Manual TiltUp

View Publication •

You ha have ask aske ed to to vie view w a publi lica cattio ion n

•

When Whe n yo you u hav have e fin finish ished ed vie viewin wing, g, cli click ck on

•

Now No w ch choo oose se on one e of th the e fo foll llow owin ing g op opti tion ons: s:

view publication chosen

in the to toolb olbar ar to ret return urn to thi this s scr screen een..

return to publications list

FOREWORD This publication was produced by the Reinforced Concrete Council (RCC). It was jointly funded by the Department of the Environment, Transport and the Regions (DETR) and the RCC under the Partners in Technology Programme. The Reinforced Concrete Council promotes better knowledge and understanding of reinforced concrete design and building technology. Its members are Co-Steel Sheerness plc and Allied Steel & Wire, representing the major suppliers of reinforcing steel in the UK; and the British Cement Association, representing the major manufacturers of Portland cement in the UK. The principal authors are: Martin Southcott, BSc, MBA, MICE. Project Director of the RCC. Alan Tovey, CEng, FIStructE, ACIArb, MIFS. Formally an Associate Director with the British Cement Association, and who now runs an independent consultancy, Tecnicom. Production of the manual was managed by Tecnicom .

ACKNOWLEDGEMENTS The authors wish to express their thanks to the many organisations and individuals around the world who have generously given both time and expertise in helping with the background material and contents of this publication. The list is long and we apologise for any error or omission. UK – Jacqueline Glass and Ray Ogden, Oxford Brookes University; Jonathan Harrison, Tilt-Up Consulting Services; Freddy Chalcroft, Chalcroft Construction Ltd; David Leach, Gatehouse Leach Training and Development; Chris Ainge, International Bible Students Association; Mike Fuller, BRC-Square Grip; Surendra Arora and Richard Moss, Building Research Establishment; Pal Chana, Sunday Popo-Ola and Naysan Khoylou, Concrete Research and Innovation Centre (Imperial College); Tony Threlfall, Concrete Design and Detailing. Republic of Ireland – Kevin McLouglin and Andrew Dixon, KML Carl Bro. USA – Ed Sauter, Tilt-up Concrete Association (TCA); David Moses, Clayco; Bob Foley and Kimberley Kayler Izenson, CON-STEEL Tilt-up Systems; Dan Doran, Composite Technologies Corporation; Joe Steinbicker, Steinbicker & Associates. Australia – Bob Potter, Cement & Concrete Association (C&CA) of Australia; Mark Turner, Steel Reinforcing Institute of Australia. New Zealand – David Barnard, Cement & Concrete Association (C&CA) of New Zealand.

Particular thanks go to the TCA, C&CA Australia and C&CA New Zealand for permission to make comprehensive use of the technical material contained in their tilt-up guides and to Jacqueline Glass for the material used in the historical overview and for cost details in Chapter 2. Also to Gillian Bond of Words & Pages for production editing. Note on diagrams Throughout this publication the diagrams are illustrative only, and do not show all the detail of construction.

Many construction activities are potentially dangerous, so care is needed at all times. Current legislation requires all persons to consider the effects of their actions or lack of action on the health and safety of themselves and others. Advice on safety legislation may be obtained from any of the area offices of the Health and Safety Executive. 97.366 First published June 1998, version 1.1 ISBN 0 7210 1533 6 Price Group J © British Cement Association

Published by British Cement Association on behalf of the Industry Sponsors of the Reinforced Concrete Council Council British Cement Association Century House, Telford Avenue Crowthorne, Berks RG45 6YS Telephone (01344) 762676 Fax (01344) 761214

All advice or information from the British Cement Association is intended for those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted. Readers should note that all BCA publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Tilt-up design and construction

CONTENTS 1. INTRODUCTION

The tilt-up technique

1-1

About this publication

1-1

2. OVERVIEW

Applications

2-1

The advantages of tilt-up

2-1

Costs Cost modelling exercise Building models Results

2-2

The tilt-up method Architectural considerations

2-4 2-5

Structural considerations

2-5

Construction considerations

2-6

Factory-cast panels

2-6

History and development

2-6

Project examples

2-7 to 2-26

References

2-7

3. PLANNING FOR TILT-UP

The design and construction team

3-1

Optimising tilt-up benefits

3-1

Layout of panels and erection sequence Casting position of slabs Cranage and crane movements

3-3

Panel sizes and tolerances Sizing and shape Tolerances

3-7

Types of panel joints

3-10

Prediction of strength development

3-10

Programme and speed of construction Case study

3-11

References

3-13

4. FLOOR SLABS AND FOUNDATIONS

Floor slab design and construction Construction method Tolerances and finish (casting surface) Weather conditions Curing, compacting and finishing Design loadings Procedure for slab design Construction programme

4-1

Foundation design and construction Details Column footings Panel seating Construction

4-3

References

4-5

i


5. DESIGN OF PANELS IN SERVICE

Principles of design

5-1

Design loadings In-service loading Construction loading Structural design

5-1

Typical structural form

5-3

Reinforcement Purpose Design Detailing

5-4

Design of loadbearing panels Suggestions for UK in-service design Simplified design procedure for slender panels Panels with openings

5-5

5-3

Design of outer leaf of a sandwich panel Sandwich panels and ties Factors affecting tie design Load transfer through ties within a sandwich panel Types of ties and anchors Stainless steel ties Composite fibre connectors Leaf thickness and cover to reinforcement Allowance for differential movements Design checks for the outer leaf and its ties

5-11

Building stability Shear walls

5-17

Fire resistance Panel thickness and cover to reinforcement Panel stability Fire growth and spread

5-18

References

5-18

5A. APPENDIX DESIGN EXAMPLES Example 1: Single-storey factory/warehouse

5A-1

General Expansion joints Foundations Erection Perimeter berm Building extension

5A-1

Loadings

5A-2

Design for in-service loads of typical panel without openings Design assumptions Loading Eccentricities of loading Load combinations First order moments at ultimate limit state Second order moments at ultimate limit state Effects of wind suction Cracking and deflection at serviceability limit state External leaf reinforcement

5A-3

Design of panel acting as a shear wall

5A-6

References – Example 1

5A-9

Example 2 - two-storey hybrid structure

5A-10

General

5A-10

Loadings

5A-10

ii

Contents

Design for in-service loads of a typical panel Basis of design Eccentricities Effective dimensions Design ultimate loading at critical section Bending moment due to first floor load Reinforcement Panel between the window openings Design of panel during erection References - Example 2

5A-11

5A-12 5A-12

6. DESIGN OF PANELS FOR LIFTING

General

6-1

Design The general principles Bending moments and flexural stresses Reinforcement Strongbacks

6-1

Lifting hardware and accessories Lifting hardware Lifting inserts

6-6

References

6-8

7. CONSTRUCTION OF PANELS

Formwork Casting surface Edge formwork Blockouts for windows, doors, etc Grooves, indents and rebates Pilasters, columns, set-backs and curved forms Mitred joints

7-1

Bond-breakers Effect of surface Bond-breaker types Selection Application Summary - bond-breakers

7-4

Panel finishes Procedure

7-6

Reinforcement Fixing the reinforcement Checklist

7-6

Embedments Pick-up points and brace attachments Weld-plates and other connections

7-7

Sandwich insulation Installation of insulation and ties

7-7

Concreting, finishing and curing the panels

7-7

References

7-8

8. PANEL ERECTION

Panel strength

8-1

Preparations for erection

8-1

Erection sequence

8-1

Cranes

8-1

Rigging the panels

8-2

Strongbacks

8-3

iii


Lifting methods Walking a panel The walk-out panel

8-3

The lift

8-4

Lifting sequence

8-4

Safety precautions

8-5

Lifting problems Panel sticks to the floor slab Panel does not hang correctly

8-5

Positioning and levelling the panels Adjustments and tolerances

8-5

Bracing Braces - general conditions for use

8-6

Release of panel

8-7

Final grouting

8-7

References

8-7

9. NON-STRUCTURAL CONSIDERATIONS

Thermal design of a building Air penetration Thermal insulation Thermal performance of sandwich panels Thermal mass

9-1

Sound insulation Mass law

9-5

Panel finishes Surface treatments and finishes Procedure Grooves and relief Exposed aggregate Surface treatments Trompe l’oeil Brick finish Combinations Mock-ups and test panels Examples of panel finishes Achieving successful finishes

9-5

Painting tilt-up panels Selection of paints Paint materials Achieving a successful paint-job Application Service life

9-9

Weather resistance of panels and joints General Joints

9-11

References

9-12

10. CONNECTION DETAILS

General criteria

10-1

Connection design Conceptual design

10-1

Types

10-2

Details Main roof and floor connections Roof or floor diaphragm connections Panel-to-panel connections

10-3

iv

Contents

Panel-to-foundation connections Connections for sandwich panels Connection details - further information References

10-14

11. SAFETY REQUIREMENTS

General

11-1

Safety checklist 1. Prior to construction 2. Prior to erection day 3. At the safety meeting 4. During the lift 5. After the lift

11-1

Individual responsibilities Structural designer Main contractor Tilt-up sub-contractor

11-3

References

11-3

12. SPECIFICATIONS

References

12-1

13. SPECIALIST SUPPLIERS AND SERVICES

13-1

v


1

INTRODUCTION

to provide the necessary information for its wider adoption in the UK. Where necessary, additional original work has been undertaken to ensure compatibility with UK codes and practice.

THE TILT-UP TECHNIQUE Tilt-up construction involves site casting the concrete walls of a building on its floor slab or on a separate casting bed and then tilting and lifting them into position by crane (Figure 1.1). The result is rapid construction arising from a well planned process more akin to a factory production line but retaining the flexibility of in-situ concrete work. Tilt-up is widely used for one- and two-storey buildings in New Zealand, Australia and particularly the United States, where some 7000 of these are constructed every year.

The contents will appeal to all those involved with the procurement of new low-rise buildings – from clients and their advisors through to contractors. In particular, Chapter 2 gives all parties a nontechnical overview of the technique including applications, benefits, costs, architectural opportunities, historical development and typical applications examined through real case studies. It also includes a summary of detailed cost modelling showing that tilt-up can be highly competitive in many sectors in the UK.

The technique is also increasingly popular for threeto five-storey structures, in particular for office and residential buildings. It is also used for a whole range of special building types as given in Chapter 2. Figure 1.2 shows a typical example of a high quality tilt-up building. In the UK many designers and developers are seeking an alternative to metal claddings, and tilt-up opens the opportunity for a more robust panel at competitive cost. Because tilt-up acts both as structure and cladding it offers major benefits compared with other constructional forms, including competitive costs, rapid construction, inherent fire resistance, security, durability, sound insulation, low air-penetration, easily sealed washable surfaces, high thermal mass and insulation, and scope for new architectural expression. These benefits are looked at in more detail in Chapter 2, which includes the results of extensive cost modelling work demonstrating the competitiveness of tilt-up construction.

ABOUT THIS PUBLICATION This publication brings together worldwide experience of tilt-up into a single document in a form suitable for the UK. The examples and details shown are selected to demonstrate the versality of tilt-up and

Figure 1.2 Example of high quality tilt-up building

Tilt-up panel lifted by crane External wall line

External face Tilt-up wall panel cast face down on floor slab

External face

Figure 1.1 The tilt-up technique

1-1

Tilt-up panel in position to form structural wall

Introduction

The efficiency of tilt-up results from careful planning and close co-operation between designers and constructors, who should both carefully study Chapter 3. Chapter 4 describes the design and construction of floor slabs and foundations, and Chapters 5 to 12 provide information on all aspects of the design, construction and erection of tilt-up panels. These chapters are intended for both designers and constructors, to impart a common understanding of their roles. However, Chapter 5 contains considerable detailed technical material on the structural analysis and design of tilt-up panels, mostly of use to the structural engineer. Chapter 13 provides additional information on suppliers of goods and services. Finally, it is worth emphasising that, although this publication deals mainly with on-site tilt-up, much of the material is equally relevant to factory-cast panels, which may prove more suitable for a particular project.

1-2


2

AN OVERVIEW

This chapter gives a non-technical overview of tilt-up construction covering applications, benefits, costs, construction method, architectural opportunities, structural and constructional considerations, and factory-cast alternatives. Finally, after a brief history of the development of tilt-up, some typical applications are examined through real case studies. The costs section includes a summary of detailed cost modelling showing that tilt-up can be highly competitive in many UK markets. types it clearly has significant benefits to offer, including:

APPLICATIONS Tilt-up is a construction method that allows great design flexibility and encourages innovation. Walls can be produced as solid panels for use with added insulation or as sandwich panels where insulation is installed during production. The use of concrete offers the designer structural capacity, fire resistance, sound insulation, thermal capacity, aesthetic qualities and durability. Tilt-up panels commonly carry the roof load and provide transverse load resistance in shear, and thus may be used for both loadbearing and non-loadbearing walls to a variety of structures (see Figure 2.1 overleaf). The following list illustrates just some of the diversity of applications for which tilt-up has been successfully used. A number of case studies showing plans and other details are given later in this chapter under the heading ‘Project examples’. Commercial and industrial construction Warehouses, workshops, storage units, offices, distribution centres, factories (including clean rooms, controlled atmosphere, meat curing and timber preservation), hotels, restaurants, shopping complexes, auditoria, service buildings and cold stores.

•

Cost of construction - Extensive cost modelling has shown tilt-up to be highly competitive with traditional construction over a wide range of building types and sizes (see the ‘Costs’ heading later in this chapter).

•

Speed of construction - This is often a major reason for choosing tilt-up. After the floor slab is placed, the typical elapsed time from starting to form the panels until the building shell is completed may be only four to five weeks. Materials for wall panels are easily procured with minimal lead times, allowing a fast start to a near-factory assembly line process, which progresses while any roof steel is fabricated.

•

Ease of construction - The uncomplicated construction method simplifies and minimises on site work and reduces labour costs.

•

Energy conservation - Tilt-up walls can be economically insulated to give whatever insulation values are required, from a normal building through to highly insulated cold stores. Sandwich tilt-up panels not only provide all necessary insulation but, by incorporating the insulation during panel production, minimise follow-on trades and build in useful thermal mass to moderate temperature extremes. Air penetration is also minimised.

•

Durability - The strength of concrete coupled with the uncomplicated method of construction offers reliable durability, as attested by the buildings dating back to the 1940s.

•

Sound reduction - Buildings in a noisy area, such as near an airport or roads, can benefit from the sound reduction properties of concrete. The mass absorbs the sound rather than letting it through as can occur with lighter forms of construction. Alternatively, noisy processes within buildings are more easily isolated.

•

Low maintenance costs – Visual concrete (fairfaced, textured, profiled, tooled and exposed aggregate finishes) and cast applied facings (inlaid stone etc.) require little specific attention and modern paint systems have been shown to have considerable life. The wider panel width

Recreational construction - Squash courts, indoor cricket facilities, gymnasia and basketball courts. Residential construction - Houses, two- to threestorey flats, town houses and halls of residence. Rural construction - Farm sheds, piggeries, dairies, tanks, drainage systems, grain stores and settlement tanks. Other construction - Churches, community halls, schools, colleges, sound barriers, retaining walls, security walls, reservoirs, water treatment structures, plant rooms, fuel tanks, prisons, and fire compartment walls.

THE ADVANTAGES OF TILT-UP For a building method to be chosen in the highly competitive construction market it has to show that its benefits outweigh those of the alternatives. As tiltup is so widely used for many different building

2-1


offered by tilt-up minimises the number of joints and length of sealant, thus reducing the cost of maintenance. Concrete walls are less subject to mechanical damage, and are easily washed down. Sandwich panels are particularly beneficial in this respect since they offer both insulation and two hard surfaces.

•

and low temperatures including freezers, and clean rooms for food, drug and electronics manufacture.

Fire resistance - Concrete is an obvious first choice for fire resistance. Tilt-up panels can be readily and economically designed for up to four hours fire resistance and are particularly cost effective as fire separation and compartment walls. A 160 mm thick wall, for example, can provide up to two hours fire resistance. Concrete sandwich panels do not suffer the fire-spread problems associated with some metal systems.

•

Lower insurance rates - The fire resistance of tilt-up concrete walls and added security may result in lower premiums.

•

Low air penetration and robust, easily sealed surfaces - Tilt-up is easily sealed, making it ideal for controlled environments such as fruit storage, meat curing, timber preservation, high

•

Architectural attractiveness - Tilt-up offers exciting new architectural opportunities that complement current building trends. There are now many stunning examples of this in practice worldwide.

•

Extendibility - By planning for the possibility of expansion, building and panel connections can be designed so that the panels can be removed, relocated or added to.

•

Security - Unlike metal-clad buildings, forced entry through concrete walls is very much more difficult. Tilt-up is frequently used for security walls and prisons.

•

Safety - With a tilt-up building, much of the work is on the ground; there is no vertical formwork, no scaffolding, and since the floor slab is poured first, workers have a safer working surface. The short and uncomplicated project cycle presents less opportunity for accidents.

COSTS Cost modelling exercise

Loadbearing tilt-up panel supporting roof

Tilt-up panel wall

(1)

This section is based on cost research undertaken by Jacqueline Glass in the School of Architecture at Oxford Brookes University, as part of a broader PhD study of tilt-up construction sponsored by the Engineering and Physical Sciences Research Council (EPSRC) and Reinforced Concrete Council (RCC).

Tilt-up panel wall

In established markets, tilt-up is often used for lowrise buildings, and this will probably also be the case in the UK. Research undertaken at Oxford Brookes analysed costs for both tilt-up and conventional construction methods for a generic building with a storage/production space fronted by two-storey offices (Figure 2.2 shows a typical building model).

Strip footing

(a) Tilt-up panel designed as loadbearing wall

Tilt-up panel attached to internal perimeter frame

Office 576 m 2 Tilt-up cladding panel

Tilt-up claddng panel

Elevation Office grid 6 x 6 m Warehouse 24 x 12 m Warehouse 2304 m 2

Internal perimeter frame Tilt-up panel may span onto isolated pads

Plan

(b) Tilt-up panel designed as cladding panel

Figure 2.1 Tilt-up panels used for loadbearing and non-loadbearing walls

Figure 2.2 Cost model building A

2-2

Section

An overview

2

within £4/m of the cheapest metal cladding option, and for building C, tilt-up is more economical for both plain and decorated panels. For external walls, all tilt-up options (see Table 2.2) cost between £39 2 and £72/m , which makes them competitive with a large range of cladding products. Tilt-up sandwich 2 panels cost about £15/m more than normal panels, which is a modest premium to pay for long-term energy and durability benefits. The cost of tilt-up panels typically increases with height due to additional lifting and material costs for taller, heavier panels, but this is offset by using tilt-up as fire2 protecting internal walls for £36/m for tilt-up, 2 compared with £72/m for proprietary fire partitions.

Building models 2

2

A range of floor sizes from 2,304 m to 18,090 m was tested for each of eight wall options. Loadbearing, decorated loadbearing, and sandwich loadbearing tilt-up panels were compared with varying qualities of conventional construction based on steel portal frames. In addition, a fourth hybrid system of non-loadbearing tilt-up panels on steel portals was considered. Building designs and specifications were of a basic standard with allowances for services and external works, but not for fit-out. For buildings C and D, fire compartment walls were required in addition to a separation wall between the warehouse and offices.

Construction programme – The next paragraph describes the programmes generated and used in the cost model exercise, see Chapter 3 ‘Programme and speed of construction’ for more specific discussions of these matters and actual programmes.

Results 2

The total building costs/m ground floor area including preliminaries, contingency and fees are given in Table 2.1.

All cost data includes preliminaries and time-related charges taken from construction programmes

Analysis of results – For buildings with an area above 2304 m2, loadbearing tilt-up is consistently 2

Table 2.1 Total building costs/m gross floor area Type of external wall

Building A 2 2,304 m Eaves height 6m

Building B 2 4,500 m Eaves height 8m

Building C 2 9,180 m Eaves height 10 m

Building D 2 18,090 m Eaves height 12 m

Loadbearing tilt-up panels

TU

£360

£324

£319

£292

Tilt-up insulated sandwich panels

TU

£374

£338

£329

£299

Decorated tilt-up panels

TU

£361

£326

£320

£292

Tilt-up cladding panels

Hybr

£355

£327

£327

£294

Built-up metal cladding system

Trad

£342

£320

£321

£288

Composite cladding panels

Trad

£387

£360

£357

£318

Aluminium cladding system

Trad

£421

£391

£384

£339

Blockwork/built-up cladding

Trad

£354

£328

£327

£292

Table 2.2 External wall costs/m2 wall area, including plant-based preliminaries Type of external wall

Building A

Building B

Building C

Building D

Loadbearing tilt-up panels

TU

£49

£50

£54

£57

Tilt-up insulated sandwich panels

TU

£62

£65

£69

£72

Decorated tilt-up panels

TU

£51

£52

£56

£59

Tilt-up cladding panels

Hybr

£39

£39

£41

£42

Built-up metal cladding system

Trad

£36

£36

£36

£36

Composite cladding panels

Trad

£104

£104

£104

£104

Aluminium cladding system

Trad

£155

£155

£155

£155

Blockwork/built-up cladding

Trad

£48

£45

£43

£43

2-3


Table 2.3 Time on site from mobilisation to completion using published lead times (weeks) Type of external wall

Building A

Building B

Building C

Building D

Loadbearing tilt-up panels *

17

23

29

33

Tilt-up insulated sandwich panels *

18

24

31

35

Metal cladding panels

13

19

29

33

* Later discussions with contractors experienced in tilt-up revealed reduced lead-in times over those published and used in this study. This results in possible reductions of at least two weeks in the tilt-up times given above. Examples of faster programmes are given in Chapter 3.

developed for the cost model. Assuming normal site conditions, published lead times and continuity of work, loadbearing tilt-up is just four weeks behind conventional construction methods for buildings A and B, but runs exactly in parallel for building C and D. Insulated sandwich panels incur a little more time, but this is more than offset by benefits gained in the long term (Table 2.3).

temporary casting bed enables the main building ground slab to be cast later following erection of the roof. Thus tilt-up offers more than one method of construction programming. A typical construction starts with the levelling of the site before foundations are dug and cast. The ground floor sub-base material is then rolled and accurately levelled; a membrane is laid and simple edge formwork fixed. This is typically laser-levelled for extreme accuracy. The main floor slab (Figure 2.3 (a)) may be laid by the long-strip method, typically 4 m wide by the full length, and finished by power float. But, increasingly, the slab may be laid in wide pours and finished by laser levelling and power float.

Conclusions The results of the cost model show clearly that tilt-up can out-perform several conventional UK methods, and provides an economical alternative for internal fire walls. There is also a definite indication that cost competitiveness of the technique could at least rival its performance in countries such as the USA, Australia and New Zealand, although it is not easy to compare UK costs data with that from established tilt-up markets overseas. (All costs are current at January 1998; data does not include landfill tax, overheads, profit, variations or VAT.)

When the slab has gained sufficient strength, the tiltup wall panels are constructed upon it. The panels may be cut to size after long-strip casting, or more commonly are formed individually. The main floor slab or previous panels act as the panel's casting face and completing the formwork requires only simple perimeter side forms. Climbing forms or full depth formwork is used when the panels are stack-cast. Before each is cast, a bond-breaker, form-liner or other material is placed on the floor slab, or on top of the previous panel in the case of stack-casting.

The kind assistance of the following UK companies is gratefully acknowledged. Curtins Consulting Engineers Davis Langdon & Everest

Finally, the panel reinforcement and fixings for the roof are placed and the panel is then concreted and finished (Figure 2.3 (b)). Careful attention is given to the casting position of panels on the base slab in order to minimise crane movement and achieve the most efficient construction sequence.

Gazeley Properties Hanscomb Partnership Laing Special Projects MACE Ltd Slough Estates

After typically two to seven days, when a panel has gained sufficient strength, props and lifting devices are attached. The panel is then gradually lifted or tilted up until it is upright (Figure 2.3 (c)). The flexural stresses during the lifting reach a maximum when the panels are at an angle of about 30 degrees. At this point, the stresses are often greater than when the panel is in place, which can give the designer added assurance in the completed structure.

Tilt-up Consulting Services WH Stephens & Sons and several other leading companies

THE TILT-UP METHOD Tilt-up construction is the on-site precasting of the walls of a building. In one method of construction, the perimeter foundations and internal ground slab are cast first, and then the wall panels are cast individually, contiguously or continuously on the slab (see Figure 7.2, Chapter 7). Alternatively, panels may be cast one on top another, as stack-casting. After panels have gained sufficient strength, they are tilted up and positioned around the perimeter. The internal frame is then constructed and the roof built. If desired, stack-casting the panels on a separate

Particular economic benefits come from the methods used to lift the panels. It is common to use a multipoint lifting system so that the bending stresses are kept below the flexural tensile strength of the concrete thus minimising and often eliminating the need to rely on reinforcement during lifting. This not only enables savings in reinforcement to be made but also allows thinner panels to be erected with 2-4

An overview

consequential savings in concrete and final wall thickness. Reinforcement is normally placed in a single central mat.

are positioned, the roof trusses and purlins are erected, and the roofing is finally fixed (Figure 2.3 (e)). This short description demonstrates the uncomplicated procedure for tilt-up construction that can be used to create a variety of panel configurations for a wide range of different building types.

