuss at contract stage the implications with the relevant parties, particularly the TWD. The detail of the design and the precast plank tolerances may have a significant impact on the vertical tolerances of walls andor columns, on cover to projecting reinforcement and on positioning of construction joints. Guidance on tolerances is given in CIRIA Publication C558 Permanentformwork in construction (Ref. 15).

PWD to consider column/plank interface

At the ECBP the columns required casting to a higher than normal standard of vertical tolerance. There are normally three solutions to the problem of the column/plank interface: (a)

48

Cast the column the exact height and bed the planks onto them (i.e. bed within the cover zone of the reinforcement).

Tolerances

5.8Cranage

Communicate tolerances to constructor

(b)

Cast columns high by, say, 20 mm. The PC unit supplier must determine the size of the box-out to give sufficient tolerance and have a method of sealing the resultant gaps.

(c)

Cast the columns deliberately low, with the precast planks on packs. This is not ideal because of the difficulties of forming the column extensions.

Unfortunately, at the ECBP, on the half of the floor which used the thicker planks ‘glued’ together, the PWD specified that Densit had to be over the entire column top area, thus solution (a) was not possible. Solution (b) was attempted, but was further complicated by the additional requirement for the bottom reinforcing bars from the planks to meet across the column heads. Tolerances on the vertical height of the columns were not communicated to the constructor, and, knowing that they were to be seated alongside units of 75 mm depth, the tight 20 mm control into the underside of soffit level was not strictly adhered to, with the result that several column tops had to be trimmed off on site to fit. Thus there was no alternative at the ECBP and the planks had to have cut-outs, which made for great difficulties in positioning and sealing the joints, plus difficulties of aligning the tops of the columns and the meshing reinforcement. The above problems highlight the need for PWDs to make the relevant allowances in their design when selecting such precast alternatives. Having early discussions with the TWD to envisage the practical problems will give significant benefits at construction stage.

Research needed into innovative edge forms when using precast planks/slabs

Precast suppliers should be encouraged to develop and supply edge forms that connect directly to precast slabs. One note of caution is that they should be so designed that it is possible to fit the slab reinforcement without the need to thread the bars under straps or supports to the stop ends. One of the problems previously encountered in the use of edge forms provided and attached to precast slabs is the difficulty of providing a true and straight edge to the building, without unnecessary ‘dog legging’ of the edge forms. Any innovative solutions should take account of alignment issues.

Research into guardrail fixings into precast construction

In a similar way to that discussed in Section 5.6, when using any precast system, whether for slabs or for precast staircases, the provision for connecting temporary guardrail posts or assemblies should be considered, preferably as part of the system. Where the precast units are relatively fragile, perhaps only 75 mm or even 30 mm thick, then the guardrail provisions may best be incorporated into the falsework support system.

ECBP had a dedicated crane working on the frame construction. The location of the crane is shown in Figure A3 in Annex A. On conventional sites other trades would share the use of the crane, e.g. brick layers, plumbers, plasterers, unloading equipment. Unrestricted crane use, such as at the ECBP, is rare. Crane needed on most generic systems at ECBP

’Hand‘s free’ radio for slinger/signaller

Team operating flying tables/forms to have basic slinger/signaller training

The crane was considered a necessity at the ECBP for most of the systems used. A much larger crane is needed for handling precast units and aluminium flying table systems, than for the strip and re-erect systems, which would use the crane mostly between loading-out platforms. The lifting radius would not have been so large on these systems; it might be possible to eliminate the crane altogether if another method of moving 1000 kg pallets were used, e.g. an external rough terrain forklift truck with a long boom. The slinger/signaller, when directing the crane using the radio, effectively works ‘one-handed’. This can create problems when trying to land awkward-shaped objects such as column forms. A radio that can be used ‘hands-free’ would make the slinger/signaller’s job easier and improve safety and performance. This applies to applications when the slinger/signaller is assisting the operatives with the formwork. Another aspect of the safe use of the crane was the position of the slinger/signaller on the aluminium flying table system. It was observed that, when the tables were being removed, the slinger/signaller controlled the crane from underneath the slab, i.e. from the floor on which the tables were being rolled along. There is an important need for this operation to be controlled by radio, because thls slinger/signaller cannot see the crane. During the removal operation there will be an operative(s) on the top of the slab as the table comes out who might also have to give instructions to the crane. It is recommended that all members of the special team operating the flying tabledforms should have received at least basic training as slingers/signallers and be aware of the correct crane signals.

49

5.8 Cranage

Care is necessary with short tables in gusty conditions when they are only attached on front slings as the natural rocking action of tall tower cranes can be significant and has been known to pull short tables out of a building. The use of the crane with flying tables or flying forms as units of formwork will have been planned and considered in the method statements and risk assessments. It is recommended that compensating clutches be used on all table and flying form lifts involving four unequal legged slings. During operation of the system, there will generally be positions in the cycle when only the front slings are attached to the unit. To connect the rearward slings, the common solution is to ‘rest’ the table onto the slab to take the weight off the front slings; this has safety implications to prevent the unit slipping, but more importantly can impart high upward forces into the newly cast slab. Some systems with supplier’s method statements allow the units to be rolled out of the building, past their centre of gravity, without the crane attached. The unit is expected to tip and the back edge of the table then wedges the unit against the previously cast sofit. The four lifting points are exposed and the crane is not required until the table is due to be moved.

Gusty wind and tower crane

Use compensating clutches if not using C-hook Tipping of tables and resting on underside of cast slab not under crane control is deprecated

With the use of thinner slabs, often struck after only 24 hours, these methods of work are deprecated. An alternative method of moving large sections of tables, as flying tables, or otherwise, is to use a large purpose-designed lifting appliance such as a C-hook. These have to be designed and routinely tested as lifting equipment (see Ref. 23). They are ideal for moving tables of similar length, but generally are not suited to handling both short and long tables. The method of handling would require a safe working procedure to be established.

Establish procedure

5.9 HmgOicatioans of chan Falsework schemes and drawings should not be altered without the approval of the TWD. On-site supervisors should also be aware that changing details on drawings and sketches without reference to the TWC can seriously affect the safety and operation of the system as designed.

Refer on-site changes to TWC

At the ECBP the sketch provided for the steel skeletal system showed three bays of frame, one on each side of the slab and one in the middle. The gap between these bays was clearly shown on the drawing as 750 mm with no connection between them. The bays were actually erected with a gap of only 600 mm between them as horizontal ties of 600 mm had been provided for another area of the work. Once the bays were spaced, the 600 mm ties were removed. The direct effect of moving the outside legs inwards by 150 mm (i.e. 750-600 mm), increased the outer standard leg load by 10.4% and increased bending in the primary beams by 6%. It also reduced the external working platform from 815 mm as drawn to only 585 mm - below the C(HSW) Regulation minimum width of 600 mm (Schedule 2, Ref. 16). This on-site amendment highlights the need for training of site staff and a control system operated by a TWC. There is an obvious requirement to make suppliers and constructors more aware of the reasons why certain equipment is specifically needed. Although, economically, the culture of accepting substitutions because they are available is tempting, the overall effect on productivity and speed may actually be compromised. The change in supply of aluminium beams to different modules had the effect of increasing by 33% the number of tables moved, with a likely corresponding reduction in overall performance. See also Section 4.4.7.

50

Train site staff

Suppliers and constructors to be aware why specific equipment needed

6 Backgr und to striking and backpropping Recommendations - Striking and backpropping flat slabs General 1. It is essential that the technical staff involved in backpropping and striking are fully aware of the implications of the striking and backpropping procedures. 2. Procedures such as the 'Permit to load the falsework', and 'Permit to strike the formwork/falsework' must be in a written format: verbal procedures are not sufficient. Checks should be established to ensure that reinforcement is correctly located. Propping, striking and backpropping should be carried out under supervision to agreed procedures. 5. The Permanent Works Designer should make available the necessary information, such as design loads and concrete strengths, to enable the TWC to evaluate striking times and propping. 6. The Permanent Works Designer may need to consider the temporary loading condition during construction, and the possible loading of a slab above its design service load. 7. The method of establishing the required equivalent concrete cube strength in the critical area of the slab should be agreed before construction starts. The LOK test is the recommended method. 8. Propping and striking procedures should be on the agenda of every construction progress site meeting. 9. The PWD and TWD should use a concrete density of 24 kN/m3 in the design of the structure and in backpropping calculations, unless another value is specified.

3. 4.

Striking 10. The method of establishing the equivalent cube strength of the concrete in the critical area a t the time of striking should be stated. (Further information is given in Section 7.2.) 11. When considering early striking, the strength of the youngest concrete in the structure is the criterion. This is likely to be predicted from testing slab concrete from the last area cast near to an internal column.

12. The critical conditions that should be checked are: the strength of each slab prior to striking, the strength of the supporting slabs when subjected to the loads of concreting the new slab. 0

Backpropping 13. The relative arrangement of falsework and backpropping has a significant effect on the force transmitted through the supporting slab into the backpropping. 14. Backpropping should ideally be fitted below the supporting slab with four props at third span points, and this should be repeated down through the structure, as required, in a similar manner. Continues overlea$..

51

6.1 General

Backpropping (Continued) 15. For backpropping calculations assume that 70% of the total load from the casting slab through the falsework will be carried by the supporting slab. 16. The strength of the supporting slab should be checked prior to casting, assuming that 70% of the relevant loading from the slab being poured through the falsework will be carried by the supporting slab. 17. I n calculating backpropping loads, subject to confirmation in the site risk assessment, no imposed construction operations loads need be considered. 18. Proprietary suppliers should have available the stiffness properties in kN/mm for each type of prop used for backpropping. 19. Where backpropping is to be pre-loaded, the method and loads should be agreed between the PWD and the TWC. 20. Steel and aluminium vertical propping must not be mixed in one support system for a given floor slab, either as falsework or backpropping.

21. It is essential that the location and arrangement of the backpropping are fully checked.

The recommendations for striking and backpropping in Chapters 6 and 7 are applicable to structures with the following characteristics: (a)

The slab is in situ reinforced concrete or a composite with participating or non-participating permanent formwork, such as thin concrete precast concrete units.

(b)

Maximum.equiva1ent thickness.of dense concrete 350 mm15.

(c)

The slab is not post-tensioned or cast on thick precast prestressed participating concrete units.

(d)

The slab is two-way spanning with or without beams, or one-way spanning with beamsl6.

(e)

The cast slab is intended to be struck and become self-supporting before any additional loads are placed on it.

(f)

The reinforcement is positioned within specified tolerances.

(8)

Where backpropping is to be used, the suspended slabs at each level are of similar construction, thickness, etc.

(h)

Where used, there are no more than two levels of backpropping.

Striking of the formwork and falsework from a cast slab needs careful consideration, including detailed procedures, method statements, and backpropping of loads through lower floors. The constructor is responsible for safe operation of the striking process: the onerous responsibility of ensuring that the risk of striking is correctly managed rests with the TWC. Obviously, the agreement of the PWD is important and the TWC should liaise on matters such as strength assessment, methods of curing, and order of striking. Both the PWD and the TWC as ‘designers’ under the CDM Regulations (Ref. 4) have responsibilities for ensuring that the overall design has taken into account the temporary works. This will include the provision of backpropping as necessary throughout the building. This Guide places emphasis on following well-planned procedures, and control of striking

15 The depth of 350 mm was chosen as representative by the research. A thicker slab has greater stiffness and the ratio of the loads (deadimposed) is likely to be different, so the steering group limited the method to 350 mm thickness.

16 Particularly stiff beams will affect the deflected shape of the structure and the method adopted may not be accurate.

52

Requirements for flat slabs: Chapters 6 and 7

TWC responsibility to manage the striking process

Designers to take account of any temporary works

6.1 General

and backpropping are critical procedures in safe flat-slab building construction. BRE Report BR 394 (Ref. 2) fixther emphasises this aspect: Slab construction in multistorey buildings to become a precision operation

“.

. . it has to be recognised that construction of the slabs in multi-storey buildings must become a precision operation where the constructor can reasonably guarantee that the props and backprops are placed in the agreed positions at the appropriate time and that they are not moved prior to the agreed time for their removal defined by the construction programme.”

Application of method for striking a flat slab see Chapter 7

This Section discusses the background to backpropping and striking and recommends a method of determining the required strength for striking slabs. This method save significant time and reduce backpropping, while still producing safe and serviceable slabs that have not been damaged during construction. The application of the method is covered separately in Chapter 7.

Crack width and serviceability as a criterion for striking

Research by the University of Leeds under PiT Task 4, published as BRE Report BR 394 (Ref. 2), verified that crack width and serviceability are more rational criteria for striking than flexural strength, and permitted earlier striking of flat slab construction. The previous CIRIA method (Ref. 31) does not consider cracking of the slab as a performance criterion. Theoretically, cracking of slabs at early age is a more onerous criterion than moment capacity. The effects of punching shear were considered in the research.

The procedure should be understood by all parties prior to use

The methods for early striking introduced in CIRIA Report R136 (Ref. 3 l), described in detail in the Formwork Guide (Ref. 5) and BS 5975 Code of practice for falsework (Ref. 6), all give safe values for the required strength of the concrete at time of striking the slab. The proposed method is based on a very different concept to the traditional idea of the ‘seven day strike’ on general building work, and the faster ‘three day strike’ on fast-track proprietary systems. The new procedure should be understood by the interested parties before use. The basis of the proposed method, subject to the limiting criteria detailed in Section 6.2, is that, providing cracks or deflections generated within the concrete slab at the time of striking are not larger than those expected when the full strength slab experiences its full design service load, the structure will not be harmed by early striking. The requirement is for a simple procedure to produce a safe and serviceable slab that has not been unacceptably damaged during construction, such as by deflection17 that can safely carry long-term design loads in service.

Punching shear and flexural strength criteria considered

Designers will be aware of the problems of punching shear in flat slabs without drop heads. Shear failure is sudden and occurs without visible warnings, so care is necessary in design. The research (Ref. 2) compared relationships for punching shear and flexural strength at ultimate limit state condition. The conclusion, by consideration of the EC2 and BS 8110 rules, was that the more onerous criteria for punching shear would be an acceptable means of limiting flexural cracking in flat slabs. This principle has also been adopted in this Guide.

Flowchart Fig. 20

The method recommended for applying these criteria for striking flat slabs in buildings is shown as a flowchart in Figure 20 and detailed in a sequential format in Chapter 7.

Construction staff to be aware of TW implications

The operations of backpropping, loading and striking slabs should only be carried out under supervision. Where early striking is to be based on the proposed method it is essential that the technical staff involved are fully aware of the implications of the procedures and method. This is not a threat to demonstrate the analytical nature of the subject and warn constructors off using the method, but is to raise awareness of the complexity and importance of the subject. Few people in construction realise the risks that may be involved in backpropping. A casual survey of site operatives and supervisors will demonstrate that, although they are aware of the use of backpropping, few realise how changes in the type, position or installation of propping can affect the structure and, more importantly, its integrity.

Risks in backpropping

Agreed procedures for propping and striking

All installation of propping, striking and shoring should be carried out to agreed procedures, as discussed in detail in Sections 7.6 and 7.7.

Risks in projecting results

It is accepted by this Guide that there are risks in projecting results from only a few studies for acceptance for general practice. What has been demonstrated by Task 4, and confirmed by other research (Ref. 37), is that the proposed method can be confidently adopted for similar structures

17 Unacceptable deflection may be defined as deflection that is harmful for the member concerned. (Clause 6.2.6.3.2 of BS 81 10: Part 1 (Ref. 36))

53

6.1 General

within defined limits of type of structure, thickness of slab, etc. One of the core objectives of ECBP was to promote the economic advantages of this type of flat slab structure, without complex downstands or drop heads to columns, and early striking is a key issue.

Promote economic flat slabs

Proposals to change loadings (see Section 3.2), and the introduction of ‘lean construction’ have brought about more economic, flexible, slab design (Section 3.3). As PWDs design for lower imposed loads, such as those used at the ECBP, loads during construction will become more critical. If a floor slab is unable to support the one being cast above it, the loads have to be distributed between the floors. On a low-rise building, say up to three storeys, it may be economic to transfer the forces directly to the ground by leaving propping in place at all times, but in multistorey work, this becomes impractical and load transfer between floors is required.

Current design imposed loads are considered conservative

It is possible that backpropping considerations from the temporary works design may give a lower limit of imposed load on a suspended slab. This could naturally limit what is achievable in terms of slender construction. This is covered in more detail in Section 6.5.

Consideration of backpropping might limit minimum imposed load on a slab Deflection considerations (Section 6.3)

Short-term and long-term deflections are critical for leaner structures - see Section 6.3. With the earlier striking times now proposed, it is recommended that each cast floor area is struck completely and allowed to take up its deflected shape under self-weight before additional slabs or areas are cast. When construction continues on higher floors without allowing the lower floor slabs to take up their deflected shape, the cumulative loads from subsequent construction will impart large cumulative forces into the temporary works, and possibly the slabs. BS 5975 (Ref. 6) demonstrates in Annex M how this could be more than 225% of a floor weight! This serious risk of overloading lower slabs by cumulative loading can be significantly reduced by adopting the recommended procedure for each floor slab cast. In practice, constructors using one set of formwork are unlikely to have this problem. To avoid excessive loads building up cumulatively in the backprops, it is also recommended that backprops are slackened off after striking a slab and then repositioned and retightened as required, to an agreed procedure. See also Section 7.7.

Allow each floor slab to be struck completely to take up deflected shape

Slacken and retighten backprops each time

6.2 PhiOosophy for Ooading and striking ffoau sOabs The principle adopted in this Guide and published by Beeby in ICE Proceedings ‘Criteria for the loading of slabs during construction’ (Ref. 38) is that the early loading of slabs either due to striking of formwork or due to other construction loading should satisfy the following two relationships:

Ref. 38

Loading factor

F,,, 4 1.0

Equation 1

Cracking factor

F,, I 1.0

Equation 2

A further criterion, from knowledge of the deflections caused by slabs cracking under load is considered as: Effective deflection factor Fe, I 1.O

Equation 3

Except under very special conditions, no slab should be subjected to a load greater than its equivalent design service load at the time loading is considered, nor exceed the actual design service load, determined from Equation 4. As the slab matures and gains strength, the load it can carry will increase up to a limit - its total design service load - at which point the characteristic concrete strength must have been achieved. The slab may not be able to carry any further load without permanent strain or cracking, in excess of that implicit in the design.

No slab subjected to load greater than design service load

It is implicit in the previous paragraph that, to load a slab to its full design load, the reinforcement has to be placed in the correct position. If the reinforcement around the columns is not at the correct depth, this will have a direct effect on the behaviour of the slab, particularly on crack formation, and may increase long-term deflections; thus the quality of workmanship may need to be considered in any striking procedures. This should include checks that the reinforcement is correctly positioned before concreting takes place.

Reinforcement location can affect long-term deflections

The PWD and TWD may, after detailed consideration and a risk assessment, permit applied loads on a slab during construction greater than the design service load as they will generally be of short duration. This is discussed in Section 6.6.

See Section 6.6 and Annex E

54

Check to verify reinforcement before concreting

6.2 PhilosoDhv for loadina and strikina flat slabs

Strike earlier than intended and slab may be damaged!

It is generally assumed that the crack width in a loaded concrete slab is proportional to the stress in the steel reinforcement, which in turn is proportional to the load. Hence if load is removed or added, there will be a proportional reduction or increase in crack width. Although the slab is designed for the ultimate limit state, the actual maximum load on the slab at the time considered will be the summation of the unfactored loads because the consideration of crack width is at serviceability limit state, not ultimate. The load on a slab at any stage of construction should not be greater than the PWD’s unfactored design service load, so the ratio of imposed loads to dead loads is important. Obviously if the concrete slab is struck earlier than intended, the structure may be permanently damaged. The measurements at Cardington have shown that the cracking around columns has been as expected, and that cracking within the spans has not been in evidence. The Task 4 report concludes that:

BRE BR 394

Cracking at internal columns critical

Consideration of cracking allowance (Annex G)

“The highest moments are likely to occur around the internal columns but careful inspection at various times failed to reveal any signs of flexural cracking in these regions. Simple calculation suggests that cracking should be expected in this region at loads well below those actually imposed on the slabs. Cracking may be ignored in flat slabs and the conclusions also hold good for solid slabs supported on beams or one-way spanning solid slabs.” The location and extent of cracking vary throughout a slab, and control measures should vary accordingly. In a flat slab there is likely to be an area surrounding the columns where the moment becomes excessive. The most critical areas will be around the internal columns. The position of the top reinforcement also influences cracking; if the workmanship is poor and the top bars are incorrectly located, this will alter the moment capacity and affect the cracking. The forthcoming Eurocode 2 (Ref. 39) gives formulae for the analysis of members that are expected to crack under load, and which behave in a manner intermediate between the uncracked and fully cracked conditions. Permanent works designers may wish to allow this cracking to be considered in their analysis; this is discussed further in Annex G. Subject to the requirements in Section 6. I , the two main criteria for ensuring that a concrete slab being loaded or struck is not overloaded are: Loading factor ( F , ) =

Equation 4

Total design construction load on slab - w -I l . 0 Total design service load on slab wser

and Cracking factor (Fer) = -

Equation 5

where Characteristic strength is generally the specified 28-day strength

I 1.0

w = total unfactored design construction load on the slab in kN/mZ w,,, = total unfactored design service load in kN/m2 fc = estimate of the equivalent cube strength of the concrete at the location in N/mm2.

f,, = specified characteristic strength of the concrete in N/mm2 By rearranging Equation 5, the required equivalent cube strength (fcreq)of the concrete in the critical area (see Section 6.4) at the time of striking to enable a particular total construction load w to be applied to a given slab, may be calculated from:

Equation 6

which can be rearranged to give:

where

fc

req

=

estimate of equivalent cube strength of the concrete at the critical location in N/mm2

55

6.2 Philosophy for loading and striking flat slabs

f,,

= specified characteristic strength of the concrete in N/mm2 w = total unfactored design construction load on slab at the time considered in W/m2 wser = total unfactored design service load in kN/ m2.

In Equation 6 the term in the right-hand expression is the concrete strength factor C;, Concrete strength factor (Csf)=

Equation 7

Thus the relationship between the loading factor and the strength factor can be expressed as: Equation 8

A graphical presentation of Equation 8 is shown in Figure 22 (page 81). It should be noted that the value of Equations 4 and 5 should remain below unity except under certain condtions where, with the approval of the PWD and the TWC, it may be exceeded. For this reason Figure 22 includes values greater than unity. The method of establishing the equivalent cube strength of a slab for striking with a particular loading using the above equations is presented in Section 7.2.

Calculating the deflection of a concrete slab is complex, and is generally outside the scope of this Guide. It is inextricably linked to the extent of cracking of the slab, so early striking, also based on crack control, may have an effect on the deflection. The spreadsheet at Annex F includes the option to allow some cracking in the slab, as permitted in the forthcoming Eurocode 2 (Ref. 39). Recommendations in design codes generally limit the ratio of basic span to effective depth for beams and flat slabs depending on the support conditions. This is based on limiting the total deflection. This gives an acceptable serviceability condition for most flat slab structures. Clause 3.4.6.3 of BS 8110: Part 1 (Ref. 36) states:

BS 8110 CI 3.4.6.3

“Basic spadeffective depth ratios are based on limiting the total deflection to spad2.50 and this should normally ensure that the part of the deflection occurring after construction of finishes and partitions will be limited to spad500 or 20 mm, whichever is the lesser, for spans up to 10 m.”

Typical deflection/span ratio 11250

Within the elastic limits, deflection is proportional to load. This proportionality will, however, be lost if serious flexural cracking develops. Studies at the ECBP during construction showed that, though some limited cracking may have occurred around the columns, the deflections could be predicted reasonably closely if the cracking was ignored (Ref. 2). The assumption of proportionality between loads and deflections therefore remains reasonable when considering the same flat slab with similar loading patterns. Obviously, changing from a distributed to, say, central point loading would affect the moment and the deflections. There remains the question whether increased creep resulting from early loading could result in long-term increases in deflection. Results from the ECBP over the first eight months or so of the life of the structure tend to confirm that the effect is not great. However, a linear relationship was found between the long-term deflection (currently 500 days) and the minimum value of the parameter cf,lfcu)~~6(w,,,/w)for a slab up to the time considered. It would thus appear that this parameter (Equation 2 ) conveniently takes adequate account of any variations in the effects of creep, elastic modulus and tensile strength (i.e. all significant mechanical properties of concre t e).

Deflection proportional to loads

Calculations on the ECBP slabs (Ref. 2 ) have been made to predict the overall change in long-term deflections at 3000 days compared to those at 180 days as measured. This consideration is necessary to predict possible long-term deflection of the slab to reduce the effect on movement of internal partitions, etc. The results show that the calculated difference in instantaneous deflection between striking the supports at 24 hours (5.7 mm) compared to striking at 7 days (4.3 mm) is only 1.4 mm on the 7.5 m spans. The calculations assumed that the slab was not cracked. In all cases the deflections of the 7.5 m spans were well within the specification limits. Research on the effects of early striking and on deflections of slabs in general is ongoing (Refs 40 and 41). It is accepted that the early striking and subsequent use of a slab to support the next slab,

Long-term creep not a problem

Calculated instantaneous deflection at ECBP: 5.7 mrn at 1 day 4.3 rnrn at 7 day

6.4 Assessment of concrete strength at early age

Early striking increases long-term deflections

will lead to some increase in long-term deflection over that predicted for a similar slab struck at seven days (the comparison did not take account of any construction operations loads).

Long-term deflection increase is about + 25% for 24 hour compared to 7 day strike + 15% for 3 day compared to 7 day strike

Early indications from the research (Refs 2, 40 and 41) for the typical floor slabs at the ECBP suggest that, when using the striking principles in Section 6.2, long-term deflections will increase by about 25% when striking at 24 hours, and by about 15% when striking at three days, compared to the predicted long-term deflections (neglecting peak construction load) had the slab been struck at seven days. The comparison to the seven day strike is given as the PWD is likely to have used the seven day strike in the initial calculation of long-term deflection. Although the instantaneous deflection caused by early striking is increased, when considered with the overall design, the statement in BR 394 (Ref. 2) is still considered to be valid: “Early loading will not have a significant effect on the total deflection in normal circumstances”.

6 4 As~ssmelmuoff cocmclreue sUrelmgUh a%earlly age Importance of reliable strength assessment

Use the 95% confidence limit

Air-cured cubes underestimate in situ strength particularly in winter

It is important for users of this Guide to understand that the principles for striking recommended are different from previous ideas, and, to obtain the full economic advantages of early striking, the reliable assessment of the equivalent cube strength of the actual concrete in the critical area at very early age is needed.

Since it is not possible to obtain the characteristic strength of the in situ concrete, an approach is adopted in which the location of the weakest concrete in the slab is identified and the lower bound to the in situ strength at this location determined. The lower bound is taken as the lower 95% confidence limit to the in situ strength at this critical location. The rate of gain of strength of concrete at very early age, i.e. in the first 48 hours, is fast, and to be able to strike the slab within this time without damage, a realistic assessment of the actual strength of the concrete is needed. Traditionally this would be obtained from testing cubes, often cured alongside the slab, and these are reasonably representative of the concrete. CIRIA R136 (Ref. 3 1) confirms that, for suspended slabs up to 300 mm think, the differences in maturity between cubes cured alongside and the in situ concrete are small, and such methods are acceptable. However, evidence from the testing programme carried out jointly by the Queen’s University of Belfast and the University of Liverpool (Ref. 42), suggests that, under winter conditions at least, air-cured cubes are likely to have much lower maturities and therefore to significantly underestimate in situ strength at very early ages.

Pull-out test: BRE preferred method

Pull-out tests involving pre-planned inserts, LOK test, in the actual concrete structure were shown to be the preferred method of very-early-age strength assessment for striking. (Ref. 32)

Where was concrete sampled?

As striking times are reduced, where and when the concrete sample is taken become increasingly important. Take the example of a slab pour started on Day One at 0900 hours and not completed until 1500 hours on the same day. Next morning at 0900 hours, cubes from the first batch of concrete will now be 24 hours old, whereas those from the last batch only 18 hours old. This difference in time can be critical when considering early striking. The strength gain relates directly to the hydration of the cementitious content of the concrete. As, in the UK, water is generally added at the concrete batching plant, the travelling time for the ready-mixed concrete truck, of possibly up to an hour, needs to be added. This adds to the time after placing and makes the ‘youngest’ concrete in the example 19 hours old.

Measure time from water added for relevant concrete (not relevant if in situ testing)

This Guide recommends that, for assessing the strength of concrete for early-age striking, time is measured from when the water is added to the cement for the relevant concrete. The time is not relevant when measuring the in situ strength of the actual structure.

Is it then prudent to strike in the same order as the concreting? This requires an understanding of what striking really means. To be able to remove the soffit formwork the falsework load-bearing members (the vertical standards) have to be destressed - this is simply done by undoing each head or base jack a few turns. (The starting point is covered in Section 7.6.) Although on a large area it might be thought practical to lower the entire formwork “at once”, in practice the sequence will be progressive, with the initial lowering by a few turns quite quickly (working from an adequate platform) followed over time by removal of the supports. Thus a large area can be struck very

57


quickly, certainly faster than it was concreted; hence the recommendation is that the weakest concrete in the new slab to be struck should govern the striking time. Other things being equal, one would anticipate that the concrete at the bottom of the slab will be stronger than that at the top. This has been shown to be the case by in situ concrete strength measurements. This difference is a function both of the maturity of the concrete and of the inherent differences caused by vibration and water movement (bleeding).

Top vs boj measuren

The underside of the concrete slab will be insulated by the sofit formwork, which will retain the heat of hydration and prevent water loss; in contrast, the top surface of the slab will be more exposed and have lower early strengths. As the days continue these differences will proportionally reduce. Further, the area most likely to crack first due to flexural bending is the top surface near an internal column, thus the top surface of the concrete near an internal column is representative of the most critical area in terms of cracking. (In terms of deflection the potential cracking of the bottom surface in the span is more important, but the strength of the concrete in this location is greater.)

On multiple spa1 concrete at the 1 adjacent to inter towards end of likely to be

Knowing the order and layout of the concrete pour, the critical areas for concrete strength determination for early striking can be assessed.

On single Spi concrete at the at mid-span is li critical, but me? at the top surfac lower boi

Access to concrete laboratories for early strength cube tests can be restrictive, so other ways of assessing early strength may be faster and more convenient. The BCA Best Practice Guide Early age strength assessment of concrete on site (Ref. 32) is based on one of the research tasks from the ECBP (Ref. 42), and recommends that early striking is determined by pull-out tests on the structure. Although it is not within the scope of this Guide to discuss the merits of the various ways of assessing the strength of concrete, relevant observations for flat slab building construction are included here:

BCA Guide Ei strength as6

Cubes cured alongside and stored on the slab under similar conditions are likely to give lower bound estimates (Ref. 32). Although this method is not recommended for very early striking, it has been included in Table 1 for site use. The results are conservative but the full benefits of early striking may not be achieved, particularly in cold weather. The number of cubes tested will depend on the confidence limits expected and the correlation achieved. Because the cubes are not testing the actual concrete, an allowance may have to be made in the correlation to obtain the estimated characteristic strength. CIRIA Report RI36 (Ref. 3 1) recommends a factor of 1.25 to cater for the uncertainty of the test method when using cubes for early-age strength assessment.

Not ideal m

CIRIA R136 fact

Temperature-matched curing (TMC) is suitable for the larger building site and is more accurate than cubes cured alongside. TMC permits earlier striking than reliance on air-cured cubes, but is dependent on where the temperature sensor is placed in the flat slab. The point selected for the sensor should be 50 mm below the upper surface and be representative of the concrete considered to be critical. It is relatively expensive. Because the cubes are not testing the actual concrete, an allowance may have to be made to obtain the estimated cube strength of the in situ concrete in the critical area. CIRIA Report R136 (Ref. 31) recommends a factor of 1.25 to cater for the uncertainty of the test method when using cubes for early-age strength assessment.

Wet-cured cubes, cured in a curing tank on site to BS 1881: Part 11 1 (Ref. 43), are not representative of the actual concrete strength and should not be used for early strength assessment for striking.

Not suita

Rebound hammers (the best known is the Schmidt hammer) need extensive calibrating against cubes from the concrete in question, and can give unreliable results if not used correctly. It is a non-destructive testing technique. CEB Bulletin 243 (Ref. 44) states:

Not relial

“The rebound hammer is easy to handle and allows testing of large areas in a short time. It is ideally suited for determining the variation of the concrete quality over the different sections of the structure. Concrete structures that show great differences in the results obtained by the rebound hammer will automatically need a greater number of strength tests. Other tests may indirectly give an indication on the compressive strength of the concrete.”

58

CIRIA R136 fact


The rebound hammer test is still used extensively, such as in proprietary tunnel form systems, but is not considered suitable for early-age strength determination. Recommended use of LOK test. Result is based on average of four readings read off correlation curveltable

(e)

The LOK pull-out test is recommended in the BCA Best Practice Guide (Ref. 32). It requires a cast-in insert and the reading from each test is recorded. The equivalent concrete cube strength is now read off a correlation curve/table for the concrete. The number of individual readings to establish the equivalent cube strength of the slab should be predetermined. This Guide recommends that the result is based on the average of four readings. The LOK test is an accepted test method and was first included in BS 1881: Part 207 (Ref. 45) in 1992. Although a reliable test, it can be operator sensitive. Operators should be trained and competent in its use. One of the undoubted successes of the ECBP on formwork was to see the change in opinion of the frame constructor over the use of the LOK tests, from initial scepticism to reliance! Inserts are tested as required. They proved very valuable as a basis for confirming the concrete strength for very early striking. A comment from the Task 2 report on the LOK test: “A further innovation adopted on ECBP, which has implications for the falsework, was the use of cast-in inserts (LOK tests) to determine concrete strength’s. At the beginning of the project, these were used as research, in order to correlate the LOK test results with cube strengths. However, as confidence in their use increased, they were used to back up the cube results.”

The correlation is not mix-specific: either the manufacturer’s correlation curves/tables can be used or curves/tables can be established on site for a specific concrete. It is important to use the correct correlation curve/table for the confidence limit assumed. There is evidence that for more specialised concrete, e.g. high-performance, air-entrained concrete, the correlation of the LOK test is not as good as for typical structural concrete used in building construction.

Not mix-specific Use correct correlation curve

Correlation curves for fewer than four readings to establish the 95% confidence levels can be established for specific concretes - this could reduce the number of tests, but might delay the time of striking if the correlation showed a higher average value was needed.

Use of less than four results

Not recommended

(f)

The Cap0 test is another pull-out test but, although it does not require an insert to be cast in, it does require a reamed hole to be drilled in the concrete after casting. It is more timeconsuming than the LOK test to carry out and is not suitable for very early striking. See also the Best Practice Guide (Ref. 32).

(g)

Pull-off tests involve attaching a metal block to the concrete and measuring the force to pull off the block; the metal block is either attached to the surface, or by coring the concrete and breaking off the core, to a level near-to-surface. The test is designed for site use and normally six tests are recommended. It was not found to be suitable for early-age striking as the epoxy resin gave problems at low temperatures, and with moist concrete. Until new developments overcome the problems, pull-off tests are not recommended for early striking strength assessment.

(h)

Other tests for strength assessment are being developed and should not necessarily be ruled out for use; what is important is to establish confidence in the strength prediction.

This Guide has demonstrated that successful striking at early age, as envisaged in Section 6.2, requires the temporary works management to assess the lower bound concrete strength with confidence in the youngest area of the just-cast slab. Although this Guide recommends the use of the LOK test as a pull-off test for early-age strength assessment, Table 1 summarises other methods of strength assessment that might be considered. In practice on site, a combination of techniques for strength assessment will be used, and engineering judgement will determine the concrete strength for a particular condition.

1s At the ECBP the LOK inserts were fixed in the softit and in the top of the slab approximately in the middle of a 7.5 m square bay. For those fixed in the softit, a 400 mm square panel was constructed in the softit which was removed to allow testing of the insert to destruction to take place. No problems were witnessed with the construction of these panels.