When the panel is in an upright position it is carefully swung to the perimeter where it is propped in place (Figure 2.3 (d)). Multi-point lifting can enable larger panels to be erected, so saving time in construction. These larger panels require the use of appropriate cranage but since they are rapidly erected it means that heavy capacity cranage is often needed for only a few days. For example, the entire external walls 2 covering an area of some 3000 m to a building can be erected in only a couple of days. Once the panels

ARCHITECTURAL CONSIDERATIONS Until recently it was contended that tilt-up could not compete with the quality of precast concrete, and should retain its inherent simplicity rather than seeking to become too sophisticated. However, the advancement of tilt-up techniques is such that it is now possible to attain consistent high quality finishes. The improved appearance of tilt-up buildings stems largely from the sensitive detailing of panels and an almost limitless range of colours, patterns and textures (see Chapter 9). The front cover and projects examples at the end of this chapter demonstrate the high quality of architecture now routinely achieved.

External wall line (a)

Wall foundation Floor slab

Designers of industrial and low-specification buildings tend to utilise simple, economical finishes, whereas more prestigious commercial markets use more flamboyant combinations of finishes and materials. However, the change in manufacturing and commercial markets from heavy industrial into high-tech industries means that aesthetic considerations seem to be more important to designers and developers than in previous years.

External wall line (b) Tilt-up wall panel cast on floor

Tilt-up construction is ideal to meet this new demand because designers can take full advantage of available colours, textures, surface finishes, and architectural embellishments in varying degrees of complexity to enhance the image of the tilt-up building.

Tilt-up panel lifted by crane (c)

(d)

These additional features included stepped profiles, trellises, polished stone or ribbon glazing. Thus the modern tilt-up building is able to provide a quality, economical, robust and durable structure that, together with attractive landscaping, can achieve spectacular results.

Tilt-up panel positioned onto foundation

The technique of tilt-up construction was developed especially for on-site use but it has also been extended to off-site precast work. Many tilt-up projects now make use of both site and factory components for optimum design and construction flexibility and this is likely to be the case within the UK.

Temporary bracing

(e)

Roof members brace tilt-up walls

STRUCTURAL CONSIDERATIONS

Structural tilt-up panel

The design of panels and stability of tilt-up buildings is well tested since their reliability has been proved in the earthquake regions of the world. Tilt-up design recommendations are fully developed in those countries where it is widely used, with many having (2) specific national codes and standards, and other (3) (4) design guides and . There are no specific UK

are braced position, the internal columns, if any, tilt-up construction sequence Figure 2.3 inTypical

2-5


tilt-up codes or standards but Chapter 5 of this publication presents design suggestions compatible with the main British structural code for concrete, BS 8110, and contains further references on design.

there are special demands on tolerances, finishes, quality or concrete mix (such as coloured concrete or special aggregates). Finally, it is worth noting that some buildings may justify installing a temporary ‘factory-casting’ facility on site.

CONSTRUCTION CONSIDERATIONS In tilt-up construction, the floor is often cast before the roof. It has been said that tilt-up may not, because of weather conditions, be as suitable in this part of the world as it is in other countries. This view is, however, misguided since tilt-up has been successfully used in Scotland, where one project was built to programme during the worst weather conditions for 100 years. Tilt-up is also used in New Zealand, and in parts of the USA and Canada, where the weather conditions are similar to our own.

HISTORY AND DEVELOPMENT Tilt-up construction was first introduced in the early 1900s in the USA. However it was not until the 1950s with the introduction of mobile cranes and ready-mixed concrete that its use really grew. At this time the first design and construction guides appeared in the USA, paving the way for pioneering contractors to capitalise on the increased sophistication of the technique in the following decades.

In Britain reliable short- and medium-term weather forecasting services specifically for construction are readily available from the Meteorological Office and others and are commonly used for programming of weather-dependent processes. Precautions such as tented enclosures or temporary edge wind-breaks can be used in the event of driving rain. Weather is not a major factor with tilt-up since the problems are not significantly different from those faced on any other construction project involving the placing of concrete. Contractors simply take precautions or adjust work to cater for prevailing weather conditions.

From the late 1970s to the present day, tilt-up’s use has grown substantially, due partly to traditional skills shortages but mainly to its speed, providing more efficient construction to meet growing economic pressures, and an increase in the real and (5) perceived quality of tilt-up buildings . Its established use has spread from the USA market, to those of Australia and New Zealand. In each case, its development followed a distinct pattern. The technique began in low-rise industrial structures where it developed a quality image. Whilst increasing its share of the market in these building types, it diversified into other commercial, leisure and residential sectors.

The method of tilt-up is now so well developed and proven that full design and construction expertise is readily available. There are consultants, contractors and material suppliers with experience of tilt-up in both the UK and Ireland and throughout the world. Thus the developer or designer can be assured that there is sufficient availability of materials, equipment and expertise to ensure competitive and reliable tilt-up construction within the UK and in the rest of Europe.

The technology, expertise, and reliability of tilt-up has improved due to improved structural engineering, availability of products specifically developed to suit tilt-up, achievement of better quality finishes, and changes in procurement routes. Currently, in the USA, some 13 million square metres of tilt-up walls are built each year, the equivalent of around 7000 low-rise buildings. This increased at a rate of 12% between 1995 and 1996. Of these buildings, around 65% are industrial or warehousing, 25% are offices, and 10% are retail development and other miscellaneous projects.

FACTORY-CAST PANELS Most of the material in this guide applies equally to wall units produced in the factory and delivered to site. However, with factory-cast panels there are some obvious differences to be considered.

•

Road transportation limits overall sizes to around 12 m long by 3 to 4 m wide (depending on whether delivered flat or upright on the lorry). There is, therefore, a maximum width of opening that can be incorporated into a single panel.

•

Different economics may apply, as many units will be cast from a single mould, justifying more complex shapes and costly formliners.

•

Transportation and handling may require extra reinforcement and lifting fittings.

•

Construction programmes alter as casting is independent of ground slabs, but lead times will be longer.

•

Factory precast may be more appropriate where

In Australia, tilt-up’s use for the industrial market took off in the 1970s because of a dramatic rise in brick prices, and a crisis of confidence in low-rise (6) metal-clad structures, often referred to as ‘sheds’ . Subsequently, it has also become popular for speculative house building in the 1990s where it has now been used for terraced houses, luxury apartments, individual villas and housing association developments. Canada has an established tilt-up market and the method has also been used in other countries such as Malaysia, Argentina, Brazil, Hungary, Mexico and South Africa.

2-6

An overview

In the UK and Ireland, tilt-up has been mainly used for industrial and warehousing structures, some incorporating office space. It is interesting that these tilt-up structures remain quite distinct, standing out from often very commonplace metal-clad sheds . In addition to advanced factory buildings in Scotland, further buildings have been constructed by the Watchtower organisation in London and in Co.Wicklow, Ireland. The latter particularly have achieved exceptional quality and appearance. The Watchtower organisation is highly committed to tiltup and has a rolling programme of around 40 new buildings under construction, throughout the world, utilising tilt-up. '

3. Cement and Concrete Association of New Zealand. Tilt-up technical manual. C&CA, Porirua, New Zealand, 1991. TM 34. 32 pp.

'

4. Brookes, H. The tilt-up design and construction manual. HBA Publications, Newport Beach, Dayton, Ohio, USA, 1997. 292 pp. 5. Spears, R, E. Tilt-up construction and design considerations. Concrete International, Vol. 2, No. 4, 1980. pp 33 – 38. 6. O’Hagan, R. The incredible rise of tilt-up construction. Australian Concrete Construction. Vol. 1, No. 1, 1989.

Recently a contractor specialising in design-build cold store projects in the UK has adopted tilt-up for its plant rooms in order to save time and reduce construction problems. There is a growing trend worldwide towards the use of tilt-up concrete sandwich panels. One supplier is now claiming that about 5 million square metres of panels have been built using its system alone.

PROJECT EXAMPLES The examples shown on pages 2-8 to 2-26 have been selected from the UK and elsewhere to give an indication of the scope and form of tilt-up construction. They show tilt-up panels used for loadbearing walls to carry vertical roof and floor loads and lateral wind forces, panels used for earthretaining structures, and non-loadbearing cladding. The examples also show panels used to create very slender walls that provide economy by maximising the nett-to-gross area of the building. The continuous run of panels also means there are no intruding columns as can occur with a portal frame. The differing panel finishes and shapes provide an insight to the architectural freedom available with tilt-up construction. The examples also demonstrate the ability of tilt-up to be used for both single- and multi-storey buildings and to be designed to support both floors and roofs. The opportunities for tilt-up should become apparent from reviewing just these few examples.

REFERENCES 1. Glass, J. Evaluation of tilt-up construction in relation to selected UK building types. Post Graduate Research School, School of Architecture, Oxford Brookes University, UK. (PhD thesis). 2. Standards Australia. Tilt-up concrete and precast concrete elements for use in buildings. Part 1: Safety requirements. Part 2: Guide to design, casting and erection of tilt-up panels. Standards Australia, North Sydney, NSW, 1990. AS 3850. 16 pp, 24 pp.

2-7


GLENROTHES 2, SCOTLAND Glenrothes 2 was the second major loadbearing tilt-up panel system constructed in the UK. (The first major structure was Glenrothes 1 built in 1984 as an advanced factory unit and is constructed with 165 mm thick solid panels, typically 7 m wide by 7.5 m high, weighing in the region of 20 tonnes each.) The second building was erected in 1986 as another advanced factory unit and is constructed with a similar internal structure and panel size and weight. The regulatory thermal standard was achieved by the use of an insulated internal lining in conjunction with external insulation provided by an earth berm at ground level. The concrete panels

are decorated with a paint finish applied directly to a textured surface created by casting against a formwork lining sheet. The structure has a constructed area of 2 3743 m but is designed to be sub-divided into two 2 1870 m units and is also detailed to allow for expansion to 7486 m2. The building has a main 5.5 m clear height single-storey factory facility and a two2 storey office of 392 m occupying one corner. Due to delays experienced with the steelwork to the office on Glenrothes I, the office elevation to this building has double-glazed aluminium curtain walling units supported by featured concrete structural tilt-up units.

Location - Glenrothes, Scotland Contract period – 22 weeks (see Chapter 3 for construction programme) 2

2

Total net floor area - 3743 m + 390 m office Typical panel size - 7 m wide by 7.5 m high Typical panel weight - 20 tonnes External finish – Paint on textured surface Thermal insulation - Internal insulated lining with external earth berm Textured panel finish

View showing provision for an office at each end

2-8

An overview

7.5 m

Typical cross section

North elevation

90 m

45 m


OLD NAAS ROAD, DUBLIN Built in 1990, this development consists of two high-spec units in the heart of Ireland's premier industrial location at Old Naas Road, Dublin. Both units are designed to accommodate production, warehouse and office use. The development 2 2 consists of one 967 m unit and one of 816 m , each 2 having a 100 m office area over two storeys, approached by a framed entrance for pedestrian use. The walls are solid tilt-up panels internally insulated and finished externally with paint featuring a distinctive inset band, and incorporating colour-coated aluminium windows. The roof decking is a metal composite insulated system

incorporating translucent sheeting to 15% of the roof area. There are no internal columns as the roof trusses are designed to span 27 metres between the loadbearing tilt-up perimeter walls. Eccentricity of load is minimised by the use of recessed steel connections within the supporting panels. Container access is provided to the 6 metre high warehouse production area by steel roller shutter entrances. The development was promoted as a new concept of building to image-conscious companies seeking industrial and office accommodation of high quality and finish.

Location - Old Naas Road, Dublin 12, Irish Republic Total floor area - 1783 m 2 in two units Contract period - 14 weeks Typical panel size – 7.88 m wide x 6.75 m high Typical panel weight - 22 tonnes External finish – Paint with feature painted band Thermal insulation - Internal insulation with protection boarding at lower levels

Inset band echoing stepped site

2-10

An overview

Typical elevation showing inset band

Floor plan

6.75m

Truss support detail Cross section

27 m

27 metre clear span 32.5 m

27 m

27 m


LABORATORY, QUEENSLAND, AUSTRALIA This project, built in 1995, shows an example of sitecast tilt-up concrete panels, incorporating a considerable degree of refinement, used to clad a two-storey reinforced concrete frame. Tilt-up panels were selected because of their economy and speed of construction. The laboratory facilities are on two 2 floors and have a total area of around 2750 m . The panels, which have an internal plasterboard finish, are typically 6 m wide by up to 7.7 m high with a

concrete thickness of 150 mm. Externally the panels have a combination of flat and horizontal ribbed surfaces and were given a high-build paint finish. The colour selection, together with the clean eaves detail and design of sun screens and entrance awning, combine to make this a stylish industrial building. The roof is of conventional steel purlins with main beams.

Location - Garnet Street, Carole Park, Queensland, Australia Total floor area - 2750 m

2

Contract period - Construction time not known, but tilt-up used for speed Typical panel size - 6 m wide x 6.9 - 7.7 m high Typical panel weight - 19 tonnes

Interest provided by use of plain and ribbed panels

2-12

An overvie


2.70 m

In-situ concrete slab and edge beam 0.25 m 0.85 m

In-situ concrete column

First floor plan

2.70 m 150 mm two storey tilt-up cladding panel

25.3 m

Typical wall section 48.5 m


MARY MCKILLOP CATHOLIC CHURCH, QUEENSLAND, AUSTRALIA This church was built in 1995. A limited budget and a practical brief provided the ideal opportunity for the architects to take advantage of the economies of tilt-up construction, as well as to use the system imaginatively to produce a memorable image for this new Roman Catholic church. The panels are only 170 mm thick and weigh up to 18.45 tonnes. The external panels contain an off-white cement and have a vigorous ribbed surface, which has been grit-blasted to reveal the light-coloured Pine River stone aggregate. The tops of the exposed walls are

provided with channels discharging onto the roof to reduce staining. The panels are butt jointed and sealed with a thixotropic joint sealant on a polyurethane base over a backing rod. Great care has been taken with the acoustic performance of the building: the ceiling is treated to absorb sound but no special treatment was required for the walls as the reflection of sound provided by the concrete gives life to the church music, especially to choral works. The designers were recipients of a Concrete Institute of Australia Excellence in Concrete Award in 1995.

Location - Birkdale, Queensland, Australia Total floor area - 1200 m

2

Contract period - Not known, but 22 panels erected in 8 hours Typical panel size - 6 m wide x 4.0 - 7.5 m high Typical panel weight - Up to 18.45 tonnes External finish - Off-white cement, grit-blasted concrete with vigorous ribbed surface Thermal insulation - Not required

Imaginative use of rugged grit-blasted ribbed panels

2-14

An overview

Section A - A

A

A

0

2

4

6

8

10 m


COLD STORE PLANT ROOM, WOLVERHAMPTON This project is the first of several cold store plant rooms to be constructed in the UK. The plant room is only about 22 m by 7 m but lies on the critical path as it is vital to the installation of the complex refrigeration plant. The use of tilt-up for this project is estimated by the contractor to have saved up to four weeks over other alternative forms of construction. The panels are typically 7.9 m high by 5.8 m wide and weigh in the order of 25 tonnes. The panels are of sandwich construction (60 mm outer leaf, 50 mm rigid insulation and a 150 mm loadbearing inner) and thus provide the necessary

insulation and structural capacity as a single constructed element. The two leaves of each panel are joined together by composite ties to minimise thermal bridging. The panels were stack cast on a separately cast slab adjacent to the plant room. This method was adopted as the plant room floor has limited dimensions and is heavily troughed for service pipes. The tilt-up panels support lateral wind loads and vertical loads from roof and mezzanine floors. Externally the building is clad with profiled steel sheets, but consideration is being given to the use of ribbed-faced concrete tilt-up panels for future projects.

Location - Wolverhampton Total floor area - 154 m

2

Contract period - Panel construction 10 days, erected in 1 day Typical panel size - 5.8 m wide by 7.9 m high Typical panel weight - 25 tonnes External finish - Profiled metal cladding on insulated tilt-up panels Thermal insulation - Sandwiched insulation in tilt-up panels (Information by courtesy of Chalcroft Construction Ltd and Tilt-up Construction Services)

Erection of insulated panels

Profiled steel cladding

Alternative ribbed concrete finish

Finished building with metal cladding

2-16

An overview

7.9 m

Elevation showing multiple panel openings

22 m

7m

Metal cladding

Ground floor plan of plant room

Plant room

Cold store

Loading dock

IBSA BURIED SERVICES BUILDING, MILL HILL, LONDON The International Bible Students Association (IBSA) has used tilt-up construction techniques for more than 40 projects worldwide. This project, constructed in 1996, is an example of the diversity of this form of construction. The tilt-up panels are used for the retaining walls of a buried services building. The main panels are designed to carry both vertical loads from the buried roof as well as the lateral loads from the retained backfill. The project uses the structural and economic benefits of tilt-up to the full. The building is approximately 22 m x 20 m. The solid tilt-up wall panels are typically 200 mm thick and vary in size up to 5.2 m wide by 4.8 m high. The

main panels support the main internal concrete roof beams carrying transverse in-situ floor slabs. Thermal performance is achieved by utilising the insulating properties of the retained soil. A membrane waterproofing is applied to the perimeter panels. A drainage layer is provided at the bottom of the panels, and discharges to an outfall. The tilt-up panels were stack cast on the basement slab which, because of the confined site, was temporarily extended locally to accommodate certain panels. Only 1 m working space existed beyond the perimeter walls.

Location - Mill Hill, London Total floor area - 440 m 2 Contract period - Not known but 13 panels erected in one day Typical panel size - 5.2 m wide by 4.8 m high Typical panel weight - 12.8 tonnes External finish - Plain finished panel with installed water proofing Thermal insulation - Provided by lightweight aggregate and retained soil (Information by courtesy of IBSA, and Gatehouse Leach Training and Development)

Arial view showing panel erection on confined site

Basement construction being roofed

Finished project, buried and laid to lawn

An overview

An overview

Geotextile

Top soil

Lightweight agg. Insulation Membrane

Subsoil Concrete roof slab Tilt-up retaining wall

4.50 m

Concrete raft

Infill strip

Typical section

Bentonite membrane Gravel Subsoil drain

Elevation (plant room at lower level)

Existing building

19.73 m


H DENNERT DISTRIBUTING, OHIO, USA The H Dennert Distributing complex is a modern white building that incorporates an interesting portico, which distinguishes it from other buildings nearby. The building is designed as a mixed-use distribution and maintenance centre and has an area 2 of around 14,285 m of varying plan form based on a 15 m grid. In addition, the building is designed to 2 accommodate a 3800 m expansion as business grows. This was a design-build project, utilising a local contractor licensed by CON-STEEL Tilt-up Systems working directly with the owners, H Dennert Distributing.

requirements by using three different types of wall panels. The panels for the office are of single-leaf construction with added internal insulation. The walls in the staging area are of sandwich construction to provide high strength, high insulation and low maintenance. A newly developed, proprietary insulated hollow-core panel, competitive with insulated masonry, is used in the drive-through and catering areas. The solid tilt-up wall panels are typically 185 mm thick and vary in size up to 5 m wide by 8.9 m high. The hollow-core panels are 225 mm thick and typically 5.9 m wide by 8.9 m. The sandwich panels have a 90 mm outer leaf, 50 mm of insulation and a 165 mm loadbearing inner leaf. Panel dimensions are up to 8.0 m wide by 10.2 m high and weigh up to 35 tonnes.

The building comprises seven function areas: office, drive-through sales, maintenance shop, interior staging, ambient case storage, keg draft drive-in cooler, and special events catering. This project shows the flexibility of tilt-up to meet varying wall Location - Cincinnati, Ohio, USA Total floor area - 14,285 m

2

Contract period - Unknown but fast-track construction Typical panel size – Varies, up to 8.0 m wide by 10.2 m high Typical panel weight - Up to 35 tonnes External finish - Painted Thermal insulation - Provided by a combination of internal, sandwich or cored insulation (Information by courtesy of CON-STEEL Tilt-up Systems)

Featured panels offset the curved portico

2-20

An overview

10.2 m

Project utilises panels of different shapes and insulation methods

Typical part section

Elevation

52 m

122 m

29 m

34 m 55 m

27 m

30 m

36 m


YMCA/YWCA, EASTERN REGIONAL CENTRE, ONTARIO, CANADA The brick appearance of this building is created by using a brick slip system incorporated during the construction of the tilt-up panels. The brick slips TM were placed onto a Brick Snap grid (Scott Systems Inc. of Denver, Colorado) placed onto a previously cast slab and backed by 75 mm of concrete. (The use of brick slips is common practice for works precast panels and has now been developed for site use.) Preformed insulation, 50 mm thick, was placed on top of this and composite ties inserted to provide a tie between the

outer leaf and a 165 mm inner leaf designed to carry internal loads. The building project was let out to alternative bids and tilt-up produced the best and most committed fast-track schedule. The building has an area 2 of some 2860 m and was constructed in just 16 weeks. This project demonstrates the flexibility and adaptability of tilt-up construction enabling rapid construction for a building with restricted access on two sides due to roadways and steep embankments. To aid construction two temporary casting beds were needed.

Location - Cumberland, Ontario Total floor area - 2860 m

2

Contract period - 16 weeks. Typical panel size – Varies, up to 4 m wide by 10.2 m high Typical panel weight - Up to 24 tonnes External finish - Brick surface Thermal insulation - Provided by sandwich insulation (Information courtesy of CON-STEEL Tilt-up Systems)

Brick-faced panels enhance the external appearance (inset shows brick slips laid face down in Grid Snap

TM

system)

2-22

An overview

10.7 m

3.8 m


North elevation

57 m

18 m

28 m


DY-4 SYSTEMS INC, KANTA, ONTARIO, CANADA Speed of construction, energy efficiency and flexibility for future expansion were the key factors in DY-4's decision to go with the tilt-up design2 build proposal for their 6900 m building in Ontario. The result is an especially attractive building completed in just 21 weeks with all 55 panels being erected in only 4.5 days. The panels are of sandwich construction consisting of a 75 mm outer leaf, 75 mm of preformed insulation and a 150 mm inner 2 leaf. This provides a U-value of 0.4 W/m K in the panels directly as constructed. The thermal capacity of the concrete inner leaf is also utilised to enhance the building's energy efficiency. The panels weigh

between 18 to 35 tonnes. Pouring and lifting schedules allowed the structural steel to be erected on one wing of the building while panels were lifted and positioned on the other wing. The concrete panels are of a high quality and have an external exposed white marble aggregate textured surface created by medium grit-blasting. Additional site-cast reveals, horizontal accent bands, and unique triangular columns on the 10 m high entrance are used to break up the flat linear surface of the building and reinforce the tower's identity as the building's signature piece.

Location - Kanata, Ontario Total floor area - 6900 m 2 Contract period - 21 weeks Typical panel size - Sandwich panels typically 6 m wide by 8.8 m high Typical panel weight - Up to 35 tonnes External finish - Exposed (grit-blasted) white marble aggregate Thermal insulation - Provided by sandwich insulation (Information by courtesy of CON-STEEL Tilt-up Systems)

General view of entrance area showing triangular columns

2-24

An overview

8.8 m


South elevation

North elevation

38 m

m 8 3

38 m

31 m

4 m

An overview

BALLARD POWER BUILDING, BRITISH COLUMBIA, CANADA This research and development facility for hydrogen fuel cell technology employs a creative combination of concrete tilt-up wall panels, exposed structural steel and glazing to achieve the high-tech look required by the client. The building has a ground 2 floor area of some 7600 m and incorporates a 2 3800 m suspended first floor. There is a total of 75 tilt-up panels that support the roof and floor loads, and serve as shear walls for both wind and seismic loads.

area. Each wall panel is the region of 14 m high by 15 m wide and, in order to achieve a finished surface on each side of the wall, it is constructed from two 190 mm thick panels separated by a 25 mm gap. Connections between the leaves were possible only at areas hidden by the suspended ceiling space or below floor slab levels. The connections were achieved by inserting steel studs through drilled holes and epoxy grouting into place. The exposed edges of the walls are finished with a 420 mm wide steel plate strip to complete the concrete and steel high-tech image.

Free-standing tilt-up panels dominate the entrance Location - Burnley, British Columbia 2

2

Total floor area - 7600 m ground floor, 3800 m first floor Contract period - Unknown Typical panel size - Up to 14 m high by 15 m wide Typical panel weight - Up to 96 tonnes per leaf External finish – Fair-faced concrete Thermal insulation - Unknown

Ballard Power Station, British Columbia, Canada

2-26


3

PLANNING FOR TILT-UP

This chapter examines the planning process vital to the effective design and construction of tilt-up structures. It then discusses the need for effective communications between the design and construction functions. This is followed by consideration of the economics of construction. Casting layout and erection sequence, cranage, panel sizes and tolerances, and types of panel joints are then presented. Finally, following prediction of strength, typical programmes and speed of construction are examined using real case studies.