59

~~~

~

6.4 Assesment of concrete strength at early age

Table 1: Summary of concrete strength assessment methods for early striking. Number of cubes or tests carried out

Comment

Average of 4 (recommended)

Use 95% confidence limit of correlation curves

Less than four

Requires specific correlation curves for confidence limits

Pull-out Cap0 test

Average of four

Use 95% confidence limit of correlation curves

Temperature-matched curing

Normal testing

Relatively expensive Select relevant location for sensor. Mean is not characteristic (’I

Cubes cured alongside and under the same conditions as the slab

Normal testing

Underestimates strength Mean is not characteristic (’)

Pull-off Limpet test

Average of six

Wet-cured cubes

-

Rebound test (Schmidt Hammer)

-

Other tests

Risk assessment

Method of concrete strength assessment (I)

Pull-out LOK test

(WC)

I

I Not suitable for early striking I Not applicable I Not recommended I -

Notes: All testing should follow the procedures in BS 1881 (Ref. 45). 1. Where testing is not carried out on the actual structure, e.g. cubes, the relationship between the maturity 2. and the characteristic strength should be established. CIRIA R136 (Ref. 31) recommends a factor of 1.25 to allow for the testing not being representative of the actual structure. The number of tests appropriate for each method of strength assessment varies, and engineering judgement is required. It is often prudent to have the facility to include a ‘spare’ sample in the unlikely event of a rogue result. So with the LOK test, casting in sets of inserts allowed one initially to be pulled; if the concrete was satisfactory the second, third and fourth could then be pulled and the average strength of four calculated; but if the first was too low, further testing was delayed. Reference to the correlation curve for the concrete would give guidance, and allow a judgement to be made on when next to test for early striking.

Test sequence used

It is not recommended that, with the risks involved with early striking and possible failure andor excessive cracking of the slab, reliance should be placed on only two or three tests.

6.5 Backprop 6.5.1 General During construction, ‘when the imposed load on a floor slab exceeds its carrying capacity, some of the load is distributed through the floor, either directly to the foundations or to other slabs. Four ways to calculate the load in backprops are given in this Guide: 9

Methodone Use a simple assumption about the percentage of load transferred through supporting slab(s). This is generally conservative and recommendations on percentages for one or two levels of backpropping are given in Table 3. This is the method most likely to be used in the initial calculations for assessing backpropping.

Method One. Table 3: Section 6.5.3 and Annex C

Method Two Use equations to predict the load transfer, knowing the stiffness of the slabs and of the backpropping. This method considers deflection in one plane only.

60

Method Two. Section 6.5.3

6.5Backpropping

Method Three This method uses the equations in Method Two but includes simplifying assumptions. It is likely to be reasonable and to give slightly conservative values in most situations.

Method Three. Section 6.5.3 0

Method Four. Section 6.5.3 and Annex F

BS 5975 Section 6.5.2

MethodFour Introducing deflection coefficients into the equations in Method Two to allow for the location of the slab and its deflected shape. This gives Method Four a three-dimensional approach to the shape, and edge panels will behave differently to internal panels of slab. This is a complex calculation and the spreadsheet on the enclosed CD Rom and in Annex F carries out this calculation. The spreadsheet also calculates the required equivalent cube strength, the loading factor, cracking factor and, where required. the effective deflection factor.

Methods for calculating loads in backprops are given in Appendix D of the Formwork Guide (Ref. 5) and Annex M of BS 5975 (Ref. 6). These give details and calculations for various backpropping arrangements based on several simplistic assumptions, which are discussed in detail in Section 6.5.2. The slender, yet economic, structures involved in commercial flat slab designs now envisaged and demonstrated for the future by the ECBP, have low values of design imposed load compared to their self-weight, as discussed in Section 3.2. This means that they have less capacity to carry additional construction loads from higher levels, and thus loads need transferring either to other slabs below or to the foundations.

Transferring loads directly to foundations depends on layout and stiffness of props and slabs. See Sections 7.4 and 7.5

Where a building has only two slabs to be cast (the top slab could be a roof), then, as each slab is built and allowed to take up its deflected state unpropped, additional loads from backpropping can be transferred direct to the building’s foundations - the slabs already cast acting as ‘packing’ between each floor’s propping. The research (Ref. 32) showed that this arrangement did not transfer all the load directly to the foundation slab, contrary to conventional thinking. Only if the backpropping is directly under every falsework standard can a reasonable percentage be transferred to the foundations. This is discussed in more detail in Sections 7.4 and 7.5.

Little advantage backpropping through more than one level

In multi-storey construction, with three or more floors to construct, it is impractical to carry the additional loads direct to the foundations. Backpropping can interfere with following trades and programming will be affected by extensive backpropping. The aim is to minimise interference by limiting the amount and number of levels that need backpropping. In this case, the floors below the one being constructed need to carry the loads. The proposed method of calculation is shown in Sections 7.4 and 7.5. It should be noted that the research (Ref. 2) concluded that “very little advantage was gained by backpropping through more than one level at Cardington”.

Strength of supporting slab is critical

At each stage of the backpropping procedure critical conditions apply. These need to be carefully considered and are discussed in detail in Section 6.5.3. The strength of each slab being used to carry load needs to be checked. The strength of the supporting slab, particularly the one immediately beneath that being cast, is nearly always the most critical, both during falsework erection when it is only just cast, and later during the concreting process. The arrangement of the backpropping, and the strength of the lower floors may also become critical and will affect the load distribution.

Excel Spreadsheet Annex F

To assist TWDs, an Excel spreadsheet for Method Four written by the University of Leeds and first published with the BRE Report BR 394 has been updated to allow for pre-loading of backprops. A CD ROM containing the spreadsheet and accompanying manual is enclosed with this Guide. The operating limits and instructions are given in Annex F. 6.5.2 Backpropping assumptions

The main assumptions currently used to design backpropping, which are generally accepted as giving conservative results, are presented and then considered in the light of the work from the ECBP (Ref. 2). The current assumptions are: Assumption 1

Slabs behave elastically.

Assumption 2

An imposed construction operations load is applied at all times.

Assumption 3

The density of concrete is 25 kN/m3.

Assumption 4

Propping between floor slabs and the falsework shoring supporting the soffit formwork is rigid. (i.e. does not change in length with load).

61

6.5Backpropping

Assumption 5

The load on the supporting slab from the falsework is uniformly distributed and the backpropping generates a uniformly distributed support system eom underneath the slab.

Assumption 6

The load transferred through a supporting slab can be proportioned pro rata to the stiffness of the floors considered.

Assumption 7

Backpropping is installed with zero pre-load.

Assumption 8

The effect of backpropping on deflections is ignored.

Assumption 9

Effects of temperature changes are ignored.

The forces that have been measured in the backpropping to flat slabs in buildings (Refs 2 and 37) are significantly lower than predicted from the above assumptions, with generally only 30% of the loads being transferred into lower floors and into the backpropping, compared to up to 50% predicted in conventional theory. A table comparing the loads transferred using the BS 5975 method and that proposed in Method One is shown in Annex C.

See Figure 27

The following paragraphs discuss the assumptions stated above, and highlight some of the new concepts that are recommended for backpropping of flat slabs in building construction, offering solutions and guidance where necessary. The completed floor slab that is directly carrying the construction of the new slab, known as the supporting slab, is shown to be critical for load-carrying capacity at several stages of construction.

Assumption 1

The slabs behave elastically

Reasonably elastic behaviour occurred with the slabs at the ECBP, which were significantly more slender than current practice. Therefore elastic behaviour is likely with similar solid slab structures, although not necessarily for waffle or trough floor slabs. The considerations for striking are generally for the slab to remain within its serviceability limits, so that it remains reasonably elastic. However, there will be some cracking in the slab which will exhibit some non-linear behaviour and provide redistribution of the moments.

Assumption 2

Supposition considered true for solid slab structures

An imposed construction operations load is applied at all times

TWDs need to consider the load applied by operatives during concrete placing, and also any imposed load on floors after casting, such as for inspection. These construction operation loads are transient. Three questions arise out of the measurement of loads in the backpropping. (a)

“Should the sofit formwork and its associatedfalsework be designed to carry the construction operations load during concrete placing?” This is currently assumed as an imposed construction operations load of 1.5 kN/m2 (see Section 4.2 and BS 5975 (Ref. 6)). This represents an equivalent concrete thickness of only 60 mm, i.e. about 18-20% of the load. The considered view is ‘yes’, and the TWD should allow for this load when designing the temporary works. The load is transient and moves with the concrete as each area of slab is cast: once the concrete is placed and the top surface finished off, it is not trafficked by operatives for several hours.

(b)

Design soffit formwork and falsework for construction operations load of 1.5 kN/m* (Service Class 2)

“Should the construction operations load during concrete placing, as discussed in (a) above, be considered when checking the carrying capacity of the supportingfloor and/or any backpropping?’ Initial thoughts would suggest that it should be included, or at least at a lower value of, say, 0.75 kN/m2 (Service Class 1 loading).

62

This is really a question of risk assessment, and should be “What is the likelihood of an actual load being applied?* If it were only one value of Service Class 1 loading, and was ignored in the calculations, but then occurred, the increase in load of only 6% (maximum) is unlikely to cause distress to the slab.

Calculations at Section 6.6 show it to be only 1.1% for the ECBP slabs

There is some sense in checking the design of the floor supporting the falsework to take a lower value of imposed load. On a rapid construction programme, simultaneous operations may lead to operatives working both under and on top of the soffit formwork. Thus the risk is considered reasonable to reduce loading at this stage. Certainly, if the area were delineated

See also the discussion in Annex E and consideration of the load combinations for rare conditions

6.5Backpropping

and access restricted, it would be possible to eliminate this load on the supporting slab. It should be noted that this loading condition may not be the most critical stage for the supporting slab, as discussed later. (c) Not necessary to design backpropping for construction operations loads unless other activities known

“Should backpropping calculations include an allowance for the imposed load on the supporting floors?” Again this is risk assessment, and this Guide recommends that, unless known activities are being carried out on the floors in question, it is not necessary to design the backpropping for imposed loads from construction operations on previously cast floor slabs. If operations such as storage or block-laying are likely to be carried out on the lower floors, then due allowances will be required.

The construction operations loads were shown at the ECBP to be non-existent. Quoting from the Task 4 Report (Ref. 2), (see also Figure 27): “The output from the various load cells (on the backprops) do not suggest that major construction loads were detected, and it is clear that most slabs below that being concreted carried very little construction load permanently”.

This does not mean that TWDs need not design for construction operation loads arising, for example, from other trades’ activity on the recently cast slabs. At the ECBP not all the panels of the building were fully instrumented. Individual buildings will have different risks depending on the structural shape, configuration, finishes, etc. For example, a building with internal block walling might require storage areas for the blocks on the floors, plus defined working areas for block laying on lower floors, giving significant construction operation loads in the building that need to be considered. Assumption 3

Density of concrete for temporary works considerations

In the design of reinforced concrete flat slabs the PWD generally assumes a concrete density of 24 kN/m3 (see Section 3.2). This value will include the reinforcement. The same value for density will also be used by the TWD when carrying out the calculations for the backpropping. There is divergence of opinion among TWDs over the value of density of concrete containing reinforcement to be used when designing sofit formwork and falsework. The steering group found that those dealing mostly with building work often use the lower value of 24 kN/m3, whereas those handling both building and heavier civil engineering structures use 25 kN/m3; the latter is the value used in most determinations of concrete pressure against vertical formwork. BS 5975 (Ref. 6) states a value for concrete density of 2500 kg/m3, i.e. 24.52 kN/m3. The higher value agrees with the recommendation in DD ENV 1991-2-1 (Ref. 11) for a density of 24 kN/m3 plus 1 kN/m3 for ‘reinforced concrete’ and a further 1 kN/m3 for ‘unhardened concrete’ (see Section 4.3). This suggests 26 kN/m3 for the density of reinforced wet concrete, which is more than the value of 24-25 kN/m3 generally accepted in the UK. Design of falsework use concrete density in the wet condition, 25 kN/m3

It is recommended that, for the design of falsework for normal flat slab construction, concrete density be considered in the wet condition, i.e. 25 kN/m3. The slabs at the ECBP, even with the heaviest level of reinforcement, had reinforced concrete dry densities slightly less than 24 kN/m3 (Ref. 2), justifying the previous recommendations. Assumption 4

Propping considered to be rigid

This is obviously not correct, as the propping members act elastically, and change in length with load. The moduli of elasticity (E) for the two most common propping materials are: Steel 200 x 106kN/mZ Aluminium

Do not mix steel and aluminium propping

Equation 9

69 x 106kN/mZ Because the prop materials have such different elastic moduli, mixing props on a single floor (i.e. steel with aluminium) must be avoided. When props are mixed there will be a complex load redistribution between the props of different materials and the slab; as load increases, the aluminium members will shorten and load will distribute through the slab and to the stiffer steel members: Change in prop length =

Increase in load x Overall length of prop Cross -sectional area of prop x Modulus of elasticity

63

6.5Backpropping

It is recommended that suppliers provide the stiffness properties (in kN/mm) for their proprietary propping. Stiffness is inversely proportional to height of propping, so a longer prop is less stiff. For example, at the ECBP, the stiffness of the 3.5 m long aluminium backprop used was 25 kN/mm. This stiffness value is used in the backpropping spreadsheet in Annex F. Typical values for elastic shortening of falsework are given in Section 5.6. The approximate stiffness of an adjustable steel prop of 3.5 m length is 35 kN/mm. Introducing this stiffness into the four spreadsheet examples in Annex F reduces the load carried by the supporting slab by 4 5 % . This correspondingly increases the load attracted to the stiffer backprops and transfers load to the lower slabs. It is therefore reasonable to assume that changing from aluminium to steel backprops in these examples does not significantly alter the load transfer. Assumption 5

Suppliers to give prop stiffness At ECBP for prop lengths of 3.5 m, values used were: aluminium 25 kN/mm steel 35 kN/mm Steel backprops reduce bad in SuPPofiing slab (approx. 4 to 5%)

Load applied and supports assumed to be uniform4 distributed

The assumption of an applied uniform load on the slab being supported by backpropping, also as a uniform load, is a convenient design concept for the backpropping.

In building construction, the falsework loading the supporting slab is likely to be a series of point loads on a 3.0 x 2.4 m grid giving leg loads during construction in the order of 50 to 60 kN. The supporting backpropping often has a larger individual leg capacity and is arranged on a larger grid. Thus the actual deflected shape of the supporting slab has an effect on the final loads transferred through the floors. The relative arrangement of falsework and backpropping is shown in Research Report BR 394 (Ref. 2) to have a significant effect on the force transmitted through the supporting slab into the backpropping. Considering first the effect in only one plane, Table 2 illustrates the proportion of load transferred through the slab, which varies from 25 to 50%, depending on the end restraint system adopted. The load transferred to the backpropping will be less than the load required for the falsework such that Case (b) is rarely justified, and can be ignored. Thus the recommendation of this Guide is to fit the backpropping below the supporting slab at third points, as illustrated in Figure 14 Case (d). It should be noted that it is not normal to include backprops in the line of the columns, so one bay of slab with backprops at third points has four props per bay. When the three-dimensional deflected shape of a slab is considered, the movement of the various members and their method of support becomes very complex - certainly far too difficult to accept as a simple method for building flat slabs. By way of illustration, Figure 14 gives an indication of the effect in one plane (two dimensions only) of two different propping arrangements. Predicting the actual load spread through a structure in three dimensions is complex and many factors are involved, such as the relative stiffness of the floors and the preload induced when backpropping is introduced, which is discussed later. When the deflected shape and different resultant loads are allowed for at the propping positions, it is possible to show mathematically that only about one-third of the assumed load is actually distributed through the supporting slab into the backprops, see Table 2 and Figure 14. Table 2: Effect of falsework and backpropping locations.

I 1

Case Falsework

I

1

(a)

Backpropping

Comment

I Uniformly distributed I Uniformly distributed I Conventional theory

(b) [As detailed (c) At 1/3 points of span

I Directly below

I

Proportion w, /

Wbl

I 50%

I Not generally practicable I 50%

At mid-span only

Not ideal on long spans

3742%

(d)

Uniformly distributed

At 1/3 points of span

Recommended

3 2-3 7%

(e)

Uniformly distributed

At mid-span only

Not ideal on long spans

25-3 1%

1 I

1

Notes: 1. The Research Report (Ref. 2) checked this ratio for simply supported members, fully fixed members and propped cantilevers. The variation in the ratios calculated is shown in this column. The lowest value of 25% occurred on the fully fixed member condition for Case (e) only; the next lowest was 30%. 2.

64

The load applied to the slab from the falsework is wp.The load in the backprops below the slab is wb,.

Fit backpropping at third points

6.5Backpropping

Falsework

Case (c) see Table 2

Case (d) see Table 2

Figure 14: Effect on deflected shape of different backpropping positions. Use Method One Table 3

It is recommended that the percentages of the load transferred through slabs for backpropping calculations are as in Table 3. This is Method One for calculating the loads in the backprops. This gives significantly lower loads transferred than the current assumption that each supporting floor takes the same proportion of the applied load, assuming similar stiffness of each slab. Assumption 6

Supporting slab carries 67% to 75% of the construction load

Load carried by a sIab is proportional to its stirness

Distribution of load through the supporting slab has been demonstrated in Task 4 not to be directly proportional to the stiffness of the floor(s). Task 4 has shown that the supporting floor is always required to take between 67 and 75% of the construction load, even when more levels of backpropping are introduced. A comparison of the original BS 5975 assumptions with the research findings on proportion of load carried by the supporting slab is shown in Athex C. The important conclusions in Table 3 is that, for one level of backpropping, 70% of applied load is taken by the supporting slab and only 30% is transferred through the slab to the backprops. Various arrangements of backpropping were used in the research in Task 4 which clearly showed that the supporting slab contributes significantly to the support of the new slab.

Appears to be a limit on PWD ratio of imposed to self-weight load

The corollary of the 70% limit (with Method One) on the load on the supporting slab is that, unless the constructor transfers all the backprop loads to the rigid foundations of the building, there appears to be a limit to the ratio of loads that PWDs can accept when assessing the self-weight and service loads imposed on suspended slabs in multi-storey construction. The recommendation of this Guide, that only 30% is transferred to lower floors, gives the relationship:

70% added load on the supporting slab I spare capacity

Equation 10 See also Section 4.3

where added load is the concrete weight of the newly cast slab. Depending on the order of backpropping, the weight of the temporary works and possibly the construction operations load may need to be added to the concrete weight. Spare capacity is the value of load that any slab can carry at the time considered, after deducting its self-weight, and possibly any other loads. The Task 4 research (see Annex A) has shown that, in most cases, it is the supporting slab that is critical, but other floor slabs require checking as their load history is established. It is important to understand that the spare capacity may not be the total of the PWD’s imposed loads on the slab. For example, if backpropping is inserted after the falsework is erected on the slab, then the net spare capacity of the slab is the PWD’s imposed load less the weight of the falsework and formwork for the next slab. There are three cases to consider: 1.

Equation 10 is true and construction can continue safely without the supporting slab being overloaded.

2.

Equation 10 is not true because the full design strength of the slab has not yet been achieved. In this case the slab cannot be loaded until the required concrete strength has been achieved. Once the concrete has gained sufficient strength Equation 10 becomes true, as Case 1.

65

6.5Backpropping

3.

Equation 10 is not achievable, even after the concrete slab has gained full design strength. In this case construction would load the slab above its design service load, and could theoretically cause damage. The risks involved in proceeding should be considered by the PWD and TWC. This is discussed in Section 6.6 and in detail in Annex E.

Theoretical load above the design service load

As an example, taking the values from the ECBP from Sections 4.3 and 3.2, using the reinforced concrete density of 24 kN/m3 (Column 2 at Section 4.3), and assuming that the concrete has achieved its full 28-day characteristic strength gives: Spare capacity 1.00 + 0.50 + 0.50 + 2.50 = 4.50 kN/mZ (partitions) (raised floor) (ceiling) (imposed load) Added load 6.00 + 0.50 + 0.75 + 0.75 = 8.00 kN/m2 (slab weight) (falsework) (2 x operations load) and, using Equation 10, gives: 70% (added load) = 70%

x

8.00 = 5.60 kN/m*,

which should be 54.50 kN/m2 (spare capacity), which is not true.

In this example, Case 3 will apply. Alternative proposals for consideration by the PWD are discussed in Section 6.6. Assumption 7

Backpropping is installed with zero pre-load

When installing backprops between the lower floors they are often fitted as ‘finger-tight’. They often do not incorporate frames or devices to keep them upright, so the operatives always ‘take up the slack’ when fitting them, to ensure that they do not become a safety risk and fall over. Loadcell measurements on backprops at Cardington and Reading (Refs 2 and 37) showed that loads up to 19 kN have been generated in these props during installation, although a more usual figure would be 10 to 15 kN. The worked example in Annex D uses the measured values of 8.5 kN as the pre-load in the props, giving about 0.60 kN/m* as a distributed load. The Excel spreadsheet (Annex F) includes the provision for pre-loading the backpropping, and an estimate of, say, 6 kN in each prop would seem to be reasonable. Compared to the likely loads of, say, 60 kN at design stage, this pre-load is significant. The pre-loaded backprops actually help the load distribution, because loading the supported slab upwards, and pushing the lower slab downwards, has the effect of reducing the 70% load in the supporting slab when the full falsework load is applied.

Estimate: pre-load in backprops of 6 kN

The physical operation of installing backprops that “do not fall over” when first fitted has been shown to induce some pre-load into the system. This is one of the reasons why Method One in Table 3 recommends 70% transfer of load in the worst case compared to the theoretical 75% (Task 4 recommendation) for stiff two-dimensional structures. See the worked example in Annex D. How then can a site effectively and safely pre-load backprops to advantage? Provided similar props are used, the change in length is directly proportional to the load, see Equation 9, so for a given storey height and type of prop, turning the adjusting collar a set amount initiates a crude measuring device for pre-loadl9. Perhaps proprietary suppliers of props should have available information on the pitch of the thread form used, as ‘mm extension per complete turn’ or by stating the individual prop stiffness (in kN/mm) per metre.

How to pre-load props?

Calculating the prop extension required to achieve a particular pre-load is complicated by the stiffness of the slabs which deflect with load, thus changing the elongation etc. The prop stiffness (in kN/mm), translated by the TWC into load per turns, and hence the number of complete turns on the adjusting screw to give this extension, may be impossible to calculate. Assumption 8

The effects of backpropping on deflections

Measurements on the slabs at the ECBP, with up to two levels of backpropping, showed that all the slabs performed satisfactorily and that deflections were within the intended design code limit of (span / 250). This gives a deflection limit of 30 mm on the 7.5m spans at ECBP. Even the deflection

19 In practice it was found difficult to control the pre-load at ECBP, even with load cells installed, because of the tendency to transfer load between adjacent props as one is tightened up, due no doubt to the thin flexible slab.

66

Deflections of ECBP floor slabs struck early are within design limits

6.5Backpropping

.-

...

of the floor slab where backpropping was prematurely struck (see Section 4 3 ,floor 620 was within specification. The BRE Report (Ref. 2) details the sequence of backpropping used, part of which is reproduced in Figure 27. Assumption 9

Ambient changes affected prop load

Eflects of temperature changes ignored

To date, temperature has been ignored in backpropping, but at the ECBP the forces in the backprops varied over time, even where no major operation (such as pouring concrete or striking the formwork) was taking place. It became apparent that there was often a correlation between changes in the backprop forces and variations in ambient temperature. The temperature of the aluminium backprops is likely to follow the ambient temperature quite closely. The concrete structure, however, changes temperature much more slowly. Thus, when a rise in temperature occurs, the props heat up and expand. This expansion will be resisted by the structure, which will not have, changed temperature significantly and hence will not have expanded, and the compression force in the backprops increases. These changes may be due in part to the effects of temperature on the instrumentation itself and the type of load-measuring device used. The change in prop length depends on the stiffness of the props, original length (the propping height), the temperature change and the material of the props. Approximate coefficients of thermal expansion are: 8 x 10-6per"C Concrete with limestone aggregate 10 x 10" per "C Concrete with granite or basalt aggregate 12 x 10" per "C Concrete with flint or quartzite aggregate 10-12 x 10" per "C Steel 22-24 x 10" per "C Aluminium It can be seen that, when adjustable steel props are used for backpropping, provided they are not in full sunlight, the concrete structure and the steel props generally expand a similar amount, whereas thermal changes in aluminium propping affects the load in each prop. It has been shown from the measurements at the ECBP that a 5°C increase in temperature caused an increase of some 10 to 15% in the backprop forces, of up to about 3.75 kN per prop. In practical terms, an on-site temperature variation of, say, 20°C is possible, with corresponding changes in prop forces. It is hard to see how this temperature effect could be anticipated, or even accommodated in any backpropping design, other than within the normal factors of safety employed, but its effects are now recorded. 6.5.3

Loads in backprops

The four methods of backpropping mentioned in Section 6.5.1 allow for one or two levels of backpropping, as shown in Figure 15. The figure also illustrates the symbols and nomenclature used in this Guide. The following Sections explain the background to the four methods Stiffness in kN/mm

The stifkess of the slab or backpropping is the load required to give a unit movement on a given part of the structure. Stiffness is generally expressed in kN/mm.

Method One - Use ofpercentages of load The use of Table 3 is the simplest method of predicting the loads transferred through the slabs on backpropping. It shows the percentages of the imposed construction load that are transmitted through the supporting floors and into the props. The table shows the percentages for a new slab cast with no backpropping, with one level backpropped, and then with two levels of backpropping (see Figure 15). Where two levels of backprops are inserted it is assumed that the backpropping of each slab is similar, i.e. props are above each other on the floor above.

Results about 24 months after casting floor 6 seem to indicate that it will have a maximum midspan deflection of 25 mtn (1 :300) after the instantaneous deflection on striking of about 10 mtn on the 7.5 x 7.5 m bays. 20

67

6.5Sackpropping ~

Stiffness

One level propping

Load

Two levels of propping

Slab to be cast Falsework

W

P

Supporting slab 1

ss1

Backprops

'b

Lower slab 2

SS2

Wbl

Backprops (when fitted) Sb Lower slab 3

Wb2

ss3

Figure 15: Diagrammatic representation of backpropping of suspended slabs.

Table 3: Method One -percentage of load distribution from temporary works loads. Location

Load

New slab being cast

Total wp

No backprops One level of backprops Two levels of backprops

On slab

On slab

100%

100%

FalseworWformwork

100%

On supporting slab 1

100% Wp

In backprops

In backprops

On slab

In props

100%

100% 70% wP

100% 65% wP

30% wp

wbl

On lower slab 2

In props

30% wP

35% wP 23% wp

wb2

12% wp

12% wp

On lower slab 3

Method Two - Use of simplified formulae The Task 4 report (Ref. 2) has established Equation 11 for the calculation of loads in backprops, assuming that the slabs have been struck individually and have taken up their deflected shape, prior to installation of the backpropping, as shown diagrammatically in Figure 15, for one and two levels of backpropping. The analysis assumes that the structure is in one plane, effectively supported on two opposite edges only. The stiffnesses of the slab and the backpropping are required.

Analysis in two dimensions

For one level of backprops, as shown on the left hand side of Figure 15: Equation 11

where

wp

=

w,, =

load in the falsework supports load in the backprops between the supporting slab and the slab below, i.e. the first layer of backpropping

6.5Backpropping

S,,

=

S,,

=

sb

=

stiffness of the supporting slab at the time considered stiffness of the lower slab under the supporting slab at the time considered stiffness of the backprops.

For two levels of backprops, as shown on the right hand side of Figure 15: Equation 12

Equation 13

where

wb2

=

load in the lower level of backpropping.

S,,

=

stiffness of the lowest slab at the time considered.

The lower slab is the rigid foundation. Equations 11, 12 and 13 can be used where the backpropping is taken direct to the foundations, in which case either the lower slab 2 for one level of backpropping, or the lower slab 3 for two levels of backpropping, is the adequate foundation, and hence either

S,,

or S,,

= CO

=

CO

as appropriate.

Method Three - Further simplified formulae Two dimensions and slabs at least twice the stiffness of backpropping

The Task 4 Report (Ref. 2) recommends that, to calculate realistic loads in backpropping, the slabs will be at least twice as stiff as any backpropping introduced.

s,

I

This makes -

ss2

2 and Sb

=

sb

2 and gives the following revised equations.

=

For one level of backprops, as shown on left side of Figure 15: Equation 14

wbl

wP

=

[3

+

2)

For two levels of backprops, as shown on the right hand side of Figurel5: Equation 15

WL,

We

P

=

I

.

Equation 16

The lower slab is the rigid foundation. Where support is taken from adequate foundations, Equations 14, 15 and 16 can be used, in which case either the lower slab 2 for one level of backpropping, or the lower slab 3 for two levels of backpropping, is the adequate foundation, and hence either

S,,

=

00

or S,, =

CO

as appropriate.

This is further discussed in Method Four below. The recommended method of applying these equations in practice is shown in Sections 7.4 and 7.5.

69

6.5Backpropping

Method Four - Three-dimensional consideration The backpropping spreadsheet in Annex F, takes into account the relevant changes in stiffness for the slabs and props, and gives a more precise calculation in three dimensions for the movement of the slabs. This is known as Method Four. The spreadsheet method generally predicts that 75% of the load will be carried by the supporting slab for internal panels. It can cater for up to 12 backprops per bay under different loading conditions.

Three-dimensional analysis

See Annex F

In comparing the measured loads on the backprops at the ECBP, it was shown (Ref. 2) that, on average, the original BRE spreadsheet tended to underestimate the backprop forces by about 6% because it did not account for any pre-load in the backprops. The spreadsheet with this Guide can allow for pre-loaded backprops.

Discussion of methods Method Two

Referring to the earlier assumptions for Method Two and for propping discussed in Section 6.5.2, and used in BS 5975, if the backpropping is rigid and infinitely stiff, and the slabs have the same stiffness, then:

S,,

= S,, = S,,

and & = CO, giving:

for one level of backpropping

wbl

= 50% wp

(from Equation 11)

for two levels of propping

wbl

= 67% wp

(from Equation 12)

and

wbz = 50% wbl

(from Equation 13)

Substituting gives

wb2 =

33% wp

This agrees with the earlier predictions for backpropping from BS 5975 with the loads being distributed equally to the three floors.

See Table C1 in Annex C

Consider the effect of the slabs still having similar stiffness, but with the backprops as stiff as the floors i.e.

S,,

= S,, = SS3= Sb

This gives: for one level of backpropping

wbl

= 33.3%

wp

(from Equation 11)

for two levels of propping

wbl

= 37.5% wp

(from Equation 12)

and

wb2 = 33.3% wbl

(from Equation 13)

Substituting gives

wb2= 12.5% wp

Method Three

If, in Method Three (the further simplified method), the slabs discussed previously are of similar stiffness, then

s,, = s,, = ss3 and the slabs are at least twice as stiff as any backpropping introduced. This gives: for one level of backpropping

wbl

= 25.0% wp

(from Equation 14)

This is the source of the theoretical value of 75% of the load into the supporting floor if the backprops take only 25% of the load. This gives: for two levels of propping

wbl= 26.7% wp

(from Equation 15)

and

wb2 = 25.0% wbl

(from Equation 16)

Substituting gives

wb2 = 6.7% wp

In practice, constructing a vertical building with work moving upwards means that the backpropping is installed similarly at both levels such that the installed capacity of the lower propping is designed for wb2 = wbl

70

6.5Backpropping

Method Three - Two-storey building on rigid foundations. If the foundations on the left side of Figure 15 are rigid, then Ss, = 00, giving: for one level of backpropping Propping all the way to the ground on two-storey building load on slab is 67% wp

wbl

=

33.3% wp

Thus the load taken by the supporting slab is 66.7% wp and, contrary to general beliefs, when propping ‘all the way to the ground’, not all the load is transferred directly to the rigid foundation. Method Three - Three-storey building on rigid foundations If the foundations on the right side of Figure 15 are rigid, and the slabs are of similar stiffness, then S,, = S,, and S,, = 00, giving for two levels of backpropping

wbl

= 27.2% wp

Propping all the way to the ground on three-storey building load on slab is 73% w,,

Thus the load taken by the supporting slab, contrary to general beliefs, is 72.8% wp even when backpropped ‘all the way to the ground’, as not all the load is transferred to the foundations.

Agreed procedure

Whenever backpropping is required, an agreed sequence should be established before work starts. See also Section 7.7. The sequence should establish the principles, and the exact layout should be understood by the operatives on the site. The significance of correct propping has been highlighted by this Guide, and the following extract from the Task 4 Report (Ref. 2) is relevant:

Importance of sequence being understood by operatives

6.5.4 Sequence and installation procedure for backpropping

“Some years ago (1991) an attempt was made by the British Cement Association to investigate backpropping forces by monitoring them on site. The results were so variable that they were effectively uninterpretable. It was discovered that one of the reasons for this was that construction operations were being carried out on floors which supported backprops and, where it was found that a particular prop was in the way, it was temporarily removed and then replaced. Clearly, such a procedure makes a complete nonsense of any attempt to calculate backprop forces with any accuracy. If the economic operation of a construction site requires that backprops should be able to be moved in this way, then the use of sophisticated calculation methods are a waste of time and the results simply misleading. Only the simplest possible approach would be justified and even this could lead to an underestimate of the loading on the slab supporting the props. It is probably not enough simply to instruct operatives not to move backprops. Sites are ‘can do’ places and operatives are used to using a considerable degree of initiative in getting the job done. If things such as backprops are getting in the way of what they are doing, they are not likely to ask permission but simply shift them, and, they will tell themselves, quite correctly, who will ever know anyway? If it is felt desirable to design backpropping systems in a more rigorous way then it has to be recognised that construction of the slabs in multi-storey buildings must become a precision operation where the contractor can reasonably guarantee that the props and backprops are placed in the agreed positions at the appropriate time and that they are not moved prior to the time for their removal defined by the construction programme.” Deflection results from sixth floor reported at Section 6.5.2, Assumption 8

At the ECBP a controlled procedure was issued which was generally followed by the site staff. However, in the absence of site supervision, the backprops between the fifth and sixth floors were removed before striking the seventh floor! In theory this should have caused additional cracking and deflection of the sixth floor, but this was not measured in practice, because of the conservative estimate of the construction loads discussed previously.

TWC to prepare an understandable, and consistent, repeatable procedure

The TWC should prepare, in advance of construction, an understandable, consistent, repeatable procedure for managing the backpropping and to take advantage of the findings discussed, particularly the controlled pre-loading of the backprops. The application of the sequence to backpropping is covered in detail at Section 7.7 together with a typical backpropping layout planning chart in Figure 27. The timing of the installation of the backpropping can be critical. If a floor has just been struck, and the falsework is being moved up directly from underneath to be seated on top of the newly struck floor, the strength of this new supporting slab can become critical. At the time of its complete striking it may have to carry not only its own self-weight, but also the weight of the temporary works plus any operative loading while they are re-erecting it. Thus backpropping might be required very quickly after striking to transfer these extra loads caused by this stage of construction to the lower floors.

71

6.5Sackpropping

In general, backpropping should be (re)installed at the earliest available opportunity to ensure maximum distribution of loads through to the slab beneath. It has already been seen that pre-loading the backpropping may be advantageous in transferring loads through the structure.

6.6 Loading a slab U 0 above the design serwice lload 6.6.1 Introduction

During the early life of a slab, there are occasions when construction loads, w,will, in theory, exceed the slab’s design service load, w,,,.Most commonly this situation occurs when the young slab supports loads arising from concreting the slab above. The problem is likely to be most apparent when self-weight is a high proportion of the characteristic design load: in other words, where live load (imposed load plus superimposed dead loads such as services and finishes) is small compared to the self-weight of the concrete. By way of illustration, using the example from the ECBP in Assumption 6 in Section 6.5.2, which used the full construction operations imposed load of 1.5 kN/m2, Equation 10 gives a theoretical loading applied to the supporting slab of 70% (8.00) = 5.60 kN/mz which has to be I 4.50 kN/mZ(spare capacity)

Calculation presupposes that supporting slab has achieved full strength at time of early loading, and that PWD has accepted such a proposal

Removing one value of 0.75 kN/mZ, i.e. reducing the construction operations load to 0.75 kN/mz, from loads gives 70% (8.00 - 0.75) = 5.08 kN/m2 Thus the theoretical increase in the design service load is: (6.00 + 5.08) = 1.06 = 6% 10.50 If there is no construction operation load on any slab, then the added loads reduce to: 70% (8.00 - 1.50) = 4.55 kN/mZ The phenomenon of loading a slab to above the design service load is not new, and many such slabs are so constructed and have been shown to perform perfectly adequately. The purpose of specifying a construction method is to limit deflections. In practice there are several ways of overcoming the apparent difficulties of theoretically overloading the slab during construction. The main issues to focus on are: *

reviewing the loads to be resisted

e

reviewing combinations of loads

0

*

considering appropriate limit states testing the in situ strength of the concrete using appropriate partial safety factors (y).

The aim is to allow contractors to deliver quality structures safely and economically. Specifically, the three proposals in Section 6.6.2 are intended to ensure that construction loading does not adversely affect slab performance at either the serviceability or ultimate limit state. 6.6.2 Proposals

Pending further research, it is recommended that the following course of action should be taken when considering young slabs supporting loads from concreting operations above. Each part of the four principles should be considered in turn by the TWD and PWD who should agree on those to be adopted. Agreement presumes that both TWD and PWD are confident that many of the uncertainties encountered at design stage become known at construction stage or are adequately provided for in the method of construction and are adequately documented. The proposals are based on theory presented in Annex E (which addresses both the serviceability and ultimate limit states). They presuppose that the slab(s) under consideration is (are) substantially

72

See fuller explanation of theory in Annex E

6.6 Loading a slab to above the design service load

the same as the one(s) above it. Changes in configuration, e.g. additional thickness, holes at one level and not above, may significantly concentrate loads, moments, etc. Such cases should be dealt with separately.

Proposal 1 To assess the temporary case at serviceability limit state (SLS) apply the two principles for loading and striking flat slabs, i.e. check:

See Section 6.2

Loading factor (F, ) =

Equation 4

Total design construction load on slab - - w I1.0 Total design service load on slab Wser

and

[

;er)($r

Cracking factor (Fer) = -

Equation 5

where

I 1.0

=

total unfactored load on the slab in kN/m2, including self-weight, a proportion of self-weight of supported slabs and construction imposed loads.

w,,, =

total unfactored characteristic design load on the slab in kN/m2, including self-weight, the weight of services, finishes, partitions etc and imposed loads used for the design of the permanent work.

w

If Equations 4 and 5 are not met, ensure that proposals 2a and 2b are both complied with.

Proposal 2a: At serviceability limit state (SLS) Use load combination factors, e.g. Eurocode 2 (Ref. 39) with frequent combination factors of 0.5 and 0.421 for independent loads. *

Use temporary load cases of: - 1.5 kN/m2 on formwork and 0 kN/m2 on supporting slab by denying access to such

areas and disallowing its use for storage during concreting operations, and - 0.75 kN/m2 on formwork and 0.75 kN/m2 on supporting slab for access and inspection. *

With respect to Equation 5, allow use of higher grade of concrete such thatf, >feu so that Equation 5 (extent of cracking and punching shear check) is satisfied. Relax Equation 4 to allow w to exceed w,,,to an extent to be agreed such that Equation 17 (below) is satisfied.

To prevent inelastic deformation/yielding at supports, ensure: W

Equation 17

Pb

~

%er

where riser is the ultimate design load (ULS: gk = 1.4, qk = 1.6)

Pb

Moment at section after redistribution = Moment at a section before redistribution

Quasi-permanent load combination factors of 0.2, 0.3, 0.6 or even 0 (relating to snow loads) might also be considered - but would make relatively little difference to the Equation 4 ratio w/w,,,.Effects of pre-load in the props are ignored

21

73


I

Proposal 2b: At ultimate limit state (ULS) Adopt a fourth criteria as

n

- I 1.0

Equation 18

riser where n is the ultimate construction load (ULS: gk = I .2, qk = 1.2)

Use Eurocode rare combination factors of 1.O and 0.5 for independent loads. Use load cases as in proposal 2a. Use a yf factor of 1.2 for ultimate loads, for both g, and Q. f , > 0.85f,, (partial safety factors, ym, reduced through elimination of uncertainties).

Disregard relatively small increases in ultimate bending moment, shear and bond capacities created by increasingf, beyondf,,.

6.6.3 Effects of proposals The effects of the proposals on the construction of multi-storey flat slabs can be illustrated by examples of where theoretical problems may occur and the increase in concrete strength required to overcome these. Figures 16, 17 and 18 are based on flat slab thicknesses in the Reinforced Concrete Council publication Economic concrete frame elements (Ref. 12) and the following assumptions: Density of concrete = 24 kN/m3 One level of propping, i.e. 70% carry-through of loads Allowance for self-weight of formwork with falsework = 0.50 kN/mZ. Proposal 1 According to Proposal 1, Equation 4, there are many theoretical cases of temporary overload. For example, the flat slabs at the ECBP span 7.5 m, and support a total imposed load of 3.5 kN/mZ (2.50 kN/mZ plus 1.00 kN/mZ for partitions: see Section 3.2). According to Figure 16, the slab would have been temporarily overloaded by just under 20%. Using the specific loads at the ECBP, Beeby (Ref. 6) estimated they were overloaded by 10% to 15%. Proposal 2 This proposal is in two parts: Proposal 2a considers the serviceability limit state (SLS), and Proposal 2b the ultimate limit state (IXS), using appropriate load combination factors, partial safety factors, etc. Proposal 2a: SLS The serviceability limit state may be satisfied by ensuring thatf, exceedsf,, by the amount indicated in Figure 17. Essentially this requirement limits the extent of cracking to that assumed in the original design. If the specification of a higher strength concrete is required, its additional cost may make alternative procedures more attractive. On the other hand, the required strength may arise through usual margins in concrete strength. Figure 17 suggests that, to avoid damage in the ECBP slabs, f , should have been greater than approximately 1lO%f,,. In the event, the one slab whose strength,f,, was less than 1lO%f,, at the time of concreting the floor above, has deflected most.

To prevent inelastic deformation it is necessary to consider construction loads in relationship to design loads and the amount of redistribution assumed in the design. In this respect wh,,,should be less than p , (as defined above). Typically, designers might use up to 20% redistribution (a p factor of 0.80), so this requirement will usually be non-critical. However, 30% redistribution is allowed in design codes of practice (a P b factor of 0.70) and as may be seen from Figure 18, this requirement might become critical.

,

Proposal 2b: ULS To satisfy the ultimate limit state it is necessary to limit ultimate loads and to ensure that the concrete is of adequate strength.

74

6.6 Loadina a slab to above the design service load -

10.0

N

E

z

7.5

D

a

3

5.0

0 W

cn

Br

2.5

~~

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

SPAN - rn

Figure 16: Extent of theoretical overloading of flat slabs due to concreting operations above - based on Proposal 1, Equation 4

10.0

N

3

5

7.5

I 0

a

9

5.0

D

W

cn

2

2.5

1.5 I

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

SPAN - rn

Figure 17: Proposal 2a: fc/fcurequired to satisfy Equation 5 at SLS.

10.0

N

3

5

7.5

I

D U

2

5.0

e

0

20

2.5

1.5

I

5.0

6.0

7.0

8.0 SPAN - m

9.0

10.0

11.0

12.0

Figure 18: Proposal Za: w/n,,, should be less than pb to satisfy Equation 17 at SLS.

Notes to Figures 16, 17 and 18 1.

2.

3. 4.

5. 6. 7.

Span = continuous multiple span IL = imposed load: 1.5 kN/m2, e.g. residential 2.5 kN/mZ,e.g. ofices 5.0 kN/m2, e.g. commercial offices 7.5 kN/mz, e.g. storage/plantrooms 10.0 kN/m2, e.g. storage A superimposed dead load of 1.5 kN/m2 has been assumed to allow for finishes, services, etc. on all but residential where no allowance was made. Construction imposed loads: for Figure 16 construction loads of I .5 kN/m2 at the level of concreting and 0.75 kN/m2 at the level of the slab supporting the false work. For Figures 17, 18 and 19 refer to proposals for values, combination factors and g factors. f, = characteristic concrete strength required (at the relevant time)

f,, = characteristic concrete strength at 28 days as assumed in design and as specified. For the purpose of illustration the charts and examples relate tof,, = 35 N/mm2

75 , ~


Situations where nln,,, > 100% are probably not acceptable without special measures being taken, such as checking specific moment and shear capacities. Figure 19 suggests that the slabs at the ECBP (7.5 m span, 3.5 kN/m2 imposed load: 2.50 kN/m2 plus 1 .OO kN/m2 for partitions) were not overloaded. Slabs with low imposed loads and long spans need special attention.

See Section 3.2

Compared to the SLS requirements, the ULS requirement forf, > 0.85f,, (based on punching shear criteria) is probably not often critical. 10.0

7.5

5.0

2.5

1.5

4.0

5.0

6.0

7.0

8.0

9.0

SPAN - rn

10.0

11.0

Figure 19: Proposal 2b: n/nxr should be 12.0 less than 1.0 to satisfy Equation 18.

6.6.4 Alternative measures Consideration of the above proposals and implicit theory may result in unsatisfactory temporary loadings, uneconomic construction, uneconomically high grades of concrete, and/or lack of agreement between TWD and PWD. In such cases several practical steps can be taken. These include: Use two levels of prop. Transfer of loads becomes 65% (compared to 70%) and overloads decrease by 3%.

Backpropping Method One (Section 6.5.3)

Design temporary case as worst case. The contractor may choose to increase reinforcement to allow construction to proceed as required. Use innovative construction methods, for example: construct column strips before middle strips - this results in more stop ends but less load is experienced by column strip below. similarly, construct beams before slabs hang formwork off columns. These alternative approaches should be discussed and agreed between the PWD and TWD and incorporated into the method statement and/or safety plan. Some may have implications on liability and all will probably have implications on cost.

In summary, whatever approach to construction is agreed and adopted it must be documented in method statements and safety plans as appropriate. Above all it must be adhered to. It is recommended that, for backpropping considerations only, subject to confirmation in the site risk assessment and in the temporary works design brief, no imposed construction operations loads need to be considered.

76

See backpropping and Section 6.5.2, Assumption 2

This Chapter introduces the method for calculating when formwork can be struck and what backpropping may be required for straightforward flat slab buildings. To assist readers, a worked example based on the ECBP to illustrate this method is included in Annex D. Check critical conditions

At each stage of construction the critical conditions to be checked are: 0

0

0

Striking should be agreed in advance

the strength of the slab before striking the strength of the supporting slab when subject to additional loads from the temporary works or concreting the next slab the arrangement of backpropping.

Striking of flat slabs should be agreed in advance, and removal of supports should not commence until permits to strike are issued. The proposed method can only be considered if the flat slab under construction meets certain criteria: (a)

The slab is in situ reinforced concrete or a composite with participating or non-participating permanent formwork, such as thin precast concrete units.

(b)

Maximum equivalent thickness of dense concrete 350 mm22.

(c)

The slab is not post-tensioned or cast on thick precast, prestressed participating concrete units.

(d)

The slab is two-way spanning with or without beams, or one-way spanning with beams.

(e)

The cast slab is intended to be struck and become self-supporting before any additional loads are placed on it.

(f)

The reinforcement has been positioned within specification.

(8)

Where backpropping is considered, the suspended slabs at each level are of similar construction, thickness, etc.

(h)

No more than two levels of backpropping are used.

Action if controls on striking not introduced

Where these criteria cannot be met, or adequate controls cannot be introduced to control the site operations, the striking and backpropping should be discussed with the PWD and the TWD to establish the requirements for the contract. Information may be obtained from the contract specification, notes on the PWD drawings, CIRIA R136 (Ref. 31), BS 5975 (Ref. 6), etc.

Construction staff to be fully aware of implications

The operations of backpropping, loading and striking slabs should only be carried out under supervision. Where early striking to the proposed method is planned it is essential that the technical staff involved are fully aware of the implications of the procedures and method to be adopted. To assist readers, a flowchart of the procedures involved in striking is shown in Figure 20.

Striking flowchart Figure 20 Control loading on slabs

Calculation of the striking strength requires the TWC to assess the imposed load on various slabs, particularly where backpropping is to be used. The TWC should ensure that the assumptions made in the calculations are applied in practice, and that site control can avoid unplanned loading of the slabs, e.g. with pallets of blocks, machinery, reinforcement. See Section 4.3. Where backpropping is required to transfer loads through the building, the TWC should prepare, in advance of construction, a clear, consistent, repeatable procedure; a flowchart of the procedures

22 The depth of 350 mm was chosen as representative for the research. A thicker slab is stiffer and the ratio of the loads (deadimposed) is likely to be different, thus the steering group limited the method to 350 mm thickness.

77

7.1 General

involved in backpropping is shown in Figure 2 1. The procedure will manage the backpropping, and take advantage of the latest findings, particularly the controlled pre-loading of the backprops. Complex instrumentation, expensive monitoring equipment, etc. is rarely justified. What is required has to reflect the expertise of the site personnel and their supervision, be practicable and conform to the designer’s performance requirements for the final structure.

Backpropping flowchart Figure 21

Whether backpropping is required, and whether it is placed on one or two levels, is discussed in Sections 7.3, 7.4 and 7.5.

Sections 7.3,7.4 and 7.5

When planning backpropping, the sequence and resultant loads on the lower slabs should be considered at an early stage. A typical example taken from the ECBP is shown in Figure 27 and discussed in Section 7.7.

Consider sequence see Figures 26 and 27

Information required when planning the striking and backpropping is summarised in the checklist in Table 4. Reference may also be made to the flowcharts in Figures 20 and 2 1.

Table 4: Checklistfor striking and backpropping procedures. Units with default values

Likely source

Section

PWD

7.1

n/a

PWD

3.1

kN/m2

PWD

3.2

y / ~What is the unfactored design imposed floor load?

kN/m2

PWD

3.2

y / ~What is the characteristic concrete strength?

N/mmz

PWD

days

PWD

Check Item y/N

I Does the slab comply with the flat slab criteria (a) to (h) above?

I

y / ~What is the column layout and number of storeys? What are the self-weight (dead) loadings assumed in the design? y/N (slab self-weight, partitions, flooring, ceiling, services, etc.)

Y/N

At what age is this strength to be considered?

n/a

I

y / ~Is it based on cube strength or cylinder strength?

PWD PWD

4.7

TWC

6.4

0.5kN/mz

TWD

4.2

kN/mZ

TWD

4.3

How will the early concrete strength be assessed? y/N (e.g. LOK test, cubes cured alongside) What is the formwork system, and what is the self-weight allowance y/N for the falseworWformwork load? Y/N

What is the design construction load during concreting?

y/N

What density of concrete will be used in the calculations?

24 kN/m3

TWD

4.3 and 6.5.2, Assumption 7

Y/N

What is the likely imposed load on a floor after striking?

0.75kN/mZ

TWD

4.3

Y/N

Is there confidence that the reinforcement has been positioned with the correct cover? In particular, too much cover to the top mat reinforcement at column positions will reduce the slab capacity to carry construction loads.

TWC

Does the risk assessment allow the construction operation loads to be

6.5.2, Assumption 2

y/N ignored in the backpropping calculations?

Will the floor be struck completely to become self-supporting and to y/N take up the instantaneous deflection? y/N

Will steel or aluminium backprops be used?

TWC

I

aluminium

Will the preloading of the backprops be controlled, and if so, what is y/N the value of the required pre-load? What is the layout of the backpropping? y/N Are one or two levels of props being installed? - y/N

Which method of calculating the loads in the backprops will be used?

y / ~Have the supervisory staff been informed of the agreed procedure? Are the site supervisory staff aware of the permit to load and the permit y/N to strike forms and their location?

78

I

TWC

TWC I

I Method 1 , 2 , 3 or4

TWc TWD TWC

6.5.2, Assumption 7

6.5.3

TWC

Figs 20 and 2 1

TWC

4.5

STRIKING FLOWCHART

or obtain advice

method of concrete trength assessment?

TWD and TWC

construction load at time of striking

* Cannot be struck and needs propping. Refer to PWDand TWD. See Section 7.2, Figure 21

More than 1.0

Less than 1.O

loading factor. Equation 4,

factor Csf Use Figure22

Agree with PWD and TWC

c

Calculate required equivalent cube strength for striking 4 req. Use Equation 19, Section 7.2

--4

I

Establish equivalent cube strength of concrete for critical area as agreed. Use correct correlation curves/tables to give the 95% confidence limit.

striking procedure. See Figure 26 and Section 7.6

Figure 20: Flowchart for striking flat slabs in buildings.

Note: It is assumed that safety during striking has been considered, including working platforms, material handling, etc.

79

7.1 General

TWD whether to proceed.

Select backpropping calculation method (Section 6.5.3) METHOD ONE: See Table 3 (recommended) METHOD TWO: Formula (2D) METHOD THREE: Formulae (2D) simplified METHOD FOUR: Spreadsheet (3D) Annex F

a

Refer to TWD and PWD

Calculate loads.

Follow backpropping procedures (install and removal) See Section 7.7

Figure 21: Flowchart for backpropping #at slabs in buildings.

80

Calculate loads in props and supporting slabs.

7.2 Method to establish required concrete strength

7.2 Method to establish required concrete strength The strength of a given concrete slab required during a particular construction operation can be calculated using Equation 6, having first satisfied the requirement that the loading factor in Equation 4 is less than unity (see Section 6.2). This approach is covered in a simpler and perhaps more understandable way in the steps below. Step 1

Calculate the total unfactored design construction load23 on the slab (w)

Step 2

Establish the unfactored design service load on the slab (w,,,)

Step 3

Check if w,,,is greater than w. If it is, the operation must not be carried out, and repropping or backpropping is required. Refer to the PWD, TWD and/or the TWC.

Step 4

Calculate the loading factor (from Equation 4): Loading factor = Total design construction load - -w Total design service load Wser

Equation 4

Using this loading factor, read off the required concrete strength factor (C,J fiom Figure 22.

Step 5 1.1 1.o

0.9 0.8

Pa0

2

0.7 0.6

(3

5

0.5

a

9

0.4

0.3 0.2 01

o

''

O!I

I

012

' ' 013

014

I

0!5

I

016

017

I

018

CONCRETE STRENGTH FACTOR Csf

' ' 019

Figure 22: Graph of I!O I I,',

loading factor against concrete strength factor.

Notes: 1. Values of ratio of loading above 1.O should be used with care, and only with the approval of the PWD and

TWC.

2. Figure 22 is a graphical representation of Equation 8.

Step 6

Ask for the characteristic design strength of the concrete slab cf,,) in N/mmz.

Step 7

Calculate the required equivalent cube strength Cf,,,) of the concrete in the critical area for performing the construction operation, using values from Steps 5 and 6:

fcreq = C,

Equation 19

where

23

fcreq

=

f,,

(i.e. Step5 x Step6) required equivalent cube strength of the concrete in the critical area at the time of striking in N/mm2

Depending on the sequence of construction, this may or may not include the formwork weight.

81

7.2 Method to establish required concrete strength

c,, f,”

Step 8

=

concrete strength factor from Figure 22

=

characteristic design strength of the concrete in N/mmz

Check that the slab has achieved the required strength in the critical area24.

If in any doubt ask the TWC

7.3 No backpropping required During the construction of a suspended slab, see Figure 23(a), the TWD will establish the total load during construction (w,)(see Section 4.3). This will include the self-weight of the new slab (generally using a reinforced concrete density of 24 kN/m3), the self-weight of the temporary works, plus the superimposed construction load during the concreting operation (1.5 kN/m*). In order to be able to cast the new slab, the supporting slab should be strong enough to carry its own weight (again calculated using a reinforced concrete density of 24 kN/m3) plus any finishes already applied and the applied load (w,).This was referred to earlier as the ‘spare capacity’. Provided the calculated (spare capacity) w, all that is required is for the concrete strength of the supporting slab to have reached the required strength, determined in Section 7.2. By inspection, if the floors are similarly designed, this will rarely occur on suspended slabs as the supported slab is already supporting itself! If the new slab is a first floor, cast on suitable foundations, see Figure 23(b), then the total load during construction (w,)is transferred directly to the foundations. In order to strike the falsework, in both Figures 23(a) and (b) the required concrete strength of the new slab will have to be ascertained from Section 7.2. It is assumed that the slab will be struck and allowed to deflect prior to the construction of any further floor slabs. New slab to be cast

Falsework Imposed load (wp)

Load ( wp)

Supporting slab 1 Foundations

Lower slab 2 (a) Cast on slab

(b) On foundations

Figure 23: No backpropping.

7.4 One level of backpropping The previously cast floor slab is now the upper supporting slab for the next level of construction. The TWC should establish whether this supporting slab has sufficient ‘spare capacity’ at its very early age to support the self-weight of the temporary works and possibly some imposed construction operations load at the time considered. As the supporting slab matures its capacity should increase up to its design service load capacity. The recommendation that the supporting slab is always considered to take the weight of the formwork and falsework for the next slab should be kept in mind. This overcomes the onerous requirement to place the backprops in position before formwork can be moved up the building. In certain fast-track systems of flying forms and large tables this will always be the case as formwork is struck from below to be immediately moved up. The intention should be to install the backpropping at the earliest opportunity.

24 Only the LOK test and the Cap0 test are tests of the actual in situ concrete, and can be considered to indicate directly the equivalent cube strength of the in situ concrete (see Section 6.4). Other methods of test may require the average result from tests to be greater than the calculated required equivalent cube strength as calculated above.

82

7.5 Two levels of backpropping

New slab to be cast Falsework Imposed load (70% wp - f p ) Supporting slab 1 Imposed load (30% wp+ Pp) Lower slab 2 Foundations to be adequate Lower slab 3

(a) Suspended

(b) On foundations

Figure 24: Backpropping one level. Note: Loads shown are from Table 3 (Method One).

In considering the backpropping for a suspended slab, see Figure 24(a), during the construction of the new slab, the TWD will establish the total load during construction (w,)(see Section 4.3). This will include the self-weight of the new slab (calculated using a density of reinforced concrete of 24 kN/m3), the self-weight of the temporary works, but with no superimposed construction load (see Assumption 2 in Section 6.5.2). The load in the backprops (wb, ) may be estimated from Method One, as shown at Figure 24, or be calculated using either Method Two and Equation 11 or Method Three and Equation 14. Knowledge of the relative stiffness of the components is of course required. The additional load on the supporting slab will then be the difference (w,- w,,,). This loading often governs the speed of construction at a critical stage. The TWC must ensure that both the supporting slab and the lower slab 2 have gained sufficient strength before casting the new slab. The more accurate method to predict the loads, once the arrangement of the falsework and backpropping is known, is to use Method Four. Use of the spreadsheet is shown at Annex F. Pre-loading the backprops by an agreed amount (P,) reduces the imposed load on the supporting slab, and increases the load on the lower slab. The concrete strengths related to load capacity at each stage of construction need to be considered in the backpropping sequence. 66.7% wp load in supporting slab. Method 3 Section 6.5.3, Assumption 3

Destress the falsework, allow new slab to deflect, then remove backprops

When a second floor is cast, such as Figure 24(b) with one level of backpropping direct to the foundations, then providing the supporting slab has been allowed to take up its deflected shape, the load in the backpropping is also calculated from Equation 1 1 with S, = CO, possibly with the pre-load added. Having established the backpropping procedure, the order of removal will affect the loading on the floors. If any backprops are removed prematurely, the full load (w,)will be suddenly applied to the supporting floor, with serious consequences. Thus the only acceptable method of removing the backpropping is to strike the falsework fnst, allow the new slab to deflect, and then remove the backpropping.

7.5 Two levels of backpropping The two previously cast floor slabs are now the supports for the new slab, with the most recently cast being the critical supporting slab. The TWC should first establish whether this supporting slab has sufficient ‘spare capacity’ at its very early age to support the self-weight of the temporary works and some imposed construction operations load at the time considered. As the supporting slab matures its capacity should increase up to its design service load capacity. The recommendation that the supporting slab is always considered to take the weight of the formwork and falsework for the next slab should be kept in mind. This overcomes the onerous requirement to place the backprops in position before formwork can be moved up the building. In certain fast-track systems of flying forms and large tables, this will always be the case as formwork is struck from below to be immediately moved up. The intention should be to install the backpropping at the earliest opportunity.

83

7.5Two levels of backpro~pinq

New slab 10 be casi

Supporting slab 1 Imposed load (23% wp) Lower slab 2 Imposed load (12% wp + Pp) Lower slab 3 Adequate foundahons

(a) Suspended

(b) On foundations

Figure 25: Backpropping two levels. Note: Loads shown are from Table 3 (Method One).

In considering the backpropping for a suspended slab, see Figure 25(a), during the construction of the new slab and placing the concrete, the TWD will need to establish the total load during construction (w, ) (see Section 4.3). This will include the self-weight of the new slab (calculated using a reinforced concrete density 24 kN/m3), but again, with no superimposed construction load (see Assumption 2 in Section 6.5.2). The self-weight of the falsework and formwork may not necessarily be carried through to the backprops, because if erection has commenced before installing the backprops, the supporting slab will already be supporting this load. The load in the two levels of backprops (wb,and wb2)may be estimated from Method One, as shown in Figure 25, or calculated using either Method Two and Equations 12 and 13 or Method Three and Equations 15 and 16. Knowledge of the relative stiffness is of course required. The load on the supporting slab 1 (Figure 25) will be the difference (wp- w,,,). This loading often governs the speed of construction at a critical stage. The TWC must ensure that both the supporting slab 1 and the lower slabs 2 and 3 have each gained sufficient strength before casting the new slab. The more accurate method of predicting the loads, once the arrangement of the falsework and the backpropping is known, is Method Four. Use of the spreadsheet is shown at Annex F. Pre-loading all the backprops by an agreed amount (P,) reduces the imposed load on the supporting slab, and increases the load on the lowest slab, but this depends on the magnitude of pre-loading. If the preloading is the same, there will be no net effect on the load on lower slab 2, but there would be an advantage for lower slab 3 because of the extra spare capacity now available - the backpropping load wbz will be less than wb,.The concrete strengths related to load capacity at each stage of construction need to be considered in the backpropping sequence. It is interesting to note that the work at the ECBP suggested that there was little benefit in backpropping through more than one floor in terms of relieving the load in the most critical supporting slab. This was discussed in Assumption 6 in Section 6.5.2. In a three-storey building, such as in Figure 25(b), with two levels of backpropping direct to the foundations, providing the supporting slabs have been allowed to take up their deflected shape, and the lower floor backprops destressed and repositioned before casting the new slab, the load in the backpropping is also calculated from Equations 15 and 16 with S,, = 00, with any pre-load added.

72.8% wp load in supporting slab. Method 3. Section 6.5.3

Having established the backpropping procedure, the order of removal will affect the loading on the floors. If any backprops are removed prematurely, the full load (w,) will be suddenly applied to the supporting floor, with serious consequences. Thus the only satisfactory method of removing the backpropping is to strike the falsework first, allow the new slab to deflect, and then remove the backpropping. The order of removal of backpropping should be from the top down, with props between the supporting slab and slab 2 removed before those between slabs 2 to 3.

Order of removal of backpropping affects loads

It should be noted that this method applies more load on the supporting slab. Inspection of Table 3 shows that the physical effect of erroneously removing the lowest group of backpropping from slab

84

7.6 Sequence of striking sofit formwork

2 to 3 will have a limited effect on slab 2, increasing the loading by about 12%. This is well below the loads likely to have been used on this slab 2 for previous construction of slab 1, and the slab is unlikely to be harmed. Table 3 also illustrates the limited value of backpropping through more than one floor as a way to reduce the loading on the supporting slab.

7.6 sequence off striking somu ffaDlrmwolrk The order of striking soffit formwork is important. The concept of the temporary works may dictate the procedure adopted - for example, flying forms will not permit any propping to be left in place - and the exact procedure needs to be considered. See the Formwork Guide (Ref. 5). There will be safety considerations, as discussed earlier. The TWD should set out the method and procedure for the removal of soffit formwork. Working platforms and material handling will have been considered in conjunction with the TWC andor the proprietary supplier. The key principles for striking flat slabs are illustrated in Figure 26 and listed here:

1.

Obtain approval to strike. This Guide recommends that the 'permit to strike' is a site-signed written document. Check that there is a relevant signed form.

2.

Check the safety precautions for operatives and other persons during striking operations, particularly guardrails and crane control.

3.

Sofit formwork to reinforced concrete slabs should be struck in two stages. First, ease all the supports by about the same amount (about one or two turns for a prop), Second, starting at mid-span, remove the supports, working towards the columns or walls. See Figure 26(a). This ensures that all the load is not left on the last few supports at mid-span, which may cause reverse bending and cracking in the new slab.

Start striking slabs at mid-span

Order of striking internal and edge panels can affect load distribution. Preferably the PWD should specify the order

4.

The action of post-tensioning flat slabs, see Figure 26(b), will tend to lift the slab off the soffit formwork. It is still recommended that, unless specified otherwise, striking of post-tensioned slabs starts mid-span.

5.

On multiple slab areas, comprising internal and edge panels between columns etc, the PWD may have made design assumptions about the order of striking the supports. These assumptions should be communicated to the site. If this is not done, it is suggested that the TWC proposes

MidsDan falsework load

(e) Post-tensioned concrete cantilever

(b) Post-tensionedconcrete slab

-'m 6,

Strike slab either beam side first

(c) Reinforced concrete slab on beam($

Figure 26: Sequence of striking flat slabs.

85

7.6 Sequence of striking sofit formwork

to the PWD to start by strilung the internal bays first, followed by edge and corner bays. This order limits the mid-span deflections and, more importantly, controls the bending moment induced in perimeter columns during construction. 6.

Where beams are involved, with either one- or two-way slabs, the procedure should be to strike the slabs on either side of the beam, before striking the beam, again starting the beam striking at its mid-point. See Figure 26(c).

With slabs on beams, strike slab first, followed by the beams

7.

Where the soflit form is part of a projecting cantilever, such as at balconies, start the removal from the tip of the cantilever and work towards the column, beam or wall. See Figure 26(d).

8.

Post-tensioned concrete cantilever slabs, Figure 26(e), will deflect the adjacent span and may increase the load on the falsework. In such cases discuss the striking procedures with the PWD and TWD.

Always strike cantilevers from the tip. Take particular care with stressed cantilevers

The practice of striking areas of soflit tables with compressed air should be very carefully controlled. Crash striking of soflit formwork should never be allowed.

7.7 Sequence of insPaQOin

ova0 offbackprop

The TWC should prepare, in advance of construction, a clear, consistent, repeatable procedure in order to manage the backpropping. The findings of the work on the ECBP, particularly the controlled pre-loading of the backprops, should be kept in mind in order to ensure the most effective procedure. The backpropping sequence will differ from contract to contract, and this Guide can only give suggestions to enable the various aspects to be considered. Before establishing the procedures, the following aspects may need to be considered. (a)

Self-weight and imposed loads.

(b)

Magnitude of construction operations loads.

(c)

Layout of the temporary works on the supporting slab. The method of removing and replacing the formwork between floors can affect the floor loading.

(d)

Will the formwork be placed on the supporting slab before the backprops are inserted? The order affects the load calculations for both the backprops and the supporting slab.

(e)

Method of striking falsework between uses.

(f) (8)

Layout and material of the backpropping.

(h)

Stiffness of the various existing concrete floors, at the time considered. This requires an estimate or results of the concrete strength in the slabs.

(i)

Pre-load in the backpropping, if any is assumed.

0)

Slab strengths required to carry the loads as a result of the backpropping arrangement, and implications for the imposed loads that might be carried on each floor.

Stiffness of the backpropping.

The presentation format of the procedure is important and a suggested diagrammatic approach is shown in Figure 27, using the calculation methods given in this Guide and also showing the results from the ECBP research. The timing of installing the backpropping can be critical. If a floor has just been struck, and the falsework is being moved up directly from underneath to be seated on top of the newly struck floor, the strength of this new supporting slab can become critical. At the time of its complete striking it may have to carry not only its own self-weight, but also the weight of the temporary works plus any operative loading while they are re-erecting it. Thus backpropping might be required very quickly after striking to transfer the extra loads from this stage of construction to the lower floors. In general, backpropping should be (re)installed at the earliest opportunity to ensure maximum distribution of loads to the slab beneath. It has already been seen that pre-loading the backpropping can be advantageous. This requires careful planning if the backpropping procedure has assumed that all the backprops in a floor have to be

86

Standard calculation Operation

Diagram

In P'OP

itage

Was on slab

h slab now

Measured In Prop

0

6.75

11.30

G

0

52

I1 days

0

6.75

6.0

24

25 h

11.30

6.75

6.0

55

12 days

2.48

0

6.75

8.00

6.7

6.75

6.75

6.0

0

0

8.00

8.00

0.7

-

Insert backpropping between first floor and second floor

0.7

1

6.1 0.6

0

6.75

6.75

0

0

0

0

8.00

10.70

6.6

G

4

A

4

0

6.52

6.00

10.32

47

13 day

8.20

65

24 day

2.20

1.80 6.75

7.80

0

0

0

8.00

6.0

23

43 h

10.70

6.75

6.0

48

15 day

7.80

6.75

6.0

65

26 d a y

0

0

G

Strike backprops 4-5 March 1998

0

2.48

1.25

Stnke formwork to third floor and move materials up onto the third floor. 28 February 1998 (1 100 h)

0

-

G

Concrete third floor 26 February 1998 (1 500 h)

0

0

1.25 Erect formwork and Fix reinforcement for third floor

[N/mmL)

-

2.48

Strike formwork to second floor 14 February (1 500 h) Strike backpropping to first floor 17 February (0800 h)

h"

On slab

6.5

8.25 zoncrete second floor 13 February 1998 :1400 h)

Zoncrete

2

G

Figure 27: Typical backpropping layout planning chart from ECBP. Notes: 1. In use on site the 'measured' values would not be included, and the concrete strength only established as the work progressed and would not be included in the initial chart. 2. The information is based on the published results from the construction of the second to third floors (Ref. 2). 3. The standard calculation values agree with the proposals in this Guide. See Annex D for the worked example and explanation of the values. 4. Construction operation loads are not considered in the backpropping calculations.

7.7 Sequence of installing and removal of backpropping

pre-loaded in order to reduce the resultant imposed load on the supporting slab. Although theoretically possible, it is difficult to pre-load props a stated set amount when positioned between relatively flexible concrete slabs. It is considered reasonable, as already discussed in Assumption 7 in Section 6.5.2, to allow in the calculations for some general value of pre-load in backprops. The removal and destressing of backpropping should be carried out with the same care as striking the formwork and falsework. The following needs considering: 1.

2.

Has approval been given to remove the backpropping? If the jacks for destressing the backprops are at the top or high up, working platforms will be required.

Approval to remove backpropping Working platform needed if jacks used at high level

3.

Start destressing the backpropping for slabs near the centre, working towards the supports for columns or beams. Where the backpropping supports cantilevers, always start destressing at the end of the cantilever and work towards the supports or beams.

For slabs, start at centre; for start at tip

4.

Where there is more than one level of backpropping, check with the TWC the exact order of removing the props - generally the upper levels should be destressed before the lower levels - but it depends on the sequence already agreed.

Check prop sequence where more than one level

The installation of backpropping is covered in Section 6.5.4, which includes a recommended procedure when pre-loading is specified by the TWD. The removal of backpropping should be to an agreed procedure, and some guidance follows: 1.

Obtain approval to remove the back props. Refer to the site written ‘permit to strikehemove backpropping’. This might be a different permit to that for the stnking of the slab soffit formwork.

2.

Check the safety precautions for operatives and other persons during removal of propping. Long props can be heavy and difficult to handle.

3.

Supervisors should especially check that the individuals removing the backpropping are aware of the floor level on which to start and the sequence agreed.

4.

Backpropping to slabs should be removed in two stages. First, ease the props by about the same amount (about one turn). Second, starting at mid-span, remove the supports working towards the columns or walls. This ensures that all the load is not left on the last few backprops at mid-span, which may cause reverse bending and cracking in the new slab.

5.

On large slab areas, comprising internal and edge panels between columns etc., remove internal bays first, followed by edge and corner bays, to limit the mid-span deflections.

6.

Backpropping to staircases may need guidance on order of removal - check with the TWC on the sequence agreed.

9.8 AccideonUaU owerUoadiung off sUabs The prevention of accidental overloading of a slab, for example, by inadvertently placing a heavy bundle of reinforcement onto the newly cast slab at mid-span, such as that recorded in the Reading site monitoring (Ref. 37), depends on the site management and control procedures. This control is now more important as flat slab structures become ‘leaner’. It was for many years a ‘rule of thumb’ on site that repropping for such an eventuality with props with a capacity of about 25% of the slab self-weight should be provided (Ref. 5). Research at the ECBP project demonstrated that the performance of the thin floor slabs has exceeded their specification requirements, but also identified that, with such structures, the process and the control become more critical as the structures become leaner. Unless the site risk assessment identifies accidental overloading of new slabs to be a risk, then repropping for such an eventuality is not recommended for normal flat slab construction. See also the worked example on the second floor at the ECBP, calculated in Annex D.

88

8 References

Note: References in the Annexes are included here

PARTNERS IN TECHNOLOGY 1996. Project: A radical redesign of the insitu concrete frame process, Department of the Environment Project Reference BRE/SDD/ECBP. BRE, Garston, 1 April 1997, 26pp. BEEBY A.W. A radical redesign of the in-situ concrete frame process, Task 4: Early striking of formwork and forces in backprops. The University of Leeds, Building Research Establish ment. Report BR 394, London, 2000, 106pp. Sir JOHN EGAN (Chairman). Rethinking construction. The Report of the Construction Task Force to the Deputy Prime Minister, John Prescott, on the scope for improving the quality and efficiency of UK construction. DETR, HMSO, London, 1998. HEALTH AND SAFETY EXECUTIVE. Managing health and safety in construction. Approved code of practice. HSE Books, Sudbury, 2001. 100pp. Publication HSG 224. (Includes the Construction and Design and Management Regulations.) 5.

THE CONCRETE SOCIETY. Formwork - a guide to goodpractice. Slough, Second edition, 1995, 320pp. Special Publication CS030.

6.

BRITISH STANDARDS INSTITUTION. BS 5975: 1996. Code of practice for falsework. London, 1996, 134pp. Including AMD 9289 December 1996.

7.

CONSTRUCT. National structural concrete specification for building construction. British Cement Association, Crowthome. Second edition, 2000, 72pp. Ref: 97.378.

8.

THE CONCRETE SOCIETY. Plain formed concrete finishes. Crowthome, 1999, 45pp. Technical Report 52.

9.

CONSTRUCT, A guide to contractor detailing of reinforcement in concrete. Crowthorne, BCA, 1997, 26pp. BCA Publication CSG/OOl.

10. BRITISH STANDARDS INSTITUTION. BS 6399: Part 1: 1996. Loadings for buildings. Code of practice for dead and imposed loads. 16pp. 11. EUROPEAN COMMITTEE FOR STANDARDISATION. ENV 199 1-2- 1, Eurocode 1: Basis of design and actions on structures - Part 2- 1: Actions on structures - Densities, seEf weight and imposed loads. 1995, 50pp. Published as BSI document DD ENV 1991-2-1: 1996. 12. GOODCHILD C.H. Economic concrete frame elements. Reinforced Concrete Council, Crowthome, 1997, 128pp. BCA Publication Ref: 97.358. 13. BRITISH CEMENT ASSOCIATION. BCA Guide: Kickerless construction. Crowthome, 1988, 6pp. Ref. 47.023. 14. BENNETT D.F.H, Advances in concrete construction technology. BCA, Crowthome, 1989, 17pp. Ref. 97.309. 15. CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION. Permanent formwork in construction. Joint CIRIAKoncrete Society Report, London, 200 1, 174pp. CIRlA Publication C558. 16. STATUTORY INSTRUMENTS 1996, No. 1592, Health and Safety - The Construction (Health, Safety and Welfare) Regulations 1996, Her Majesty's Stationery Office, London, 2 September 1996, 24pp. 17. HEALTH AND SAFETY EXECUTIVE. Protecting the public -your next move. Health and Safety series booklet HS(G) 151, HSE Books, Sudbury, 1997, 52pp.

89

References

18. BRITISH STANDARDS INSTITUTION, BS 5973: 1993. Code of practice for access and working scaffolds and special scaffold structures in steel. London, 1993, 108pp. 19. BRAGG, S.L. Final Report of the Advisory Committee on Falsework. Her Majesty’s Stationery Offce, London, June 1975. 151pp. 20.

BRITISH STANDARDS INSTITUTION. Draft prEN 128 12. Falsework - performance requirements and general design. Draft for Public Comment 97/102975DC, London, April 1997, 40pp. (When implemented will partially supersede BS 5975: 1996.)

21.

BRITISH STANDARDS INSTITUTION. BS EN 1065: 1999. Adjustable telescopic steel props -product specijkations, design and assessment by calculation and test. London, 1999, 35pp. (Replaces BS 4074.)

22.

BRITISH STANDARDS INSTITUTION. Draft prEN 13377. Prefabricated timber formwork beams - Requirements, classification and assessment. Draft for Public Comment 98/109758DC, London, November 1998,24pp.

23.

HEALTH AND SAFETY EXECUTIVE. Safe use of lifting equipment. Lifting Operations and Lifting Equipment Regulations 1998, Health and Safety series booklet L 113. HSE Books, Sudbury, 1999,62pp.

24.

PRECAST FLOORING FEDERATION. Code of practice for the safe erection of precast concretejooring and associated components. Leicester, 200 I , 99pp.

25.

BRITISH STANDARDS INSTITUTION. BS EN 1263-2: 1998, Safety nets. Safely requirements f o r the erection of safety nets. London, 1998. 12pp.

26.

BRITISH STANDARDS INSTITUTION. BS EN 1263-1: 1997, Safety nets: safety requirements, test methods. London, 1997, 30pp.

27.

The Manual Handling Operations Regulations. HMSO, 1992. SI 1992/2793.

28.

THE CONCRETE SOCIETY. Checklist f o r erecting and dismantling falsework. Crowthorne, November 1999,24pp. Ref. CS 123.

29. THE CONCRETE SOCIETY. Checklist for assembly, use and striking of formwork. Crowthome, April 2003. 28pp. Ref. CS144. 30.

BRITISH CEMENT ASSOCIATION. Concreting f o r improved speed and efficiency, Best Practice Guide for In-situ concrete frame buildings. Crowthorne, July 2000,4pp. Ref. 97.502.

31.

CONSTRUCTION PIDUSTRY RESEARCH AND INFORMATION ASSOCIATION. Formwork striking times - criteria, prediction and methods of assessment. London, 1995, 60pp. Report R136 (supersedes CIRIA Reports 67 and 73.)

32. BRITISH CEMENT ASSOCIATION. Early age strength assessment of concrete on site. Best Practice Guide for In-situ concrete frame buildings, Crowthorne, May 2000, 4pp. Ref. 97.503. 33.

PRICE W.F. Curing concrete. Concrete, Vol. 32, No 8, September 1998. pp. 9-10. Current Practice Sheet 1 12.

34. HEALTH AND SAFETY EXECUTIVE. Health and safety in construction. HS(G) 150 Rev. I , HSE Books, Sudbury, 200 1, 124pp. 35.

The Health and Safety at work etc. Act. HMSO, 1974. ISBN 0 10 537774 3

36.

BRITISH STANDARDS INSTITUTION. BS 81 10: Structural use ofconcrete Part 1: 1997. Code of practice for design and construction. London, 1997, 122pp inc AMD 9882 September 1998.

37.

KAMALA N., DICKENS J, and PALLETT P.F. Site measurement of loads in shores and reshores, Concrete. Vol. 30, No. 2, March/April 1996. pp.16-17.

38. BEEBY, A.W. Criteria for the loading slabs during construction. Structures and Buildings, Proceedings of Institution of Civil Engineers. Vol. 146, No. 5, May 2001. pp.195-202. 39.

90

EUROPEAN COMMITTEE FOR STANDARDISATION. ENV 1992-1-1, Eurocode 2: Design of concrete structures. Part I : General rules for buildings. 1992. Published as BSI document DD ENV 1992- 1-1,274pp.

References

40.

HOSSAIN T.R. and VOLLUM R.L. Modelling the deflection of reinforced concrete slabs under time varying loads. Concrete Communication Conference 2000, University of Birmingham, June 2000, Proceedings. BCA, Crowthorne, Ref. 98.003. pp437448.

41.

BRITISH CEMENT ASSOCIATION. Influence of serviceability on the economic design of concrete structures. DETR Project 3913536 cc 1824, Crowthorne, 2000. (To be published as Defections in concrete slabs and beams. Concrete Society Technical Report, 2003.)

42.

BUNGEY J.H., LONG A.E., SOUTSOS M.N. and HENDERSON G.D. Early age acceptance of concrete (Improved quality management). CRC Ltd, London, 2000. BRE Report BR 387.

43. BRITISH STANDARDS INSTITUTION. BS 1881: Part 111: 1983. Method of normal curing of test specimens (20°C method). 8pp. 44.

COMITE EURO-INTERNATIONAL DU SETON. Strategies f o r testing and assessment of concrete structures. CEB Information Bulletin No. 243, Guidance report from the Task Group 5.4 on Assessment, Maintenance and Repair, Lausanne, 1998, 185pp.

45.

BRITISH STANDARDS INSTITUTION. BS 1881 : Part 207: 1992. Testing concretes. Recommendations f o r the assessment of concrete strength by near-to-surface tests. London, 1992,20pp.

46.

SADGROVE B.M. The strength and deflection of reinforced concrete beams loaded at early age. CIRIA, London, 1971. Technical Note 3 1.

47.

REYNOLDS G.C. Bond strength of deformed bars in tension. Cement & Concrete Association, Slough, 1982, 23pp. Technical Report 548.

48.

INSTITUTION OF STRUCTURAL ENGINEERS. Appraisal of existing structures. London, 1996. 106pp.

49.

HIGHWAYS AGENCY. BA 44/95. Assessment of concrete highway bridges and structures.

91

ANNEX A - The Cardington Project

A 1 Construction As part of the European Concrete Building Project (ECBP), a seven-storey in situ reinforcedconcrete-framed building was constructed at the Building Research Establishment (BRE) Cardington Laboratory (Figure A 1). The construction and its associated process research were performed under the then Department of the Environment’s Partners in Technology (PiT) programme through a contract led by BRE (Ref. 2). The process research was divided into six Tasks: Task 1: Re-engineering the concrete frame business process University of Cranfield. Task 2: High performance formwork University of Birmingham Task 3: Improved rebar information and supply University of Loughborough Task 4: Early striking of formwork and forces in backprops (Ref. 2) University of Leeds Task 5: Process efficient concreting Imperial College Task 6: Early acceptance of concrete (Ref. 42) University of Liverpool and Queens University, Belfast

Figure A I : View of completed building ---

Previous page is blank

93

Guide to flat slab formwork and falsework

The building had seven floors (six floors and a roof). It represented an imaginary office block in central Bedford, with one side facing onto ‘Bedford High Street’. The overall plan area was 22.8 x 30.3 m (690 m’). Each floor was of flat slab construction, 250 mm deep, with service voids and stair areas at each end of the building. A cross-section and plan of the building are shown in Figures A2 and A3. Columns were on a wide 7.5

x

7.5 m grid, giving a structure 3 bays wide by 4 bays long.

To suit modem commercial buildings the soffit height and column length for each floor was kept constant at 3.5 m. The soffit height was different on the ground to first floor because the building was built on the ‘strong floor’ in the laboratory and pad foundations approximately 700 mm deep were used. Thus to maintain the column length at 3.5 m, the actual ground to first floor soffit height was about 4.2 m. The first six floors were constructed using in situ construction; the top floor (floor seven - the roof) used two thicknesses of precast concrete planks as permanent formwork with an in situ concrete topping. The thicker concrete planks, incorporating the bottom reinforcement, had a 75 mm-deep x 200 mm-wide joint filled with ‘Densit’. Each floor of the building was cast in one pour necessitating formwork for the whole slab area to be erected. Two sizes of column were used: internal columns were 400 mm square and perimeter columns were 400 x 250 mm. Twenty columns per floor were used, six square and 14 rectangular. The columns were the same size all the way up the building: high-strength concrete was used in the first three sets of columns, i.e. up to the underside of the third floor slab. Although a stair void was designed at each end of the building, only one stairway was actually installed in precast concrete; it was in a ‘Z’ to half landings and ‘stitched’ together on the half landing with the proprietary material Densit. As no walls were to be used, lateral stiffness was considered a problem by the PWD and, after striking the floor’s formwork, steel tension cross-bracing was installed in two directions to provide horizontal stability to the building. A general arrangement of the building is shown in Figure A3. For the first six slabs, three formwork systems were supplied, each being used on two floors. Observations were made by on-site researchers of the use of each of the systems and comments were obtained from the contractor’s operatives, the contractor’s site management and other construction professionals. The intention of the study was not to compare the systems used, but to consider each of the systems on its own merits. A brief description of the systems follows.

A2

Eqwipment for Floors 1and 2 - Props and timber beams

This equipment was used to model the traditional approach to concrete slab construction. The props were aluminium rather than conventional adjustable steel props, and were fitted with a proprietary four-pronged forkhead that held a 240 mm-deep proprietary primary timber lattice beam. Another layer of similar proprietary timber beams, known as secondaries, was placed transversely on top of the primaries, and the plywood was fixed to the secondary members. The props were stabilised during initial erection using a proprietary tripod. First floor propping height was about 400 mm higher than the second floor because of the height of the column pads. This required props of extension length from 3000 mm to a maximum of 3985 mm. The spacing of the secondary beams was determined by the 488 mm safe span of the plywood. The centres of the props were determined by the capacity and layout of the primary beams, and were generally on a 2050 x 1283 mm grid. The total weight (plywood + beams + props) = 7.4 + 16.0 + 7.4 = 30.8 tonnes

A3

Equipment for Floors 3 and 4 - Ischebeck Titan Ltd

The Ischebeck Titan system comprises vertical aluminium props, called Size 4 Titan Legs, supporting two levels of aluminium beams which support the plywood. The primary beams were twin-web

94

Annex A - The Cardington project

I*

Figure A2: Typical cross-section of ECBP.

C

0

A 7500

I:

Hanger floor to fall

7500

-

. D

I(

7500

T

I

I

I

22.8 rn x 30.3

Figure A3: Plan of building.

95


225 mm-deep Titan beams and the secondaries were twin-web 150 mm-deep Titan beams. Jacking was accommodated at the bottom of the system. The system was generally made up into six- and 12-legged ‘tables’ with horizontal frame sections bolted to the Titan Legs. The complete arrangement of legs, frames, beams and plywood were handled as ‘tables’ between floors, the largest being 8.0 x 6.4 m in plan. The total weight (plywood + Titan beams + Titan legs) = 7.4 + 14.6 + 9.1 = 3 1.1 tonnes

A4

Equipment for Floors 5 and 6 - Kwikstage Shoring 55

The Kwikstage Shoring 55 kN system is a steel skeletal system. It comprises a head and base adjustable jacking system with vertical uprights, and is laterally stabilised by two levels of ledgers and transoms known as Shoring Ties. The grid arrangement used (1.8 x 1.8 m) was not the most efficient use of the equipment but it was all that the project could procure for the contract. The arrangement of supports was also dictated by the continued use of the on-site Ischebeck Titan aluminium beams used in construction of two of the lower floors. Total weight (plywood + Titan beams + Kwikstage) = 7.4 + 15.0 + 20.0 = 42.4 tonnes

A5

Floor 7 - Birchwood Omnia Ltd - Precast plank and Densit

Precast concrete planks 75 mm thick and 2.0 x 7.2 m in area were used on half of the ‘roof‘ floor. These thick planks incorporated the bottom mat of reinforcement. A gap between planks 200 mm wide and 75 mm deep was filled with a proprietary cementitious product, called ‘Densit’ which was mixed on site, to effectively lap the reinforcement at this location. A top mat of reinforcement was fitted and the remaining 175 mm topping concrete placed conventionally. The other half of the ‘roof‘ floor utilised 30 mm-thick precast concrete planks, known as thin planks, supplied in 2.4 x 7.4 m sizes and regarded as non-participating; thus reinforcement and concrete was conventional on this section of the floor with the planks replacing the face contact material and acting as permanent formwork. The falsework used to support the two types of precast plank was an adaptation of the Ischebeck Titan system on a slightly different grid. It comprised Titan legs and beams in one direction, plus some extra beams on the edge of the building. Weight (Titan beams + Titan legs) = 4.2 + 7.8 = 12.0 tonnes

96

~

~-