THE DESIGN AND CONSTRUCTION TEAM

resulting in reduced risk of conflict and in increased efficiency.

The simplicity of tilt-up results from thorough planning for construction during design. Planning for tilt-up requires the involvement of every member of the design/construction team if tilt-up’s advantages and versatility are to be fully exploited and its cost and speed benefits are to be maximised. Co-operation should begin at the planning stage and continue through to the completion of the project.

Using a franchised or licensed tilt-up contractor belonging to one of several schemes run from the United States and operating abroad may attain further benefits. Finally it should be noted that a number of individuals and suppliers are able to offer tilt-up planning and consultancy in the UK through experience gained both here and abroad (see Chapter 13).

Typically, the team comprises the architect, building and panel designers, contractor, specialist subcontractor, lifting contractor and lifting accessories supplier. The lifting operation is vital to any successful job and the lifting contractor should be consulted as early as possible in the planning process.

OPTIMISING TILT-UP BENEFITS The most suitable configuration for tilt-up is a large low-rise modest building with few openings, allowing a near production-line process of forming and erecting almost identical panels. Buildings such as warehouses, distribution centres and some industrial and retail buildings can come close to this ideal. However, many other forms are also suitable and the following criteria may be used to help identify them.

It is important that each member of the team is aware of the constraints of the tilt-up method and of the broad implications of any planning decision. Compromise will often be necessary; the participation of all members of the team in the decision-making process is therefore required if the best solution is to be found, particularly for the casting and erection sequences.

Wall to floor ratio - Ideally this should not exceed 70 to 80% to allow walls to be cast individually on the floor, with space for the crane to operate. However, stack-casting of some panels or the use of adjacent temporary casting beds is not uncommon, permitting a higher ratio, or even removing the need to use the ground slab. When the slab is used for the construction of the panels it must be designed for both in-service loads and for any cranage loads during erection.

Circumstances such as project location or tendering procedures may dictate that the full team is not known at the design stage. In such cases, the designer should make every effort to compensate for missing input by soliciting advice from specialists with local knowledge. These could include equipment suppliers, crane operators and specialist sub-contractors.

Configuration – Construction efficiency will be maximised by a building largely composed of walls that permit a large number of similar sized wall elements, but more variable building forms are also viable.

Changes made during construction must be very carefully considered, since many decisions depend on or affect other operations. As with other forms of construction, reversing one decision may start a chain reaction that could necessitate the reconsideration of all subsequent decisions.

Panel size - A typical site-cast panel for a low-rise building can be around 7.5 to 9.0 m high and 7.5 m wide and weigh between 25 to 35 tonnes. However, between 15 and 25 tonnes is a more typical weight for a tilt-up panel in order to make optimum use of cranage. Economic construction of buildings of three or more stories is not uncommon. However, these may require a more sophisticated arrangement of lifting rigging than for panels under around 9 m in

The above points reveal that tilt-up is ideally suited to design and build contracts. However, other forms of procurement are successfully employed. In all cases it is important that the ground slab (and ideally the foundations) are constructed by the same contractor as the tilt-up panels to avoid problems with finish and tolerance. In this way, tilt-up reduces the number of trade packages and interfaces,

3-1

Planning for tilt-up

Alternatively, one day’s crane usage, with 15 to 30 panels at 5 m to 7 m width, gives a sensible minimum enclosed floor area of between 700 and 1400 m².

with pad foundations, when used. A decision must be taken early in the design process, taking into account the above factors, and optimising the crane size and number of crane set-up positions. Some buildings, otherwise suitable for tilt-up, may have a wall to floor area greater than the economic maximum of 70 to 80%. There are several methods for overcoming this:

Confined sites - Contrary to common belief, tilt-up can prove ideal for confined sites where access around the building is limited. An excellent example of this is the basement structure in Chapter 2 (page 2-18) where careful planning and stack-casting allowed all panels to be cast and erected from the slab with only a metre or so of working space beyond this.

•

Stack-casting up to six panels deep releases space for crane access. The upper faces of box-outs for openings are often set low to contain a thin sacrificial layer of concrete. This facilitates float finishing of the whole surface giving a good finish for forming the next layer. Careful planning and execution is necessary to maintain tolerances and finishes.

•

Temporary casting beds, typically of 75 mm concrete, can be used outside the building footprint. After the construction of the panels they may be broken-out, buried, or incorporated in permanent works.

•

Multi-stage casting and lifting allows the floor to be used several times.

•

External crane positioning may release sufficient floor space for casting.

LAYOUT OF PANELS AND ERECTION SEQUENCE To optimise crane capacity and usage, wall panels are usually cast face down on the floor slab, as close as possible to their final erected position. Their tops will generally be close to the perimeter slab make-up strip with sides touching neighbouring panels. This allows the crane driver to have full sight of the lifting rigging whilst proceeding down the line of panels. It is essential, therefore, that the layout and casting order should be planned around the proposed erection procedure with access for concreting and finishing in a production-line process. Occasionally the lifting position is different, resulting in ‘reverse pick’ or ‘blind’ lifting and increased crane capacity for the longer reach required, eg. from a set-up outside the slab.

s d i u R a m 1 2 1 1 1 0 9 8 7

Panels are normally cast face-down to allow easy lifting, with inserts hidden from sight on the inside face. This permits easy formation of an architectural finish on the outer face, using form-liners or feature strips placed on the floor slab.

Max. panel weight 9.5 tonne 11.4 13.5 16.5 20.0 22.6 5.4 m

m 7 . 3

Casting position of slabs Figure 3.1 shows a typical casting layout of wall panels before erection on a building with sufficient floor area to accommodate panels without stack casting. Panel location is best decided by trial and error using a physical model of the building and the wall panels. This might take the form of a plan of the ground floor slab, including movement joints, column box-outs, perimeter make-up strip and any other features affecting the slab surface forming the casting bed. Wall panels can be formed to scale in card or even thin plywood and marked up with all features and inserts relevant to the casting and erection procedure. A plan of the crane on transparent film, with its outriggers and capacity at varying radii (Figure 3.2) completes the model.

Figure 3.2 Crane capacities at different lifting radii

Several of these options may be combined, but all must be considered carefully as they can affect speed, economics (especially crane time and capacity), and finish quality.

Panel sizes and weight, crane capacity and reach, wall and slab configuration, and wall features such as openings, are all inter-related. Panel size is often determined by the availability and cost of cranage. Thickness and height are the next most important parameters, which affect both in-service and lifting design as well as cranage. Panel width should also co-ordinate with doors and architectural features and

To ensure efficient operation and to minimise errors, drawings should clearly present all the necessary information in a form to suit site operations. The panel layout (Figure 3.1) provides a unique numbering system reflecting the erection sequence,

3-3


6.14 m 1.30 0.76

1.54

1.54

0.76

1.54

3.54

1.30

Joist seating embedments

1.59

0.2 0.19 panel thickness

1.85

0.1

0.18 panel thickness

1.23

1.84

1.84

2.46

Bracing inserts

0.1

2.31

Lifting inserts

10.36

3.54

3.14 0.3

6.90

12 x 90 mm reveal at door head

7.28

1.0 2.92 1.23

12 mm coil inserts for reinforcement connection to slab

External elevation

Internal elevation

Figure 3.3 Drawing showing opening, thickness and position of inserts and fixings

4413

660

1314

2642

2920 4258

2920 2260

9093

9500

6452 6045 3404

305

3099

2642

Figure 3.4 Typical panel setting-out information

and shows both casting and final positions of the walls in relation to the slab and foundations.

Good dimensional control (Figure 3.4) is best achieved using a setting-out drawing with both panel diagonals and running chainages, making minor variations self-compensating and providing checks for squareness.

Each panel will have its own drawing (Figure 3.3). To suit face-down construction these also show the view from the inside, with details of all openings, features, fittings and inserts to allow construction of the panel. Reinforcement may be shown on a separate sheet or view.

Cranage and crane movements Ideally, planning should involve the crane contractor and should mock-up all operations including

3-4


bracing. A key objective is to optimise crane hook time and set-up time with lifting capacity. To illustrate this point the reader should imagine the crane locations and reach necessary to erect and brace the panels illustrated in Figure 3.1, whilst minimising the crane capacity.

2

1

During planning it must be decided whether panels will be contiguous, share side forms, or be spaced apart. Having chosen an erection sequence and panel numbering to suit, panel casting positions will loosely reflect erection, with adjustment to suit corners and bracing (Figures 3.5 and 3.6). Provision must be made for crane access and exit. The last panel is often set vertically in a temporary position next to the exit whilst the crane moves outside before completing the envelope.

3 4

5

Bracing affects casting location (particularly at corners) see alternative solutions below

1

2

3

6 4

Where floor space dictates that stack-casting is necessary, the order (Figure 3.7) and number of panels per stack should reflect the maximum that can be handled by the crane without relocating, and the constraints of placing and finishing the concrete to tolerance.

1

2 3

5 6

6

4 5 3

(a) All panels cast on floor slab

1

2

3

1

2

3

Corner panel can oversail

4 1

2

1

2

5 6+5 +4+3

6

6

4

4

5

5

6 Panels cast on external slab

Stack-cast panels

(b) Panels cast on floor and stack-cast on adjacent panels

(c) Panels cast on floor and on external casting slab

Figure 3.6 Influence of bracing on casting layout

The rating of a crane is the maximum load that can be lifted at its minimum radius. The radius is measured from the centre of rotation of the crane. The greater the radius, the lesser the load. For example a crane rated at 40 tonnes will carry 40 tonnes at its shortest reach, but at 6 metres radius will lift only about 18 tonnes (Figure 3.8). For this size crane, the lowest operational radius is around 6 metres but this will increase for larger cranes.

Erecting towards opening permits absorption of tolerances

Many factors come into the selection of crane size, and this should have been determined at the planning stage along with panel sizes and casting layout. The crane operator should be involved at this early stage, and should visit the site before the day of lifting to inspect access, restrictions and ground conditions below the crane and outrigger positions.

Opening

When assessing panel working radius, 1.5 metres should be added to the final panel position to allow for the tilt of the panel when on the hook. Also when evaluating crane capacity the weight of rigging gear

Figure 3.5 Influence of corner detail on casting layout and erection sequence

3-5


and any strongbacks plus an allowance for suction (see Chapter 8, ‘The lifting sequence’, item 3) need to be added to the weight of the panel.

and move between lifts. Also, a large crane will generally not be able to get as close to a panel, and rigging of large panels will be more complicated. Certainly a larger crane required for only a few larger panels in a contract is an uneconomical solution. Whatever size crane is used, a check is necessary to ensure it can get onto site and can manoeuvre into all the set-up positions required.

The use of a larger crane with fewer panels of greater size will not always be economical. The additional crane costs need to be balanced against the reduced casting costs. A larger crane will take longer to set up

High point loads will be imposed on a slab from the outriggers of a mobile crane. This load should be spread over the slab by using timber bearers to keep bearing stress to a reasonable level (The rule of thumb sometimes used in the USA is 10 t/m² for a 125 mm thick slab.) Crawler-mounted cranes impose lower bearing stresses on the ground and can be useful when erection from outside the building is possible.

Direction of placing

1 2 3 4 Crane position 1

5 6

The lifting limitations (height, reach and load capacity) of the chosen crane should be carefully examined. As a rough rule of thumb, crane capacity should be two or three times the maximum panel weight, rising to as much as ten times where external casting beds are used. Dismantling, moving and setting up in a fresh location takes considerable time and is completely unproductive. Therefore, the more panels that a crane can erect from a given position the more efficient the operation. It may sometimes be necessary to move the crane on cast walls still to be lifted, and this can result in tyre marks that are difficult to remove. If this is critical, the running surface should be protected with paper, hardboard or tarpaulins. (Note that some tilt-up advocates will not contemplate heavy plant running on wall panels.)

7 8

1 2 3 4

Crane position 1

5 6 7 8

Stack-casting sequence

Stack-casting sequence

Figure 3.7 Stack-casting sequence

Radius for erected panel True radius whilst placing panel Minimum of 1.5 m extra to be allowed when assessing project

Figure 3.8 Practical crane working radius

3-6


(a) Examples of use of strongbacks

Blocked-out for panel thickness and height of opening

Bolt with plate-washer Steel channel

Insert in panel

(b) Detail of heavy-duty steel strongback

Figure 3.9 Example of use of strongbacks

The availability of all rigging, lifting beams, shackles, etc. should be confirmed from specialist hire companies or tilt-up specialists. With modern quick-release inserts it is common to use only one set of rigging/lifting gear and there seems to be no great speed advantage in using multiple sets.

bottom tie is best used to close the opening in the buried portion of wall (Figure 3.30 Separate spandrel/lintel panels are time consuming to set on columns, tying up the crane. It is worth considering combining these panels with their supports despite the need for more complex reinforcement or even thickening the inner face. L-shaped panels with narrow legs are best avoided as they may require strongbacks for strength at lifting (Figure 3.9). Where used they should be stable laterally.

Rate of erection will vary with the size of the panels, layout, complexity of bracing, etc. As a guide, competent contractors aim to erect one panel every half hour and frequently achieve a cycle time as low as 15 to 20 minutes.

Weight - 25 to 35 tonnes per panel is a good working weight, requiring a crane capacity in the region of 80 or 100 tonnes for a working radius of around 8 metres (see previous section). Up to 55 tonnes is feasible, but will involve more complex rigging and the penalty of a larger crane and longer set-up times. However, placing a larger panel usually takes no more crane time than placing a smaller one.

PANEL SIZES AND TOLERANCES Sizing and shape The preceding sections have illustrated how panel size and weight are inter-related with layout and erection. The following points are worth considering when sizing panels:

Thickness - Typical ratios of panel thickness to height (slenderness ratios) between effective points of support in service vary between 1 to 30 and 1 to 50 or even 60 occasionally. The panel must resist the

Shape - Rectangular panels are most economical and where openings, such as doors, start at floor level, a

3-7


Correct panel start position

Variation absorbed in reduced joint

Designed joint width and position

Designed joint width and position

Variation absorbed in enlarged joint

Each panel starts on designed position

OPTION 1 (Not preferred)

Designed joint width maintained

All variations taken up at this point

Designed joint width maintained

OPTION 2 (Preferred) Oversail corner or opening

Figure 3.10 Absorption of tolerances at joints

stresses not only in service but also at lifting, which is often the most critical case (see Chapter 5 for panel in-service design and Chapter 6 for lifting design). Too thin a section will require complex lifting arrangements, which may make tilt-up uneconomic. Too thick will make panels overly heavy and produce cost penalties in cranage and foundations. For planning purposes, a thickness equal to the panel’s effective height divided by 50 is often used. Other factors to consider are cover to double-layer or, more normally, single-layer reinforcement, and the size and location of any rebates that reduce the effective section size.

Tolerances It is of the utmost importance that the specified panel and joint tolerances are realistic. Once established they must be maintained. In general, variations in size have a tendency to increase overall wall length. Depending on their size, joint details may be used to absorb these variations either progressively at each joint or collectively at one location, eg. at an oversail corner or doorway (Figure 3.10). If tilt-up panels are being used in conjunction with in-situ construction, then the tolerances for tilt-up panels should not be used to absorb the construction errors of the in-situ (1) work .

Rebates - Rebates are used architecturally to break up panels, to hide joints, to demarcate areas for painting and texturing, or for other aesthetic devices. They are a powerful way to modify the look of wall panels but effectively reduce the section thickness available for structural use. Where possible, it is best to avoid horizontal rebates within the centre third of a panel’s effective length for slenderness so as to prevent the need for increased section thickness or reinforcement to compensate.

Construction tolerances - There are no British Standards that deal specifically with tilt-up construction, although BS 5606 (2) provides guidance on tolerances and deviations for both precast concrete and in-situ concrete which might be used as a guide to assess suitable tolerances for tilt-up panels. Alternatively it would perhaps be better to adopt the tolerances used in those countries where tilt-up is more prevalent. The Australian Standard AS 3850.2(3) 1990 gives the tolerances shown in Table 3.1. Those recommended by the Tilt-up Concrete Association (4) are shown in Table 3.3. If panels are carefully formed, their foundation pads checked thoroughly, and all elements properly checked, it is straightforward to attain these tolerances.

Rigging - Rigging arrangements should be kept as similar as possible to avoid time lost in changing rigging, even where this means fittings are underused structurally. Width - This should be decided by considering foremost building geometry and roof truss spacing and then weight, wall height, lifting fittings, and architectural requirements. A width of 7.0 m is not unusual for wall panels and up to 12.0 m for spandrel/lintel panels.

Joint width between panels (tolerance) - The design joint width between two panels will depend upon the panels’ width, the specified joint tolerance and on the shape and strain capacity of the sealant to be used.

3-8


Table 3.1 Recommended tolerances (3) Panel size(m)

Tolerances (mm) 1

Squareness 2

Edge straightness 3

Thickness 3

Width

Height

Planeness

< 3.0

+0, -5

±5

±5

±5

±5

±10

> 3.0 < 6.0

+0, 10

±10

±5

±15

±7

±10

> 6.0

+0, 12

±10

±5

±15

±10

±10

1. Deviation of any point on the face from the intended line. 2. Measured as tolerance in length of diagonal. 3. Provided that in any 3 m, the deviation from the intended line does not exceed 5 mm.

Table 3.2 Panel and erection tolerances Type of tolerance

Casting

(4)

Item and details

Up to 6 m

Height and width of basic panel

6 m to 9 m Each additional 1m

Thickness

1

Skew of panel 2 or opening

Per 1.8 m

Openings cast into panel

Size of openings

Location/placement of embedded items

Inserts, bolts, pipe sleeves

Maximum difference

Location of opening

Lifting and bracing inserts Weld plate embedments (lateral bracing) Weld plate embedments (tipping and flushness)

Erection

Joint width variation

3

Panels up to 6 m tall Each additional 3 m height

Joint taper 4

Maximum for entire length Panels up to 6 m tall Each additional 3 m height

Panel alignment

Alignment of horizontal and vertical joints Offset in exterior face of adjacent panels

1. The average variation of panel thickness through any horizontal or vertical cross-section of the panel. 2. Measured difference in length of the two diagonals. 3. Measured between panels at the exterior face of the panels at the joint. 4. The measured differences in joint width indicating panel edges are not parallel.

3-9

Tolerance (mm)

±6 ±8 ±3 ±5 ±3 ±12 ±6 ±6 ±10 ±12 ±25 ±6 ±6 ±3 ±10 ±6 ±3 ±6 ±6


This width may be in the order of 12 to 15 mm for a narrow panel (3 m wide) with a high transverse movement sealant or 25 mm or greater for larger panels (6 to 7 m wide) with a less accommodating sealant. Joint tolerances are important for the performance of the joint sealants (most of which have movement capabilities of around 25%) and are critical for weatherproofing.

able to accommodate both movement in-service and tolerances in panels and erection. Alternatively, a one-stage joint can be formed using a preformed gasket (Figure 3.11(b)). The multiple-stage joint - (Figure 3.11(c)) one-stage joint and should be used in severe climate conditions, as may occur in some parts of the UK. Because this joint makes use of a minimum of two lines of sealant, a minimum panel thickness of 100 mm is recommended. The interior line of sealant is usually applied from the inside surface of the wall and acts as the continuous air seal between the interior and exterior. The exterior sealant acts as the rain barrier and prevents direct entry of most airborne water. A third inner sealant is sometimes used on panels with a permeable external layer. Any water that does enter the joint is drained in the airspace and out through drainage holes at the bottom of vertical joints.

Information on the application of joint sealants is given in Reference 5. Maintenance of the designed joint width as shown on Figure 3.10 is the preferred (3) option , with dimension variations taken out at doorways and/or oversail corners. Also the joints must allow the panels to move relative to each other as the temperature or humidity changes.

TYPES OF PANEL JOINTS The joint detailing is very important when considering the cost, appearance and performance of a tilt-up building. The detail must be compatible with the:

• • • • •

Special care should also be taken when designing and detailing interfaces between different building materials such as window-frames, door-frames, roofing and flashing. To ensure satisfactory performance, details must account for differential movement between materials caused by temperature changes or structural loading. If necessary, special grooves, dovetail slots, and embedded items can be cast into the concrete to attach window frame assemblies or roof flashing.

Structural design assumptions Forming and placing methods Erection procedure Fixing detail Construction tolerances

Proper detailing of vertical and horizontal joints between panels is important because this is where the wall is most susceptible to rain penetration. There are three basic types of weather-resistant joints used for tilt-up panels: the one-stage sealant joint, the multiple-stage sealant joint and the dry-baffle joint. In each case the top of the panels is normally finished with a capping to prevent rain entering the joint at the head of the panel.

The dry baffle joint - (Figure 3.11 (d)) by a continuous sheet of elastomeric material slotted between rebates in faces of vertical joints. Although it requires a more complex side form to panels, the joint has proved effective in some precast panels in the UK with no maintenance over a 20 year span, although it would be prudent to provide access at the top for replacement. The difficulty of producing such an edge profile on site should be considered carefully.

The one-stage joint - This is economical and the most common joint used in North America (Figure 3.11 (a)). It performs satisfactorily in most climates. Typically, a foam backer rod is placed in the joint from the exterior and a field-moulded joint sealant is then installed. Because this joint provides only a single line of defence against weather, and is exposed to the deteriorating effects of weather and ultraviolet light, it requires the following:

• • • •

A variation to joint details shown in Figure 3.11 (a) to (d) is where the gasket or sealant is substituted by a precompressed impregnated sealing tape. The resulting seal is achieved by a precompressed foam which is less sensitive to joint construction tolerances, widths and movements. Chapter 9 Weather resistance of panels and joints - considers joints in greater detail.

A good overall wall design Proper site installation

PREDICTION OF STRENGTH DEVELOPMENT

High quality materials Regular maintenance

The sealant must provide a completely airtight and water-tight seal. Poor adhesion of the sealant may allow water penetration. The design of the seal for the joint is complex and involves the consideration of a number of factors, eg. expected movement, width of the joint, type of sealant, and width-to-depth ratio of the sealant. In general, for a given width-to-depth ratio, wider joints are preferable as they are better

3-10

The speed of construction of tilt-up is affected by the time taken for the cast panels to reach sufficient flexural tensile strength to resist the lifting stresses (this is dealt with in more detail in Chapter 6). Typically this will be achieved in between two to seven days, depending on weather and concrete grade used. Insulating blankets can be used to speed up the curing process if critical. The prediction of the strength development of the concrete can be obtained from Reference 6. Although intended primarily for


Continuous sealant and backing

Neoprene cruciform gasket

INTERIOR

INTERIOR

Flashing

Flashing

(a) One-stage face-sealed joint

(b) One-stage gasket joint

Rain drainage zone Continuous sealant and backing

Baffle strip Continuous sealant and backing strip

INTERIOR

INTERIOR

15 - 25 mm

Flashing

Flashing

(d) Dry baffle joint

(c) Multiple-stage sealed joint (shown two stage)

Figure 3.11 Basic types of weather resisting joints used on tilt-up panels used on tilt-up panels

Figures 3.12 and 3.13 show two construction programmes for buildings in the USA, and Figure 3.14 shows one for a project built in the UK. Note the overlap of main activities in Figure 3.13 and that tiltup panels are mostly completed during the roof steel fabrication period. In considering the three sites, some differences in approach are evident, but by combining these with experience of other sites the following guidelines are obtained:

concrete in suspended formwork, predictions should be generally applicable for slabs cast on the ground.

PROGRAMME AND SPEED OF CONSTRUCTION Speed of construction is a key benefit of tilt-up construction. To make best advantage of this, the design and construction team need to be aware of all the major activities, their precedence and the scope for overlaps and parallel processing. In the United States, contractors specialising in tilt-up are able to strip the site, form foundations and slab, and cast and erect the tilt-up walls to a 9,300 m² warehouse in only five to six weeks.

3-11

•

Tilt-up allows a rapid start on site, due to the short lead-time for reinforced concrete cast on the ground.

•

Because wall panels are formed on the ground floor slab, for fast construction the earliest possible start on the slab is required, consistent with achieving a casting quality finish.