~~~~~~~

~

ANNEX B - Temporary works design brief checklist This checklist covers the basic information likely to be needed by a TWD starting a design. Not all items are required on all contracts, and the list is not intended to be exhaustive. Reference should be made to the design risk assessment that will be prepared as the design is formulated.

GENERAL REQUIREMENTS Administration and contract 1.

Contract name, company reference number, and TW reference number as appropriate.

2.

Is it a tender or a confirmed contract?

3.

Is the equipment to be purchased, hired, or a combination of both?

4.

If a firm contract, give details of the frame contractor, principal contractor, andor main contractor as necessary.

5.

Name, address and contact details of the PWD.

6.

Name, address and contact details of the Planning Supervisor.

7.

Name, address and contact details of the TWC.

8.

What insurance cover is required?

9.

Apart from Reg. 13, do the CDM Regulations apply? (Generally if the work is not notifiable.)

10. Date when the temporary works design is required. 11.

Up-to-date detail and general arrangement drawings.

12. Copies of existing programmes and relevant specification clauses: if none, assume the National structural concrete specifcation for building construction (Ref. 7). 13. Copies of (or extracts from) the Health and Safety Plan. 14. Structure for which a TW scheme is required, together with brief description. 15. Location of site: access, exposure, height, surroundings, etc. 16. Site restrictions: overhead cables, noise limits, enclosure requirements, etc. 17. Statutory restrictions on the site: local air rights, pavement licences, etc. Construction

18. Types of surface finish required. 19. Are the sofits flat? Details of any residual cambers required by the PWD. 20.

Structural dimensions on curves and gradients so that they can be identified and easily set out, and checked, on site. Can the required curved surfaces be formed in straight lines, as individual facets? If so, state the allowable offsets to aid setting out.

21.

Details of available cranage and maximum planned lift (in kg) over sofit area, and outside limits for handling tables. LOLER (Ref. 23) gives specific requirements for lifts.

22. Is there a requirement for external scaffolding, andor total enclosure? 23. Has provision been made in the original planning for debris protection fans and have relevant approvals been sought for their use on site? 24.

Has the reinforcement been detailed by the contractor? (See Ref. 9)

25.

What are the design arrangements for the stair flights? Can they be used to speed up the temporary works?

26.

Any details by the PWD of safety guardrail connections during construction?

97


27.

Method of placing and compacting the concrete, e.g. pumping, crane, skip.

28.

Type and position of specified Construction joints, and accepted methods of forming joints.

29.

Information from the PWD on design service loads, density of the reinforced concrete, assumed characteristic strength of concrete, etc.

30.

Included in the PWD information, what tolerances have been allowed for in the PWD design. This is particularly necessary where precast units are involved in the TW process.

3 1.

Safe load capacity of the foundation slab? (as bearing pressure in kN/m2).

32.

Agreed method of concrete strength assessment, e.g. LOK test, cubes cured alongside.

33.

Sequence of concreting, and limitations on size of pours.

TEMPORARY WORKS Information and procedures 34.

Does the PWD andor the TWC have a preconceived concept for the temporary works?

35.

Is an independent check required on the TW calculations?

36.

What material or equipment is available for use?

37. What is the likely level of competence of site operatives, training required, and quality of supervision? Possibly include statements on past performance. 38.

Method of material handling envisaged between formwork uses?

39.

Work at height requires legal consideration of access and the provision of suitable working platforms, for all operations at height. Protection can also be achieved by keeping operatives away from exposed edges.

40.

Suitable working platforms and edge protection on all sofit formwork, and on column forms preferably on all four sides, should always be shown on drawings.

41.

What attachment points for temporary access and fall prevention equipment are required?

42.

Will temporary enclosure of the structure be provided?

43.

Details of loading-out platforms envisaged and planned. Including likely equipment to be used on the platforms. Design for imposed loads from, say, 4.5 to 20 kN/m2.

44.

Material likely to be used for face contact material to give surface finish required.

45.

Where proprietary suppliers are involved, they should be requested to submit calculations as part of their final contract to supply, generally only after receipt of order.

46. Number of copies of each drawing required. Ideally the format should also be specified, e.g. A4 black and white. 47. Procedure to be adopted to ensure that the specified construction sequences are observed. Information may be needed to assist the TWD and the TWC to write the procedures for striking and controlling backpropping. This could include the permit systems. 48.

Any training required for operatives in strength assessment testing? e.g. LOK test.

Columns and walls 49.

Column kickers - minimum 100 mm - recommended for perimeter columns.

50. Are the columns square or round? 5 1.

Do the columns have heads? This will restrict the type of column formwork suitable. If heads are required, presumably square columns have square heads, circular columns have round heads, etc.

52. What are the details of the slab fixings for the temporary column stability props, and have tripping hazards been reduced in the design? 53. Unless stated otherwise it should be assumed that column concrete will be compacted using internal vibrators. The use of external vibrators dictates significantly heavier and stronger column forms together with special fixing devices for such vibrators. 54. Are the supporting columns/walls to be cast 20 mm into the new slab?

98

Annex B - Temporary works design checklist

Soffits, slabs and falsework 55.

Weights of handled units of tables, column form sections, etc., should be stated. LOLER (Ref. 23) gives specific requirements for lifts.

56. Category of sofit form release agents to be used. 57. Early consideration of handling lengths of beams, etc., such as on manually struck areas of sofits, should be encouraged.

58. Include information on the limitations to any backpropping, e.g. number of levels, location assumed for backprops, programme requirements for installation, any pre-loading required. 59. What are the backpropping assumptions with regard to loads on slabs, construction operations loads (risk assessment on operations loads, if any applied), one or two levels backpropped, etc? 60.

What density of reinforced concrete is to be used in the falsework design? (The value for the backpropping will generally be established at 24 kN/m3.)

61. Attention should be paid in designing table forms at the edges of structures. 62. Position and details of any clean-out openings, access panels for LOK tests, etc.

99

ANNEX C - Comparison of load transferred Predicting the actual load spread through the structure in three dimensions is complex, and was discussed in detail in Assumption 5 in Section 6.5.2. When the effects of the deflected shape and resultant different loads are allowed for at the propping positions, it is possible to show mathematically that only about one-third of the assumed load will be actually distributed through the supporting slab into the backprops: see Table 3 and Figure 15. This is significantly less than.the assumption made in BS 5975 (Ref. 6), and previously assumed for flat slabs. Table C1 reproduces the information in Table 3 for Method One (Section 6.5.3), with the addition of the values assuming that each supporting floor takes the same proportion of the applied load, assuming similar stiffness of each slab, as suggested in BS 5975 (Ref. 6). Table C1 shows the percentages of applied load distributed from casting a slab: with no backpropping, with one level backpropped, and then with two levels of backpropping - assuming that the two levels of backpropping are identical, i.e. exactly above each other on the floor.

Table C1: Percentage of load distribution from temporary works loads.

_N_ o Location

Load

New slab cast on falsework

wp I

Supporting slab 1

I

backprops

100%

Previous BS 5975: 1996 On slab In prop

1

100%

100% 100%

I

One level of backprops

Now proposed Method One On slab In prop

I

100%

-

I

70% wp

I

100%

-

100%

33% wp

-

Backprops Lower slab 2 Backprops Lower slab 3

I

Notes: 1. Assumes lower and supporting floors have been struck, have taken up their deflected shape and are carrying their own weight.

2. Floor loading from imposed loads and self-weight not considered. 3. Strength of slabs to carry applied loads not considered.

4. All floors are suspended.

100

100%

100%

100%

100%

50% wp

Two levels of backprops Previous Now proposed BS 5975: 1996 Method One On slab In prop On slab In prop

65% wP

-

I

ANNEX D - Worked example of striking and backpropping to second floor at the ECBP D1

General

This example considers both the striking of the ECBP second floor and its subsequent use with backpropping to support the casting of the third floor. The stages of the operation are shown in Figure 27. The second floor was cast using a specified characteristic strength C37N concrete with accurate strength determination for striking from the averages of LOK tests in situ and temperaturematched cube results. The measured loads in the props and the actual concrete strength determinations are also shown in Figure 27. The flat slab was a 250 mm-thick solid reinforced concrete slab on a column grid of 7.5 x 7.5 m. The project required each floor slab to be fully struck and allowed to take up its instantaneous deflection prior to construction of subsequent floors. Thus it conforms to the criteria in Section 7.1, and the proposed method of establishing concrete strength in Section 7.2 can be used, and is shown diagrammatically in the flowchart in Figure 2 1. The TWC had agreed that construction operations loads were not to be considered in the backpropping considerations. (See Assumption 2 in Section 6.5.2.) When the third floor was cast, this second floor became a supporting slab and was backpropped down one level with four props. The calculations of backpropping loads for three of the four methods of calculating backprop loads are discussed in Section D10. The method follows the nomenclature and order given in Sections 7.2 and 7.4. The calculations for both the floor loading and the backprops are considered in similar terms as load per unit area to assist direct comparisons.

D2

Loadings

The total load on the floor slab when it is struck is its self-weight (calculated using a reinforced concrete density of 24 kN/m3) plus the construction operations load for access on top of the slab. This is one of the loads used to check the striking requirement, as summarised below: 24 kN/m3 )

Self-weight of concrete slab

6.00 kN/m2

(i.e. 0.25

Superimposed construction load

0.75 kN/m2

(i.e. Service Class 1 loading)

Self-weight of formwork

nil

x

Step l a Hence: total second floor load at striking24 (w) =

6.75 kN/m2

(see Figure 27 Stage 6)

If formwork is to be erected on this slab then: Additional imposed load (BS 5975)

0.75 kN/mZ

Self-weight of the formwork

0.50 kN/mZ

Step Ib Hence revised load at striking (w) =

(see Section 4.3)

8.00 kN/m2 (see Figure 27 Stage 7)

24 This assumes no additional loading directly from any formwork moved straight on to the struck floor from beneath, such as being struck out and moved immediately upwards using table or flying form systems.

101


Second floor slab struck

. . . . 7

Load (w) 6.75 kN/m*

Falsework

Heads lowered and slab allowed to deflect

First floor slab

Backprops

f

f

\Load

(w) 8.00kN/m2

Foundations

Figure D1: Situation aRer striking second floor.

The PWD's design service loadings were as follows: Self-weight of concrete slab (floor 2)

6.00 kN/m2

partitions

1.OO kN/m2

raised floor

0.50 kN/mZ

ceiling services

0.50 kN/m2

Superimposed office floor load

(i.e. 0.25 x 24 kN/m3 )

2.50 kN/mZ

Step 2 Hence: PWD's design service load (wSer)= 10.50 kN/m2

D3

To establish the criteria for striking

Step 3 Having established the values for both w at Steps l a and l b and wserat Step 2 above, the first check confirms that:

w,,,(10.50 kN/m2) is greater than w (6.75 kN/mZ) Step 4

OK

w - 6.75 Loading factor = Total design construction load -- -= Total design service load wser 10.50

0.643 = 0.64

The loading factor F, is less than 1, so the operation can proceed. Step 5 From Figure 22 look up the concrete strength factor, C,, for a loading factor of 0.64. This gives a Csfof 0.47 Step 6 For the second floor, the specified characteristic strength C37 i s h u= 37 N/mmz Step 7 Hence the required equivalent concrete cube strength in the critical area from the method of strength assessment from Equation 19 is

f,req = Csffcu= 0.47 x 37 = 17.39 N/mm2 Although the formulae used are based on the cracking factor, for completeness, the previous values can be used to verify that the cracking factor is less than or equal to 1 from:

= 0.642 x 2. 127°.6= 0.642 x 1.572 = 1 .OO

Hence both striking criteria F, I 1.O and F,, I 1 .O are satisfied. The second floor was actually struck on 14 February 1998 at a strength of 24 N/mmz (Figure 27 Stage 6).

102

Annex D - Worked example of striking and backpropping to second floor at the ECBP

D4 Further considerations - strength assessment As stated in Section 6.4(e), the use of the LOK test based on an average of four readings gives the direct value of the equivalent concrete cube strength of the in situ concrete at the test location. Using LOK tests, the site would need to obtain the average of four readings (Ref. 32) by reading off the equivalent cube strength from the correct correlation curve or table. It is, of course, important to ensure that the test location represents the critical area of the concrete slab for striking. In this example at, say, an average of 18 N/mm2 from the four results, the slab could be approved for striking. If the concrete is to be struck using cubes cured alongside as the only method of strength assessment, because they are not directly representative of the in situ concrete, as discussed in Section 6.4 (a), the mean of the cube results needs to be increased to obtain the lower bound strength: Mean of air-cured cube results

= required

equivalent cube strength

x

factor

At the time of writing, information on the magnitude of this factor was not available. This would need to be discussed between the PWD and the TWC. Indications from earlier Reports, such as BR 394 (Ref. 2) and CIRlA 136 (Ref. 3 l), suggest that this factor might be between 1.10 and 1.25, but will never be less than unity.

Step 8 If the factor was 1.25, the revised equivalent cube strength required is

fcreq= 1.25 x

0.47 x 37 = 21.7N/mm2

In this case it had already been determined that the slab had reached this strength at the time of striking. Note that it may be necessary to also check the strength requirement of the first floor in order to establish whether the backprops could be removed. In this case the floor strength had reached 55 N/mm2 at time of striking out the second floor (Figure 27 Stage 6).

D5 Second floor erection

- Backpropping consi

erations for falsework

Considering different stages of construction will arrive at the required minimum concrete strength needed for operating the system. The next operation on the second floor will be to erect the temporary works for the third floor. See Stage 7 in Figure 27. This will require the full construction operations load and the weight of the formwork to be considered. From Step 2 in Section D2, the total load on the second floor slab (w)is now 8.00 kN/m2. Thus

w Loading factor = Total design construction load - -- 8'oo = 0.762 = 0.76 Total design service load wser 10.50 From Figure 22 look up the concrete strength factor C,, for a loading factor of 0.76. This gives a Csfof 0.63 If the strength is assessed with the LOK test based on the average of four readings, the required equivalent cube strength is the average of the four results: Required equivalent cube strength is

fCreq

= C,, f,,= 0.63 x 37 = 23.3 N/mm2

This slab had actually reached a strength of 24 N/mm2 at the time of striking, so the operation of erecting formwork on the second floor, shown as Stage 7 (Figure 27), could have started immediately. However, had striking been carried out at the lower bound value, i.e. if the average results had only been 17.4 N/mmz (Section D3), construction work on erecting and assembling the falsework and formwork for the third floor could not have started on this floor slab until it had been backpropped, or the slab concrete had matured and reached 23.3 N/mm2.

103


D6 Second floor third floor

- Backpropping considerations for casting the

The temporary works arrangement was to insert one level of backpropping from the underside of the second floor to the first floor slab, shown diagrammatically in Figure D2 and also in Figure 27, Stage 8. The TWC needs to verify the loads that are to be carried by the supporting slab 1, i.e. the ECBP second floor slab, from concreting the new slab (third floor) scheduled for 26 February. It is normal to assume that the backprops have been fitted after the formwork has been erected. This was the case at the ECBP. This means that the weight of the formwork is already being carried by the supporting second floor. Self-weight of concrete slab (third floor) Formwork self-weight Construction operations load Hence falsework imposed load (w,)

6.00 kN/m2 (i.e. 0.25 x 24 kN/m3) nil nil (see Assumption 5, Section 6.5.2) =

Supporting slab self-weight (second floor) Formwork self-weight on supporting slab Hence total construction load w

6.00kN/mZ 6.00 kN/mZ 0.50 kN/m2

=

(on unpropped second floor)

12.50 kN/m2 > 10.50 kN/mZ( W J

This is larger than the PWD’s total design load, so backpropping is confirmed as essential. As constructed, one level of backpropping was used and the following sections calculate the results assuming Methods One, Three and Four. See Section 6.5.3 and Figure 15. The results from the three methods are compared alongside Figure D2 in this annex.

D7 Method One

- Backprop loads by w e (ob percentages of load

Using Table 3 for one level of backprops gives the load in the supporting slab, i.e. the second floor slab, when casting the third floor of 70% of w,.This is Stage 9 at Figure 27. Thus the imposed load on the supporting slab (second floor) is 70% of 6.00 kN/mZ= 4.20 kN/m2 and the total construction load on the second floor will be 6.00 + 0.50 + 4.20 = 10.70 kN/m2. This is greater than the PWD’s design service load of 10.50 kN/mZand approval will have to be sought to load the slab above the service load. See Section 6.6 and the discussion in Section D12. The load in the backprops is 30% of wp= 30% of 6.00 kN/mZ= 1.80 kN/mZ,which gives a total load in the first floor slab of (6.00 + 1.80) = 7.80 kN/mZ.

D8 Method Three formulae

- Backprop loads by use of further simplified

Using the assumption that the slabs will be at least twice as stiff as any backpropping introduced, Equation 14 (Section 6.5.3) can be used to calculate the backprop loads. The relative stiffness of the first floor (SSJ and the supporting slab (&,) are needed to calculate the backpropping forces. The lower floor is likely to be stiffer than the supporting slab as ‘it will be about three weeks old at the time of casting the third floor. Records show that the relevant cube strengths were 47 and 65 N/mmz, and elastic moduli ( E ) 36.01 and 40.12 kN/mmZ,respectively. S 36.01 Let us assume that the ratio SI = 0.90 say Ss, 40.12 ~

104


Equation 14 gives the load in the backpropping directly under the second floor as:

Thus the imposed load on the supporting floor slab is 6.00 - 1.54 = 4.46 kN/m2 This assumes that the falsework was placed on the slab before the backpropping was installed. Thus the total construction load on the second floor using Method Three will be 6.00 + 0.50 + 4.46

=

10.96 kN/mZ

Again, this is greater than the PWD’s design service load of 10.50 kN/mZ and approval must be sought to load the slab above the service load. See Section 6.6 and the discussion in Section D12.

D9 Method Four

- Backprop loads from spreadsheet

Using the values of load from the known arrangement of falsework and backpropping used to cast the third floors, the Method Four spreadsheet results for an internal panel, an edge panel and a corner panel with one level of backpropping are shown as Examples One, Two and Three in Annex F. The results show the loads in the backprops as 1.29 kN/mZ for the internal panel and 1.53 kN/m2 for the edge panel, increasing to 1.74 kN/mZfor the stiffer corner panel. Note that in calculating the load in the backpropping the value of 6.00 kN/mZ was used, i.e. excluding self-weight of the falseworWformwork. The self-weight is included in the design construction load when considering the supporting slab. In the worst case, the supporting slab 1 (the internal panel), i.e. the second floor slab, has an imposed load when casting the third floor of: (wp - wb,)= (6.00 - 1.29 + 0.50) = 5.21 kN/m2

This is greater than the ‘spare capacity’ 25 of the supporting slab, i.e. the second floor, at 4.50 kN/m2 (assuming that the second floor has reached its full 28-day strength). The calculated total construction load on this second floor, excluding any construction operations loads, is now shown in the output26 as:

w,,, = 5.21 + 6.00 = 11.21 kN/m2 The value of 11.21 kN/m2 represents a theoretical load above the total service load (wser)of about 11.21- 10.50 ~ 1 0= 0 6.76 = 6.8% 10.50 giving a loading factor F, = 1.07. However, no account has been taken of any pre-load in the backprops or cracking in the slabs. When the fourth floor was cast, backpropping was increased to two levels. The analysis at Example Six in Annex F shows, yet again, that, without pre-loading the backprops, the supporting slab was theoretically loaded to above its service load. Example Six shows w,,, = 11.11 kN/mZ, which represents an increase as follows: 11.11-10.50 10.50

~10= 0 5.81 = 5.8%

giving a loading factor F,

25

=

1.06.

The realistic spare capacity is the design service load less the actual self-weight, i.e. 10.50 - 6.00

= 4.50 kN/m2. 26

Note that the self-weight here is the second floor slab on its own, without any construction loads.

105


D10 Discussion A summary of the results from three methods discussed is shown in Figure D2. Summary of loads (kN/m2)by Method:

Third floor being cast

Slab

ONE

THREE

FOUR

6.00

6.00

6.00

( Falsework designed in all cases for 8.25 2 7 )

Supporting slab i.e. Second Floor

Lower slab i.e. First Floor

In second floor

10.70

10.96

10.76 to 11.21

In backprops

1.80

1.54

1.29 to 1.74

In first floor

7.80

7.54

7.29 to 7.74

Foundations

Figure D2: Backpropping one level.

These calculations also show that the third floor slab cannot be cast until approval has been received from the PWD to load the slab above its design service load. This also implies that the supporting slab has gained, at least, its full 28-day strength. This is discussed in Section D12 of this Annex and in Annex E. Note that it may also be required to assess the strength of the concrete in the lower slab, i.e. the first floor, as well, before casting the third floor, though because of the small amount of load transferred it is rarely a problem, unless the pre-loading is excessive. If backpropping were extended down to the rigid foundation level andor pre-loaded, then the parameters would be changed and casting might be possible earlier. This demonstrates the complexity of the backpropping considerations, and emphasises the need for careful control and use of detailed procedures.

D11 Second floor - Pre-load of backprops and allowance for cracking Perhaps the reason why the supporting slab is rarely overloaded in practice is because of the pre-loading of these props when installed. The backprops were placed at third points in a slab, and therefore four props took the load. The approximate area supported by each prop is 7.5ox7.50 = 14.,m2 4 Using Method One and percentages of load (See Section D7) gives the load per backprop as approximately 14.1 x 1.80 = 25.38 kN per prop. The aluminium prop used for the backpropping had a stated pitch of 36 mm per turn of the collar, and the assumed stiffness was 25 kN/mm, so to pre-load the supporting slab by, say, 25 kN requires 1 mm of prop extension, i.e. only 1/36th of a rotation or 10". This takes no account of the stiffness of the slab above and below, and assumes them to be infinitely rigid. This small amount of movement in the prop is most likely to be nearly always taken up by 'fingertight' and, as already stated, pre-loads of 12-1 5 kN were regularly recorded. This is one reason why the measured values of load were low, and the floor slabs were not actually shown to be distressed. Assuming that the pre-load is distributed as a uniform load gives an equivalent floor load of 0.60 kN/mZ; this reduces the second floor loading. This equates to an estimate of 8.50 kN per 27 The design of falsework is recommended for the wet condition of reinforced concrete density of 25 kN/m3 with full construction operations load of 1.5 kN/m2 (see Assumption 3 in Section 6.5.2).

106


backprop. The Method Four calculation for one level of backpropping with pre-loaded props is at Example Four in Annex F. Comparing Examples One and Four shows that the load reduces on the supporting slab and increases on the first floor, giving: revised load on supporting floor

=

6.00 + 0.50 + 4.10

=

10.60 kN/m2

revised load on first floor

=

6.00 + 1.90

=

7.90kN/m2

The value is still greater than the designer’s service load of 10.50 kN/mZ and construction could only continue with the approval of the PWD. Where the PWD allows some cracking in the slabs, as recommended in Eurocode 2 (Ref. 39), the values will change. Assuming a distribution coefficient of 0.2 and a ratio of curvature of 0.3 gives the revised calculation at Example Five in Annex F. This shows that, for the same slab, with only one level of backpropping, the loading factor F, = 1.O, allowing construction to proceed. The actual loading is as follows: w - 10.51 Loading factor = Total design construction load - - --= Total design service load w,,, 10.50

1.001

From Equation 7, the concrete strength factor C, for a loading factor of 1 .OO 1 gives a C,, of 1.002. If strength is assessed with the LOK test based on an average of four results, the required characteristic concrete strength is:

fcreq= C, xfcu= 1.002x37 = 37.08 kNlmm2 (as stated at foot of Example Five) The calculation shows that with one level of backpropping a negligible stress of 0.2% is recorded. From Figure 27 Stage 9, it is noted that the equivalent cube strength in the supporting slab 2 was 47 N/mm2 on the day of casting the third floor (26 February). If a value is to be entered for the distribution coefficient other than zero then a value for the uncrackedkracked ratio of curvature should also be entered. It would be safest to enter a default value of unity (1) for the ratio of curvature - in practice this ratio will usually be in the range 0.25-0.3 3. Note that the load in the backpropping actually measured on site during installation of backpropping on the ECBP was 0.60 kN/mZ(Figure 27 Stage 8). This seems to vindicate the assumptions made in this section, and explains why such structures may not have been unduly overloaded during construction in the past. When the similar exercise is carried out on the fourth floor construction with two levels of backpropping, shown by comparing Example Six with Examples Seven and Eight in Annex F, it is noted that the loading factor reverts to unity both when pre-loading the props to 8.50 kN (Example Seven) and also when allowing for cracking and pre-loading to 6.0 kN (Example Eight). The calculation shows that with two levels of backpropping a negligible stress of 0.8% is recorded. It is noted that, in all the cases discussed, the supporting slab is required to have achieved the equivalent 28-day cube strength of 37 N/mmz.

D12 Second Floor - Discussion on loading to above the service load At the ECBP advantage was taken of the fact that the actual strengths achieved even at relatively early ages (say 14 days) were much higher than the characteristic cube strength. This is discussed at Section 6.6 and also explains how the concrete slabs were able to be cast. Considering the worst case from Method Four of load on the second floor (Example One) gives: w 1 1.2 I Loading factor = Total design construction load - - - - = 1.067 = 1.07 Total design service load wser 10.50

107

Guide to flat slab forrnwork and falsework

From Figure 22 look up the concrete strength factor C,, for a loading factor of 1.07. This gives a CSfOf1.10. Hence the concrete strength required is fcreq

2

= Csfxfc, =1.10~37=40.7N/mm

This load above the design service load was checked at the ECBP and approval was given to proceed. As already stated, the concrete strength at the time of casting the third floor was 47 N/mmz (see Figure 27 Stage 9). This approval is reasonable, particularly when considering that no pre-loading or allowance for cracking had been included in the calculations above. The use of the spreadsheet vindicates the approval in Examples One to Six. In practice, the principle of loading the slab to above the service load should be considered by the PWD in discussion with the TWD. The considerations at serviceability and ultimate limit states are discussed as Proposals 2a and 2b in Section 6.6.2. A fuller treatise is given in Annex E. The PWD will need to check the area of tensile reinforcement, as well as the compressive strength of the concrete at the time considered. Reinforcement areas may be more than the minimum required, and this is often the reason why approval to proceed can be given. The construction operations load is the load that is or is not going to affect the slab. It was not measurable at the ECBP, yet it needs to be seriously considered. The calculations have assumed that it may be ignored for the purposes of checking the loads in the slabs and the backprops. It is for the TWD and the PWD to determine the magnitude, if any, to be allowed. For example, the assumption that the construction operation loads may be reduced is considered at the serviceability limit state (SLS) in Table E l in Annex E. It shows that, considering loading combinations, the likely load is 0.53 kN/m2 SLS on the supporting slab. Thus adding in the likely percentage of the concrete load (at 70% from Method One), and assuming the falsework already on the supporting slab, would give a total construction load on the supporting slab of: (6.00

x

70%) + 0.53 + 0.50 + 6.00 = 11.23 kN/mZ

This is in excess of the total construction load stated 'at Method Four of 11.21 kN/mz. Thus the PWD needs to be involved to verify whether the slab can accept this loading at early age. Perhaps pre-loading the backprops, as discussed in Section D 11, together with strictly followed procedures, are the solution in such cases.

D13 Second floor - Backpropping considerations for accidental overloading To avoid overloading a new slab accidentally, props with a capacity of 25% of the slab self-weight should remain in place. This gives a propping capacity of 25% of the supporting 6.00 W/m2 = 1S O kN/m2. As this is generally less than the installed capacity of the backpropping, no extra backpropping is needed for accidental loading.

D14 Fourth floor - Distributed load from falsework The construction of the fifth floor, supported from floor 4, used one level of backpropping. The falsework was considered as providing a uniformly distributed load onto the fourth floor, and nine backprops were used, positioned at quarter points. The analysis, in Examples Nine and Ten in Annex F, shows that the loading factor and cracking factors are less than unity. Where pre-loading is not considered, the full 28-day strength of the fourth floor has to be achieved before casting. When the backprops are estimated as pre-loaded to only 6.0 kN each, the required equivalent cube strength is only 3 1 N/mmz. It should be noted that the effect of pre-loading the backprops can influence the load required to be carried by lower slabs. In Example Ten the program delivers a warning on the screen that the load on the middle slab (slab 2) of 9.00 kN/m2 is approaching the value of the supporting slab load of 9.50 kN/m2. This recommends that a manual check on the capacity of the lower slab 2 should be carried out. In practice, it should be older and hence stiffer than the supporting slab, and may not be a cause for concern. The printed output from the spreadsheet gives values for the supporting slab.

108

ANNEX E - Theory of loading a slab to above the design service load Introduction

El

This annex explains the reasoning behind the proposals in Section 6.6 for dealing with temporary overloading of flat slabs. CIRIA Report 136 (Ref. 31) lists the factors that need to be considered when setting criteria for striking formwork (collapse, deflection, freeze-thaw damage, mechanical damage, moisture loss, colour variation, durability, thermal cracking and shock, and site requirements). However, with respect to theoretical overload of slabs, as with striking of slabs, the main concerns are deflection and collapse. The two principles proposed in Section 6.2 for early striking of slabs are: - - w I 1.0 Loading factor (F, ) = Total design construction load on slab Total design service load on slab Wser

Equation 4

and

[Cr)[gj

Crackingfactor (Fer) = -

Equation 5

where

I 1.0

f,

=

total unfactored design construction load on the slab in kN/mZ total unfactored design service load in kN/m2 characteristic concrete strength at the time of striking in N/mm2

f,,

=

characteristic strength of the concrete in N/mmz.

w w,,,

= =

As has been shown in Section 6.6, using these two principles directly to check the slab when it is supporting concreting loads above can lead to many situations where the slab is theoretically overloaded. Such slabs could be classed as being unbuildable without being redesigned for the worst load case of construction. While this is an option, the intention here is to investigate more considered approaches that allow design and construction teams to deliver economic concrete structures safely. In order to deal with these theoretical overload situations it is necessary to first look more closely at theory and the underlying assumptions used in the above criteria and apply engineering judgement. These judgements should be agreed between the PWD and the TWD as giving safe and satisfactory performance.

E 2 Theory: base criteria Equations 4 and 5 are based on work by Beeby (Ref. 2) and essentially deal with deflection and collapse. It is proposed that these same principles form the basis for checking a slab during the early part of its life when it is used to support falsework, formwork and concreting loads to the slab above in multi-storey construction.

Summary One Use Equations 4 and 5 as the basis for checking temporary overload of flat slabs during construction.

109


E3

Loads

E3.1 Load factors: independent loads Currently, in line with BS 5975 (Ref. 6), a construction operations load of 1.5 kN/m2 is applied to the floor being concreted and 0.75 kN/m2 on all other slabs. For structures (cf. falsework), it is generally accepted that, where several statistically independent forms of loading are applied simultaneously, the probability of them all being at their maximum value at the same time reduces with the number of loads to be considered. This is not covered explicitly by BS 8110,(Ref. 36) but Eurocodes apply this concept and apply factors to imposed loads depending whether they are considered rare, frequent or quasi-permanent. With ultimate loads, it may be in order to use the most onerous factor and consider the rare loading case. Considering Eurocode 2 (Ref. 39), Beeby (Ref. 2) suggests that combination factors of 1.0 and 0.5 may be appropriate: these would give the rare load case, the greater of 0.70 + (gks + 0.5 + qks)

Equation 20

either

(gku+qk, 0.5)

or

(gku + qku)

0.70 + (gks + 0.5 + qks

where

gk,

=

characteristic dead load of the upper slab (being concreted)

qks

=

characteristic imposed load of the supporting slab

0.5 kN/m2

=

general industry allowance for weight of formwork and falsework (Ref. 5)

0.70

=

Method One carry-through factor (assuming one level of backprops) from Table 3 in Section 6.5.3, i.e. 70% of the loads from the slab being concreted are carried by the supporting slab. If two levels of backprops are used this reduces to 0.65 (i.e. 65%).

Equation 2 1

0.5)

The case is less clear for deformation. With long-term deflection, quasi-permanent loads are taken: combination factors of 0.2,0.3,0.6 or even 0 (in the case of snow) are used depending on the nature of the loading and use of the structure. In the case of temporary overload affecting cracking and deflection it may be prudent, until proven otherwise, to use the more onerous frequent factors. Beeby (Ref. 2) suggests that factors of 0.5 and 0.4 may be appropriate for a frequent load case. The load to be considered would now be the greater of either

(gku+qk,

or

(gk,

+ qk,

x

0.5)

x

0.4)

0.70 + (gks+ 0.5 + qks x 0.4)

Equation 22

+ (gks + 0.5 + qks

Equation 23

0.70

0.5)

E3.2 Load duration The duration of overload is likely to be short. It should be stressed that the problem occurs only for about 12 to 18 hours until the slab being poured becomes self-supporting. At the ECBP no significant construction imposed load was detected in the props. See Figure 27. Duration is not a mitigating factor for the ultimate limit state - an overload is an overload. However, in the serviceability conditions there are many factors to take into account that are the subject of continuing debate. These considerations had a bearing on the rare and frequent factors proposed above.

E3.3 Load allowances. It may be argued that it is unreasonable to allow both 1.5 kN/m2 at the upper level and 0.75 kN/m2 at lower levels during concreting operations. For instance, it is usual to cordon off the area below concreting operations and make it inaccessible during concreting and indeed until the slab is struck. Provided the area is cordoned off and not used for temporary storage it may be argued that, for the duration of the 1.5 kN/m2 load, a concurrent allowance of 0.75 kN/mZfor construction loads may be inappropriate. The concern here is with the structure and the structure has much more continuity and robustness than the temporary works for which the ‘concreting’ imposed loads are intended. Indeed it is difficult to envisage the 1.5 kN/mZimposed load co-existing on a moving front of concrete operations with the full dead weight of concrete on anything other than a local scale.

110

Annex E - Theorv of loadinu a slab to above the desisn service load

E3.4 Temporary load patterns Temporary load patterns may differ from permanent load patterns and should be considered. BS 8 110 (Ref. 36) load patterns are for all spans loaded or alternate spans loaded. The fact that the 'live' load (n - gk) experienced by the slab is likely to be applied as a wave of point loads as concreting operations on the slab above proceed makes little material difference to bending moment coefficients. Usually the supporting slab will be complete and the degree of continuity in the slab will be as assumed in the design. However, if this is not the case, e.g. if only three of five spans are complete, special consideration must be given to temporary moments, e.g. end span bending moment coefficients may be different and internal spans may become temporary end spans. There is also likely to be less redundancy during the construction phase than when the structure is complete. In terms of the permanent works design, the temporary overload might be considered as being a load test on a structure that has yet to be accepted by the main contractor or client, i.e. it is at the specialist contractor's risk until accepted. Obviously safety must not be compromised and all parties must be satisfied with methods of construction proposed, but specialists should know their business and be well placed to make suitable judgements. Collapse is unlikely to be an issue as considerations of serviceability are generally more critical. E3.5 Loads: self-weight of formwork and falsework Although it is general in the industry to allow 0.5 kN/m2 for the self-weight of the formwork and falsework to flat slabs up to 350 mm thick, some systems may be different. Precast permanent formwork suppliers have used a value of 0.25 kN/mZ to allow for the falsework. (See Ref. 15.) E3.6 Pre-loads in back props The effect of pre-loading backprops is to transfer a load equal to the pre-load from the slab above to the slab below. Controlled pre-loading of backpropping may be feasible, but it requires strict procedures to ensure correct installation and maintenance. Nonetheless the normal installation of backprops and handtightening instills a pre-load in the props which at the ECBP equated to between 0.3 and 0.7 kN/m2, i.e. approximately 6 kN per prop. One proprietary supplier's aluminium props are stated as having a stiffness of 25 kN/mm at 3.5 m extension and one full turn equivalent to 38 mm extension. In theory, to obtain 6 kN in such a prop would entail only a 2.4" turn. This is entirely feasible, but not normally measurable without the use of load cells and special methods. While pre-load should not normally be relied upon, it may certainly be used in mitigation. E3.7 Load combination factors Tables El and E2 show the effects of using load combination factors. Summary Two

For ULS use Eurocode 2 (Ref. 39) rare combination factors for independent load factors of 1.0 and 0.5. For SLS use frequent combination factors of 0.5 and 0.4. Summary Three

If access below concreting operations is denied during concreting operations then, so far as the permanent structure is concerned, a concurrent allowance of 0.75 kN/mZfor access below formwork might be ignored. Two load cases should then be considered: 1S O kN/mZabove with 0 kN/m2 below, and

0.75 kN/m2 above with 0.75 kN/mZbelow.

E4

Deflection: Serviceability Limit State

Deflection of concrete sections is inextricably linked to the extent of cracking and the degree to which the cracking moment is exceeded.

111


Table El: Loads and combinationfactors at serviceability limit state (SLS). Construction imposed load (kN/m*)

Upper

I

Combination factor

Carry through of loads

Construction imposed load for supporting slab qkss (m/m2)

Notes

Lower

TOP

Bottom

Max

1.5

0.75

1.o

1.o

70%

1.80

1.80

1.5

0

0.5

0.4

70%

0.53

0.53

1.5

0

0.4

0.5

70%

0.42

1 level of props and deny access below formwork

0.59

1 level of props

I Between concreting operations and striking 0.75

0.75

0.5

0.4

70%

0.56

0.75

0.75

0.4

0.5

70%

0.59

Table E2: Loads and combinationfactors at ultimate limit state (ULS). Construction imposed load (kN/m2)

Upper

I

Combination factor

Carry through of loads

Construction imposed load for supporting slab qkss (kN/m2)(char)

Lower

TOP

Bottom

0.75

1.o

1.o

70%

1.80

1.80

1.os

1.5

Notes

Max

f i o L r e i n g operations 1.5

0

0.5

1.o

70%

0.53

1.5

0

1.o

0.5

70%

1 .os

)cretin;

operations and striking ~~

~

0.75

0.75

0.5

1 .o

70%

1.01

0.75

0.75

1.o

0.5

70%

0.90

1.01 1 level of props

Flexural cracks occur when moments produce tensile stresses that exceed the concrete’s tensile strength. According to the most recent studies tensile strength is proportional to (cube strength)o.6. Moment is proportional to load. Hence w,,,lfcu0.6 is a measure of the cracking implicit in the design. Beeby (Ref. 2) showed that crack width and curvature are related to (1 - k Cf,,,Wlw>’); thus the factor (wlfc 0.6) helps describe stiffness and hence deflection. Equation 5 ensures that the extent of cracking (and instantaneous deflection) induced during construction does not exceed the value implicit in the design of the member under service conditions. This criterion may be considered as being a little onerous as long-term deflections, e.g. to Eurocode 2 (Ref. 3 l), are usually based on quasi-permanent loads (see Load factors: independent loads in Section E3.1) and allow for duration of load. In the proposals adopted in Section 6.6, some recognition of the temporary nature of the load has been allowed in the loading (by adopting the frequent factors of 0.5 and 0.4 for construction imposed loads).

112

1 level of props and deny access below formwork

Annex E - Theory of loading a slab to above the design service load

Equation 4 is seen as being not unreasonable for members significantly cracked under service loads. In effect it also limitsf, tof,,, which guards against normal consideration of ultimate failure due to bending. Although based on lower levels of load, Sadgrove (Ref. 46) concluded that early loading of slabs made little difference to long-term deflection. Repeat loading tends to reduce creep so it may be argued that an initial overload induces deflection, normally associated with creep, before the finishes are applied. It is generally accepted that early loading (i.e. early striking) does not have a significant effect on total deflection in normal circumstances. Likewise, it may be argued that a temporary overload will have little effect on long-term deflections. (Creep is influenced by age at loading but the effect of creep on deflection is small in comparison to the effects of cracking.) In the case of a young slab supporting concreting operations above, the excess loads are temporary only. Beeby remarks: “If the [construction] load did exceed the service load, the cracks would exceed the values implicit in the design while this load was applied but would reduce in size on removal of the load. This would probably not have a significant long-term effect on performance.” (Ref. 2)

If w > w,,,,cracking and deflection can be restricted by using a higher grade concrete, i.e. by increasingf,, such that principle 2 is satisfied. “It might be argued that exceeding the design service load during construction was not serious and that the inequality [criterion I ] above could be relaxed. It is doubtful that designers would consider significant overload during construction as being reasonable and so major violation of inequality 1 should be avoided.” (Ref. 2)

The question arises: “but what is major?”, is it 10% or, say, 30%? If, as stated above, Equation 4 is there for structural members that are assumed to be substantially cracked due to service loads, any overload will cause more cracking and deflection. However, iff, is allowed to exceedf,, to the extent that Equation 5 is satisfied, then the serviceability limit states of cracking and deflection should be no worse than that implicit in design. This situation will of course be temporary and from practical experience largely recoverable and not detrimental to the long-term performance of the member. Obviously excessive overload would cause irrecoverable deformation, which would be apparent before the member is offered for acceptance. To ensure no inelastic deformation at supports, the temporary serviceability moments should not exceed the design moment (at ultimate limit state). Presuming similar support conditions, this can be done by ensuring w/n,er < pb (Equation 17). This limit becomes critical when levels of redistribution approach 30%. At 20% redistribution (as assumed in the moment factors in Table 3.12 in BS 81 10 (Ref. 36)), this limit is likely to be less critical. Summary Four 0

0

For SLS, use a higher grade of concrete and allowf, to exceedf,, such that Equation 5 is satisfied. Providing Equation 5 is satisfied, ensure ultimate capacities are not exceeded. i.e. check ULS.

Ensure

W

-<

pb

nserv

E5.1

Bending

It can be shown quite easily that the bending moment capacity of a lightly reinforced section is relatively unaffected by concrete strength. Increasingf, abovef,, does not significantly increase the moment capacity of the section, Mcap. For more highly reinforced members with concrete strengths belowf,, the relationshipf, o.6/fCu 0.6 bears reasonable comparison to the relationship of moment capacity to designed moment capacity, Mcap/Mcapfcu. Figure E l shows the relationship for a C28/35 concrete.

113


150 I

I

*

200 mm solid slab 0.28%, T12 @ 2008 250 mm2/mT, cover 25 mm 200 mm solid slab 0.50%,

+T16 @ 2008 250 mm2/mT, cover 25 mm 200 mm solid slab 0.75%,

+T20 @ 2008 250 mm2/mT, cover 25 mm

---

Beam 300 x 450 mm deep, 1.78%, 3 T32B and 2 T20T, 30 mm cover all round

+'cu/35 I

20

10

30

50

40

f , (N/mm2)

Figure E l : Typical ultimate bending moment capacity graph for a C28/35 concrete.

Abovef,,, there is little increase in moment capacity. Very often sections may be detailed with excess capacity, but without reverting to the design calculations on every section it is not possible to rely on this being the case.

E5.2 Shear The most critical condition for the ultimate limit state is likely to be shear. Figure E2 shows the relationship for a typical C28/35 concrete. Eurocode 2 (Ref. 39) assumes that shear strength v, is proportional tofc0.666 ,whereas BS 8110 (Ref. 36) assumes that shear strength is proportional to fC0.333. Thus forf, If,,, the use of Equation 5 is close to the more onerous condition for checking shear. Abovef,,, assuming shear strength is proportional tof, capacity.

0.666

may lead to an overestimation of shear

140 120

100

80 60 40

L

2o 010

20

30

40

50

& (N/mm2)

Figure E2: Typical ultimate shear capacity of a C28/35 concrete.

E5.3

Bond

Ultimate bond strength is, according to Clause 3.12.8.4 in BS 8 1 10: Part 1 (Ref. 36), proportional tof,,o3. BS 8110 Table 3.26 is based on original work by Reynolds (Ref. 47) whose work is based

114

Annex E - Theorv of loadincl a slab to above the desicm service load

on Tepfers’ bond expression, which in turn relates bond strength directly to concrete tensile strength. As stated in Section E4, concrete tensile strength is taken to be proportional tof, 0.6. Table 3.26 in BS 8 1 10 is derived from an equation that includes a term (0.5 + c/D),where c is cover and D is the diameter of the bar. The table assumes that c equals D, whereas, particularly for slabs, c > D.For example, using a 16 mm bar with 20 mm cover would lead to a 17% increase in ultimate bond stress using these expressions. Local bond checks are not required in BS 8110.

’ L

Equation 18

It would appear prudent to ensure that the ultimate performance of the member is not compromised, and further to ensure that capacities are not exceeded in the temporary case. This can be done by checking that & n - 2 1.0 nser

where

n

=

riser =

ultimate construction load (ULS: gk = 1.2, qk = 1.2) ultimate design load

(ULS: g,

=

1.4, qk = 1.6)

Summary Five

Relatively small increases in ultimate bending moment, shear and bond capacities created by increasingf, beyondf,, should be disregarded in the assessment of the temporary overloads. To ensure capacities are not exceeded, ensure n I riser. E5.4 Load factors: temporary nature of load CIRIA Report RI36 (Ref. 31) proposed that, for lightly reinforced sections, a safety factor of 1.2 should be used for dead and construction loads. A precedent has been set for the reduction of the partial safety factors for loading in the Institution of Structural Engineers’ Appraisal of existing structures (Ref: 48) and CEB Bulletin 243 Strategies for testing and assessment of concrete structures (Ref. 44).

The factor of safety for load y is made up of three components, such that Equation 24

Yf

where

=

YfI

Yn Yn

yf,

=

load variation factor

yn

=

load combination and sensitivity factor

yn

=

structural performance factor

If dimensions and densities are known so that it is possible to calculate the self-weight accurately then it may be possible to justify a reduction in yfl to 1.05. The yn factors should not change, as the probabilities remain the same for assessment, i.e. yn = 1 .O. If measured dimensions, including any eccentricities due to construction inaccuracies, are used in the assessment process and realistic or conservative assumptions are made about the mechanisms of load transfer then it may be possible to reduce yn to 1.15. Putting these together gives a possible yr of 1.2 for dead load. CEB Bulletin 243 also suggests the possibility of a reduced safety factor for limited periods. This is primarily aimed at the period between inspections. However, it may be possible to draw an analogy between the temporary period between inspections and the temporary period before further strength is gained. This should not be seen as an additional reduction in addition to the yf of 1.2, but as a further justification for the yr value of 1.2. This reduction in the partial safety factor maintains the same reliability overall. Summary Six

A value of 1.2 may be used for yf in the temporary situation provided both TWD and PWD are confident their assessment is sufficiently rigorous that many of the uncertainties encountered at design stage are known.

115

I


E5.5 Material factors of safety The material factor of safety (ym) is made up of two components: Ym

Equation 25

= Yrn1.Ym2

where yrnl takes account of possible reductions in the strength of the material in the structure as a whole compared with the characteristic value deduced from control specimens and ymztakes account of possible weaknesses of the structure arising from any other cause. Traditionally, factors for concrete assume cube tests rather than tests on concrete in the structure. Where an average of up to four tests on the in situ concrete is used (e.g. LOK tests) a smaller margin is in order. A precedent has been set for reducing partial safety factor for materials. BD 44/95, the Highways Agency Advice Note Assessment of concrete highway bridges and structures (Ref. 49) and the Institution of Structural Engineers’ Appraisal of existing structures (Ref. 48) allow reduced yrnfactors when used with worst credible strengths. The worst credible strength is defined in BD 44/95 as the lower bound to the estimated in situ strength. It is essentially the worst value of that strength that the engineer, based on their experience and knowledge of the material, realistically believes could be obtained in the structure under consideration. This value may be greater or less than the characteristic strength of the materials assumed at design stage. Since this value eliminates some of the uncertainties associated with the use of characteristic strengths appropriate to the design phase, reductions may be made in the partial safety factor for materials. For the purposes of this report, an analogy is drawn between the worst credible strength for the assessment of bridges, etc. and the in situ characteristic strength used in and required for assessing striking times andor temporary .overload. Reductions can also be made in ymzto give the values of ym presented in Table 4A of BD 44/95, reproduced in Table E3 below. In other words, the moment capacity of a section with a concrete strength 80% of the specified characteristic using ymc= 1.2 is the same as a section with a concrete of the specified characteristic using ymc= 1.5, as used in normal design. ([ 1.2/1.5] = 0.80) According to BS 81 10, shear strength is a function o f f , l ’ 3 / ~ ~So ~ . the shear capacity of a section with a concrete strength of 80% off,, and using ymc= 1.15 is the same as that of a section with concrete atf,, using ymc=1.25 ([ 1.15/1 .2513 = 0.78) With respect to punching shear, the requirement to limit face shear to 0.8fCu%will be satisfied by using yrnc= 1.15 and a concrete whose strength is 85% off,,, presuming the permanent works design used allowable shear based onf,, with ymc= 1.25. ([ 1.25/1.1 512 = 0.85) The absolute limit of 5 N/mm2 will be unaffected. Ultimate bond strength is a function off,%. So the ultimate anchorage bond capacity of a bar in a concrete whose strength is 80% offu, using ymc= 1.25, is the same as that of a section with concrete atf,, using ymc= 1.40. ([1.25/1.40]2 = 0.80) Table E3: Values of materials safety factor y,, at the ultimate limit state. (Rej 49) Application

Reinforcement I Concrete in flexure and compression Shear in concrete Bond

I I

Value for use with Characteristic strength

1.15 1S O

1.25 1.40

I

Worst credible strength

1.10 1.20 1.15 1.25

Notes: 1. For buildings, BS 8 1 10: 1997 (Ref. 36) gives yms= 1.OS.

2. May be reduced to 1.05 if measured steel depths are used in addition to the worst credible steel strength

116

I I

Annex E - Theory of loading a slab to above the design service load

Summary Seven

Providing the assessment of the characteristic in situ strength of concrete is regarded as ‘the worst credible strength’, a concrete with a characteristic in situ strength of 85% of the design characteristic strength,f,,, may, for the ULS, be equated to the design characteristic strength. (Partial safety factors, ym, may be reduced through elimination of uncertainties.)

117

ANNEX F - Backpropping spreadsheet

Operation of Excel spreadsheet for calculation of backprop loads F 1 Conditions of use The spreadsheet calculations for backprop loads on the enclosed CD-ROM are subject to the following conditions of use, and purchase of this Guide will be assumed to be agreement to the terms of the use by the vendor. The spreadsheet is only considered applicable for buildings incorporating solid slabs where the following criteria are met: (a)

The slab is in situ reinforced concrete or a composite with either participating or nonparticipating permanent formwork, such as thin precast concrete units

(b)

Maximum equivalent thickness of dense concrete 350 mm28.

(c)

The slab is not post-tensioned or cast on thick precast stressed participating concrete units.

(d)

The slab is two-way spanning with or without beams, or one-way spanning with beams. Note: where heavy stiff beams support the slab, the method may not be accurate.

(e)

The cast slab is intended to be struck and become self-supporting prior to any additional loads being placed on to it.

(f)

The reinforcement has been positioned within specification.

(8)

The suspended slabs at each level are considered to be of similar construction, thickness etc.

(h)

There is either one or two levels of backpropping.

F2

Spreadsheet liability

A fundamental condition of use is that the user accepts responsibility for the input and output of the computer and how it is used. As with all software, users must be satisfied with the answer the spreadsheet gives and must be confident in its use. The spreadsheet can never be fully validated but has been tested, both formally and informally, through members of the steering group. However, users must satisfy themselves that the use to which the spreadsheet is put is appropriate. The original Excel program was first issued by BRE in early 2000 as CPROP.XLS with BRE Report BR 394 (Ref. 2) and amended by Eur Ing Peter Pallett with assistance from several members of the steering goup. The technical content of the original spreadsheet has been updated to provide a basis for making allowances for cracking. The operating manual is to clarify its use following experiences, with operating and testing the spreadsheet. The initial spreadsheets were checked to try to guard against values outside those that might be encountered in practice being used. By its very nature, this exercise could not be all-encompassing. Future developments, such as adoption of European standards with modifications to formulae and different philosophy, might lead to properties strange to UK users. The spreadsheet is supplied as an Excel protected spreadsheet.

The depth of 350 mm was chosen as representative for the research. A thicker slab has greater stiffness and the ratio of the loads (deadimposed) is likely to be different, thus the steering group limited the method to 350 mm thickness. 28

118

.

I

.

Annex F - Backproppins spreadsheet

F3

Introduction

The backpropping spreadsheet gives information for temporary works designers (TWD) and permanent works designers (PWD) while carrying out assessments of various construction procedures for the loading of in situ concrete slabs on multi-storey buildings. Principally, the spreadsheet is used as a tool for understanding the load distribution when loading slabs with falsework andor backpropping. The use of varying positions of falsework supports and numbers of levels backpropped during construction made previous determination of the loads transferred through the building a complex task. The Excel program, designed and researched from work carried out on the European Concrete Building Project (ECBP), allows designers to ascertain the effects of their likely actions and predict the effects on the sequence of operations. It will lead to a greater understanding and safer approach to backpropping and construction of flat slab buildings. The primary aim is to construct the flat slab building without detriment to the permanent work. This will involve knowledge of the specification and performance expected from the structure under consideration. The essential criteria for the loading of slabs during construction are satisfied if: Equation 1

Loading factor

F,,, 5 1.0

Equation 2

Cracking factor

F,, I 1.0

A further criterion, from knowledge of the deflections caused by slabs cracking gives Effective deflection factor

Equation 3

Fe, I 1 .O

The spreadsheet outputs these factors taking into account the known or expected concrete strengths, type of loading conditions etc. Criteria exceeding unity are displayed in red. Research found that preloading of the props was normal, and an allowance for this is included. The spreadsheet also allows advanced users to consider in more detail the effects of allowing some degree of cracking in the structure, as proposed in Eurocode 2 (Ref. 39) This is discussed in more detail in Annex G . If in any doubt about cracking considerations, refer to the designers. Knowledge of the design and construction loads applied to the structure is required. The program will be used by TWCs to provide advanced information on sequencing, expected concrete strengths to perform certain operations etc. The methods of backpropping flat slabs generally allow for one or two levels of backpropping as shown in Figure F 1. The figure also illustrates the symbols and nomenclature used. One level of backpropping

New slab to be cast Falsework Supporting slab 1

m

Load

1 1 1 1 From casting slab

. L ”

Backprops

Upper level wb,

Lower slab 2 Backprops (when fitted)

m U Two levels of backpropping

Lower level wb2

Lower slab 3

Figure F1: Diagrammatic representation of backpropping.

F4

Operation of the Excel spreadsheet

The basic program is called Backpropcalc.xls (425 kb). It is STRONGLY suggested that users copy the program and create a working copy of the spreadsheet for everyday use, by using “file,

119


save as” etc. When the program is saved it over-writes the existing version leaving the titles and layout as the last time used, hence the suggestion that you need to keep the source program. The program is saved in Microsoft Excel 97-2000 and 5.0/95 97 (*.xls) workbook format. The Excel program contains four macros built in; these are safe to open. These spreadsheet written instructions are broadly based on those published in BR 394 (Ref. 6) and have been modified to suit nomenclature such as “supporting slab”, “backpropping levels”, and the references and numbering are as in this guide. See also Figure F 1. The operations are in stages, as shown on the right hand side notes in the spreadsheet:

Start up On loading the program, the first display is a dialog warning about macros, select “enable macros” and continue. The spreadsheet is displayed as shown opposite, with a series of operating instructions and grey buttons down the right-hand side. Although the worksheet is protected, cells with blue numbers require input of information.