APRIL 1996 1

8

15

MAY 22

Design coordination with

02 Apr

29

6

JUNE 13

20

27

3

JULY 10

17

Grading 30 Apr

24

1

8

15

22

29

AUGUST 5 12

Ground slab

Erect steel

18 Jun

30 July

19

26

SEPTEMBER 2 9 16

23

30

Snagging 24 Sept

Design development

Footings and foundations

Lay out tilt-up panels

Building M & E

09 Apr

21 May

02 Jul

20 Aug Project completed 01 Oct

Steel fabrication 22 May Permit review

Pour tilt-up concrete

07 May

09 Jul Roofing

Erect tilt-up panels

27 Aug

Structural steel shop

External works

07 May

04 June Exterior doors and overhead

2

Figure 3.12 Construction programme for a 19,000 m warehouse in the USA

WEEKS

ACTIVITY

3

2

5

4

6

7

8

Site layout Substructure and slab Ground slab cured Cast tilt-up panels

Procure reinforcement

Crane on site Tilt and brace panels Erection of steelwork

Allocate steel subcontract

Steel fabricated off site

Roof surfacing Caulk joints/remove bracing Internal finishes External works Hand-over to client 2

Figure 3.13 Construction programme for a 11,000 m warehouse in the USA

ACTIVITY

WEEKS 2

4

6

8

10

12

Strip site/cast foundations Cast floor bays Cast tilt-up panels Tilt panels and brace Services Steel frame / roofing Joinery / windows Wall / floor finishes Earth berms / landscaping Panitwork Cleaning / hand-over

Figure 3.14 Construction programme for Glenrothes 2, Scotland (3743 m 2)

3-12

14

16

18

20

22


•

• •

•

•

•

•

Panel preparation may start as little as 24 to 48 hours after sufficient slab is cast. Typical average rates for forming and casting vary between three and 15 panels/day, increasing with size of building. Sandwich panels will add only one to two days to the total panel casting period.

Slab construction and roof erection followed on in a phased manner. Erection of panels to the workroom proceeded the rest of the walls to allow early fit-out.

Typical average panel erection rates vary between five and 14 panels/day, but the fastest rates can apply equally to smaller buildings.

Table 3.3 Project data for mail sorting buildings

Further information on tilt-up projects in America is given Reference 7.

Item

For a large site, tilting-up of panels may be scheduled to take several weeks and the first panels may start to be erected before the final panels are cast, provided that the panels have gained sufficient strength at lifting. This allows an early start on the roof steel, but increases crane time. For fast construction, roof steelwork fabrication may well be a critical activity. However, with tiltup, erection of the walls is nearly complete before roof steel is required (unlike with a traditional portal frame), so allowing a shorter programme. Roof steel erection is made easier by the firm working platform of the floor slab and may be started as soon as there are sufficient load-bearing wall panels in place. Roofing can follow on close behind.

Building 1

Building 2

Total building area (m2)

19,881

23,550

Construction start date

Jan 1992

Feb 1992

Substantial completion date

May 1992

Sept. 1992

Total number of tilt-up wall panels

195

156

Average/maximum number of panels cast daily

15/19

12/17

Average/maximum number of panels erected daily

12/16

14/18

REFERENCES 1. Cement and Concrete Association of New Zealand. Tilt-up technical manual. C&CA, Porirua, New Zealand. TM 34, 1990. 32 pp.

As a mature floor slab already exists before roofing is completed, M&E and other fit-out activities, such as racking, can start early and proceed unhindered. A fast finish is achievable.

2. British Standards Institution. BS 5606 , Guide to accuracy in building. BSI, Milton Keynes, 1990. 56 pp.

In general, the larger the building, the greater the scope for overlapping sequential activities to speed construction.

3. Standards Australia. AS 3850.2, Tilt-up concrete and precast concrete elements for use in buildings, Part 2: Guide to design, casting and erection of tilt-up panels. Standards Australia, North Sydney, 1990. 24 pp.

Case study The following case study outlines details of two quite complex buildings in the USA, including reasons for the choice of tilt-up.

4. Tilt-up Concrete Association. Tolerances for tilt-up panels. TCA Newsletter, USA, Vol. 3, No. 4, December 1995. pp 1-2.

Table 3.3 provides details of two large low-rise mail sorting buildings requiring reception, dispatch areas and a workroom for sorting, with special fire rating and separation requirements. The roof is metal deck on a truss and joist system. The application of value engineering resulted in the choice of tilt-up over concrete masonry, steel frame and metal cladding, and factory precast options. Criteria considered for this fast-track project included winter working, fire, risk of programme slippage, security, ease of modification, architectural scope and cost.

5. CIRIA. Manual of good practice in sealant application. CIRIA, London. Special publication 80, 1991. 58 pp. 6. Harrison, T, A. Formwork striking times criteria, prediction and methods of assessment . CIRIA, London, 1995. Report 136. 71 pp. 7. Tilt-up Concrete Association. Video: Tilt-up concrete construction. Published in the UK by the Reinforced Concrete Council, Crowthorne, 1996.

Tilt-up was the cheapest alternative and the fastest by approximately six weeks. Fast construction was achieved by careful co-ordination of the slab castings, and the casting and erection schedules for tilt-up panels. The crew for tilt-up panels was able to start on preliminary work for panels within 24 to 48 hours of slab casting. As soon as a further slab was cast the process continued.

3-13

3-14


4

FLOOR SLABS AND FOUNDATIONS

This chapter presents those aspects of the design and construction of floor slabs and foundations particularly relevant to tilt-up. For floor slabs, the inter-related issues of construction method, tolerance and finish are examined, followed by weather, curing, compacting and finishing. Then design loading, design procedure and the construction programme are discussed. Finally, the chapter covers foundation design and construction issues, including details, column footings, panel seating and construction. developed to predict the effects on the performance of pallet handling equipment rather than on the visual quality of the finish or the dimensional accuracy of components cast in contact with floor slabs. However, inspection of the recommended tolerances in Chapter 3 suggests that tolerances suitable for tiltup may be achievable with several of the floor classifications given in Reference 1, depending on the relative casting position of adjacent panels on the slab and the attention paid to the particular requirements of a casting-bed.

FLOOR SLAB DESIGN AND CONSTRUCTION The detailed design and construction of groundsupported concrete floor slabs are specialised activities beyond the scope of this publication. The sector has undergone major efficiency changes over the last few years with trends towards capital intensive mechanisation, fewer joints in the slab, and the use of steel fibre reinforcement. However, recent (1) publications by the Concrete Society and the (2) Institution of Civil Engineers are standard works giving detailed guidance on the subject. The scope of this Chapter is therefore confined to specific items related to the special requirements of tilt-up construction on the ground floor slab

(3)

Brookes cites a US tolerance of 6 mm in 3 m, which loosely translates to FM2 in Reference 1. UK industrial floor contractors may consider FM3 more realistic for forming slabs in the open. Experienced US and Australian contractors producing high quality tilt-up buildings report that modern slipform pavers and laser levellers can produce a floor slab with consistently high quality suitable for tilt-up. Finishing is normally by power float, possibly with hand finishing at edges (4). It is important that the slab surface finish is free of visible float marks or other blemishes likely to affect the chosen panel finish.

Construction method The intended or predicted final use of the floor will give in-service tolerances and joint requirements suggesting the likely method of construction. This then needs to be examined for any additional requirements for forming the panels. Reference 1 outlines a variety of UK floor slab construction methods generally characterised by increasing bay width and hence speed and economy of construction, balanced by decreasing constructional accuracy in terms of surface flatness.

When openings must be left in the floor for pipes, utilities, or the erection of interior columns or walls at a later date, a 20 to 40 mm coat of concrete over a sand fill can be used to close the opening temporarily. The concrete can be knocked out after the panels have been tilted. An alternative system is to form up the opening using form-ply or polystyrene and place a 20 mm coat of concrete over the formed surface.

However, experience abroad is that it generally requires very little extra effort to ensure that the normal floor finish will form a suitable casting surface for tilt-up. But bay width, joint spacing and flatness can influence the visual quality of the finished tilt-up panels, and co-ordination of the tilt-up panel dimensions to avoid floor joints may influence the chosen method of floor slab construction. (See Reference 1 for typical floor construction details, bay sizes, surface flatness, etc.) Bay widths and floor joint spacing will normally co-ordinate with column grid lines. Where co-ordination of panels with slab joints is not possible, the joint may be flush filled with, for example, a silicone sealant, or hidden by a feature rebate designed into the panels.

Formwork to provide recessed areas in the panel face should be robust enough to remain plane under the application of concrete and associated construction loading. Since the panel will reflect imperfections in the casting surface, any pre-located floor bracing points or floor joints in the casting area will need to be masked out before the panels are cast. Suitable materials for patching and joint filling include silicone sealants and hot wax.

Tolerances and finish (casting surface)

Weather conditions

It is important to note that the commonly used measurements of flatness in Reference 1 have been

Both the floor slab and the tilt-up panels are normally cast in the open, and weather conditions such as

4-1


wind, rain and heat must be taken into account. Tiltup is popular in the USA, Canada and New Zealand where all weather conditions met in the UK are routinely catered for, especially in New Zealand’s climate which is very similar to our own. Whilst respectful of the weather, both designers and contractors abroad do not consider it to be a (4) significant problem .

of 6 mm or less) and any bleed-water has dissipated.

In addition, detailed, localised weather reports are readily available in the UK, allowing contractors to plan concreting activities around extremes and take sensible precautions. Generally, apart from normal good practice under usual weather conditions (see Reference 5), only two conditions require extra care when not constructing under cover: Hot windy weather - Ensure that the fresh concrete does not dry out during finishing and apply the curing membrane as soon as possible. ‘Fog’ spraying is successfully used in the USA.

Curing, compacting and finishing

Start using the vibrating screed as soon as possible, ensuring that the ‘fat’ does not creep under it where it rests on the form, thus increasing the thickness of the floor.

•

Magnesium floats are best to for bringing up the ‘fat’ prior to a steel float finish.

•

Use the bull float in both directions for the best finish and use it before the bleed water starts to rise.

•

Do not attempt to power float until the concrete is hard enough to walk on (footprint indentations

When hand trowelling at edges, use a straight edge to ensure that the concrete is flat, as it is easy at this stage to hump it at the edges.

•

Do not use a steel trowel too early; this will slow the drying, as it seals the surface of the concrete.

•

Where joints are sawn remove slurry before it can dry.

•

Bending stresses due to applied loads. Bending due to differential horizontal movements due to moisture and thermal gradients through the slab. Tension due to moisture and thermal contraction being restrained by sub-base friction.

These aspects are covered fully in Reference 1, but for tilt-up, the key difference for slab design is the loading imposed on it during construction. Table 4.1gives the typical tilt-up constructional loads which may be compared with maximum in-service loads for a typical warehouse. Whilst the loads are not all directly comparable due to the differences in loaded area, it can be seen that crane loads for large tilt-up panels may govern slab design. In many cases, however, timber bearers can be used to reduce construction loads to that for which the main floor slab is designed in-service. It is essential that the maturity and hence strength of the slab is taken into account when considering constructional loadings.

A few general tips on compacting and finishing are given below.

•

•

• •

It is essential to review and confirm the compatibility and suitability of chemicals for curing and bondbreakers, which must also take account of any requirement for subsequent paint finishes to panels.

Use a poker vibrator at the sides and ends of floor.

The first float pass should be at right angles to screeding ridges and subsequent ones at right angles to the previous pass.

Slabs on ground are subject to stresses arising from (1) three sources

The normal way to ensure good curing is to apply a special spray curing agent immediately after float finishing. This will also act as a bond-breaker between slab and panel (see Chapter 7). General guidance on curing of concrete is given in Reference 5.

•

•

Design loadings

Ground floor slabs and tilt-up panels require careful attention to curing. This ensures high quality concrete, without drying shrinkage cracks, which performs well as a casting bed for panels formed on top. Additionally, panels rely on developing good tensile strength of concrete at lifting. This requires strong crack-free concrete and hence good curing.

Do not wait until all the concrete is placed before starting vibration.

The power float operator should ensure that flatsoled footwear is used as treads will cause indentations that are difficult to remove. Ride-on operators should avoid sharp turns which reduce flatness.

In the USA, there is a move towards pan floats on power trowels, with reports of better tolerances and increased productivity. (Reference 6 gives detailed guidance on the use of pan floats and on achieving a high degree of flatness.)

Heavy rain - Avoid concreting in standing water and protect the surface from excessive water by sheeting over if necessary.

•

•

Procedure for slab design The typical approach for the design of the main slab design is as follows.

4-2

•

Fix tilt-up panel sizes and weights in accordance with Chapter 3.

•

Determine crane size and maximum outrigger/axle loading in discussion with the lifting contractor

Floor slab and foundations

Table 4.1 Typical loadings on ground slabs Period

During tilt-up construction

Loading type

Load onto slab

Concrete mixer truck

2 x 9.5 t axles

Full truck

Tilt-up crane capacity 140 tonnes

Outrigger load 60 tonnes

25 t panel @ 10 m radius

70 tonnes

28 t panel @ 16 m radius

200 tonnes Warehouse in-service operations

Notes

Forklift wheel (max)

6.6 tonnes

Mezzanine stanchion footing

6.5 tonnes

Narrow isle racking stanchion footing

Up to 30 tonnes

•

Determine the maximum in-service loading applied to the slab.

•

Choose a suitable slab thickness to suit the worst loading case, allowing for slab maturity.

•

After final co-ordination of panel casting layout and slab joint positions, design reinforcement to suit shrinkage requirements.

Dynamic wheel loads

Racking bases are not normally designed to distribute vertical load

reinforcement is also minimised. Pad footings may be justified where circumstances dictate, such as to allow buried services to pass under the panel, or where tilt-up is used for cladding a portal frame and combined footings are suitable. In poor ground, piles may be necessary to support footings.

Tilt-up floor slab experience is summarised by (3) Brookes who recommends a minimum thickness of 150 mm where the crane loads the floor slab and 125 mm where it is placed outside the slab. References 7 and 8 give guidance on the design of floors that takes into the effect of point loads such as high bay storage and cranes. Where plastic fibres are incorporated, temporary casting beds may be as thin as 50 mm. In this case a blowtorch should be used before applying the bond-breaker to burn off any protruding fibres which could increase lifting ‘suction’ forces.

Floor line

Grouted after panels erected

Continuous foundation

(a) Continuous strip foundation

Construction programme The floor slab of a tilt-up building is normally on the critical path for construction (see Chapter 3). Where speed is important, the slab should be laid as early as is compatible with economic operations and achievement of the desired quality of finish. Unlike conventional framed low-rise buildings, the slab will normally be constructed in advance of the structure unless alternative casting beds are used. However, one benefit is that this allows earlier unhindered access for fitting-out and M & E later in the programme.

Floor line

Grout setting pads

Pad foundation

(b) Isolated pad foundations

Figure 4.1 Continuous and isolated foundations

Details Foundation details will vary, depending on the degree of base fixity required for the tilt-up panel, whether the panel forms an internal or external wall, relative ground and floor levels, and the proximity of any boundaries. In general, footings will be placed symmetrically under the panel except at boundaries. Bearing friction should not be considered to carry significant horizontal forces.

FOUNDATION DESIGN AND CONSTRUCTION Foundations to tilt-up panels are generally of continuous strip form although isolated pad foundations have been used (Figure 4.1). Continuous footings simplify excavation and minimise the encroachment of excavation into the floor slab. This then reduces the size of slab make-up strip, releasing space for panel casting and reducing crane lift radii 4-3


300

Figure 4.2 suggests methods for transfer where moment fixity is not required. Figure 4.3 gives details of various forms of cantilevered panels with moment (3) (9) fixity from the USA and New Zealand . It should be noted that Figure 4.3 (c) provides restraint only against outward panel rotation. Base restraint may be required to cater for the effects of fire (see Chapter 5).

Temporary concrete topping

400 Polystyrene block cast-in to form cavity

300 600

Column footings

Piles as necessary

Column footings are generally cast before the ground floor slab. They are set with their upper surface below the underside of the slab (Figure 4.4). A diagonal box-out in the slab co-ordinates with floor slab joints and allows later fixing and concreting over the column base bolts. Where tilt-up panels are to be temporarily cast over the column box-out, a thin layer of sacrificial concrete can be placed on a sand bed and finished flush with the floor slab for breaking out later.

(a1) Cantilevered party wall Temporary concrete topping cut away and polystyrene removed. Tilt-up panel positioned and gap filled with non-shrink grout

Shims

Bolts screwed into cast-in inserts at 600 crs

(a2) Cantilevered party wall

Grouted anchor dowel

Grouted recess

Cast-in continuity strip This section of slab completed after erection of panels

Shims for initial support and leveling

Shims

This part of the foundation poured after erection of panel

50 mm nominal

Figure 4.2 Seating arrangement for simple support

Cast-in continuity strips or bolts in cast-in anchors at 600 crs

(b) Cantilevered boundary footing

Cast in continuity strip Tilt-up panel with bars projecting from base

Floor slab

Concrete pad placed each end before placing panel and main footing Shims Continuous foundation

Footing poured last

(d) Free-standing wall footing

(c) Simple boundary footing

Footing for for cantilevered cantilevered party, party, boundary boundary and free-standing walls Figure 4.3 4.3 Footing Figure and free-standing walls

4-4

Floor slab and foundations

REFERENCES

Column

1. Concrete Society. Concrete industrial ground floors. A guide to their design and construction, Concrete Society, Slough, Technical Report No. 34, 1994. 148 pp. Plus supplement to TR 34 , Specification and control of surface regularity of free movement areas. 1997. 32 pp.

Diamond pattern opening, filled with sand and topped with thin concrete to form casting surface for panel forming. Removed and filled afterwards

2. Institution of Civil Engineers. Concrete industrial ground floors, design and practice guides. ICE, London,1996. 56 pp

Floor slab

3. Brooks, H. The tilt-up design and construction manual. HBA publications, Newport Beach, USA, 1997. 229 pp.

Base plate and anchor bolts

4. Glass, J. Ph.D. Thesis. Evaluation of tilt-up construction in relation to selected UK building types. Post Graduate Research School, School of Architecture, Oxford Brooks University, 550 pp.

Foundation

Figure 4.4 Interior column footing

5. British Cement Association. Concrete on site: No.6. Curing, No. 11. Winter working. BCA, Wexham Springs (now Crowthorne), 1993.

Panel seating

6 Surprenant, B and Simonelli, B . Using pan floats. Concrete Construction, Oct 1997. pp 781787. 7. Chandler, J. W. E. Design of floors on ground . Cement and Concrete Association (now British Cement Association), Wexham Springs (now Crowthorne), 1982. Technical Report 550. 22 pp.

To speed panel erection, the tops of foundations are set slightly lower than the installed base level of the panels. This allows bearing pads of around 40 mm depth and 600 mm length to be formed in grout prior to erection. Panel setting out and identifying marks are then transferred to the pads, which generally support the ends of two adjacent panels. High impact plastic shims are then used to support the panels as steel shims can result in hard spots, causing diagonal cracking in panel ends due to shrinkage frictional (3, 9) forces . Later, the entire length of panel is grouted underneath to distribute forces evenly.

8. Chandler, J. W. E. and Neal, F.R . The design of ground-supported concrete industrial floor slabs. British Cement Association, Wexham Springs (now Crowthorne), 1988. Interim Technical Note 11. 17 pp 9. Cement & Concrete Association of New Zealand. Tilt-up technical manual. C&CA New Zealand. Porirua. 1991. TM 34. 32 pp.

Construction Placement of foundation concrete to strip footings is simple, as mixer trucks can generally place concrete directly from their chutes. The typical construction sequence of slab and footings is shown below (see also Chapter 2). 1.

Services under the slab and footings are laid and backfilled.

2.

Column footings are cast.

3.

The slab sub-base is placed and compacted.

4.

The floor slab is cast, finished and cured.

5.

Exterior footings are excavated and cast, and bearing pads are subsequently formed on them.

6.

Tilt-up panels are formed and cast on the floor slab (possibly in parallel with operation 5).

7.

Wall panels are erected and grouted under their bases.

8.

Once the roof structure is sufficiently advanced to allow removal of panel-bracing, the make-up strip of the floor slab is cast.

9.

Column bases and the boxed-out floor slab are completed.

4-5

4-6


5

DESIGN OF PANELS IN SERVICE

This chapter provides guidance on the design of tilt-up panels in service. Prior to this it considers overall structural design, including erection conditions - which are often critical and are covered more fully in Chapter 6. Typical structural form and the use of a central layer of reinforcement are examined. A method for the design of loadbearing panels is presented and then illustrated in a design example in Appendix 5A. The design principles of sandwich panels are presented for information, as this aspect of design will normally be undertaken by the sandwich tie system manufacturer free of charge. Finally, overall building stability and fire design are considered.

PRINCIPLES OF DESIGN

In-service loading

This chapter gives information on the design of tiltup panels that are commonly provided with only a single central layer of reinforcement. Tilt-up panels are used typically in tall single-storey buildings, where vertical loading is limited to roof loads plus self-weight. The panels are normally slender (height/thickness between 30 and 50) and lightly loaded.

The design loading for the erected situation will depend on the building type, how the element is used, the support and fixings used, and other conditions. In general the design loading requirements of BS 8110 (3) Part 1 will be appropriate for tilt-up construction. However, BS 8110 does not specifically cover tilt-up walls and therefore this chapter sets out more specific suggestions for design that have been developed (6) following an assessment of international practice .

Design of such highly slender panels is not specifically covered by BS 8110 but a compatible approach is suggested later in this chapter under ‘Design of loadbearing panels’. These panels are analysed for first order and second order moments, resulting from vertical and lateral loads, but typically by using a simplified procedure.

Vertical loads - By incorporating connections to either the top, face or within the panel as shown in Figure 5.1, tilt-up panels can be designed to carry roofs, intermediate floors, gantry cranage loads and (1) building services loads .

More comprehensive construction details are given in Chapter 10, Figures 10.4 to 10.8. In some situations with tilt-up the greatest vertical load can be the weight of the panel itself.

Tilt-up panels may also be used for domestic or office buildings where they support intermediate floors in addition to roof loads and self-weight. Thus they are more heavily loaded, particularly in the sections between window openings. Since the panels are laterally supported by the floors at a normal domestic/office storey height, the slenderness ratio will usually be less than 30. In this case the normal design procedures for loadbearing walls in BS 8110 will be appropriate.

Lateral (wind) loads - The wind loads should be considered as acting laterally to the plane of the panel. It is important that the walls provide a sufficient resistance to the lateral loads applied. For panels up to approximately 6 m high, loads can be taken at ground level by cantilever action alone (Figure 5.2 (a)).

However, it is more common to design the panels as propped cantilevers (Figure 5.2 (b)) or simply supported members (Figure 5.2 (c)) with the roof designed to function as a diaphragm to carry the lateral load applied on one set of panels to those at right angles (Figure 5.2 (d and e)). The latter can act as a shear wall to resist the applied load (Figure 5.2 (d)).

DESIGN LOADINGS Tilt-up wall panels are not only designed for the loading and conditions to be experienced in the final structure (in-service loading), but also for loads (1,2) during erection and when temporarily braced (construction loading). Typically, the engineer and contractor respectively will assess these aspects although the engineer may assume responsibility for both. In many cases the construction loadings are more dominant and actually govern panel design.

The connections and fixings between any interacting units and between the units and the foundation must be designed to carry the induced forces. Further construction details are shown in Chapter 10. Volumetric movements - To avoid cracking due to concrete shrinkage and thermal movements, wall panels should not be rigidly fixed together to form a (1,2) long wall . Long walls should be broken up by the introduction of movement joints and/or connections

The effects of concrete shrinkage and temperature should also be taken into account as indicated later in this chapter. The following types of loading are to be considered during the design and analysis of tilt-up panels.

5-1


Construction loading

that will permit some movement to occur (see ‘Allowance for differential movements’ later in this chapter on page 5-16).

In considering the load encountered during the construction stage, both lifting and bracing are examined. Design for lifting can be the most critical design state, and in some situations may dictate the design of the panel. It is normally based on an uncracked section using the concrete’s tensile capacity to develop flexural strength, whereas design for in-service loadings uses normal reinforced concrete concepts.

Roof truss bearing onto plate cast into panel

Lifting - The loading experienced by a panel during the lifting process is influenced by a number of

Wind load

Panel

Straight cantilever

(a) Flush roof

(a)

Propped cantilever

Simply supported

(b)

(c)

Note: Stability in case (a) is by straight cantilever action. For cases (b) and (c) see below Anchor plate cast in panel Roof designed to transfer wind load to end panels

Angle seat welded or bolted to cast-in plate

Wind load acting on side wall of building

(b) Parapet

End panels act as shear walls to resist wind loads

Note: A similar detail can be used to support a floor

(d) Principle Trusses in roof plane to transfer lateral loads to end panels (may be in top or bottom chord of roof

Intermediate floor slab

Roof trusses

Angle seat welded or bolted to cast-in plate

(c) Floor slab

(e) General arrangement

Figure 5.1 Methods of supporting vertical loads on a panel

Figure 5.2 Transverse load resistance

5-2

Design of panels in service

factors that must be allowed for in design. The main factors are:

• •

Self-weight of the panel

•

The dynamic loading which occurs when the panel is separated from the casting surface and lifted by the crane.

H

The suction between the panel and the casting surface

The effect of these forces must be considered firstly on the panel and secondly on the lifting system over a range of inclinations for tilt-up lifting operations. The flexural tensile strength of the concrete itself is used to resist the load, with reinforcement being utilised only when absolutely necessary. The stresses imposed during lifting will depend on the panel thickness and the lifting configuration. The concrete strength and panel thickness are generally chosen so that the section remains ‘uncracked’ during lifting. The design of panels for lifting is explained in more detail in Chapter 6.

W

h Area between panel and footing must be fully grouted

Although reinforcement is not generally relied upon for lifting it is provided to control shrinkage and temperature effects and to resist in-service loads. The size and thickness of typical panels ensure that early thermal cracking does not occur.

V L

Shims (not less than 300 mm from end of panel)

V= + -

In a typical tilt-up panel, such reinforcement is often placed centrally in the thickness of the panel. This is different from normal reinforced concrete design, where the reinforcement is placed near to the outside surface to carry the tensile force due to bending.

Hxh L

+

W

V Shear carried to floor through bars cast in panel or to footing through dowels

2

Figure 5.3 Lateral shear resistance mechanism

•

Bracing - Braces are attached to the panel to provide temporary support during erection. The temporary bracing loading needs to be determined so that the bracing and inserts can be checked for adequacy, thus ensuring stability of the panel. Also, the braces themselves may require support to prevent buckling, and so knee braces may be required for very tall panels. Bracing is covered in more detail in Chapter 8.

Construction: the design for erection of the element provides for the temporary forces to which the panel is subjected during lifting and bracing

It is desirable, but not essential, that the structural design of the load panel meets the requirements of BS 8110, Structural use of concrete, Parts 1 and 2. A rational method of analysis must be used to determine axial forces, bending moments and shear forces in the wall. The effect of lateral deflection should be considered, with due allowance for cracking and creep. A suitable method is described under ‘Design of loadbearing panels’ later in this chapter.

STRUCTURAL DESIGN Tilt-up concrete panels are commonly used as loadbearing walls. These can be designed to carry all vertical loads, such as roof and floor loads, along with lateral loads including wind, earthquake and earth loads. Because of their inherent in-plane stiffness, some of the tilt-up panels are normally designed to act as shear walls for the overall lateral stability of the building (Figure 5.3).

Lateral deflections, resulting from the combined effects of lateral loading and eccentric vertical loading, cause additional bending moments. This is sometimes referred to as the P-∆ (P-delta) effect where P is the vertical load and ∆ is the lateral deflection (Figure 5.4).

The elimination of external roof supporting columns (4) and beams around the building’s periphery increases the nett usable area, with a resulting reduction in costs.

TYPICAL STRUCTURAL FORM A typical one-storey warehouse building with included offices will often have a floor-to-roof height of 6.5 to 9.0 m. Tilt-up loadbearing panels for buildings of this type typically have a wall panel thickness of 140 to 200 mm.

There are two separate design stages. These are:

•

End section of panel acts as column to carry induced compression.

In-service: the in-service design provides for the performance of the element as part of the complete structure.

5-3


Reinforcement often placed centrally in panel

Moment taken as zero when it is beneficial, ie. when it would reduce the wind moment

e

As

P M = Pe

Outer face

qlat

Inner face

lu Figure 5.5 Typical single-leaf panel section

REINFORCEMENT The correct quantity of reinforcement, properly located and securely tied, is an important ingredient in successful tilt-up structures. Whilst it is rarely needed to cater for stresses during lifting of a panel, reinforcement is required to resist the varying environmental stresses imposed on a wall in its final position. The bulk of the reinforcement is normally placed in the centre of the section. Properly designed, detailed and fixed, the reinforcement will maintain structural integrity for the life of the building.

P (a) Wind positive

e

P M = Pe

Purpose qlat

Reinforcement is provided in tilt-up wall panels to resist bending and axial stresses in the panel in service and to control temperature and shrinkage cracking.

lu

It is not normally used to resist bending stresses during lifting of the panel, as these are usually kept within the tensile capacity of the concrete alone (however, see Chapter 6). Although not often required for strength at this stage, the reinforcement provided for in-service design will prevent the panel breaking up if it is accidentally cracked during handling.

P (b) Wind negative

Extra reinforcement should be provided around panel edges and openings and across re-entrant corners. This will resist shrinkage stresses and control cracking in such vulnerable areas.

Figure 5.4 Forces, moments and deflections

The effective height-to-thickness ratio is generally 45 to 50 for simply supported panels, though they have (5) been designed and tested up to a value of 60 . Panels of this thickness typically will have one layer of reinforcement with vertical reinforcement on the panel centre line and horizontal reinforcement fixed to it, towards the outside face (Figure 5.5). In a solid panel, 6.5 to 9 m high, the vertical reinforcement may be only T10 or T12 bars at 300 to 400 mm centres. Horizontal reinforcement, which acts as the shrinkage and temperature reinforcement, varies with panel size and location.

Suitably placed reinforcement can also improve shear capacity around lifting points, but the reinforcement itself should not normally be used as a lifting point. However, some designers have successfully developed reinforcement details to act as end lifting points for smaller panels.

Design The design loading for the erected panel will depend on the building type, how the panel is used, the restraint conditions and the type of fixings adopted.

5-4


While the general design requirements of BS 8110 must be satisfied, some specific situations encountered in tilt-up panels are not covered by the code. A variety of design approaches from abroad for these situations are reviewed in Reference 6 and a panel design method consistent with BS 8110 is developed later in this chapter, together with a worked example.

These can be accommodated easily in most panel sections and can be cut and bent on site if required. Some panels may incorporate relatively highly stressed elements, such as mullions beside door openings. These may require extra reinforcement in the form of bars and links both for the lifting and service conditions. Such areas should be detailed and checked carefully, as congestion of the reinforcement can cause difficulties with the placing of concrete.

A minimum area of reinforcement should be provided for the control of shrinkage and temperature cracking, depending on the panel thickness and the grade of reinforcement used.

Adequate cover must be provided over the reinforcement to meet the requirements of BS 8110 for durability and for fire resistance. Centrally placed reinforcement will usually have sufficient cover except in very thin panels or when deep rebates are formed in one face. In such cases, cover must be measured from the bottom of the rebate and it may be necessary to increase the panel thickness.

Detailing A single layer of reinforcement placed at the middepth of the panel will usually meet the design requirements. Two layers may be necessary in panels over 200 mm thick, or to cater for concentrated loads from elements such as beams or alongside openings, or to increase shear capacity around lifting inserts.

A five-layer system of reinforcement notation may be used to deal with most requirements, with layer three being the central vertical bars most often used and layer two being the minimum horizontal steel (Figure 5.7). This simplifies reinforcement, as it requires chairs to one layer of reinforcement only with other layers fixed directly to this layer.

Placing a single layer of reinforcement off-centre to resist the bending moments during lifting is not recommended, as the resulting non-uniform restraint of shrinkage may induce warping of the panel. Draping of the reinforcement should also be avoided, as it is difficult to maintain the draped profile. Either fabric or bar reinforcement may be used. Bars give greater flexibility in providing the required cross-sectional area, especially in irregularly shaped panels. On the other hand, fabric costs less to place and fix and is generally the preferred option.

Optional layer 5 Optional layer 4 Optional layer 3 (Normally central)

The extra reinforcement required at edges and corners to control cracking can most conveniently be provided typically in the form of T16 bars (Figure 5.6).

Diagonal bars used to control stresses at openings and internal corners

Optional layer 2 Optional layer 1 Note: Bars are continuous and reinforcement chairs normally support layer 2 from the casting slab

Figure 5.7 Layering of reinforcement

DESIGN OF LOADBEARING PANELS Tilt-up panels may be used structurally as well as architecturally in loadbearing and non-loadbearing (or curtain) walls, both externally and internally. They may also have to be designed to resist earthquake and other natural forces. The loadbearing element may be either a single leaf or one of the leaves, typically the inner, of a sandwich panel.

Perimeter bars used to control shrinkage

Figure 5.6 Reinforcement at edges and openings

5-5


dominant and design is controlled primarily by bending due to lateral wind forces.

The following design method, which is used in a Design Example in Appendix 5A , has been developed from consideration and observation of the various international design methods (6) and test results for tilt-up wall panels, together with consideration of the current UK design codes of practice for concrete.

Also failure could not realistically occur without involving the whole of the panel’s reinforced width. Since the distribution of such loads is not specifically covered by BS 8110, it is suggested that the recommendations of BS 5628 (7) for masonry are adopted, which allow the vertical point loads to be o distributed through an angle of 45 . In most cases this will mean that the vertical loading at the midheight of the panel may be assumed to be fully distributed.

Suggestions for UK in-service design In general a tilt-up panel should be designed to follow the recommendation of BS 8110 in respect of materials, specifications and construction, and design and detailing (eg. concrete quality, cover etc.).

Horizontal load may also be applied at the top of the wall where the panel acts as a shear wall to resist the effect of wind load on the building as a whole. This will affect the eccentricity of the vertical load.

However, assessment of the design practices of other countries where tilt-up has been used for many years, shows there are areas where some departure is necessary. These are discussed below in relation to braced walls.

The distribution of vertical loading in the plane of the wall may generally be assumed to vary linearly along the length. If N is the vertical load and M is the inplane bending moment for all loads above the midheight of the panel, the maximum and minimum load intensities at the ends of the panel are given by ( N/L ± 6 M/L2 ) where L is the panel length.

Slenderness limits - BS 8110 Part 1 imposes slenderness limits for loadbearing walls (30 for plain walls and 40 to 45 for reinforced walls) that are unduly restrictive for tilt-up construction. Tilt-up panels are commonly elements that are subjected to small axial thrusts but where the dominant action is lateral loading.

Investigations by the ACI-SEAOSC task committee, on walls with height/thickness ratios up to 60, have shown that arbitrary slenderness limits are unnecessary provided that P-∆ effects are properly assessed and taken into account.

Transverse eccentricity of vertical loads - Vertical loads at the top of the wall tend to be applied eccentrically either by design or due to construction (3) irregularities . These eccentricities must be assessed by the designer and allowed for in design. They may be considered to reduce linearly to zero at the lateral support below.

Tilt-up wall test results (5) (further examined in Reference 6) show that walls with height to thickness ratios up to 60 can continue to sustain combined axial and lateral loads at stages well beyond cracking and first yield of the reinforcement.

It should be noted that deflections due to lateral loads and buckling can act either inwards or outwards. In this respect adverse eccentricities must be fully accounted for, but any beneficial eccentricities should normally be taken as zero (Figure 5.8 )

Therefore, a slenderness limit of 60 is proposed for tilt-up wall panels, which is the same as that applied to columns in BS 8110. However, this would require appropriate checks for cracking and deflection at the serviceability limit state (SLS), and may require the use of two layers of reinforcement.

The load due to the weight of the outer leaf of a sandwich panel may be transferred to the inner leaf entirely at the mid-point of the panel or distributed over the whole area of the panel depending on the installation details of the ties used. The eccentricity of the load due to the weight of the outer layer of a sandwich panel should be taken as the distance between the centrelines of the leaves. When the load is concentrated at the mid-height of the panel, the maximum bending moment is Ne /2 (see Figure 5.9 (a)) where N is the weight of the outer leaf and e is the distance between the centrelines of the leaves. When the load is distributed over the height of the panel at n equally spaced positions, the maximum bending moment is Ne /2n (see Figure 5.9 (b)), and may be ignored as very small.

Such checks are considered generally unnecessary provided that the height-to-thickness ratio does not exceed 50 (see ‘Cracking and deflection under service loads’ on page 5-7). Distribution of vertical loads - Vertical loads applied at the top of a panel may be concentrated at the position of a roof truss or effectively distributed where the panel supports several equally spaced purlins.

Concentrated loads are sometimes taken to be spread through an angle of 30 o to the vertical. This approach may be necessary in the case of a very heavy load on a continuous wall in order to cater for the possibility of a localised failure. However, in the case of slender tilt-up panels, the vertical load is not normally

Panels on isolated footings - The use of isolated footings at each end of the panel rather than continuous footings will increase the vertical compressive stresses and induce horizontal tensile stresses in the bottom of the panel. The reinforcement provided should satisfy the requirements of an appropriate deep beam analysis.

5-6


P ep

Take e = 0 in this case since it would otherwise reduce the total moment

Moments

Loads Ne l

ep

N e

+ Ne - 2 e

Wind

Buckling Eccentricity Combined

(a) Where eccentricity of load reduces moment

Ne l

P ep

(a) Load concentrated at mid-height of panel

Take e = ep in this case since it will increase the total moment

Ne l

ep

l 2n

N n

l n N n

e

l n

N n

l n e

N n

Wind

l n

N n

B uckling Eccentricity Combined

(b) Where eccentricity of load increases moment

+ Ne - 2n

l n

Ne

l 2n

l

(b) Load distributed over height of panel

Figure 5.8 Applied moments on panel Figure 5.9 Bending moments due to outer leaf

The design of deep beams in reinforced concrete is examined in CIR IA Guide 2 ( 8) and stress distributions are given for various panel geometries and load arrangements. Using this, effective bandwidths at the mid-height of the panel for a uniformly distributed vertical load applied at the top may be derived from Figure 5.10.

If the flexural tensile strength of concrete is taken conservatively as 2.0 N/mm 2, the moment at midheight of a simply supported panel will be less than the cracking moment if 1.0 ( L2/8) < 2.0x10 3 (h2/6) or L/h < 51.6. The deflection at mid-height = (5/384)1.0(12 L4/ Eh3) which gives = L/1250, when E = 27 kN/mm2.

Cracking and deflection under service loads – It may be assumed that visible cracking is unlikely if the flexural tensile stress in the concrete is no greater than is allowed for a Class 2 prestressed member in BS 8110. In this case, no special measures are needed for crack control and the properties of the uncracked section may be used in deflection calculations. The limiting flexural tensile stress given in clause 4.3.4.3 of BS 8110 is 0.36 √ f cu for post-tensioned members.

Thus, it is reasonable to assume that neither cracking nor deflection need be checked where the nett pressure due to wind does not exceed 1.0 kN/m 2 and L/h ≤ 50, which is the basis of the suggested simplified design procedure below. In other cases, the nett flexural tensile stress in the concrete due to both lateral and vertical loads should be determined. If the maximum stress exceeds 0.36√ f cu, the crack width and deflection should be calculated. The calculated values should be limited to 0.3 mm and panel height/250 respectively.

The maximum bending moment at the mid-height of a wall is almost entirely due to the wind load, and the cracking criterion may be conservatively checked by ignoring the vertical load. Apart from panels at corners of buildings, the nett pressure due to wind, inwards or outwards, is typically ≤ 1.0 kN/m2.

The BS 8110 calculation methods are inappropriate for centrally reinforced sections, and the methods

5-7


Uniformly distributed load

Centre of horizontal compression

0.3Ha

Compression band

0.3Ha

0.2Ha

0.2Ha

Effective support width C1 or 0.2L o whichever is the lesser

Tension band

C1

Lo

C2

Figure 5.10 Idealised stress pattern in walls on isolated footings with UDL at top

given in EC 2: Part 1 (9), 4.4.2.4 and Appendix 4, are recommended. In these calculations ß 2 = 1 is used for a single short-term loading and 0.5 for sustained loads or many cycles of re peated loading. The duration or frequency of the maximum design wind loading would be likely to lie between these two conditions and a coefficient ß 2= 0.75 might reasonably be taken when assessing wind dominated deflections

adequately tied to a ground slab, the effective height may be taken as the distance between the ground slab and the lowest effective connection with the roof.

Simplified design procedure for slender panels The following suggested design procedure is based on the slenderness limits and other factors as given a bove.

Minimum percentage of reinforcement - The values given in Table 3.25 of BS 8110 for sections subjected to flexure were derived for a section with a lever arm of (5/6) h on the basis that flexural cracking is likely at a tensile stress in the concrete of 3.0 N/mm2. This leads to the relationship l00 As/bh > 60/ f y for a rectangular section. Where a single layer of reinforcement is placed centrally in the section, the lever arm is nearer to (5/12) h and the minimum percentage should be doubled to 120/ f y giving 0.26% for grade 460 steel.

1. Determine height of wall panel between lateral supports at top (roof) and bottom (footing or ground slab). Select a panel thickness not less than height /60 in general, or height /50 where the serviceability checks in 8 opposite are omitted. 2. Determine characteristic values of dead, imposed and wind loads in accordance with BS 6399: Parts 1, 2 and 3 (10). When determining wind loads, the worst possible combinations of external and internal pressure coefficients should be considered, taking due account of funnelling between buildings and the effects of openings in the walls. Panels at the corners of buildings may need special consideration.

Effective height of panel - Panels should normally be considered as pinned at the lateral supports provided by a roof or a footing. Where a panel is

5-8


Determine, where necessary, either representative or mobilised values of earth pressures in accordance with BS 8002 (11).

Analysis to determine second order moments at ULS - The following analysis is valid where the strain at the outermost compression fibre, due to the application of the ultimate loads, does not exceed the value at the end of the parabolic portion of the stressstrain relationship given in BS 8110 or EC2. If this condition is satisfied at the stage when the tension reinforcement reaches yield, compression-type buckling is not a consideration. In this respect the BS 8110 relationship imposes a more conservative limit than EC2.

Sandwich panels should be designed as noncomposite with either all loads supported by the inner leaf or with the vertical loads supported by the inner leaf and lateral loads proportioned between the leaves according to their stiffness. Allowance should be made for any increase of axial load intensity resulting from the distribution of vertical loads and where the wall is supported on isolated footings. The eccentricity of loads applied at the top of the wall may be considered to reduce linearly to zero at the bottom.

1. The strain and stress in the tension reinforcement are taken as the values at the end of the inclined portion of the bilinear stress-strain curve in BS 8110 with γ m= 1.05 or EC 2 with γ m = 1.15. The 2 BS 8110 values are εs = 0.0022 and f s = 438 N/mm for grade 460 steel.

3. Determine the ultimate limit state (ULS) design loads for all necessary load combinations in accordance with BS 8110: Part 1, except that the value of γ f to be applied to mobilised earth pressures may be taken from BS 8110: Part 2.

2. The strain distribution in the concrete in compression is derived from the assumption that plane sections remain plane.

4. Determine the maximum co-existent values of axial load and bending moment for all necessary load arrangements. For a simply supported panel this would normally be determined at mid-height of the wall. Walls will generally be required to resist lateral loads due to wind acting inwards or outwards. Care should be taken over the relative directions of the bending moments when combining the effects of lateral loads and eccentric vertical loads.

3. The stresses in the concrete in compression may be derived from the parabolic portion of the stress-strain curves in BS 8110 or EC 2, with γ m = 1.5. The parabolic relationships and simplified linear relationships for both Codes of Practice are shown in Figures 5.11 (a) and (b). The simplified linear relationships are obtained by putting E c,1 = (2/3) E o, where E o is the initial tangent modulus to the parabola, so that when εc = εo the triangular area is the same as the parabolic area.

5. Determine reinforcement to resist the bending moments only, in accordance with BS 8110: Part 1, clause 3.4.4. Walls may be provided with reinforcement at each face or with a single layer placed centrally in the section. The minimum area of reinforcement to be provided for a grade 460 steel as a proportion of the concrete section is 0.13% at each face or 0.26% at the centre.

4. The tensile strength of the concrete is ignored Relationships derived from Figure 2.1 of BS 8110: Part 1

6. Analyse the section in accordance with one of the procedures given under ‘Analysis to determine second order moments at ULS’ which follows, to determine the resulting moment of resistance and the second order moments due to the vertical loads, for all necessary combinations of axial load and first order bending moment.

ε o = 0.0002 E o

= 4.5

E c,1

f cu

f cu kN/mm2

= 3.0

f cu kN/mm2

Parabolic f c

7. Combine the first and second order moments and compare with the moment of resistance. Where necessary, modify the reinforcement and repeat the analysis of the section until the moment of resistance is adequate.

Linear f c

= 0.45 f cu ( εc / εo )( 2 − ε c / ε o )

= 3000

f cu

εc

0.60 fcu

8. Where the height to thickness ratio has been chosen to be between 50 and 60, check cracking and deflection under service loads in accordance (9) with the procedures in EC 2: Part 1 , Clause 4.4.2.4 and Appendix 4, with β2 taken as 0.75, say, for infrequent applications of the maximum wind load. The cracking moment should be based on a concrete flexural tensile stress of 0.36√ f cu and where this is exceeded the calculated crack width and deflection should be limited to 0.3 mm and panel height/250 respectively.

Eo 0.45 fcu

Ec

εo

Figure 5.11 (a) Stress-strain relationship, BS 8110

5-9


Fc

Relationships derived from Figure 4.2 of EC 2: Part 1

N

= 0.002

εo

As f y / g m

E o = 0.567 f ck kN/mm 2 E c,1 = 0.375 f ck ε c (1 − 250ε c )

h /2 d

Parabolic f c = 567 f ck ε c (1 − 250 ε c ) Linear f c = 375 f ck ε c

e c

e s d c

0.75 fck Eo

0.567 fck

Figure 5.12 Strains and forces acting on section

(b) Ec

Figure 5.11(b) Stress-strain relationship, EC 2

d c

c

c

s

( 2b)

= d  α 2 + 2α − α   f cu bd ε s )

= F c (d − d c /3) − N (d − h /2)

(3b)

The lateral deflection at mid-height of the wall may 2 be calculated as KL times the curvature, where K may be conservatively taken as 5/48 for a wall with pinned ends. In this case the second order moment at the mid-height of the wall is given approximately by:

Parabolic stress-strain relationship

M = (5/48){ε s /(d − d c )}( N 1 + N 2 /3) L

2

f c

(4)

Where: N 1 is the design load applied at the top of the wall

(2 a )

Equating (1) and (2a) simultaneously provides a cubic equation in d c that requires a trial and error solution. The moment of resistance is given approximately by:

= F c (d − 0.375d c ) − N (d − h / 2)

}

In cases where the reinforcement is placed centrally in the section, the second term in equations (3a) and (3b) becomes zero.

The following equations are based on the stress-strain relationships derived from the curves in BS 8110. Similar equations may be derived for EC 2.

M u

{

(1)

= d c ε s /(d − d c )

= 0.45 f cu (εc / εo )(2 − εc / εo ) F c = 0.45 f cu bd c {d cεs /( d − d c )εo } × {1 − d cεs / 3(d − d c )εo }

f cu bd c2ε s /(d − d c )

M u

Where:

(a)

= 1500

The moment of resistance is given by:

= N + As f y / γ m

εc

F c

α = ( N + As f y / γ m ) /(3000

bf c dx

εc

0

f cu ε c

where

∫ = ∫ {bf (d − d ) / ε }dε 0

= 3000

d c

Equilibrium of the forces acting on the section, as shown in Figure 5.12 provides the following equation:

=

f c

Equating (1) and (2b) simultaneously provides a quadratic equation in d c giving:

εo

F c

Linear stress-strain relationship

N 2 is due to the self-weight of the wall between the top and the mid-height L is the height of the wall between lateral supports

The foregoing analysis is valid for values of:

ε c ≤ ε o or d c ≤ { ε o /(ε o + ε s )}d

(3a )

which for a grade 30 concrete and grade 460 steel gives d c ≤ 0.33 d

5-10


Panels with openings Openings in panels impose secondary loads and concentration of stresses. Full account would involve the use of finite element methods or other complex calculations which are rarely justified. Indications are (12) that a simplified analysis as proposed by Brooks gives results that are sufficiently accurate for most designs.

Roof line

A

By this method, such panels are subdivided into vertical strips, spanning between the lateral supports (Figure 5.13). The width of each strip is limited to 12 times the panel thickness. The strips are then designed to support all the lateral and vertical loads transferred to them. Due to the increased loading on the strip, reinforcement will generally be required at each face to provide adequate strength and stiffness. In exceptional cases, the panel thickness could be increased adjacent to the opening to provide stiffening piers.

C

B

Door

600

3000

600

1000

2040

Figure 5.13 Panels with openings - division of panel into strips

An example of a panel with openings is shown in Figure 5.13 where the strips are designated A, B and C. It is usual to assume that doors span horizontally so that the wind load on the vertical strip may be taken as uniform over the height of the opening. For example, the characteristic wind load on strip B could be taken as (3.0/2 + 0.6 + 1.0/2) wk = 2.6 wk per unit length over the full height of the panel.

Insulation

Reinforcement often placed centrally in panel

The distribution of vertical loading may be derived on the assumption that the stress pattern above the opening is similar to that on a wall on isolated footings as discussed earlier in this section.

Panel tie

Further detailing reinforcement may be used to control stresses around openings (Figure 5.6).

DESIGN OF THE OUTER LEAF OF A SANDWICH PANEL

Outer face

Inner face

Sandwich panels and ties Sandwich panels are tilt-up panels cast in two leaves with rigid or semi-rigid insulation between them (Figure 5.14). This makes concrete buildings energy efficient while retaining the economy and structural advantages of tilt-up.

Figure 5.14 Typical sandwich (double-leaf) panel

restrained and supported by the inner leaf. However, the outer leaf may be designed to carry a proportion of the lateral wind load. Freedom of movement of the inner and outer leaves is important, so as to avoid induced stresses due to creep, shrinkage and temperature effects that would otherwise occur.