General The program operates on a rectangular ‘panel’ equal to a single span of the slab in the x and y directions between columns or beams. The input information and resulting output are colour coded, basic input in yellow boxes, information and output for the permanent works designer are in green boxes. The in situ actual concrete information and coordinates of the support positions required are shown in blue boxes. Optional information, related to whether or not the completed concrete slabs are allowed to crack when loaded is shown in grey boxes. Operation of the program is in the stages described below.

STAGE ONE - Input headings and panel type ( I ) Input Date:, Made By: and Page: (blue)

SPACE ... ....

Insert the date, made by and page information manually by clicking on the relevant cell and amending using the normal Excel formula bar at top of page. The date format is currently set at 24-05-2002 to become 24 May 02 on the spreadsheet. When printing, the spreadsheet is set to print the date printed at the foot of the page.

(2) Input of title and selection ofpanel type (red) Click on the right-hand button labelled “Insert andor change the title, then the panel type” This will first display a dialogue box, which requests a title for the particular analysis. The size of the title is limited to the length of the box; this ensures that parts of it are not overwritten by later actions of the program. If your title is correct click “continue”, otherwise alter the title and click “OK’. The size, colour and underlining of your title are built into the program. Automatically the next dialogue box displays the possible choices of panel type. The program will handle four types of panel:

1

An interior panel of a flat slab (continuous over all edges)

2

An edge panel of a flat slab (discontinuous over one edge)

3

A corner panel of a flat slab (discontinuous over two adjacent edges)

4

A panel supported on four sides by walls or beams.

Either click “No change” to leave the selection unaltered, or highlight whichever of these is the most appropriate and click on ‘‘Amend”29

29 Earlier versions of Excel may display a warning or error at runtime when you amend the Panel Type. It is due to how the visual basic in Excel 2000 and above handles the drawing of the panel layout on the screen. It does not affect how the program calculates the backpropping loads etc.

120

'I

CONSTRUCT: Guide to Flat Slab Formwork 8 Falsework

Title: The Excel Backproppinq Spreadsheet concrete structures group

I

Made By: P.F.P.

Date: lIAuglO2

I

I

Basic Data (Required)

Falsework a n d Backpropping

Number of falsework supports Number of backprops per floor Number of levels of backpropping

Lx (m)

Dimensions of panel:

1 of 1

Page:

I

Thickness of slabs

25.00

Stiffness of backprops (kNlmm)

----

Estimated preload in each backprop (kN) Data from Designer of Permanent W o r k

Is falsework erected before backprops?

Characteristic strength of concrete (feu )

NlmmZ

40

Assumed concrete density (if not 24 kN/m3) Unfactored self weight of slab

kNlm3 kNlm2

24 6.00

Unfactored weight of partitions (w

kNlm2

1.oo

Unfactored weight of finishes (wfin)

kNlm2

1.oo

Unfactored superimposed load on slab kNlm2 NOTE: Partitions are considered as "live load"

2.50

Lower slab (2) Lowest slab (3)

-

Panel

Type: Flat slab internal panel I

I

Allow cracking Ratio of Distribution coefficient uncrackedl (default = 0) cracked

45

35.49571793

0

1

52

37.24828153

0

1

65

40.12449494

0

1

Co-ordinates of backprop locations

I 6 7 8 9

0.50

1 1 1 I

1

Properties of concrete Equivalent cube strength Modulus of elasticity, E 'lab of slab (f,) (kNlmmz) (N/mmz) Supporting siab (1)-

(YorN)

If Yes, enter the unfactored weight of falsework for casting new slab (kN/m2)

6.15 1.35 3.75 6.15

3.75 6.15 6.15 6.15

Force in backprops

X

Backprop No.

(m)

(m)

Upper level w b l (kN)

Lower level wb2 (kN)

1 2 3 4

2.55 4.95 2.55 4.95

2.55 2.55 4.95

29.64 29.64 29.64

14.53 14.53 14.53

I

I

4.95

I

29.64

I

14.53

I

39.06 39.06 39.06 39.06

Backprop Calculation OUTPUT total load from qagbelngcast load applied30 Supporting Slab(1)

I

load carried by Upper backprops load applied to Lower slab (2)

wbl

load carried by Lower backprops load applied to Lowest slab (3)

wb2

Load (kN)

Load per m2

percent %

351 56 233 00

6 25 4 14

100 00 66 28

118.56 60.43 58.13 58.13

I I I

2.1 1 1.07 1.03 1.03

I 1

I

33.72 17.19 16.53 16.53

Criteria for t h e loading of t h e supporting slab (1) during construction Unfactored design construction load Unfactored design service load

(w con ) see note

(w

Ultimate design load on slab ((LL 1.6 DL 1.4)

10.64 Loading Factor

10.50 (w

Cracking Factor

( F, ) ( Fcr)

15.40

kN/mZ

Characteristic strength of slab (28 days) ( f cu )

40.00

Nlmm'

Effective Deflection Factor (FeH)

Required equivalent c u b e strength (fcreq)

40.91

Nlrnrn'

(Allowing for cracking)

Note : Assumes no construction operations load on any completed slabs at the time of casting considered. Date printed 28/10/02

Version 2.12

121


The title and panel type will now appear on the spreadsheet. The arrangement of your panel is shown in the diagram on the spreadsheet. Any assumed discontinuous edges are shown as solid black lines, and are shown in red when assumed supported on the edge.

STAGE TWO - Input basic data (3) Input basic data at start ofprogram (yellow boxes) The appropriate information must now be keyed into the two boxes with the yellow title blocks. The information required is:

The overall panel dimensions (Lx and Ly) in m For a flat slab panel, the dimensions of the panel are the centre-to-centre spacings of the columns in the two directions. For a panel supported on four sides, the panel dimensions are the distances between the centrelines of the supporting beams or walls in the two principal directions. When deciding which dimension to choose as the span in the x and y directions, it should be noted that, where one edge is discontinuous, the discontinuous edge is assumed by the program to be the edge lying along the x-axis (see sketch on spreadsheet for co-ordinate system). The dimensions of Lx and Ly are shown on the sketch. Where two edges are discontinuous (a corner panel), then the discontinuous edges are the edges along the x- and y-axes.

The overall thickness of the slab (h) in mm The overall slab thickness in millimetres should be keyed in. The maximum thickness is queried if you input a value greater than 350 mm. The depth is used to calculate the slab stiffness on the assumption that the slab is solid. For other forms of slab, such as coffered slabs, an effective value of thickness could be used such that the second moment of area is correctly calculated.

The load applied through the falsework to the supporting slab 1 in kN/m* This is the distributed load applied through the falsework supports after erection of the backprops. Thus, if the falsework and formwork for the new slab are erected before the backprops are put in place, then the load keyed in should not include the weight of the falseworWformwork but only the weight of the wet concrete and any imposed construction loads likely to be applied during or after the casting operation. The program later requests the weight of the formwork, if not included in this load. Obviously the sequence of construction will affect the magnitude of this load. Although the concrete density of the completed slabs is requested later in the program, the TWD may wish to allow for a value for density of cast ‘wet’ concrete greater than that for ‘set’ concrete.

The number of falsework supports supported by the supporting slab 1 (maximum 12) The program will handle up to 12 falsework supports. The limit of 12 was chosen because where more than 12 supports are used on a panel, the resulting backprop loads are very close to those calculated assuming a uniformly distributed load applied to the slab30. To assume a uniformly distributed load, key in 0 (zero) for the number of falsework supports.

The number of backprops provided below the supporting slab 1 (maximum 12) The program will handle up to 12 backprops on a panel. This also is a reasonable practical maximum. The program does not include any option for treating situations where more than 12 backprops per floor panel are used.

The number of levels of backprops (maximum 2) The number of levels of backprops is limited to a maximum of TWO levels. The research results from Cardington (Ref. 2) suggest that in most circumstances further levels of backpropping will achieve practically nothing. The complexity of the calculations would, however, increase greatly.

30 It can be demonstrated by running the program first for 12 props and, then repeating for a uniformly distributed load, that this is a reasonable assumption for most circumstances.

122

Annex F - Backpropping spreadsheet

The program will also run correctly if you have no backprops by entering a zero (0). In this case, it removes any backpropping previously entered! This option is useful to justify reasons for inserting backpropping. The stiffness of the backprops in kN/mm As discussed in Section 6.5.2 Assumption 4, the loads in the backprops will vary with the stiffness of the backprops. Prop stiffness information may not be readily available. As a guide, the aluminium backprops used at ECBP Cardington and fitted over a storey height of 3.5 m had a stated stiffness of 25 kN/mm. It was calculated that adjustable steel props of similar length would have had a greater stiffness of 35 kN/mm. Estimated preload in each backprop in kN The program can allow for preloading the backprops on installation. When a value is inserted it assumes that all the backprops will be inserted in a similar way, to an assumed preload value. Load cells measurements on backprop preloads at ECBP Cardington (Ref. 2) gave equivalent distributed loads on the floors from 0.3 to 0.7 kN/m2.This equates to individual prop preloads from 4.2 to 9.8 kN per prop. Other research at Reading (Ref. 37) recorded values of preload up to 19 kN per aluminium backprop. It is unrealistic for operators on site to install propping under slabs with zero load, as the props would fall over. Some degree of preloading is considered the norm. The amount of preload is a function of the type of prop used and the operator’s method of inserting the prop. The program requests an estimated value for each prop. It is reasonable to make an assumption that a preload of say 6 kN in each prop would be inserted. Is the falsework erected before the backprops? (Y/N)

Reply Y (Yes) The usual operating condition on site is that the falsework for the new slab that is to be cast is erected on the supporting slab 1 before any backprops are fitted. The weight of the falsework and formwork is then directly carried on the supporting slab 1. When the backprops are subsequently inserted, prior to casting, the load from the new slab is then distributed through the floors, using the calculations in this spreadsheet. When checking the strength of the supporting slab 1 to carry out this operation, the total construction load ( w ) applied will be the sum of the slab self-weight, the weight of the falsework, plus the percentage of load distributed through the backpropping from the calculations. Reply N (No) Occasionally, the backpropping will have been inserted under the supporting slab before the falsework is erected. In this case the weight of the falsework and formwork for the new slab should be included in the total load from the casting of the slab. The program inserts a query to check that you have allowed for this in your loads as “load from casting new slab”. Click OK if you have allowed for it, else click CANCEL to enter a new value, then RESET. Weight of formwork and falsework in kN/m2 As previously discussed, enter the self-weight of the formwork and falsework, if it has not been included in the “Load from casting new slab”. In the absence of precise weights, the recommended value is 0.50 kN/m*.

STAGE THREE - Slab designer’s data (4) Input data from designer ofthe permanent work (green box) The critical slab in any backpropping calculations is the supporting slab 1, i.e. the one directly below the newly cast slab. Inputting information about this slab allows the spreadsheet to compute the required equivalent concrete cube strengths. The information should be readily available from the designer of the permanent works (PWD).

123


Characteristic strength of the concrete

vcu) in N/mm2

The designer’s assumed characteristic strength of concrete in N/mmz assumed at 28 days is required.

Assumed concrete density kN/m3 The default value recommended is 24 kN/m3. If you have a known assumed density, enter the value. The unfactored self-weight of the slab is automatically calculated from the density and thickness ( h ) assumed. If you had entered an equivalent thickness, the self-weight will be calculated incorrectly - this may be overcome by entering the unfactored self-weight manually, before clicking CALCULATE.

Unfactored weights and superimposed load in kN/mZ Enter the unfactored loads from the PWD’s stated values. Note that the weight of partitions and finishes are entered separately because it is normal practice to consider partitions as “live load”. Note that these entries are all the unfactored design service loads. Having keyed or altered the information in the yellow and green boxes at Stages Three and Four above, click on the SET or RESET button located on the right hand side of the spreadsheet. This will cause the program to carry out some elementary checks on the data and reformat the spreadsheet. It will clear the main tables identified by the blue headings and remove all your previous entries.

STAGE FOUR - Enter concrete properties

(5) Concrete properties and prop and backprop locations (blue boxes) Data relating to the concrete properties and the location of the falsework and backprops must now be keyed in. Remember to either press Return on the keyboard after every entry, or click on another cell after entering you< numbers. This is carried out as follows:

Concrete strengths U,) in N/mm2 or elastic moduli (E) in kN/mm2 of the slabs There are two ways by which the stiffness of the slabs may be defined: 1.

The equivalent cube strengths31 of the slab supporting the falsework, i.e. the supporting slab 1, and the properties for one or two lower slabs which support the backprops, are keyed into the first column of the blue “Properties of concrete” table. The modulus of elasticity (E) will ,automatically be calculated by the spreadsheet from the equivalent cube strengths and will be written into the second column.

2.

Elastic moduli may be keyed directly into the second column of the table and the first column left blank. The elastic modulus must be given in kN/mm2. In this case, any numbers keyed into the first column will be ignored. Note: If numbers have been keyed into the second column directly and it is then desired to use option (a) you need to click the RESET button to re-instate the equations in the second column. This will require you to re-enter all the co-ordinates and concrete strength again!

(6) Allow cracking (grey boxes)

Distribution coefficient

(c) and ratio of curvature

The ECBP research results, on which this program is based, did accept some limited cracking of the flat slabs at the columns. This spreadsheet can be used to include a further allowance should the designer accept cracking relative to the midspan position for the worst case. Eurocode 2 (Ref. 39) and Prof. Beeby’s paper (Ref. 38) introduce these concepts in detail. A short resume of the theory is included at Annex G of this guide. In the absence of information from the PWD, it is recommended to use the default values for the distribution coefficient (&= 0) and ratio of curvature as one (1). The method of strength assessment, such as the LOK test, will provide equivalent cube strengths. See Section 6.4 of this Guide and the BRE Best Practice Guide (Ref. 32) on methods of concrete strength assessment. 31

124


I

I

STAGE FIVE - Enter co-ordinates (7) The co-ordinates of the falsework supports and the backprops (Blue numbers)

The coordinates for the falsework supports and the backprops must be keyed into the appropriate tables. Co-ordinates are given in metres relative to the axes shown in the sketch on the spreadsheet. It must be remembered that the panel has to be aligned so that where one edge is discontinuous, the discontinuous edge is the edge lying along the x-axis (see sketch on spreadsheet for co-ordinate system). Where two edges are discontinuous (a corner panel), then the discontinuous edges are the edges along the x- and y-axes. If a panel type assumes a slab supported on four sides, then backprops should not be located on the edges. Backprops in these positions will attract zero load and their specification leads to a matrix which cannot be inverted - the program will fail! To avoid this, the program will not permit you to insert backprops closer to the edge than 1/12 of the span.

STAGE SIX - Calculate loads and forces Once all the preceding information has been keyed in, click on the CALCULATE button. If you have keyed in a coordinate outside the overall panel size, you will be asked to correct the value before continuing. Re-enter correct value and click CALCULATE again. The forces in the upper, and any lower, backprops are inserted into the right-hand table. It will be seen that the average loads applied in each falsework support is also entered in the righthand column of the falsework co-ordinate table. These are simply calculated by assuming that all props carry an equal load from the applied load ‘added’ to the supporting slab. If, from knowledge of the falsework design, this is clearly unreasonable, then revised values may be keyed into the table. Note though that it will not recalculate the backpropping from your manual entries. If this is done, it should be checked that the sum of the prop loads remains equal to the load entered into the ‘basic data’ table multiplied by the area of the panel.

(8) Backprop calculation OUTPUT The total applied load, distributed load per square metre and the percentages of the total load applied from the newly cast slab load, are shown for all slabs and backprops. This is the loading from the applied load from the casting of the new slab that has been distributed through the floors. The values will vary depending on the stiffness of the slabs, the backprop stiffness and the layout. (9) Supporting slab I OUTPUT (green box) The backpropping calculations are used by the TWC and the TWD to establish criteria for loading the structure. The important information is the performance of the critical slab onto which the majority of load is applied. This section of the output calculates the information required by the construction team in order to assess the risks. It also states the critical factors outlined at the start of this manual for operation of the relevant loading of the slab.

Unfactored design construction load ( w ) This is the total load applied to the supporting slab. It is the summation of the slab self-weight and the load applied from the backpropping calculations; shown in green on the backprop output. The load is reduced by the amount of any preload in the backprops averaged over the panel area. Where the falsework is erected before the backprops are inserted, the weight of the falsework will be included in the total construction load. Note that no construction operations load is assumed on the supporting slab 1. This is a recommendation for backpropping from this Guide. Where the amount of preload to the backprops is large, there is a possibility that the total load on the lower slab 2 might become critical. A warning notice is displayed when this load exceeds 90% of the unfactored design construction load on the supporting slab. As the spreadsheet only calculates the factors for the supporting slab 1, users will need to manually check whether the strength of the lower slab 2 is critical.

125


Unfactored design service load (wser) The total unfactored load assumed by the designer from self-weight, finishes, partitions and superimposed loads. All the relevant data was entered manually at Stage Four.

Ultimate design load (w,,J Although not used in the backpropping calculations, the ultimate loads are given for information. They are calculated assuming a partial safety factor of 1.6 on the live load, and 1.4 on the dead loads. Note that the partitions are considered as a ‘live load’.

Required equivalent cube strength Cf,res) The required equivalent cube strength of the supporting slab 1 to perform the operation of applying the load from the casting of the new slab is: Equation 6 =

estimate of equivalent cube strength of concrete at the location in N/mm2

w

=

total unfactored design construction load on the slab in kN/m’

wse,

=

total unfactored design service load in kN/mZ

=

specified characteristic strength of the concrete in N/mm2

where: f,req

f,,

When the required value of equivalent cube strength exceeds the concrete slab strength inserted at Stage Four, the value off ,req is red and a warning notice is displayed.

Loading factor (F,) The criteria for considering whether a slab can be safely loaded is: Loading factor

W

F, = -51.0

Equation 4

W ser

where: w,,,

=

total unfactored design service load in kN/m2

Cracking factor (Fer) The criteria for considering whether a slab can be safely loaded without excessive cracking is Equation 5

where

f,

=

estimate of the equivalent cube strength of the concrete at the location, and previously entered in the “properties of concrete” box in N/mm2

fCu

=


w,,

=

total unfactored construction load on the slab causing cracking in kN/mz

Effective deflection factor (Fern ) Allowing for the cracking in a slab, Prof. Beeby recommends (Ref. 38), see also Annex G, that the safe limits of striking are satisfied if the effective deflection factor32 is limited below unity as

Equation 26

where

32

<,,

=

distribution coefficient dependent on the ratio of stress cracked to actual design stress

Criteria for loading of slabs during construction (Ref. 42) Equation 8.

126


STAGE SEVEN - Printed output Clicking the PRINT button on the bottom right-hand side of the spreadsheet will produce a printout of the page of the spreadsheet on the default ‘Windows’ printer. The printer options and the printer should have been set for A4 paper in portrait mode in Windows (default setting). If you have changed anything since last printing, it is suggested to re-CALCULATE before printing your page. To print on another printer, set the paper size, margins, and also to input your own headers and footers use the “file, page setup” on the top left of the Excel toolbar. This will allow you to change the margins, headers and footers. Note that if you print using this method you should only select the first page to print as

page 1 o f 4

or

page 1 to 1

STAGE EIGHT - Save your work When the program is saved it over-writes the existing version leaving the titles and layout as the last time used. It is suggested that you rename your working copy of the spreadsheet for everyday use, by using “file, save as” etc. The program is saved in Microsoft Excel 97-2000 and 5.0/95 97 (*.XIS)in workbook format.

Hints and amending data (10) Amend basic data

If you want to amend: Dimensions of panel Number of falsework supports Slab thickness Number of backprops per floor Load from casting slab

Is falsework erected first? Weight of falsework. Enter the new value, click a blank box outside of the load box, then click on RESET. This will cause the program to carry out some elementary checks on the data and reformat and clear the main tables identified by the blue headings. It will remove all previous entries and, unfortunately, you will then have to re-enter your data for falsework and propping co-ordinates. ( I l ) Amend number of levels of backpropping

Change TWO levels to ONE level: Either: To compare the effect of using two levels of backprops with using one level, run the program first for TWO levels of backpropping. Leave the number of levels of backprops at 2, but alter the ‘properties of concrete’, by inputting a very low value of modulus of elasticity (E) for the lowest slab (3) and re-calculate.

Do not enter zero for E as this will result in an overflow failure in the program. Or Enter 1 in the yellow box on number of levels, then click RESET. Unfortunately you will then have to re-enter your data on falsework and propping co-ordinates.

Change ONE level to TWO levels: If you want to increase from one level to two, having run the program on one level, insert the number ‘2’ in the yellow box, then click on RESET. This will cause the program to clear the main tables and remove all previous entries. Unfortunately you will then have to re-enter your data for falsework and propping co-ordinates.

127


Do not click CALCULATE before RESET as this will result in an error message and failure in the program. Change ONE or TWO levels to NO backprops: If you want to decrease from one or two levels to having NO backprops, insert the number ‘0’ in the yellow box, then click on RESET. This will cause the program to clear the main tables and remove all previous entries. Unfortunately you will then have to re-enter your falsework co-ordinate data again.

Do not click CALCULATE before RESET as this will result in an error message and failure in the program. ( I 2) Amend title and/or panel type To change the title andor the panel type without changing the concrete properties or the co-ordinates of the props and backprops, this may be done simply by clicking the ‘Insert and/or change title panel type’ button, keying in a new title andor selecting a new bay type and then simply clicking CALCULATE.

(13) Amend stiffness of backprops To investigate a different stiffness of the backprops without changing any other parameter, just change the prop stiffness in the ‘Falsework and backpropping details’ table. Remember to click a blank cell after entering the new stiffness, then click CALCULATE. (14) Effect of different slab stiffness or backprops resting on groundfloor

The program assumes that all slabs are the same thickness and that the only factor affecting their stiffness at the various levels is the elastic modulus of the concrete. If, however, there are differences in slab thickness, this may be taken into account by adjusting the moduli of elasticity. It is suggested that the correct E and thickness are specified for supporting slab 1 and that effective moduli (Eer)are calculated for lower slabs using the relationship at Equation 27: where

h

=

the overall thickness of the slab considered in mm

Equation 27

hupper = the thickness of the supporting slab 1 in mm. This approach may be used to model situations where the backprops are resting on foundations or a ground floor slab which might be considered to be rigid. In this case, simply key in a large value for E for the lowest slab.

(15) Examplesfrom ECBP Ten examples of use of the spreadsheet based on the results from ECBP (Ref. 2) are shown on the following pages. Table F1 lists the Examples. The examples are also referred to in the worked example in Sections D9 and D11 at Annex D.

128

Annex F - Backpropping Spreadsheet

Table F1. Schedule of sample A 4 outputs from spreadsheet.

1 I 4-1 Example

Panel type

Floor being cast at ECBP

ONE

Internal AB-23

Floor 3

No. of falsework supports

Location of propping

of back Props

Page No

Number of back props

Third points

130

Floor 3

Third points

131

Third points

~~

THREE

Corner AB- 12

Floor 3

FOUR

Internal AB-23

Floor 3

Internal AB-23

Floor 3

12

1 I

Internal BC-23

Floor 4

12

I

SEVEN

Internal BC-23

Floor 4

12

EIGHT

Internal BC-23

Floor 4

12

I I

* FIVE

SIX

12

Floor 5

TEN

1

Internal BC-23

Floor 5

I I

I

I I

2

I

1

2 2

I I

Third points

I I 4

Third points

I

Third points

Third points Third points Quarter points

u.d.1.

Quarter points

- I -

4

I I

134

135

-

4 4 4

133

-

136

137

+6.00kN

I

Yes

138 139

6.00 kN

140

129

-

Example ONE D.9 Internal Panel Floor 3 concrete structures group

Date: lISeplO2


Lx ( 4 LY (m) h (mm)

Thickness of slabs

Load from castina new slab f kN/m2

Made By: P.F.P.

7.5 7.5 250


6.00

Stiffness of backprops (kN/mm)

(E, )

Assumed concrete density (if not 24 kNlm3) Unfactored self weight of slab Unfactored weight of partitions (w part)

Nlmm2 kNlm3 kNlmZ

j

37 24

!

6.00

kN/m2

,

1.00

1

1.oo 2.50

I

Unfactored weight of finishes (wfin) kN/mz Unfactored superimposed load on slab kN/m2 NOTE: Partitions are considered as "live load"

Supporting slab (1)

Equivalent cube strength of slab (f c) (Nlmm2)

Modulus of elasticity, E (kN/mm2)

A7

36.01 39767

I

0.00 Y

(

If Yes, enter the unfactored weight of falsework for casting new slab (kNlmZ)

Panel

T

Or

0.50

-

~ Flat~ slab ~ internal : panel I

I

I

Properties of concrete Slab

12 4 1 25.00

Estimated preload in each backprop (kN) .--------------------~ Is falsework erected before backprops?

Data from Designer of Permanent Work Characteristic strength of concrete

1 of 1

Page:

Allow cracking Ratio of Distribution coefficient uncracked, (default = 0) cracked

Lower slab 40.12449494 Lowest slab n/a

n/a

nla

Co-ordinate System

(default = 1


load in each falsework support

2 3 4 5

1.35 3.75 6.15 0.25 1.35

0.30 0.30 0.30 2.70 2.70

3.75 6 15

5.50

in 11 12

3.75 3.75

I

I

28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13

Backprop No.

X (m)

(m)

Upper level wbl (kN)

Lower level wb2 (kN)

1 2 3 4

2.50 2.50 5.00 5.00

2.50 5.00 2.50 5.00

20.53 15.88 20.53 15.88

nla n/a nla nla

7R 13

I I

2.70 7.50

28.13 28.13

I

total load from slab being cast

__

.-

load applied to Supporting Slab (1)

I

wbl

load carried by Upper backprops load applied to Lower slab (2) load carried by Lower backprops load applied to Lowest slab (3)

Unfactored design construction load Unfactored design service load

(w con ) see note

Characteristic strength of slab (28 days) ( ,f

,

)

Required equivalent cube strength (f rea)

337.50 264.67 72.83 72.83

I ;;::A1

I

I

6.00 4.71

I

1.29 1.29

(w

)

15.40

kNlm2

37.00

NlmmZ Nlmm'

41.24

I

I

100.00 78.42

I

21.58 21.58

nla

wb2

(w ser )


I

I,

nla

I

1 vaa

Criteria for the loading of the supporting slab (1) during construction .

Name

Loading Factor Cracking Factor

Symbol ( F,)

( F cr )

Effective Deflection Factor (Few)

0.92


Note : Assumes no construction operations load on any completed slabs at the time of casting considered.

130

Force in backprops

Version 2.12

-

Example TWO D.9 Edge Panel Floor 3

BrnziH3I concrete structures group

Date: 1ISepl02

Made By: P.F.P.

I

Basic Data (Required) Dimensions of panel:

Lx (m)

-,

7.5

Thickness of slabs Load from casting new slab ( kN/m2)

I

6.00

I

Data from Designer of Permanent Work

Characteristic strength of concrete

(E, )

Nlmm2

37

Assumed concrete density (if not 24 kNlm3) Unfactored self weight of slab

kN/m' kNlm2

24 6.00

Unfactored weight of partitions (w &)

kNlm2

1.oo

Unfactored weight of finishes (wnn) kNlm2 Unfactored superimposed load on slab kNlm' NOTE: Partitions are considered as "live load"

1.00 2.50

1 of 1

Page:

Number of falsework supports Number of backprops per floor, Number of levels of backpropping Stiffness of backprops (kN1mm)

12 4 1 25.00

Estimated preload in each backprop (kN) .---------------------

0.00


Y

If Yes, enter the unfactored weight of falsework for casting new slab (kNlm2)

(

)

Or

0.50

-

Panel

I

I

Falsework and Backpropping details

Type: Flat slab edge panel I

I

Y

slab (1)

T

36.0139767

Lower slab 40.12449494 Lowest slab nla

nla

nla

nla (default = 1)

I

6.15 0.25 1.35

4 5 6 7 8 9 10 11 12

0.30 0.30 2.70 2.70

1

1

6.15 7.25 1.35 3.75 6.15 3.75 3.75

I

I

I

2.70 2.70 5.50 5.50 5.50 2.70 7.50

I

1

I I

28.13 28.13 28.13 28.13 28.13

I wbl


wb2

Supporting Slab (1) OUTPUT ) see note

(w ser) (w

Load (kN) 337.50 251.20



T

()o

1

2.50 2.50 5.00 5.00

1

I

Force in backprops Upper level wbl (kN)

2.50 5.00 2.50 5.00

25.66 17.49 25.66 17.49

I

LOWerleVell wb2 (kN) nla nla nla nla

28.13 28.13


Characteristic strength of slab (28 days) ( f,, )

I (i)

28.13 28.13 28.13 28.13 28.13

total load fromskb-beEg cas!

Unfactored design service load

a

4

I

(w

- x Co-ordinate System


B

Backprop Calculation OUTPUT

Unfactored design construction load

1 I I


edge

)

86.30 86.30

I

Load per m21 percent %

I

I

6.00 4.47

I

I

100.00 74.43

I

1.53 1.53

25.57 25.57

nla nla

Value

Units

10.97

kNlm2

10.50

kNlm2

15.40

kNlm2

37.00

N/mm2

(1) during construction Name


Symbol ( F,) ( F cr )


39.78 Nlmm2 (Allowing for cracking) Required equivalent cube strength (f c r e q ) Note : Assumes no construction operations load on any completed slabs at the time of casting considered.

Value

1.04 0.90 0.90 Version 2.12

131

-

Example THREE D.9 Corner Panel Floor 3

Emmm

concrete structures group

Made By: P.F.P.