Sandwich panels are generally classified as either composite, where both leaves act compositely to contribute to the structural resistance of the panel; or non-composite, where the inner leaf carries the vertical loads and where the lateral loads are distributed to each leaf in proportion to their stiffness or where all loads are carried only by the inner leaf. Generally non-composite action, rather than composite, is assumed because of the unequal thermal movements between the two leaves that can (13) occur on large panels .

The ties, which connect the two layers, can be made of the following:

• • •

Stainless steel Composite fibre rods Other non-corrodible materials.

These ties transfer the loads from the outer leaf onto the structural leaf whilst allowing the outer supported leaf to move independently in response to temperature and moisture changes.

Thus tilt-up sandwich panels usually consist of an inner layer that is the primary loadbearing leaf, a layer of insulation and the outer leaf which is

5-11


Factors affecting tie design

Tensile forces - Tensile forces acting at right angles to the panel are caused by:

Most manufacturers are able to provide a free design and advisory service in respect to sandwich panel ties (see Chapter 13), but it is worth noting the factors affecting tie design and performance. The static checks required in a typical sandwich panel design, are as follows.

• • •

•

•

Hence spacing, depth, and position of ties within the panel typically need to take account of the following factors:

• • •

Transport and erection

•

Permanent loading, and temporary forces during lifting, from the facing layer

•

Different mean temperatures of the facing layer and the loadbearing layer.

The distribution of the shear forces onto the anchors and the insulation layer is time and load dependent. It is considered in manufacturers’ catalogues, and needs to be taken into account in the design of the outer leaf.

Adhesion to mould Wind pressure and suction

Types of ties and anchors

Eccentricities for asymmetrical elements Temperature gradient within the facing layer

Shrinkage

The connectors used for sandwich panels may be stainless steel ties, glass fibre rods or other noncorrodible materials (Table 5.1). This part describes some of the sandwich ties and their claimed advantages in use.

Stiffness and orientation.

Stainless steel ties

Temperature difference between the middle layers of the facing and the loadbearing layer

High strength sandwich panel tie/anchor – The (14) Frimeda / Burke panel ties are used to hold the two layers of a sandwich panel firmly together during lifting and placement of the panel. The highly effective insulating layer is encased and protected on both sides by strong, low-maintenance concrete. Burke sandwich panel anchors (Figure 5.15) tie all three components into an integrated unit that is as safe and easy to lift and effectively behaves as a monolithic concrete panel during construction.

Load transfer through ties within a sandwich panel As mentioned earlier, connections are made within a sandwich panel that ensure transfer of loads from the outer leaf onto the inner structural leaf. These include: Compressive forces - Compressive forces acting at right angles to the panel are caused by:

• • •

Lifting the sandwich panel from the casting bed.

Shear forces - Shear forces occurring within the panel plane are caused by:

Bending loading of the facing layer from wind load and temperature gradient.

Self-weight of the panel

Temperature gradient within the facing layer

Negative pressure or suction forces at right angles to the panel plane are transferred solely via the connector anchors from the facing layer to the loadbearing layer.

Tensile and compression loading within the panel plane caused by different temperatures in the facing and loadbearing layers and the mutual restraint to movement.

• • • • • •

Wind

Wind Temperature gradient within the facing layer

Round or sleeve connector anchor - The anchor is a round metal tube fabricated from grade 304 or 316 stainless steel. This material provides the long-term corrosion resistance required for the anchor. Both ends of the anchor are provided with round and oval holes (Figure 5.16). The round holes are for inserting

Live loads at right angles to the panel.

These compressive forces are transferred, via the connecting anchors and the thermal insulation, according to their compressive strength. Table 5.1 Sandwich panel ties Material type

Stainless steel ties, grade 304/316

Manufacturer/supplier

Types of ties/anchors

Frimeda /Burke

High-strength sandwich panel tie/anchor

DEHA/Dayton Superior

Round anchor or sleeve connector anchor Flat anchor Retaining ties such as: L-type connector pin, clip-on pin, clip-on stirrup Torsion anchor such as crossed connector pin

Composite fibre connectors

Thermomass

Thermomass PC connector 5-12


layer. The diameter is determined by the weight of the external leaf. Sandwich panel anchor 4700

Sizing is carried out quite easily using the tables provided by the manufacturer. Sleeve connector anchors are generally positioned on the centroidal axis of the external leaf.

Used to stiffen narrow areas and to cater for eccenticity in the loading of the outer leaf during panel handling

The circular connector has uniform loadbearing properties in all directions. Out of balance forces or extra overloads are transferred to the inner leaf by non-rigid connections (see below). This ensures that not only are static equilibrium conditions retained, but also constraining forces induced by the bending stiffness of the ties are minimised. Flat anchor - The flat anchor (Figure 5.17) is a 1.5, 2 or 3 mm thick plate fabricated from grade 304 or 316 stainless steel. Holes are provided along each long end for use as described above for the sleeve (round) anchor. The anchor is used in conjunction with the round anchor to carry the load from the outer panel. It can also be used as a torsion anchor to resist eccentric loads between the round anchor and the outer leaf of a sandwich panel. The anchor offers vertical load carrying capacity along its length, but when correctly positioned and orientated, its thin section does not restrain the outer layer from horizontal movement caused by environmental changes.

Sandwich panel anchor 4710 Sandwich panel connector tie 4000, 4600, 4610 Used to prevent curvature of outer leaf and to resist wind forces whilst allowing lateral movement

Used singly or in multiples close to centre of gravity to support the deadweight of the outer leaf

(15)

DEHA’s design manual gives information for determining the anchor’s minimum embedding depth, its dimensions, permissible load, maximum spacing and installation method.

Figure 5.15 Burke sandwich panel anchors and ties

Sleeve anchor

Internal loadbearing leaf Insulation

Flat anchor

Outer facing leaf

Figure 5.16 DEHA sleeve connector anchor

Figure 5.17 Flat anchor

special rods which tie the anchor into each layer of reinforcing mesh, while the oval holes are provided (15) to assist in bonding the anchor with the concrete .

Retaining tie - Retaining ties such as connector pins, (15) clip-on pins and clip-on stirrups (Figure 5.18) are used to tie the two concrete layers of a sandwich panel together, and prevent bowing and separation of the layers. The pins work both in tension and compression to resist wind pressure or suction. They are fabricated from grade 304 or 316 stainless steel and are available in bar diameters of 2.8, 4.0 and 5.0 mm. The ties are

The depth and diameter of the sleeve connector anchors are determined by the particular construction requirement. The element depth is selected according to the thickness of the thermal insulation or additional air layer and the thickness of the external 5-13


flexible due to their small diameter, and do not offer significant resistance to movement from thermal stresses or shrinkage that may build up in the tilt-up panel. Connector pins, with a maximum spacing between of 1200 mm, are arranged in a square, or rectangular grid typically with a side ratio of 3:4. (15) DEHA’s design manual gives information for determining pin size and loading capacity, etc.

Figure 5.19 Crossed connector pins (a) Connector pin

rotation and other likely movements. Figure 5.20 (b) and Figure 5.20 (c) are similar but use is made of supplementary anchor plates to cater for out-ofbalance forces due to asymmetry of the panel. The anchor plates are positioned with their flexible axis on a plane normal to a radial line through the sleeve anchor. This minimises restraint against shrinkage and thermal movements but enables the plates to carry vertical or rotational forces. (b) Clip-on pins

Composite fibre connectors The Thermomass building system uses the unique properties of composite materials to create an (16) efficient method of sandwich wall construction . The Thermomass fibre composite connectors (Figure 5.21) are said to be non-corrodible, extremely resistant to aggressive chemical exposure, three times as elastic with twice the tensile strength of mild steel, and offer minimal heat loss.

(c) Clip-on stirrup

Figure 5.18 Sandwich fixing pins

Fibre composite

Torsion anchor (crossed connector pin) - The (15) crossed connector pin consists of two connector o pins set at 45 , and inserted crosswise through the layers of sandwich panels (Figure 5.19). They are fabricated from grade 304 or 316 stainless steel. The pins take up forces from the eccentricity and prevent a rotation of the facing layer around the supporting anchor. They provide additional protection against external leaf warping.

Seal

Provision of steel ties in panel - The various ties indicated above are combined to carry the self-weight and to prevent twisting of the outer leaf, and to cater for shrinkage and other stresses occurring in the sandwich panel. In Figure 5.20 (a) the sleeve anchor is positioned in the centre of the panel and carries the major vertical load (self-weight) of the outer leaf, while additional pin ties and torsion anchors cater for

Long angular cut each side for retention

Moulded collar

Figure 5.21 Thermomass fibre composite connector

5-14


0 0 1

0 0 3 1

0 0 2 1

(a) Sandwich panel and sleeve

0 0 6 2

connector anchor with hairpin as the torsion anchor and connector pins in the outer area

0 0 2 1

100

1200

1200

1200

1200

0 0 3 1

140

100

2500

2500

60 50

5000 0 5 2 0 0 5

0 0 2 1

(b) Sandwich panel with a window opening with sleeve connector, flat anchor and connector pins

0 0 9

3010 120

1200

1100

1100 2320

0 0 6 2 0 5 0 1

0 5 2

600

160

300

1190

1190

0 0 2 1

70 60

2680 5000

0 0 2 1

(c) Sandwich panel with a door

0 0 0 0 1 0 2 1

0 0 0 3 0 0 0 5 0 0 0 2 0 1 2 1

opening with sleeve connector anchor, flat anchor and connector pins

120

1200 720

120

960

2400

120

120

1200

0 5 2

720

140

70 60

3840

Key

Sleeve anchor

Flat anchor

Figure 5.20 Typical usage of sleeves, flat anchors and connector pins

5-15

connector pins

torsion anchor


An indication of the properties of this composite material is given here as the material is not as well known as steel. This information is given for guidance only and the manufacturer should be consulted to confirm or provide precise values for design. The claimed properties of the connector are given in Table 5.2

200

Table 5.2 Properties of Thermomass fibre composite connector Manufacturer

Composite Technology Corporation

Connector material

Glass fibre in chemical resistant thermal set polymer. Keyed for maximum retention in concrete.

400 mm centres

400 mm centres

200

200

2

Effective sectional area

47.6 mm

Tensile strength

840 N/mm (minimum)

Flexural strength

840 N/mm2 (minimum)

Coefficient of expansion

(8.6 ± 1.7) × 10 / C

Thermal conductivity

0.1192 W/m2 /K

Figure 5.22 Typical layout of composite connectors 2

the drying out of the concrete and becomes more apparent on large panels of more than 5 m in height or width. This drying proceeds from the exterior inwards and thus creates opposing curvatures strains in the two leaves; these are controlled by tension in the sandwich ties. Rapid drying of the concrete in the first few days must be prevented by keeping it damp.

-6 o

Pull-out tests carried out on the fibre composite connector show that, when embedded to the manufacturer’s requirements, the connectors have a capacity of 8.0 kN for a Series 15 connector and 11.34 kN for a Series 20 connector. The shear capacity of each connector is approximately 4.04 kN. The ultimate strength of the connector can exceed that of other materials commonly used.

A low water-cement ratio should be used. The maximum size of the aggregate is chosen according to the workability, reinforcement and dimensions of the sandwich panel. American experience suggests that the use of concrete additives, especially wetting agents, airentraining agents, damp-proofing, permeability reducing agents and retarders can have a detrimental (15) effect on the shrinkage behaviour of the concrete .

Fire tests carried out on a sandwich panel constructed with fibre composite connectors showed no degradation after the inside leaf was subjected to o (16) 1090 C for 4 hours .

However, admixtures have been successfully used with many concrete construction projects in the UK, and advice should be sought from UK suppliers and designers. The provision of reinforcement and the effect of panel ties will help to control the effects of shrinkage, particularly in large panels.

The connectors are typically distributed uniformly over the area of the panel and are strong enough to cater for vertical loads and out of balance forces (Figure 5.22).

With increasing external temperature, especially with direct solar radiation, the external leaf moves more than the internal leaf.

Leaf thickness and cover to reinforcement To satisfy durability requirements, which should meet the recommendations of BS 8110, the thickness of the outer layer is generally in the range of 65 to 75 mm in order to provide sufficient cover to the centrally placed reinforcement. (See design example in Appendix 5A at the end of this chapter.)

Choice of positioning of the connectors, anchorages, geometrical shape of the panel and, most of all, the dimensions of the panel, have a great influence on the applied loads and the effects of moisture and thermal movements. The connector supplier normally assesses this.

Allowance for differential movements

The outer leaf is normally assumed to be permanently supported by the panel tie system, which is usually required to limit vertical displacement to about 2.0 mm relative to the inner leaf.

Differential movements caused by shrinkage and temperature should be allowed for in the design of sandwich panels. Shrinkage is mainly dependent on

5-16


The combined stiffness of these walls is usually far greater than is needed to provide stability and only some of the panels are used to carry the wind forces. A couple or so panels at each of corner of a building is often sufficient for this purpose (Figure 5.23) thus providing scope for removal of sections of the external walls for future extensions.

Design checks for the outer leaf and its ties The responsibility for the design of the panel, both inner and outer leaves, rests with the project structural engineer, but the supplier usually carries out the design of the tie system. The design checks typically carried out for the outer leaf are given below. The project engineer determines:

•

B

The concrete grade and thickness from durability requirements, assuming reinforcement sizes

•

The required reinforcement size and spacing for crack control and checks this with the assumed sizes

• •

The required insulation thickness

A

A Corner walls 'A' resist lateral load X direction

X

The permitted vertical displacement of the outer leaf.

The supplier of the tie system determines:

• • •

•

B

Corner walls 'B' resist lateral load Y direction

The tie/anchor size and spacing Whether the vertical displacement of the outer leaf is within criteria set by the project engineer

B

A

A

That the tie system has sufficient strength to carry vertical and any asymmetrical loads (leaf dead load) and lateral loads (wind loads and suction forces during tilting)

Y

That the tie system can withstand temperature and displacement strains in the outer leaf.

Figure 5.23 Shear walls

The wind loading carried by each shear wall is normally taken to be in proportion to its in-plane stiffness. The distribution of additional stresses within the panel can often be determined from a simple elastic analysis of the form:

BUILDING STABILITY The general loadings and strategies for ensuring the stability of the tilt-up panels were described earlier in this chapter. The following design assumptions are made when designing for overall stability of the building (see also ‘In-service loading’ on page 5-1).

•

For most buildings the roof is designed to transfer wind load to end walls

•

In such cases the end walls act as shear walls to resist wind loads

•

Roof trusses (and roof bracing) in the roof plane are designed as diaphragms to transfer lateral loads to shear walls

•

In multi-storey buildings, the floor slabs are designed to act as diaphragms (plate action) to transfer lateral loads to shear walls.

B

f =

N u A

±

MY I

As in the design procedure given earlier, the capacity of the wall is then checked for the prevailing combination of loads. The walls must also be checked for overturning. These shear walls are thus designed to resist in-plane loads transmitted to them by the floors and roof and, consequently, transfer these forces to the foundations. The connections between the panels at the floor and roof levels must be sufficient to transfer shear forces between panels.

Temporary bracing is also needed during the construction stage and this needs to be designed to resist both lateral and accidental or unexpected construction loading. The effects of fire on the stability of the panels must also be considered.

These shear forces are usually transferred directly into the foundations by dowel action or friction. Alternatively the shear forces can be transferred from the panel by dowel action into the floor slab, which is then used to transfers the forces into the ground by friction between the slab and the earth.

Shear walls

It may be necessary to tie panels together so that they act as a group to resist the overturning moments. Holding-down anchors at the ends may also be required, for example, on single panels.

The tilt-up panels are used to carry vertical and lateral loads applied to the face of the panels and also to provide shear walls for building stability.

5-17


Some shear walls may be isolated, such as those in the interior of the building. In addition to checking for overturning moment these will require appropriate bracing to transfer lateral forces into them.

Research Station, which concludes that spalling is unlikely to cause a problem with tilt-up construction and that there is no need to provide additional protection against spalling in cases where the cover to the reinforcement in a tilt-up panel exceeds 40 mm.

FIRE RESISTANCE

Panel stability

The fire resistance of tilt-up buildings has been extensively studied and reviewed. Tilt-up panels have been shown to perform well when designed and detailed to comply with accepted specifications and practice developed in the major tilt-up-using countries, notably the USA, Canada, Australia and New Zealand. There are some variations, but each country essentially sets out the same principal requirements.

A notable paper by Potter (18), of Cement and Concrete Association of Australia, reviews the Australian Code requirements and illustrates a series of details used to meet these requirements. The essence of this review may be considered when developing fire resistance requirements for a tilt-up project in the UK.

Fire growth and spread

Panel thickness and cover to reinforcement

There has recently been concern over the possible fire spread through certain lightweight metal sandwich cladding panels and in particular in those systems that incorporate air gaps between the insulation and the structural element. Even those systems that contain no air gaps can create problems when fire breaches the outer skin. This has been was highlighted by reported failures in these systems.

The thickness of a panel to provide a given nominal fire resistance period may be determined by reference (3) to basic concrete codes (eg. BS 8110 Parts 1 or 2 in the UK). The requirements of BS 8110: Part 2 are summarised in Table 5.3 for elements with 0.4 to 1.0% reinforcement, which would be typical for many tilt-up panels. BS 8110: Part 2 also provides alternative recommendations for thickness and cover in respect to aggregate type and reinforcement percentage.

Such fire spread is not a problem with tilt-up sandwich panels. Consultation with the Fire Research Station has confirmed that concrete tilt-up panels, in which the insulation is sandwiched between and in close contact with two concrete leaves, pose no significant risk either to fire growth or spread of fire. Care should, however, be taken in detailing if there are services within or penetrating the wall.

However, these Code recommendations apply to heavily loaded single-leaf walls and some relaxation of thickness may be possible to take account of the greater stiffness of sandwich panels, where used, and because of the relatively light vertical loads carried by tilt-up panels. The requirements for thickness and (9) cover differ somewhat in EC 2 .

APPENDIX TO THIS CHAPTER Appendix A - Design examples (see page 5A.1).

Table 5.3 Fire rating requirements Fire rating (hour)

Minimum panel thickness (mm)

Minimum cover to reinforcement (mm)

0.5

100

25

1.0

120

25

1.5

140

25

2.0

160

25

3.0

200

25

4.0

240

25

REFERENCES 1. Cement and Concrete Association of Australia . Tilt-up technical manual. C&CA Australia, Sydney, 1990. 24 pp. (Amended to a series of data sheets 1997). 2. Cement and Concrete Association of New Zealand. Tilt-up technical manual, C&CA New Zealand, Porirua, 1990. TM34. 32 pp. 3. British Standards Institution . BS 8110, Structural use of concrete. Parts 1 and 2. BSI, Milton Keynes, Part 1, 1997 121 pp. Part 2, 1985, 52 pp. 4. Spears, R. E. Tilt-up construction - design considerations. - An overview. Concrete International, Vol. 2, No. 4, April 1980. pp 3338.

In addition to setting minimum covers, BS 8110 also has certain recommendations to cater for the effects of spalling when the cover to the main reinforcement exceeds 40 mm. This poses some conflict with tilt-up panels that traditionally contain one layer of reinforcement thereby invoking requirements for supplementary protection. This matter was evaluated (17) in a report submitted to and accepted by the Fire

5. Azizinamini, A, Glikin, J. D, Oesterle, R. G. Tilt-up wall test results. PCA, Skokie, USA,1994. 16 pp.

5-18


6. Reinforced Concrete Council. A review of international tilt-up design methods. RCC, Crowthorne. To be published 1998. C/27. 7 British Standards Institution . BS 5628. Structural use of masonry. Part 1, Structural use of unreinforced masonry. British Standards Institution, Milton Keynes, 1992. 57 pp. 8. CIRIA. The design of deep beams in reinforced concrete. CIRIA, London, 1977. Guide 2. 131 pp. 9. British Standards Institution . DD ENV 1992-11: 1992. Eurocode 2. Design of concrete structures. Part 1. General rules and rules for buildings (together with United Kingdom National Application Document ). British Standards Institution, Milton Keynes, 1992. xvi, 254 pp. 10. British Standards Institution . BS 6399. Loadings for buildings. Part 1, Code of Practice for dead and imposed loads. Part 2, Code of Practice for wind loads. Part 3, Code of Practice for imposed roof loads. British Standards Association, Milton Keynes, BSI, 1992. Part 1, 16 pp, 1997, Part 2, 102 pp, 1988, Part 3, 32 pp. 11. British Standards Institution . BS 8002. Code of Practice for earth retaining structures. British Standards Association, Milton Keynes, BSI, Milton Keynes, 1994. 116 pp. 12. Brooks, H. The tilt-up design and construction manual. HBA Publications, Newport Beach, USA. 4th edition 1997. 360 pp. 13. Portland Cement Association. Tilt-up concrete building. PCA, Skokie, USA, 1989. 16 pp. 14. The Burke Group. Burke sandwich panel system. Burke, San Mateo, USA, 1983. 37 pp. 15. DEHA. Connector design manual and catalogue, DEHA. Square Grip Ltd. 16. Composite Technology Corporation. Thermomass architectural/engineering manual. CTC, Ames, USA, undated. Various inclusions. 17. Reinforced Concrete Council. Evaluation report - Fire resistance of tilt-up panels with one layer of reinforcement . RCC, Crowthorne,1997. 9 pp. C/29. 18. Potter, R. J. Behaviour of precast walls in fire. Constructional review, Feb 1996. pp 50-55.

5-19

5-20


APPENDIX DESIGN EXAMPLES

5A

This Appendix gives two design examples. The first gives the basic design and analysis of a typical slender 2 single-storey loadbearing panel for a 3500 m high-bay building, with a two-storey office space, designed for light factory or warehouse use. A fuller version including calculations for wind forces, shear wall analysis and foundation design may be found in Reference 1. The second example is of a two-storey loadbearing tilt-up panel suitable for a two-storey hybrid office building with long-span floors.

Example 1: Single-storey factory/warehouse east-west extension of the building without detriment to the stability of the present structure. The junction between panel and foundations is grouted after panel erection and the aggregate was exposed on this interface so that maximum advantage is taken of shear friction restraint arising from the weight of the panel.

GENERAL The single-storey structure for this design example consists of steel roof trusses supported on loadbearing tilt-up concrete perimeter walls, with internal steel beams and columns. The tilt-up panels are typically 7.5 m high × 6.98 m wide, some with openings (Figure 5A.1). The panels are taken down onto the foundations and the internal concrete floor slab abuts the walls on the inside face. An insulated concrete sandwich panel construction is used (75 mm outer concrete leaf, 75 mm - insulation, 150 mm inner concrete leaf). The panels for this example are designed to retain 1.5 m of soil to emphasise the structural capability of tilt-up.

Expansion joints An expansion joint has been introduced on the centreline of the roof and caters for temperature effects in the roof steel. A 20 mm joint separates the tilt-up panels and a polysulphide sealant is provided, designed to accommodate the environmental movements of the panels.

Lateral stability is provided by portal frame action of the internal steelwork in the E - W direction and by the internal office walls where they coincide with the frame (Figure 5A.2). Internal tilt-up walls are designed as permanent elements of the building. In the N - S direction, stability is provided by roof bracing spanning onto the tilt-up walls at the corners of the building, which act as shear walls. These wall panels have a reduced width of 3.5 m. A system of secondary bracing is provided in the roof on the perimeter of the building so as to ensure effective lateral restraint to the top of the panels. As requested by the client the walls are thus braced about their minor axis. The use of portal frames allows for future

Foundations The tilt-up panels are supported on concrete strip foundations and the steel columns are supported on reinforced concrete pads.

Erection The sequence of construction assumes casting of the floor slab followed by panel casting and erection. The panels will be temporarily braced to resist wind/construction loading. Erection of the steel frame starts with the the central portal frames, which are key locational and stabilizing elements of the building.

3 6980

20

6980

5

4 20

6980

6980

20

20

6980

20

6980

Roof soffitt

Parapet 1000 500 500 1300 1300

6500

7500

Finished floor level 2500

PART EAST ELEVATION

Figure 5A.1 Typical part elevation

5A - 1

300 Top of foundation


Perimeter beams and trusses will be erected and a site-welded connection made to the bearing pockets in the panels. These pockets are designed to accommodate the dimensional tolerances required between the concrete and steel-framed systems.

2.

Perimeter berm A 1.5 m earth berm surrounds the building. A filter membrane is fixed to the wall to act as a drainage zone and is linked to a drainage system at the base of the wall which traverses the perimeter of the building. The filter includes a backing layer of PVC, which acts as a waterproofing membrane.

Lateral earth pressure Perimeter walls are taken as being backfilled to a maximum height of 1.5 m above foundation level (Figure 5A.3).

It is assumed that there is free-draining granular material against the face of the wall panel with a perforated drain at low level. Therefore, assume pore water pressure is nominal and pressure distribution is triangular. Maximum lateral pressure (at base of triangle) 2 = 8.1 kN/m

Building extension Provision for extension is included on the rear wall of the building. End panels (3.5 m wide) will be retained to act as permanent shear walls. These particular walls are tied at their ends to the foundation by a reinforced in-situ connection. Columns will be erected to support beams on this elevation and expansion can take place in modules of 42 m x 42 m. An expansion joint will separate the existing building from all new structures.

1000

6500

LOADINGS

1500 max

The loadings for the building were calculated as follows.

300

Roof Gk = 0.65 kN/m2 Qk = 0.75 kN/m2

1.

Figure 5A.3 Panel retaining earth berm

First floor office 2 2 Gk = 4.3 kN/m Qk = 3.5 kN/m

Note: Shaded area indicates two-storey office space, all other areas are high-bay production space 1

14000

2

14000

3

14000

4

Trusses at 7 m centres 5

14000

6 14000

14000

7

E S

N

A

W 14000 Tilt-up wall panels B Portal frames at 14 m centres 14000

C

Purlins at 1.2 m centres

14000 Shear wall panel D Area for future extension

Figure 5A.2 Plan view of building

5A - 2

Roof bracing carrying lateral (N-S) forces to shear walls

Design example 1

Wind loading to BS 6399: Part 2 (standard method)

4. Concrete cover requirements are to BS 8110 Tables 3.2 and 3.3, and are summarised in Table 5A.1 below.

Typical maximum positive wind pressure: pmax = 0.75 kN/m2

Loading

Typical maximum negative pressure (suction) on wall panel: pmin = 0.86 kN/m

a) Roof ( N 1) The loadings are as shown on page 5A.2

2

From rafter:

DESIGN FOR IN-SERVICE LOADS OF TYPICAL PANEL WITHOUT OPENINGS

Gk = 0.65 x 14 x 3.5 = 31.9 kN Q = 0.75 x 14 x 3.5 = 36.8 kN k

From purlins:

Gk = 0.65 x 3.5 = 2.3 kN/m Qk = 0.75 x 3.5 = 2.6 kN/m

A typical sandwich panel has dimensions of 6.98 m wide by 7.5 m high. For practical reasons and to tie in with design for lifting, a panel with an inner leaf thickness of 150 mm will be used. This example continues with this thickness but further refinement could be carried out to determine the optimum thickness to carry design loads; this is likely to be somewhat less than 150 mm.

Assume dispersion of rafter reaction into wall panel produces a uniform stress at mid-height of the panel Figure 5A.4 (BS 8110, clause 3.9.4.13). Gk = 31.9 ÷ 6.98 = 4.6 kN/m Qk = 36.8 ÷ 6.98 = 5.3 kN/m

Therefore total roof load (taken as uniformly distributed):

Therefore, try panel with 75 mm outer leaf, 75 mm insulation and 150 mm inner leaf. Height of panels above foundation = 7.5 m

Gk = 4.6 + 2.3 = 6.9 kN/m

Floor slab not taken as propping the panel.

Qk = 5.3 + 2.6 = 7.9 kN/m

Roof designed to prop panel 1.0 m from the top of the panel.

b) Suspended first floor Assume this panel does not support first floor office units for simplicity. However, such support is likely to be cost effective.

Panel taken as being simply supported at foundation and roof. Therefore effective height = 7.5 – 1.0 = 6.5 m

Design assumptions Reaction from portal rafter

Purlin UDL

1. The sandwich panel is assumed to act noncompositely, with the outer leaf attached to the inner leaf by a central sleeve anchor. All vertical and lateral loads are assumed supported by the inner leaf. (Alternatively, the wind load may be shared between inner and outer leaves in proportion to stiffness 8:1)

0.5 h h = 6500

2. Requirements for erection process are considered elsewhere.

6980 Portal rafter load uniformly distributed at mid-height

3. Panels are designed in accordance with BS 8110: Part 1 and using the recommendations for design contained in Chapter 5 of this publication, including second order moments.

Figure 5A.4 Distribution of loads on panel

Table 5A.1 Cover requirements Panel

Environment

Cover (mm)

Concrete grade

Outer leaf external face

Severe

30

C45

Outer leaf internal face

Moderate (possible condensation)

25

C45

Inner leaf external face

Moderate (possible condensation)

35

C35

Inner leaf internal face

Moderate (contact with soil)

35

C35

5A - 3


c) Wall panel self weight

Load combinations

Weight of inner leaf at mid height ( N 2)

C1 : 1.4Gk +1.6Qk +1.4 E k

G k = 0.15 × 24 × 7.5 ÷ 2 = 13.5 kN/m

C2 : 1.0Gk +1.4W kp+1.4 E k

Weight of outer leaf (full height) ( N 3)

C3 : 1.2Gk +1.2Qk +1.2W kp +1.2Ek

G k = 0.075 × 24 × 7.5 = 13.5 kN/m

(Note: combinations with W ks less onerous than W kp when earth pressure is present.)

(Ignore nominal weight of insulation layer) It should be noted that the concentrated load due to the outer leaf self weight is assumed to be immediately dispersed and, as such, is applied as a line load over the entire width of the inner panel. Design of the sleeve anchor is to ensure that stress in concrete directly under the anchor does not exceed 0.6 f cu (BS 8110, clause 3.9.4.13). d)

Wind loading

The wind load has been determined as + 0.75 kN/m 2 and – 0.86 kN/m and has been applied as shown in Figure 5A.5.

2

First order moments at ultimate limit state ( M 1) The loadings and first order moments M 1 are given in Table 5A.2. Maximum first order moment M 1 = 8.6 kNm/m width

Assume reinforcement is placed centrally within panel d = h ÷ 2 = 75 mm f cu= 35 N/mm

e) Earth pressure ( E k) as item 3 on page 5A-2.

Eccentricities of loading (Figure 5A.6) 1)

Roof loads e

1

z

= 75mm (adverse)

⇒

e1 = 0mm (beneficial, ie. positive wind)

2)

K =

Therefore,

As

=

2

8.6 × 10 6 1000 × 75

2

× 35

= 0.0437

= 0.949d = 71.2 mm

8.6 × 10

6

438 × 71.2

= 276 mm2 per m width

Outer leaf e3 = distance between centrelines of leaves

= 187.5 mm

As, min

=

0.26 × 150 × 1000 100

= 390 mm 2

per m width

Therefore, adopt T10 @ 200c/c or A393 mesh fabric

Panel supported by beam or truss depending on position

W kp

W ks

= 0.75 kN/m2

= - 0.86 kN/m2

(a) Positive

(b) Negative (suction)

Figure 5A.5 Wind load on panel

5A - 4

Design example 1

Table 5A.2 First order effects at ULS (load cases C1, C2 and C3)

N 1

Load combination

N 2 + N 3

N T

Moments (kNm/m)

Loading (kN/m)

Lateral N 3 e3 loads 2

Total M 1

22.2

37.8 0.0

C1: 1.4Gk + 1.6Qk + 1.4 E k

22.2

37.8

60.0

2.1

1.8

3.9

7.3

1.3

8.6

6.1

1.5

7.6

11.3 6.9

27.0 1.1

C2: 1.0Gk + 1.4W kp + 1.4 E k

27.0

6.9

33.9 11.3 37.8

17.7

0.9

C3: 1.2 (Gk + Qk + W kp + E k )

17.7

32.4

50.1 9.7

Notes:

1. Moment due to roof load ignored (beneficial). 2. Critical moment at mid height. 3. Ignore parapet cantilever

Second order moments at ultimate limit state

e1 N1

M = N1 e1

Using the method given in Chapter 5 (analysis to determine second order effects). le

= 1.0 × 6500 = 6500 mm

α= d c (a) Roof loads

M u

e3

M 2 M=

N3 e3

M u

2

N3

N + As f y

3000

γ m f cu bd εs

= d  α2 + 2α − α   = 1500

 d − d c / 3     d − d c 

f cu bd c εs  2

 ε  N  = 0.104  s   N 1 + 2  le2 3   (d − d c )  ≥ M 1 + M 2 ⇒ panel adequate

The above is valid if, and only if:

 ε  ≤  o   d  ε o + εs  ε s = 0.00219

d c

Now (b) Outer leaf loads via sleeve anchor

Figure 5A.6 Eccentricities of load

εo = 0.0002 ∴ 5A - 5

d c

f cu

= 0.0002

0.0012  d ≤     0.0012 + 0.00219 

35

⇒

= 0.0012 d c

≤ 0.375d


For combination C1:

α

d c

=

60 × 10 3 + 393 × 460 / 1.05 3000 × 35 × 1000 × 75 × 0.00219

 = 75 

0.08 2

M u

= 0.080

Pressure due to wind = +0.75 or –0.86 kN/m 2 Slenderness ratio,

 + 2(0.08) − 0.08 = 24.6 mm 

0.375d = 28.1 > d c M 2

Cracking and deflection at serviceability limit state

⇒ OK

6500 150

= 43.3

External leaf reinforcement Provide central layer of reinforcement

35 × 1000 × 24.6 2 × 0.00219

A s,min

 75 − 24.6    3  10 − 6 ×  75 − 24.6      = 15.6 kNm/m M 1

h

=

As this is less than 50, and as wind pressure does not 2 exceed 1.0 kN/m , cracking and deflection need not be checked (see Chapter 5, ‘Cracking and deflection under service load’, and ‘Recommended simplified design procedure’).

0.00219   18.9  3 = 0.104     22.2 +  10 (6.5)2 3   75 − 24.6   = 5.4 kNm/m

= 1500 ×

l

=

0.26 × 75 × 1000 100

= 195 mm 2 /m

Therefore, adopt T6 @ 150 c/c or A193 mesh fabric Note: With reinforcement placed centrally in panel, internal and external cover is nominally 30 mm. Using C45 concrete, this complies with durability requirements.

+ M 2 = 3.9 + 5.5 = 9.4 kNm/m ≤ M u

For other loads see Table 5A.3

DESIGN OF PANEL ACTING AS A SHEAR WALL

Effects of wind suction

Lateral stability in N-S direction is achieved by roof bracing spanning onto the tilt-up wall panels at each corner which act as cantilever shear walls (Figure 5A .7 and Figure 5A.8).

Consider two further load combinations (Tables 5A.4, 5A.5) in the event of removal of retained earth. C4: 1.2 (Gk +Qk +W ks)

Note: Cl.3.9.2.1, of BS 8110 allows design of shear wall panel to exclude all forces other than static reactions due to horizontal forces. However corner panels should be checked for enhanced wind suction as a separate load case.

C5: 1.0Gk +1.4W ks 2 where W ks = wind suction = -0.86 kN/m applied to

full height of wall panel

Table 5A.3 Combination of first and second order effects (load combinations C1, C2 and C3) Load combinations

M 1

N T

α

d c

d c < 0.375 d

M 2

M total

M u

C1: 1.4Gk + 1.6Qk + 1.4 E k

3.9

60.0

0.080

24.6

Yes

5.4

9.3

15.6

C2: 1.0Gk + 1.4W kp + 1.4 E k

8.6

33.9

0.071

23.4

Yes

2.1

10.7

13.9

C3: 1.2 (Gk + Qk + W kp + E k )

7.6

50.1

0.076

24.1

Yes

4.4

12.0

14.9

5A - 6

Design example 1

Table 5A.4 First order effects (load combinations C4 and C5) Load combinations

N 1

N 2 + N 3

Moments (kNm)

N T

N 3 e 3 2

M lateral C4: 1.2 (Gk +Qk +W ks)

N 1 e1 2

M 1

17.7

32.4

50.1

5.5

1.5

0.66

7.7

6.9

27.0

33.9

6.4

1.3

0.26

8.0

C5: 1.0Gk +1.4W ks

Table 5A.5 Combination of first and second order effects (load combinations C4 and C5) Load combinations

M 1

N T

α

d c

d c<0.375 d

M 2

M total

M u

C4: 1.2 (Gk +Qk +W ks)

7.7

50.1

0.076

24.1

Yes

4.4

12.1

14.9

C5: 1.0Gk +1.4W ks

8.0

33.9

0.071

23.4

Yes

2.1

10.1

13.9

Corner panels provide stability in N-S direction

3500

N

20

S

Expansion joint Corner panel provides stability E - W stability provided by portal frame action

7500 Panel reinforcement lapped with starter bars

3500

400

H

1000 1250

In-situ portion of panel with starter bars lapped with panel reinforcement

6500

Reinforced concrete base to shear wall Panel acting as shear wall

Figure 5A.7 Shear walls to building

Figure 5A.8 Shear wall tied to foundation

Loading

a)

Gk = 2.3 kN/m

Two critical load combinations (i) 1.0 Gk + 1.4 W k + 1.4 E k (ii) 1.4 (Gk + W k + E k )

Roof loading (purlins only)

Qk = 2.6 kN/m

b)

Panel self weight (full height) Gk = (0.15 + 0.075) 24 x 7.5 = 40.5 kN/m

5A - 7


c)

Wind load and earth pressure

M = 1.4 x 75.6 x 6.5 = 688 kNm

Reactions at roof and base due to W k and E k (Figure 5A.9)

Check compressive stress for C2:

=

f t

* hk1 = 3.6 kN/m

210 × 10

3

3500 × 150

688 × 10

+

6

150 × 3500 2 /6

= 2.7 N/mm2 〈 0.35 f cu

hk2 = 7.0 kN/m

* Note: Reaction at roof due to wind suction load case is also 3.6 kN/m

Therefore, OK by inspection. Check tensile stress for C1: f t

hk1

=

150 ×10 3 3500 × 150

−

688 × 10 3 150 × 3500 /6 2

= −2.0 N/mm 2 0.75 kN/m2

Total tensile force = 0.5 f t Lt h From stress diagram (Figure 5A.10). T = 0.5 × 2.0 × 1520 × 150 × 10

= 228 kN

hk2

8.1 kN/m2

−3

Figure.5A.9 Lateral loads on panel 2.6 N/mm2

Shear wall supports 42 ÷ 2 metres of elevation (See Figure 5A.2).

C T

Therefore reaction at roof due to wind and earth pressure. H k = 3.6 ×

42 2

1980

2.0 N/mm2

1520

= 75.6 kN

Shear wall acts as a cantilever beam: span/depth = 1.86. As span/depth ratio exceeds 1.0, the wall may be designed using simple beam theory. Alternatively the approach proposed by Section (2) 4.6.4. of the ISE/ICE Manual can be adopted. L = 3500mm; h = 150mm

Extreme fibre stress; f t =

Figure 5A.10 Stress diagram

Reinforcement to be placed within 0.5×1520 of end of wall = 760 mm

As,reqd N

hL C1: 1.0Gk + 1.4 W k + 1.4 E k :

±

= 228

×103 ×1.05 460

M

Tension member As, min

2

hL / 6

C2: 1.4 (Gk + W k + E k ) : N = 1.4 (2.3 + 40.5) 3.5 = 210 kN

= 0.45

× 150 × 103 100

= 675 mm2 /m

N = 1.0 (2.3 + 40.5) 3.5 = 150 kN M = 1.4 x 75.6 x 6.5 = 688 kNm

= 521 mm2

Therefore, provide 2T20 anchorage bars 2 (As = 628 mm ) centrally within panel at each end @ 150 c/c and provide B385 structural fabric each 2 face (As = 770 mm ) (Figures 5A.11 and 5A.12). Tension lap with starter bars ≥ 20 x 38 = 760 mm

5A - 8

Design example 1

REFERENCES – EXAMPLE 1 1.

Reinforced Concrete Council. Detailed design example for high-bay tilt-up building. RCC, Crowthorne, 1998. C/28.

2.

Institution of Structural Engineers/Institution of Civil Engineers . Manual for the design of reinforced concrete building structures. ISE, London, 1985. 88pp.

B385 structural fabric each face

4 T20 @ 150

Figure 5A.11 Reinforcement in panel acting as shear wall

75

75

150

B385

T20 bar from panel

T20 starter bars alternately positioned

Figure 5A 12 Shear wall starter bars

This completes the design example for a typical tiltup panel for the high-bay building. A more detailed version including calculations for wind forces, shear wall analysis and foundation design may be found in Reference 1.

5A - 9


Example 2: Three-storey hybrid structure 4.

GENERAL

i) Panel in-service

The three-storey structure for this design example consists of hollow-core floors spanning onto three storey tilt-up panels (Figure 5A.13). The roof is of steelwork and also spans onto the tilt-up panels. The inner leaf of the panel is 175 mm thick and the clear height between floors is 2.8 m; thus the panel is stocky rather than slender. The hollow-core units are supported on angles attached to an embedded weld plate. Unlike Example 1, the tilt-up panels in this example carry significant vertical load and will be designed directly in accordance with BS 8110. The building is 37.5 m long by 15 m wide and two stories high. The panels are 10.15 m high by 7.5 m wide, with a 175 mm inner leaf plus 50 mm insulation and a 75 mm outer leaf incorporating 12 mm brick slips (Figure 5A.14).

Worst positive pressure: p = + 0.98 kN/m2 Worst negative pressure: p = - 0.90 kN/m

50 mm composite topping

Roof 2

2.

2

Qk = 1.5 kN/m

50 mm insulation

Embedded weld plate and attached angle

Floors (composite slab)

Gk = 8.6 kN/m2 3.

75 mm outer leaf incorporating 12 mm brick slips

450 mm hollow-core slab

Gk = 1.2 kN/m

2

175 mm inner leaf

LOADINGS 1.

Wind loading to BS 6399

Qk = 5.0 kN/m2

Panels

Figure 5A.14 Typical section through panel

Gk = 6.0 kN/m2

7.5 m 0.60

2.70 0.90 2.70

6.3 m

0.90 1.50 Brick facing 10.15 m

1.80

Continuous glazing

1.50

5.4 m

1.80 1.50 1.15

Panel elevation

6.3 m

450 mm hollow-core units with 75 mm topping

7.5 m

7.5 m

7.5 m

7.5 m

7.5 m

7.5 m

Typical structural floor plan Figure 5A.13 Building plan and typical panel

5A - 10

7.5 m

7.5 m

Design example 2

ii) Panel during erection 0.10

Considered as an open lattice allowing for window openings. Net surface pressure (applied to gross area) p = 0.77 kN/m

3.30

2

0.10 wl2 w

DESIGN FOR IN-SERVICE LOADS OF A TYPICAL PANEL

3.30

kN/m

0.50

0.10 wl2

Basis of design The panel may be considered as braced by the wall structures surrounding the stairs and lift shafts with lateral support provided by the floors and roof. These may be considered initially as props, resisting lateral movement at the level of the support angles welded to the embedded wall plates. The floors and roof may be considered to be simply supported on the ledger angles at this stage. After the walls have been tied into the structural screed, the floors may be considered to resist both rotation and lateral movement.

Eccentricities

3.30

2.80

0.15

0.15 Loads

Moments due to wind load

Figure 5A.15 Loads and moments on panel in-service

Design ultimate loading at critical section Roof (1.4 x 1.2 + 1.6 x 1.5) x 7.5 x 3.6

The eccentricities of the loads applied at roof and floor levels may be considered to reduce linearly to zero at the next level below (BS 8110, Clause 3.9.4.12). If the outer leaf of the wall panel is supported uniformly over the whole area of the inner leaf, no bending moments are caused in the inner leaf due to the self-weight of the outer leaf. The bending moments due to the wind load are very small compared with those due to the eccentric vertical loads, so that the critical load combination is dead and imposed (Figure 5A.15). Consider the horizontal cross-section between the window openings where a 3.6 m width of panel is supported by a 900 mm wide pillar. Assess the bottom storey, where the total load is greatest, and a position at the top of the window openings, where the reduction in the eccentricity of the floor load is negligible. Take the eccentricity of the floor load to be 200 mm, corresponding to a uniform bearing of 75 mm on the ledger angle.

Moments due to vertical load

= 110

2nd. floor fl oor (1.4 x 8.6 8.6 + 1.6 x 5.0) x 7.5 x 3.6 3.6 = 541 1st. floor (A (As second floor)

= 54 541

Wall Wall 1.4 x 6.0 x (3 (3.6 x 4.5 + 0.9 x 3.0) .0)

= 159 159 N = 1351 kN

Bending moment due to first floor load Eccentricity of load = 200 mm M = 541 x 0.2 = 108 kNm

Reinforcement Determine reinforcement conservatively using charts in BS 8110:1985: Part 3: Chart 36 d = 175 - (20 + 8 + 20/2) = 137 mm d/h = 137/175 = 0.78 3

N/bh = 1351 x 10 / 900 x 175 = 8.6 N/mm 6

2

2

M/bh2 = 108 x 10 / 900 x 175 = 3.9 N/mm

2

Effective dimensions

2 100 100 Asc / bh bh = 2.0, Asc = 0.02 x 900 x 175 = 3150 mm

From the ‘Basis of design’ given earlier, the effective height of the wall panel may be determined as for a plain wall (BS 8110, Clause 3.9.3.2.2).

Provide 5T20 - 200 (each face) with T8 - 240 as links (panel designed as a column with reinforcement in compression on inner face ). (See Figure 5A.16.)

Clear height between top of ground floor and underside of first floor l = 2800 mm Effective height le = 0.75 x 2800 = 2100 mm (BS 8110, Clause 3.9.4.3) Effective thickness h = 175 Effective height / thickness le / h = 2100 / 175 = 12 < 15

(stocky wall, BS 8110, clause 1.3.4.8))

Consider panel between the window openings as a two-span continuous beam (BS 8110, Clause 3.4.1.1(deep beam)) Clear span / effective depth l/d = 2700 / 1760 = 1.5 < 2.0

5A - 11

Design example 2

Take effective span as 1.2 x clear span le = 1.2 x 2700 = 3240 mm

and lever arm as 0.5 x overall height Then As = M / (0.95 f y ) (0.5h)

Dead load

8T8 - 240 (each face) plus 1T20 (bottom) [prpoortional for other beam strips]

1.4 x (6.0 x 1.8 + 8.6 x 7.5) = 105 kN/m Imposed load 1.6 x (5.0 x 7.5) = 60 kN/m Maximum sagging moment M = (0.07 x 105 + 0.096 x 60) x 3.24

2

= 138 kNm

5T20 - 200 (each face) [proportional for other column strips]

6

As = 138 x 10 / 0.95 x 460 x 0.5 x 1800

= 351 mm

T8 - 240 as links

2

Figure 5A.16 Typical main reinforcement for panels

Maximum hogging moment M = 0.125 x 165 x 3.24

2

= 217 kNm 6 As = 217 x 10 / 0.95 x 460 x 0.5 x 1800

= 552 mm

2

1.50

Provide 8T8 - 240 (each face) plus 1T20 (bottom) (see Figure 5A.16). w kN/m

DESIGN OF PANEL DURING ERECTION The load due to the self-weight of the panel is very small and may be ignored (BS 8110, Clause 3.4.4.1). Consider the panel to be propped at a position 8.5 m above the ground slab (Figure 5A.17). Maximum vertical moment due to design ultimate wind load on a 3.6 m wide panel area is M = 0.085 x 1.4 x 0.77 x 3.6 x 8.5

2

8.50

0.085 wl2

0.15 Loads and bracing

Moments due to wind load

= 24 kNm

This is much less than the moment due the floor load in the completed building. Therefore in-service load design governs.

Figure 5A.17 Loads and moments after propping

This completes the design example for a typical twostorey tilt-up panel carrying a dominant vertical load. A more detailed version including calculations for wind forces may be found in Reference 1.

REFERENCES – EXAMPLE 2 1. Webster, R, Chang, P, S, and Vollum, R . Hybrid concrete structures for the UK market: Outline designs for six hybrid schemes. Reinforced Concrete Council, Crowthorne. To be published, 1998. C/25. 58pp.

5A - 12


6

DESIGN OF PANELS FOR LIFTING

This chapter provides guidance on the design of panels to cover the temporary lifting condition, which often is the worst load case. Much of the material is presented for information only, as detailed lifting design will be carried out by lifting specialists using bespoke software. Lifting design principles, the number of lifting points and their locations are examined, as are their effects on panel concrete stresses at lifting. The need for additional reinforcement or strongbacks to cater for lifting forces is considered. Finally, lifting hardware and inserts are discussed with further reference to Chapter 10. etc.). This information is needed in order to enable the fixing supplier to determine an insert layout. A panel that would require only a few inserts, based on purely the load of the panel, may need more inserts because of its width or height or openings.

GENERAL In most tilt-up projects, design of the panel lifting, and indeed the bracing, is carried out by the hardware accessory suppliers in conjunction with the contractor. The reason for this is that, although the contractor could carry out analysis for lifting, it is a complicated process and most accessory suppliers have computer programs to analyse the panels for lifting stresses, enabling them to keep the design economical and efficient. They may also offer other engineering services (eg. panel bracing) for the tilt-up contractor. These services deal primarily with safe construction of tilt-up panels and have nothing to do with the structural integrity of the wall, which is normally the responsibility of the design engineer (1). However, engineers are encouraged to evaluate the expertise of the accessory supplier and, if appropriate, agree that they will bear the responsibility, in conjunction with the contractor, for safe lifting and bracing during construction. Even if such responsibility is delegated, the design engineer should be familiar with the lifting process and the possible problems (see Chapter 10, Safety requirements).