Date: 1lSepl02

1 of 1

Page:


250


6.00


Lx (m) h (mm)

Estimated preload in each backprop (kN) .---------------------

Data from Designer of Permanent Work Characteristic strength of concrete (fcu) Assumed concrete density (if not 24 kNlm')

37 24

Unfactored self weight of slab

kNlm2

6.00

Unfactored weight of partitions (w part)

kNlm'

1.oo

Unfactored weight of finishes (wnn) kNlrn2 IUnfactored superimposed load on slab kNlmZ NOTE: Partitions are considered as "live load"

Equivalent cube strength of slab (f

Slab

Lower slab (2) Lowest slab (3)

I

1.00 2.50

36.0139767

0

1

65

40.1 2449494

0

1

I

I

(

Or

)

0.50

-

Panel

Type: Flat slab corner panel

Ratio of Distribution curvatures coefficient uncrackedl (default = 0) cracked

47

nla

If Yes, enter the unfactored weight of falsework for casting new slab (kNlmz)

Y

Allow cracking

MOdUluS Of elasticity, E (kNlmm2)

(N/mm2) Supporting slab (1)

0.00


N/mm2 kNlm'

Properties of concrete

12 4 1 25.00

I

n/a

n/a

~~

I

n/a

I

- x

(default = 1)

Co-ordinate System


3

1.35 3.75 6.15

0.30 0.30 0.30

5 6 7 8 9 10 11

1.35 6.15 7.25 1.35 3.75 6.15 3.75

2.70 2.70 2.70 5.50 5.50 5.50 2.70

1

I

I

28.13 28.13 28.13

I

28.13 28.13 28.13 28.13 28.13 28.13 28.13

I I

I


I

total load from slab beingcast load ap@ecJ to SupportLng Slab (1)

I


wbl


wb2

Supporting Slab (1) OUTPUT Unfactored design construction load Unfactored design service load

(w

H, )

see note

(w ser)

Ultimate design load on slab ((LL 1.6 DL 1.4) Characteristic strength of slab (28 days), ( ,f Required equivalent cube strength (f

132

Value

(wull)

) ,eq

).

337.50 239.38

100.00 70.93

98.12 98.12

29.07 29.07

Units

10.76

kNlm2

10.50

kNlm'

15.40

kN/mZ

nla n/a

I

Criteria for the loading of the supporting slab (1) during construction Name Symbol Value Loading Factor Cracking Factor

1.02

( F, ) ( F,

)

37.00

Nlmm2

Effective Deflectiy Factor (Fen )

38.52

Nlrnrn'


0.89 0.89

-

Example FOUR D.11 Internal Panel Floor 3 Preload Date: IlSepl02

Made By: P.F.P.



,.(

Number of falsework supports Number of backprops per floor Number of levels of backpropping Stiffness of backprops (kNlmm)

12 4 1 25.00

.--------------------~ Is Estimated preload in each backprop (kN)

8.50

falsework erected before backprops?

Y

Dimensions of panel: Thickness of slabs

250

Load from casting new slab ( kN/m2 )

6.00


(E, )

Assumed concrete density (if not 24 kN/m3)

kN/m3

37 24

Unfactored self weight of slab

kNlm2

6.00

Unfactored weight of partitions (w part)

kNlm2

1.oo

Unfactored weight of finishes (wfln)

kN/m2

1.oo

Unfactored SUDerimDOSed load on slab kN/m2 NOTE: Partitions are considered as "live load"

2.50

Characteristic strength of concrete

Nlmm2


If Yes, enter the unfactored weight of falsework for casting new slab (kN/mZ)

Panel

T

I

Allow cracking

Modulus of elasticity, E (kN/mm')

Supporting slab (1)

47

36.0139767

0

1

Lower slab (2)

65

40.12449494

0

1

Lowest slab (3)

nla

nla

nla

Ratio of Distribution curvatures coefficient uncrackedl (default = 0) cracked

o

!

I (0,O)

nla

-I

I

x Co-ordinate System


Force in backprops

wbl

t

I

3 4 5 6 7 8 9 10 11 12

6.15 0.25 1.35 6.15 7.25 1.35 3.75 6.15

I I

I

I

3.75 3.75

I

I

I

)

Or

0.50

-

(default = 1;

I

(

~ Flat~ slab ~ internal : panel I

Equivalent cube strength of slab (f,) (Nlmm2)

'lab

1 of 1

Page:

I

0.30 2.70 2.70 2.70 2.70 5.50 5.50 5.50

I

I

I

I I I I

I

2.70 7.50

I

28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13

3 4

I

5.00

I

6.00 4.10

I

2.50 5.00

I I

(kN)

29.03 24.38

wb2

I I

(kN)

nla nla

28.13 28.13



337.50 230.67


I

I I


wbl

load carried by Lower backprops load aDDlied to Lowest slab f3)

wb2


(w

(w con ) see note

)

106.83 106.83

I

I

I

I

100.00 68.35

1

1.90 1.90 n/a n/a

1

I

31.65 31.65

I I

I

Criteria for the loading of the supporting slab (1) during construction Symbol . Value

10.60

Loading Factor

10.50


I

I

Cracking Factor

( F,) ( F c, )

15.40

kN/m2

,. )

37.00

N/mmz


Required equivalent cube strength (f, req)

37.60

Nlmm'


Characteristic strength of slab (28 days) ( f

(wult)


Version 2.12

133

-

Example FIVE D.11 Internal Panel Floor 3 Preload

Emana concrete struclures group

Date: llSepl02


Made By: P.F.P.



250


Load from casting new slab ( kNlmz )

6.00


(E,)

I

N/mm2 kN/m3 kN/m2

Assumed concrete density (if not 24 kNlm') Unfactored self weight of slab

'

37 24 6.00

Unfactored weight of partitions (w palt)

I " '

;

1.00


kNlm2

I

1.00

Unfactored superimposed load on slab kN/m2 NOTE: Partitions are considered as "live load"

'

2 3 4 5 6 7 8 9

I

I

11 12

1.35 3.75 6.15 0.25 1.35 6.15 7.25 1.35 3.75 3.75 3.75

I

0.30 0.30 0.30 2.70 2.70 2.70 2.70 5.50 5.50

I

2.70 7.50

1

I

1 [

Panel Type: Flat slab

I -I

28.13 28.13

I

- --

--

load carried by Lower backprops

wb2

Supporting Slab (1) OUTPUT (wmn ) see note

__

Load (kN)

Load per mz

percent %

337.50 225.74

6.00 4.01

100.00 66.89

111.76

1.99

33.1 1

nla

Value

Units

10.51

kN/m2

(w ser )

Criteria for the loading of the supporting slab (1) during construction Name Loading Factor

(wull)

Cracking Factor 37.00

N/mm2

Svmbol ( F), ( F cr )

Effective Deflection Factor (Fa" . 1 (Allowing for cracking)

Note Assumes no construction operations load on any completed slabs at the time of casting considered

134

I

I

wbl

Characteristic strength of slab (28 days) ( f ~~, )

I x Co-ordinate System

,



I

28.13

total load from slab being cast load applied t o Supporting Slab (1) -

Unfactored design service load

0.50

- internal panel I


IUnfactored design construction load

Y (YorN)

2.50

Averaged load in eact falsework support (kN) 28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13

8.50

If Yes, enter the unfactored weight of falsework for casting new slab (kNlm')

(default = 1)' Co-ordinates of the falsework supports

12 4 1 25.00

.Estimated ----preload ---in-each --backprop ----(kN) -Is falsework erected before backprops?

Data from Designer of Permanent Work Characteristic strength of concrete

1 of I

Page:

I.. I

I

1

Value

1.oo 0.87 0.88

Version 2 12

-

Example SIX D . l l Internal Panel Floor 4 concrete structures group

Made By: P.F.P.

Date: 1/Sep/O2

I

Basic Data (Required) Lx (m) LY (m) h (mm)


Load from casting new slab ( kN/m2 )

I

1 of 1

Page:


250


6.00


12 4 2 25.00

Estimated preload in each backprop (kN) 0.00 .-------------------Is falsework erected before backprops? Y

(YorN)

If Yes, enter the unfactored weight of falsework for casting new slab (kN/m2)

Panel

0.50

-


Properties of concrete Equivalent cube strength of slab (f cl

Slab

I

Lower slab (2) Low ;;;

slab

I

Ratio of Distribution curvatures coefficient uncracked/ (default = 0) cracked

45

35.49571 793

0

1

50

36.76448285

0

1

67

I

40.53187807 I

0

I

1

-I-.-.-

I

-I


3 4

9 10 11 12

1

1

I

I

I

6.15 7.25 1.35 3.75 6.15 3.75

0.30 0.30 0.30 2.70

2.70 5.50 5.50 5.50 7.50

Backprop

28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13 28.13

I

I

I

I


(default = 1)

1.35 3.75 6.15 0.25

-.-.-.-

Allow cracking

Modulus of elasticity, E (kN/mmZ)

(N/mm2) Supporting slab (1)

I

2.50 2.50 5.00 5.00

2 3 4

2.50 5.00 2.50 5.00

21.88 17.12 21.88 17.12

5.93 5.18 5.93 5.18


I

totalload from slab being cast

loadapplied to Supporting Slab (1)

I


wbl

load carried by Lower backprops

wb2


(w con ) see note

(w ser )

Ultimate design load on slab ((LL 1.6 DL 1.4) Characteristic strength of slab (28 days) ( ,f

)

Required equivalent cube strength (fcreq)

(w

)

33750 25951

I I

6.00 4.61

I I

100 00 76 89

77.99 55.78

I

1.39 0.99

I

23.1 1 16.53

22.21

11.11

kNlm2

10.50

kNlm2

I

0.39

6.58

Criteria for the loading of the supporting slab (1) during construction Name Symbol Value Loading Factor Cracking Factor

( F, ) ( F cr )

15.40

kN/m’

37.00

Nlmm’

Effective Deflection Factor (FeH)

40.68

Nlmm’


1.06

0.94 0.94

135

-

Example SEVEN D.11 Internal Panel Floor 4 Preload Date: llSeplO2


I

I

I


Made By: P.F.P.


1o

Page:

1


Thickness of slabs


Load from casting new slab ( kNlm’ )


I

I

12 4

2 25.00

--------------- ---]a:

If Yes, enter the unfactored weight of falsework for casting new slab (kNlmZ)

Panel

-


I

Properties of concrete Equivalent cube strength of slab (f c)

Slab

Allow cracking Ratio of Distribution curvatures coefficient uncrackedl (default = 0) cracked

Modulus of elasticity, E (kN/mmZ)

(Nlmm’) Supporting slab (1) Lower slab (2)

I

Low ;;

slab

I

45

35.49571793

0

1

50

36.76448285

0

1

40.53187807 I

0

67

I

I

I

1

I -I

(default = 1)

I

Co-ordinates of the falsework supports

Support No. 1 2

X

Y

(m)

( 4 0.30 0.30

(kN) 28.13 28.13

1.35 3.75

I Co-ordinates of backprop locations I

I

cl%%:hl falsework support

I

6.15 3.75

I

I

5.50 7.50

I


__

load applied to Supporting Slab (1) _ I I _

_

1 2 3 4

I

I I I

Force in backprops Upper level

(m) I

2.50 2.50 5.00 5.00

I

wbl

I

2.50 5.00 2.50 5.00

I I

I

I

I I I

Lower level

(kN)

I

I

wb2

(kN)

I

30.38 25.62 30.38 25.62

1

1 1 I

14.43 13.68 14.43 13.68

28.13 28.13

I


~

X (m)

Backprop No.

I

11 12

I x Co-ordinate System

_

I

_

_

_

-

Load (kN)

Load per m2

percent %

337.50 225 51

6 00 4 01

100 00 66.82

1.99 0.99

33.18 16.53

~


wbl

111.99 55.78


wb2

56.21 56.21

I I

1.00 1.00

I

16.66 16.66

I

I Criteria for the loading of the supporting slab

Supporting Slab (1) OUTPUT

Value

Units

10.51

kNlmZ

10.50

kNlm2

15.40

kNlmZ

)

37.00

Nlmm’


Required equivalent cube strength (f c r e q )

37.05

Nlrnrn’



(w con ) see note

(w ser )


136

(wuit)

(1) during construction Name


Symbol

( F,) ( F cr )

Value

1.oo 0.89 0.89

-

Example EIGHT D . l l Internal Panel Floor 4 Preload concrete structures group

Date: l/Sep/OZ

I

Basic Data (Required) Dimensions of panel: Thickness of slabs

I

Made By: P.F.P.

1 of 1

Page:

Falsework a n d Backpropping -

d

r

l


LY (m) ' h (mm)

Load from casting new slab ( kN/mz )

6.00

25.00


----

.--------------Is falsework erected before backprops?

Data from Designer of Permanent Work Characteristic strength of concrete (& )

N/mmZ

37

Assumed concrete density ( inot 24 kN/m3) Unfactored self weight of slab Unfactored weight of partitions (w par()

kN/m3 kN/m2

24 6.00

kN/m2

1 00

Unfactored weight of finishes (wfin) kN/mZ IUnfactored superimposed load on slab kN/mz NOTE: Partitions are considered as "live load"

100 2.50

:

-

Panel


Ratio of uncrackedl

slab (1) Lowerslab (2) Lowest slab (3)

45

I

29.28396729

0.25

0.3

50

30.33069835

0.25

0.3

67

33.43879941

0.25

0.3

I

-I

I




~

3.75 6.15 0.25 1.35

2 3 4 5

0.30 0.30 2.70

28.13 28.13 28.13

2.70

28 13

I

Backprop Calculation OUTPUT tzkad~from 2lcb being cast Fad applied_!oSuppprting Slab (1)

I


wbl


wb2

Force in backprops

X

Backprop No.

(m)

(m)

Upper level w b l (kN)

Lower level Wb2 (kN)

1 2 3 4

2.50 2.50 5.00 5.00

2.50 5.00 2.50 5.00

30.17 24.73 30.17 24.73

13.13 12.15 13.13 12.15

I

I

I

I

Load (kN)

Load per mz

percent %

337.50 227.70

6.00 4.05

100.00 67.47

109.80 59.24 50.56 50.56

I

1.95 1.05

I

0.90 0.90

I

I

32.53 17.55

I

14.98 14.98

I

I I

I

I

( I ) during construction Unfactored design construction load Unfactored design service load

(w con ) see note

(w ser )

Ultimate design load on slab ((LL 1.6 DL 1.4) Characteristic strength of slab (28 days) ( ,f Required equivalent cube strength (f

)

,req )

(w

)

10.55

Symbol

10.50

Loading Factor

15.40

Cracking Factor

( F,)

( F cr )

37.00

Nlrnrn'

Effective Deflection Factor (Fd)

37.28

Nlmm'



Value

1.oo 0.89 0.91 Version 2.12

137

-

Example NINE D.9 Internal Panel Floor 5 UDL I


Date: 1lSepl02

Made By: P.F.P.

1 of 1

Page:


Thickness of slabs

250


Load from casting new slab ( kNlm2 )

6.00


Lx (m)


0

I:

25.00

Data from Designer of Permanent Work Characteristic strength of concrete (fcu)

Nlmm' kNlm3

Assumed concrete density (if not 24 kNlm3)

I

Unfactored self weight of slab Unfactored weight of partitions (w part)

kNlmZ


I " ', kNh'

Unfactored superimposed load on slab NOTE: Partitions are considered as "live l o a d

I

,I

"m'

I

ProDerties of concrete

(

If Yes, enter the unfactored weight of falsework for casting new slab (kNlm2)

37 24 6.00

~

1.00

Panel

Or

)

0.50

-

Type: Flat slab internal panel

1.00 2.50

I

Allow crackina

Equivalent

I

Ratio of curvatures

38.62908825 37.00796335 Lowest slab nla

nla

nla

nla

-I

(default = 1)


I

1

I


I

I

I

I

Assumes a distributed load

n/a

4 5 6 7 8 9

Backprop Calculation OUTPUT total load from slab being cast load applied to Supporting __ _ _ _ _ _Slab (1)

__ - __

-..

_ I _

I


wbl


wb2


(w Mn ) see note (w

)

3.75 3.75 5.63 5.63 5.63

I

1.80 3.75 5.63 1.88 3.75 5.63

Load (kN)

Load per mz

percent %

337.50 222.74

6.00 3.96

100.00 66.00

114.76 114.76

[

I I

(w ser )


I

2.04 2.04 n/a n/a

I

[

13.91 9.21 13.81 12.51 13.81 12.56

nla nla nla nla nla nla

34.00 34.00

I I

Criteria for the loading of the supporting slab (1) during construction Name Symbol Value

10.46

kNlmZ

10.50

kN/mz

Loading Factor

15.40

kN/m2

Cracking Factor

( F, ) ( F cr )

Characteristic strength of slab (28 days) ( f cu )

37.00

Nlmrn'

Effective Deflection Factor (Fe,, )

Required equivalent cube strength (f

36.76

Nlmm'


req )

[

1.oo 0.76 0.76

-

Example TEN D.11 Internal Floor 5 UDL Preload concrete structures group

Made By: P.F.P.

Date: llSeplO2

Basic Data (Required) Dimensions of panel:


Lx (m) LY ( 4 h (mm)

Thickness of slabs

Iof 1

Page:

250

Load from casting new slab ( kNlm2 )

6.00

Number of falsework supports Number of backprops per floor Number of levels of backpropping Stiffness of backprops (kN/mm)

0 9 1 25.00

.-------------------Is

6.00

Estimated preload in each backprop (kN)

I

I


(E,

Characteristic strength of concrete ) Assumed concrete density (if not 24 kN/m3)

N/mm2 kNlm3

Unfactored self weight of slab Unfactored weight of partitions (w palt)

kN/m2

37

I

Unfactored weight of finishes (wfin) kN/mz Unfactored superimposed load on slab kNlm2 NOTE: Partitions are considered as "live load"

'lab Supporting slab (1) Lower slab (2) Lowest slab (3)

Y

(

If Yes, enter the unfactored weight of falsework for casting new slab (kN/mZ)

24 6.00

kNlm2


falsework erected before backprops?

1.00

)

Or

0.50

Panel Type: Flat slab internal panel

-

1.oo 2.50

Allow cracking

Equivalent cube strength of slab (f,) (N/mmz)

MOdUlUS Of elasticity, E (kN/mm2)

58

38.62908825

0

1

51

37.00796335

0

1

n/a

n/a

n/a

nla

Ratio of Distribution coefficient curvatures uncrackedl (default = 0) cracked


I

I


I

4 5 6 7 8 9

I

Load (kN)


wbl


wb2

168.76 168.76

I

3.75 3.75 5.63 5.63 5.63

I

1.80 3.75 5.63 i,813 3.75 5.63

Load per mz

percent %

3.00 3.00

50.00 50.00

I

1 1

19.91 15.21 19.81 18.51 19.81 18.56

I

I

nla nla n/a n/a nla nla

nla

I

n/a

1

Criteria for the loading of the supporting slab (1) during construction Unfactored design construction load Unfactored design service load

(w

(w Mn ) see note )


Required equivalent cube strength (f

9.50

Loading Factor

10.50

) req )

(wull)

15.40

kNlm2

Cracking Factor

( F, ) ( F cr )

37.00

N/mm2

Effective Deflection Factor (Fen)

31.30

Nlrnrn'



0.69 Version 2.12

139

- Allowance for cracking in eet

1 I

This annex discusses the theory used in the Excel spreadsheet on cracking allowances, as proposed by the Eurocode 2 (Ref. 39) and incorporated in Prof. Beeby’s paper Criteria for the loading of slabs during construction (Ref. 38).

6 1 The disUribution coefficient The forthcoming Eurocode 2 (Ref. 39) gives formulae for members that are expected to crack under load, and behave in a manner intermediate between the uncracked and fully cracked conditions. These give ways of more realistically assessing slab stiffness by inserting a distribution coefficient ( ‘zeta’) based on the ratio of stress in the tension reinforcement calculated on the basis of a cracked section compared with that condition to produce the first cracking.

c,

The distribution coefficient is stated as Equation 28

where

6

=

os,= os

=

distribution coefficient stress in the tension reinforcement under the loading conditions to produce the first cracking stress in the tension reinforcement calculated on the basis of a cracked section

The program default value assumes uncracked conditions, i.e. the stress in the steel is less than or equal to cjsrthe stress under the loading to.produce the first cracking, and the distribution coefficient is zero (0). As the stress in the reinforcement (os) increases relative to that causing the first cracking (os, ) the ratio (osr /a,) gets smaller with the distribution coefficient approaching a value of unity (1). The value of distribution factor, if cracking is to be considered, is entered as Equation 28 above. /os) can be replaced by (M,, / M ) where M is the cracking moment and In Equation 28 the ratio (os, M,, is the moment for the loaded condition causing the first crack.

6 2 Ratio of curwature An assessment of the effective stiffness (achieved by adjusting the effective E value) can then be made knowing or estimating the ratio of curvatures associated with the uncracked and fully cracked conditions. The spreadsheet adjusts the effective E value automatically if values are entered for this parameter, in combination with a distribution coefficient other than zero. The default value of ratio of curvature is unity (1). True values lie between 0.25 and 0.33. Refer to the Concrete Society report on ‘Deflections in concrete beams and slabs’ (Ref 41). The ratio of uncrackedkracked curvatures will have no effect if a default value of zero is entered for the distribution coefficient.

63 Effective deflection factor (Few) Provided the loaded slabs remain within the elastic limits, a reasonable assumption in backpropping calculations, then the ratio of values causing cracking to those actually designed for accepted cracking will remain the same.

140

Annex G - Allowance for cracking in spreadsheet

This leads to the following relationships:

os,- Cracking load moment o s Actual cracked moment

w,, --

Equation 29

w

where

w,,

=

total unfactored construction load on the slab causing cracking in kN/m2

w = total actual unfactored construction load on the slab in kN/m2 Rearranging Equation 28 with Equation 29 gives

%=

Equation 3 0

,/-=CL

W

W

os

In the criteria of loading flat slabs (Ref. 42), Prof Beeby redefines the distribution coefficient as

Cb

Equation 3 1

= 1-

(5)

Combining Equations 30 and 3 1 leads to

Equation 32

Incorporating the distribution coefficient into Equation 26, gives the value for the effective deflection factor as Equation 33

where

w,,,

=

total unfactored design service load in kN/m2

fc

=

estimate of the equivalent cube strength of the concrete at the location in N/mm2

fcU

=


The calculation for effective deflection factor assuming cracking given in the output of the spreadsheet uses Equation 33. If there is no cracking assumed, the value is automatically entered for the non-cracked section.

141

D abbreviations xii access 11, 22, 28, 33 aluminium support systems 13, 15-017, 25 anchorages 19-20

backpropping 4, 77-88 assumptions 61-67 loads 67-71,82-85, 104-105 methods 6&61,67-67,77-80,8688 recommendations 5 1-52 worked examples 101-I08 beams, formwork, length 34 bending 113-1 14 bond 114-115 building design considerations 6-7

c camber 9 Cap0 test 59-60 Cardington project 93-96 characteristic strength, concrete 9 clearances see tolerances collapse, SLS 113-1 17 columns 41, 44-45, forms and equipment 10,2l-22,33,38 recommendations 4 1 heads 10 kickers 6, 9-1 1 reinforcement 6 columdplank interface 7 concept selection 6-7, 13 concrete density 6 , 9 , 5 1,63 strength assessment 39, 5 7 4 0 , 81-82, 103 type, effect on temporary works 12-1 3 concreting process 38 construction joints 38 construction loads 28-30, 62-63 cracking 54-55, 14&141 cranes and cranage 15, 16,25,38,42,49-50 CRC Jointcast 11 see also Densit creep 56 cubes cured alongside 5 8 4 0 curing 13, 45,47 cycle times 16, 17, 22

142

definitions xiv deflections 9, 54 effects of backpropping 64-67 serviceability limit state 111-113 slabs struck at early age 56-57 Densit 11, 17 density of concrete 6, 9, 51, 63 design and build contracts 22 design buildings 6-9 kickers 10-11 staircases 11 design brief checklist 97-99 design service loads 72-76 detailing, reinforcement 6, 7, 12 distribution coefficients 140 downstand beams 6, 12, 16, 17,26 drawings 2 I , 24, 3 1

E early striking 53, 5 5 , 56-57 economic aspects 6, 11, 13, 16,22, 25 edge distance of standards 32-33 edge protection 6, 19-20, 21, 25, 26, 27, 31-32,47 effective deflection factor 140-141 elastic behaviour of slabs 62 enclosure considerations 4,6, 18-20 external protection 18-20

F falsework co-ordinator 22 fans 19, 41, 46 fibre cement products 17 fibres, polypropylene 12 fixings cast into slabs 22 flowing concrete 6, 7, 79, 80 flying form systems 17, 21, 26-27, 35, 41

guardrails 20, 26, 32

w harnesses 46 health and safety plans 7, 23

Index

high-strength concrete 10 holes, trimming 6, 12

implications of changes 3 50 individual props and beams 14,24-25

kickerless construction 9-1 I , 46 kickers columns 6, 9-1 1 walls 46

L

precast concrete 7, 22 blocks for kickers 10 planks (non-participating) 18, 28 planks (participating) 17, 27-28 plank systems 17-1 8, 22, 27-28, 3 I , 41 stairflights 6, 11-12 preconstruction planning stage 2 I , 40 pre-contract stage 6 2 0 pre-load of backprops 66, 106-1 07 procurement of temporary works 2 1-22, 30-36 production rates 13 progress meetings 2 I, 5 1 prop adjustment devices 33-34 prop and beam systems 1 4 , 2 6 2 5 propping considered to be rigid 63-64 proprietary equipment 14-1 8, 24-28 pull-off testing 57-60 pull-out testing 5 7 4 0 punching shear 53

labour costs 13, 14 length of formwork beams 34 level adjustment 33-34 limit state design methods 24 loads and loading 7-9 above design service loads 72-76, 107-1 08, 109-1 17 assumptions 64-66 considerations 7-9, 54-55, 101-102 during construction 28-30 factors 115 in backprops 67-7 1 loading-out platforms 4 I , 4 3 4 4 LOK tests 39, 5 I , 57-60 'lozenging' 28

radio communication 41, 49 ratio of curvature I40 rebound hammers 58-60 reinforcement 5 1 couplers 10-1 I detailing 6, 7, 12 recommendations 3,6,21-22,41,51-52 responsibilities 3-4, 22-23 retarded concrete I3 risk assessments 27

M

s

management of temporary works 3 - 4 , 2 I , 22-24,3637 material factors of safety I 16-1 17 material handling 42-43 method statements 1 I , 24, 25, 27, 39-40

safety 3 4 , 18-20,26,27 at design stage 3, 7 staircases I I safety nets 19, 3 1 self-weight of materials 6, 9, 28-29 sequence of installing and removing backpropping 71-72 sequence of work 37-38 serviceability limit state 1 1 1-1 13 shear 114 shortening, falsework 47 skeletal systems 14-15, 25, 36 skill levels 23-24, 25 slinger/signallers 41, 49 sofit formwork, striking 85-86 specifications 24 speed of construction 16, 30 spreadsheet, backpropping 61, 66, 105, 118-141 staircases 6, 11-12 standard calculations 23-24 steel skeletal systems 14-15, 25, 36 storage of materials and equipment 12 strength gain 57-60

0 over-flying 16, 17 overloading of slabs 72-76, 107-1 08, 109-1 17

P permanent formwork 17-18,27-28 permanent works designers 3-5,6-7 permits to load and strike 2 1, 36-37, 5 1 philosophy for loading and striking flat slabs 54-55 planning considerations 16, 39 planning supebisors 7 polypropylene fibres I2 pour sizes 13 precamber 9

R

143

Guide to flat slab forrnwork and falsework

striking 4,77-88 criteria 51, 53, 5 7 4 0 , 102 planning considerations 2 1, 39 procedures 77-79, 85-86 recommendations 5 1 surface finishes 7, 10

thickness, slabs 9 timber beams 25 toeboards 20 tolerances 22, 27,49 trimming of holes 6, 12 tripping hazards 22, 27

B table systems 15-16, 21, 25-26, 35-36, 41 temperature effects 67 temperature-matched curing 58-60 temporary works calculations 2 1 concept selection 13 works co-ordinators 3-5, 2 I , 22-23 design brief 2 1,2&28,30-3 1,97-99 columns 44-45 precast slabs 4 7 4 9 slabs 1 4 1 8 , 4 6 4 7 staircases 11 walls and lift shafts 4 5 4 6

ultimate limit state 113-1 17 upstand beams 6, 12

w wall kickers 9-11 wet-cured cubes 5 8 4 0 worked examples 101-108, 129-139 working platforms 2 1, 24, 3 1-32 work sequence 37-38 written confirmation procedures 3 6 3 7

,

144

BACKPROPPING SPREADSHEET (backpropcalc.xls1Version 2.12 The spreadsheet on this CD ROM provides information for assessing construction procedures for loading in situ concrete slabs in multi-storey buildings and should be used in conjunction with this Guide. The spreadsheet is in Microsoft Excel 97-2000 and 5.0/95 97 (*.XIS) and is accompanied by a manual in pdf format.

Guide t o Flat Slab

Formwork and Falsework Eur Ing P. F. Pallett This Guide explains the procedural and technical aspects of the

formwork, falsework and striking operations for constructing fiat slab in situ concrete buiidlngs. it will be of vital interest to all in the building industry involved in concrete construction. Guide to Flat Slab Formwork and Falsework will help ail members of the construction team to improve their understanding of the construction process, right through from concept by the client to construction. This will enable improved designs and construction techniques to be adopted, leading to faster, safer and more economlc construction.

New principles for early striking of flat slabs are introduced, together with recommended methods of assessing concrete strength. Four methods of assessing backpropping loads are presented to help designers and constructors to plan efficient construction schemes. One of these is included as a spreadsheet on a CD ROM. Other topics covered include temporary works management,approvals, guidance on applying the CDM Regulations,design, loading slabs above their design load, edge protection, working plafforms, temporary cladding, enclosures and other construction details. The formwork construction methods described are 'state-of-the-art' for single- and multi-storeyconstruction. The Guide is based on the study of the seven-storeyin situ

concrete frame building built as part of the European Concrete Building Project a t BRE Cardington. It is published by The Concrete Society on behalf of CONSTRUCT, and is the result of a collaborative programme between CONSTRUCT and the Universities of Birmingham and Leeds, which was jointly funded by the Department of Trade and Industry under the Partners in Innovation scheme and by industry. The multi-disciplinarysteering group for the Guide was chaired by Eur Ing P.F. Paiiett, Temporary Works Consultant. and

included consultants, contractors, temporary works specialists, researchers, and representatives of government and other authorities.

R e f CS 140 ISBN 0 946691 90 8 The Concrete Society Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS. UK Tel: +44(0) 1344 466007 www.concrete.org.u k

,

Flat Slab Formwork & Falsework

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