The lifting inserts need to be placed symmetrically about the centre of gravity in the horizontal direction so that the initial panel lift will be level, and above the centre of gravity in the vertical direction in order to enable the panel to tilt during erection (Figure 6.1). If the inserts are placed symmetrically about the centre of gravity in the vertical direction, the panel will not tilt but will lift flat. A tentative insert layout is assumed and the panel dimensions and insert locations are entered into a computer program to carry out an insert loading check and a flexural stress analysis. If the flexural tensile stress in the concrete in any part of the panel exceeds its flexural tensile strength, the lifting inserts may be repositioned or increased in number, and again entered into the computer for another analysis. If the tensile stress in the concrete is again too high and another redesign is not possible, extra reinforcing steel or some type of strongback may be used to control the flexural stress in the concrete. If extra reinforcing bars are required, the amount needed can be determined by applying normal reinforced (2) concrete design techniques as given in BS 8110 .

Accordingly, this chapter explains the engineering principles of lifting tilt-up panels, giving guidance on the lifting points and selection of inserts/fixings and the use of strongbacks.

Flexural strength of concrete - A permissible flexural strength of concrete of 0.4√ f cu cu (where f cu cu is the compressive strength at the time of lifting) is typically taken when assessing design for lifting.

DESIGN The general principles As a first step in the lifting design, it is useful to have a sketch of each type of panel involved in the project (see Chapter 3, Figure 3.3). The weight of each type of panel will need to be computed, taking account of openings and varying panel thickness. Also, the vertical and horizontal centres of gravity of the panel should be found by taking the moment about one corner of the panel. This information is necessary in order to determine the design procedure for lifting (and for bracing, see Chapter 8).

Determining the number of lift points required The number of inserts required depends upon the panel size and the panel’s strength to span between lift points as it is raised. Some panels require only two inserts, most require four and some eight (four pairs). Very large panels sometimes use as many as 16. Figure 6.1 illustrates the nomenclature for describing the number of lift points. For example, a 2 x 4 arrangement means two points per vertical row and four per horizontal row. Likewise, 4 x 4 means four high and four wide. The required number of lifting inserts will also depend on their type. A general approach for determining the number of lifting points is as follows:

The contractor would normally provide the fixing supplier with a set of drawings giving information about each panel (width, height, thickness, openings,

6-1


EDGE LIFT

The dead weight of the panel is typically increased by about 40% to make allowance for sticking or initial suction when the panel is lifted from the floor. For simplicity, this load is often taken to apply throughout the lift but sometimes is reduced to a 20% increase after release, when only dynamic and impact loads need to be considered.

•

This weight is divided by the supplier’s recommended capacity of the pick-up inserts in tension or shear, as applicable, to determine the number and type of lifting inserts.

•

The lifting arrangement will generally be determined to have a nearly equal load on each fixing but, because of the manner in which load is distributed, the load may vary by about ± 15%.

•

The lifting and bracing inserts and systems should be designed to provide a factor of safety of between 2.0 to 2.5 against failure.

0.21 0.58

(a)

•

0.21

0.29 0.71

The number of lifting fixings may also be affected by the limitations on their position. SINGLE ROW (2 pt)

Positioning the pick-up points on the panel - The location of the inserts is determined by the need to control both panel stability and stresses during the lifting operation. The main points for determining this are:

0.21 0.58

(b)

0.21

0.29

•

Judgement is initially used to determine the general pattern of lifting inserts (for example, 2 high by 2 wide Figure 6.1 (d)).

•

As indicated earlier, the pick-up points’ centre of gravity must coincide with the panel’s horizontal centre of gravity (Figure 6.2).

•

Also, in the vertical direction, the centre of gravity of the lifting points is positioned slightly higher than the panel’s centre of gravity to allow the tilting action about the lower edge to occur during lifting (Figure 6.2).

•

When a preferred pick-up point is located in an opening, it will need to be shifted together with its twin so that their centre of lift remains the same.

•

The lifting inserts are adjusted to achieve, as far as practicable, equal positive and negative bending moments in the panel during lifting.

•

The flexural stresses caused by the moments are checked against a specified allowable value (typically about 0.4√ f cu).

•

Inserts should generally not be positioned closer than about 300 mm to the edge of a panel or an opening.

•

If possible, the contractor will seek to use the same lift arrangement throughout the job since switching rigging between, say, a 2 x 4 arrangement and a 4 x 2 arrangement can be disruptive.

0.71

SINGLE ROW (4 pt)

0.10 0.26 0.28

(c)

0.26 0.10

0.18 0.40 0.42

DOUBLE ROW

0.21 0.58

(d)

0.21

Figure 6.1 Principles of lifting set-up

6-2

Design of panels for lifting

Forces in rigging cables - Stresses need to be checked at various degrees of rotation with respect to the horizontal (1). The most critical stress during o lifting will normally occur somewhere between 20 o and 50 rotation (Figure 6.3). The forces in the cables change as the panel is rotated.

Further guidance on design for lifting and on lifting accessories and equipment can be found in manufacturers’ catalogues (3, 4, 5). These catalogues will recommend pick-up points for various lifting configurations that will minimise bending stresses in the panel during lifting.

However, the calculations for determining the stresses at varying angles of rotation are extremely complex due to the cable geometry and the method of structural analysis required, and can best be accomplished by utilising the accuracy and speed of a (6) computer .

For a simple rectangular panel, the position can be determined from tables obtained from the supplier. For complicated panels, a computer program can be (6) used to determine the position of pitch-up points . When the locations of the lifting points have been finalised they must be fully marked on the panel production drawings, which should also specify the position of inserts, braces etc.

Centre line of pick must be directly in line with the centre of gravity of panel

A

A 200 - 500

B

B

C

C

Figure 6.3 Critical position for maximum bending moments and stresses

However, the following example, for a 2 x 2 arrangement (two points per vertical row and two points per horizontal row (Figure 6.1 (d)), is included here to show the mathematics involved, which need to be solved in order to determine the forces in the rigging cables.

Centre of gravity of panel

Figure 6.2 Lifting gear centralised over panel’s centre of gravity

It will be seen from this why it helps to make use of a computer program, which is normally available through the supplier of the lifting accessories.

Bending moments and flexural stresses The bending moments and flexural stresses in a panel are constantly changing as the panel rotates from zero 0 degrees (horizontal) to approximately 90 (vertical). As the cable changes its angle during rotation, the force components on the insert will vary.

Figure 6.4 (next page) shows the general lifting conditions through one vertical set of rigging cables, and can be used to illustrate an approach to determining the forces in the cables and lifting inserts.

When the tensile load on one insert increases, the tensile load on the other may decrease: this is what causes the bending moments and flexural stresses to (1) vary throughout rotation of the panel .

If the equilibrium at the top pulley is considered it can be seen that the angles α1 and α2 formed between the cable and the centre of lift must be equal since the horizontal component of Tα1 and Tα2 are equal and opposite.

It is this variation in forces that needs to be properly determined in order to produce the most efficient panel design.

6-3


But from properties of triangles P a

c

b

T

T

α1 α2

= L1

Which rearranged gives

sin β 2

β2

β1

=

B

sin 2α

Therefore

C

W

β2 sin 2 α

B sin

L2

L1

L1

Y

L1

 B sin β2    sin α  sin 2α 

b = 

B

=

P

A

T

α1

2 cos α

Substituting the values of a and b into the expression for T above gives:

R T

B sin β 2

α2

T = WY cos

= WY cos Figure 6.4 Lifting conditions through one set of cables

The values of α, β1, β2, T and b can be determined from the above equations, from which the values of R, P and the forces on the insert forces, both normal and parallel to the panel, are readily calculated. The equations for more complicated rigging become much more complex, hence the advice to use computer programs.

Considering the panel rotated through an angle φ b = L1 sin α c = L2 sin α

Determining bending stresses - The determination of stresses during lifting of an uncracked panel section is a fairly complex procedure since the analysis is complicated by the changing reactions at the pick-up points as the panel rotates to a vertical position (7) during lifting . Also the number and position of the lifting inserts affect the actual distribution of the total bending moment and a rigorous analysis would be even more complex. However, when determining bending moments and stresses, it is usually sufficient to assume that the panel is divided into vertical and horizontal strips passing through the pick-up points.

b + c = B cos φ L1 + L2

= L (length of cable )

Thus L sin α = B cos φ Therefore

B  α = sin −1   cos φ   L 

β1 = 900 − (α + φ) β 2 = 1800 − (2 α + β1 ) WY cos

φ = P(a + b)

Therefore P

=

WY cos

A single central layer of reinforcing, which is often sufficient for the in-service loads, is not enough to resist bending stresses for lifting. As a result, the lifting design is based on the assumption that the panel is uncracked and therefore the flexural stress controls the design. During lifting of the tilt-up panel, the allowable flexural tensile stress is limited to about 0.4√ f cu. To avoid cracking of the panel during lifting and to save time during construction, the contractor’s engineer must specify the concrete strength required for construction. The bending moments in the panel change significantly from those obtained when the panel is horizontal. The forces and moments outlined above are computed as shown in Figure 6.5 (opposite page).

φ

(a + b)

For vertical equilibrium at the pulley P = 2T cos α T = (WY cos a

 B sin β 2  φ ÷ ( A cos φ) +  2 cos α 2 cos α   φ ÷ (2 A cos φ. cos α + B sin β 2 )

φ) ÷ [(a + b) × (2 cos α )]

= A cos φ

b = L1 sin α

6-4


Initial Lift C X _ Y

W

R

1.

Draw load diagram

2.

Calculate vertical C.G. from panel bottom ( Y )

3.

Calculate the panel weight (allowing for suction/impact) = W

4.

Select trial lift-point quantity and location

5.

Calculate X (line of action of force P)

6.

P = W ÷ X

7.

Draw shear diagram

8.

Draw moment diagram

Cable O

Pa

C

Pb

D

h

analysis procedure (Figure 6.5 (a))

Having determined the moment diagram the stresses 2 may be computed from f = M ÷1000ht , where M is the moment in kNm, and the panel width and t , the panel thickness, are in metres.

J

(a) Tilt-up analysis procedure

Analysis at rotation(Figure 6.5 (b))

P c

b

a

Due to cable movement, the stresses will change as the panel rotates, and Pa and Pb will not remain equal so must solve for line of action of P (dimension F ).

Lb a

1. Assume critical angle from movement diagram

a

La

2. Assume cable length (2 D or greater). 3. Calculate D ′ = D cos φ F

_ Y

4. Calculate E = D sin φ

E

5. Solve for α in (sin α = D ′ ÷ cable length) 6. Solve for La in ( La - Lb = E ÷ cos α ) Note: La + Lb = cable length

D

7. F ′ = La sin α , then F = F ′ ÷ cos φ

φ

D'

R n _ Y

Rn

C

8. Using cable = 2D this reduces to

F'

C

  D  F = 2 + 4   

Wn

Pan

D

Pbn

   sin φ  cos 2 φ  1−  4 

Analyse the perpendicular components of loads only, which means the weight is reduced by cos φ. Lengths o are the same as 0 except centreline of lift = C +F 9.

W n

= W cos φ

(b) Analysis at rotation

Pn

W-R=P

P/2

=

C + F

Rn

= W n − Pn

Pan

=

Pbn

= Pn − Pan

P/2

10.

W n Y

( C + D − Y W n − ( C + D ) Rn

(c) Horizontal analysis

11. Draw shear diagram 12. Draw moment diagram

Figur 6.5 Tilt up stress analysis

6-5

D


Having determined the moment diagram the vertical tensile stresses in the concrete may be computed from:

Lifting insert Bracing insert

M 6

( panel width ) t 2 (1000) Where M is the moment in kNm, and the panel width and t , the panel thickness, is in metres. Assume uniform over full panel length .

Additional reinforcing bars in bottom face

In addition to moments and stresses developing in the vertical plane they will also occur simultaneously in the horizontal plane. These are determined in a similar manner as shown below. Horizontal analysis (Figure 6.5 (c))

(a) Panel with extra reinforcement

1. Disregard all panel weight below zero shear o between R and Pa (dimension J in 0 analysis) 2. Select trial horizontal lift-point locations

Lifting insert

3. Draw load diagram

Bracing insert

4. Draw moment diagram Having determined the moment diagram, the horizontal tensile stresses in the concrete may be computed from:

Strong-back

M 6

( panel height ) t 2 (1000 ) Where again M is the moment in kNm, and the panel height and t , the panel thickness, is in metres. Stresses are assumed uniform over full panel height.

Reinforcement

(b) Panel with strong-backs

When the panel’s uncracked strength is insufficient, additional reinforcement above that required to control in-service stresses and to control stresses due to shrinkage and thermal movements might be necessary. In such circumstances reinforcement can be provided in one or both faces to control the bending stresses (Figure 6.6 (a)). Alternatively the panel may be strengthened for the purpose of lifting by use of strongbacks.

Figure 6.6 Tilt-up panels strengthened with reinforcement and strongbacks

LIFTING HARDWARE AND ACCESSORIES Lifting hardware (3,4,5)

Lifting hardware includes items embedded in or attached to the panel to allow connection of lifting cables or attachment of braces. Rigging components include spreader beams, bars and cables. The crane or tilt-up contractor customarily provides the latter, whereas the hardware supplier provides the lifting hardware.

Strongbacks A strengthening element (a strongback, see Figure 6.6 (b) also Figure 3.9 in Chapter 3 and Figure 8.2 in Chapter 8) can be used to resist the forces during lifting in place of the additional reinforcement mentioned above, and when the panels are oddshaped or contain large openings. Strongbacks are generally steel or aluminium channels. The strongback must have sufficient stiffness to ensure that the stresses in the concrete section to which they are attached do not exceed the concrete’s permissible flexural strength, otherwise the panel may crack. The connections attaching the strongback to the panel must be capable of transferring the stresses set up during lifting.

Lifting inserts Lifting inserts are used as indicated earlier to reduce the bending in the panel as it is being lifted because excessive bending could cause cracking or failure of the panel. The lifting inserts are always paired: one cable will be attached to two inserts and looped through a sheave (or pulley) at the top to ensure equal pull on each insert.

6-6


P

P

(b) Only partial shear cone developed, hence lower capacity

(a) Insert in tension. Full shear cone developed.

P

Plastic cap box-out for lifting clutch. Set flush with surface

T

P W

(c) Insert in combination of tension and shear

Possible spalling

T

(d) Lower capacity than facelifting arrangement. Also need to consider bending effects

(a) Examples of anchors

Figure 6.8 Loads on inserts

The embedded hardware for cable attachment has developed since the time when the standard insert was a large sleeve nut, filled with paraffin to keep concrete out of the threaded hole, and with reinforcement welded to the nut to anchor it into the concrete. Modern attachments (Figure 6.7) are designed to minimise the time needed to attach and detach the cables. A quick release lanyard cable tugged from ground level eliminates the previous requirement for workers to climb ladders to detach cables. Plastic caps on the inserts prevent concrete entering the attachment and plastic antennae indicate the cable attachment’s location after the concrete pour.

DEHA

Attachments for braces can also be embedded in the panel. These too are supplied by the hardware supplier, and are generally of the coil or socket type (see Chapter 10) so that a bolt can be screwed into them. The critical loading condition for the insert may occur when the panel is either horizontal or tilted at some angle. The loading on the insert will be direct tension, shear or a combination of the two (Figure 6.8). Any failure may occur in either the steel anchor or in the concrete base: both should be checked to ensure adequacy during lifting. The fixings are also linked or lapped with reinforcing bars so that if failure occurs it will be ductile.

FRIMEDA

The factors that will be taken into account when the supplier of the lifting sockets is determining the required size and location of sockets are given below.

(b) Examples of quick release mechanisims

• •

Figure 6.7 Modern quick-release anchors

6-7

Size and weight of panel Concrete strength at lift

Tilt-up design andfor construction Design of panels lifting

• • • • • •

Type of concrete (lightweight, etc.)

• • •

Presence of recesses, etc

Size and location of openings Configuration of preferred rigging Panels cast face-up or face-down Size of crane Presence of architectural treatment such as exposed aggregate, form-liners, etc. Position of panel on the job Potential obstructions.

The placing of the lifting inserts is a very important step in the construction of the tilt-up panel. Although this small item is used only for the short time of lifting the panel into the vertical position, its location is critical. If a lifting insert cannot be placed in its exact position, the person who designed its location should be notified so that another location can be found that will not over-stress the insert or panel.

REFERENCES 1. McKinney, S. A. Certain details of tilt-up wall panel construction. Concrete International, Vol. 2, No. 4, April 1980. pp 52-57. 2. British Standards Institution . BS 8110 Parts 1 and 2: Structural Use of Concrete, 1985. Milton Keynes, BSI. 3. The Burke Group. Burke sandwich panel system. Burke, San Mateo, USA, 1983. 37 pp. 4. Dayton Superior. Tilt-up construction handbook , 1983. TU-5. 5. Conac Ltd (Now Halfen). Frimeda fastening technology manual. Halfen, Harlow, Essex. 6.

Payne, E. H. Computer assisted flexural stress analysis of site cast reinforced concrete tilt-up wall panels during erection. Concrete International, Vol 2, No. 4, April 1980. pp. 6470.

7. Cement and Concrete Association of Australia. Tilt-up technical manual. C&CA Australia, Sydney, 1990. 24 pp. (Amended to a series of data sheets 1997).

6-8


7

CONSTRUCTION OF PANELS

This chapter gives practical guidance on the construction of tilt-up panels. It covers formwork - including architectural features - in some detail, as well as bond-breakers and their specification, selection and application. The installation of reinforcing steel, embedments (for panel lifting, structural connection and bracing), and sandwich panel insulation is then presented. Finally, concreting, finishing and curing of the panels are discussed with further reference to Chapter 4. Prior to the construction of the panels, the casting surface must be checked to ensure that any slab joints, and predrilled or cast-in fixings within the casting area are masked out. The method of slab construction will have been selected previously to obtain appropriate tolerances.

FORMWORK Formwork for tilt-up construction is, at its simplest, limited to perimeter formimg of the panel. However, the extent and sophistication of the formwork and any form-lining will depend on the amount of modelling or texturing of the external surface, the requirements of panel joints and head and toe details, whether panels are cast contiguously, and whether they are stack-cast. The choice of materials for formwork and the accuracy of its construction play a vital part in ensuring that the erection process goes smoothly and efficiently, and that construction quality is adequate.

The finished casting surface must be well cured to make it sound and impermeable. If the curing agent is not the same as the bond-breaker to be applied later, it must be checked for compatibility before use. When cast directly on the floor slab with the correct attention to detail and quality, the resulting panel finish is virtually unrivalled by any other cladding material (Figure 7.1).

Casting surface

Edge formwork

The ground slab normally acts as formwork to the visible outer surface of the tilt-up panel and therefore needs to be of a suitable finish (see Chapter 4)

Timber formwork robust enough to withstand the rigours of construction is normally used. The size of

Figure 7.1 High quality cast surface

7-1


edge-formwork members and the spacing of the supports need to be related to the tolerances of construction, to construction methods, to the thickness required for structural loading and, ideally, to standard timber sizes. The type of edge formwork used for floor casting will depend upon whether the units are cast individually, contiguously or continuously (Figure 7.2).

A straight and true bottom edge to a panel may be particularly important if this surface is relied upon for the panel to be erected plumb on pre-levelled shims. If a gasket type seal is to be used in the panel joints, the side forms will need to be more accurate to meet the tighter joint width tolerance required. To facilitate this, special fixings are available for holding and positioning divider forms Where panels are formed contiguously, they can be formed on their mutual edge by a standard section timber form fixed in place by round wire nails acting as dowels into the slab (Figure 7.2 (b)). The latter are simply and accurately located by pre-drilling a tight fit hole in the floor slab. When casting, panel concrete should be brought-up equally on either side.

Edge form board Floor slab

Another efficient method of casting a long line of panels is to form up and cast every second panel and then to cast the panels in between. The sides are formed by gluing polystyrene, the thickness of the panel joint width, to the previously cast panels, and the end forms are just moved along by one panel width.

Edge profiling strip

Bracing Base board fixed to casting slab

Robust and stiff proprietary aluminium angle forms are available are. They also have the advantage that they can be fixed at wide spacings. The fixing of formwork to the floor can be important depending on the floor’s subsequent serviceability requirements or if that part of the floor is to be subsequently used for panel casting. The use of explosive fastenings may lead to spalling of the floor and should be avoided.

(a) Formwork for individually cast panels

Floor slab

Edge form timber

The use of chamfers on the edges has many benefits. They reduce spalling on the edges during removal of the side forms or during lifting, particularly at the pivot bottom edge of panels. They are more flexible than side forms and thus can be deflected and fixed during construction to take up variations in flatness of the slab, so minimising leakage. They also help mask visually any variations in joint width and make installation of joint sealants easier.

Round wire nail acting as a dowel

(b) Formwork for contiguously cast panels Panels cut during construction

For stack-cast panels more sophisticated edge formwork will be required. The full height formwork illustrated in Figure 7.3 produces reasonable dimensional control in plan but accuracy of panel thickness and finishing inevitably suffer. A better result is achieved with proprietary climbing formwork bolted into the previous panel in the stack or supported by steel RHS strongbacks raised by plywood boxes (Figure 7.4). This allows positive thickness control by screeding to the top of the form and unfettered access for float finishing. It allows for identical reproduction of panels ensuring parallel formed joints.

Floor slab Shutter also forms edge rebate

Timber base board fixed down to casting slab

With all formwork, great care must be taken at the joints. In all cases, a small bead of silicone rubber sealant should be applied to joints between forms and concrete to prevent grout loss and produce clean edges with no discoloration. Edge forms should be coated with form release agents to permit easy stripping.

(c) Flat formwork for continuously cast and cut panels

Figure 7.2 Edge formwork for single casting

7-2

Construction of panels

Battens at panel thickness

Panel

Timber or EPS groove form-strip stuck down to casting slab. Tapered for easy striking

Full height braced formwork Casting slab

SECTION

Panel feature grooves

Ground slab

Consecutively cast panels

Note: With this formwork system it is more difficult to accurately control thickness, finish and flatness of panel

Figure 7.3 Full depth formwork for stack casting

ELEVATION

Climbing RHS steel side form

Figure 7.5 Groove formers

Ground slab

securely fixed to the base slab to avoid displacement during concreting. However the blockout formwork may need to be released as early as possible to avoid shrinkage cracks developing.

Initial cast panel

Grooves, indents and rebates Grooves, indents and rebates are most easily formed on the face-down surface as mentioned previously. Timber strips for forming grooves should be tapered as shown in Figure 7.5 and sealed to prevent swelling. These can be fixed with double-sided adhesive tape or with gun- or spray-applied adhesive. Proprietary extruded polystyrene sections are successfully used for single casts in the USA; they allow crisp, accurate detailing to be achieved with minimum effort. However, care has to be taken with these as they are more fragile than timber and can be easily damaged by reinforcing bars, or by foot traffic or other heavy loads.

Plywood packing of exact panel thickness

Consecutively cast panels

Figure 7.4 Rising steel-backed formwork

Large-area shallow rebates, such as those used to create continuous window lines, are simply formed by detailing in standard thicknesses of ply sheets (single or multiple layer) with tapered edges to facilitate their removal (Figure 7.6).

Blockouts for windows, doors, etc Blockouts for major openings can be treated in a similar fashion to edge forms. They should be

7-3


Panel

Pilaster for strengthening the section

Casting slab

SECTION

Panel recessed to create continuous

False column/mullion

Dummy window

Window opening

Figure 7.7 Forming architectural features and increased sections Normal glazing

Curved external corner panel Casting slab

ELEVATION

Figure 7.6 Creating continuous glazing lines

Pilasters, columns, set-backs and curved forms

Figure 7.8 Forming curved panels

Local increases in thickness of panels can only sensibly be formed by increasing the depth of section away from the slab casting face, resulting in thickening outstands on the inner face. Providing this rule is respected, many structural and architectural forms are achievable on site (Figure 7.7). One USA contractor/developer has successfully differentiated his buildings by economical site casting of curved corner and bay panels of a high quality Figure 7.8).

Edge formwork with square section or reverse fillet to form square edge to mitre Mitred tilt-up panel

Mitred joints

SECTION

Mitred joints are sometimes used to avoid showing square butted panels at corners of buildings, but they are more difficult to construct and edges can be more easily damaged during erection. They are formed by braced sloping formwork which is usually overangled in order to allow tolerance for erection (see Chapter 10, Figure 10.19 – Mitred tolerance joint). It is much better to create a squared edge to the mitre (Figure 7.9) rather than leave as a sharp edge as this will ease construction and the resulting detail is less prone to damage. It may be formed by incorporating a square section or reverse fillet into the formwork.

Mitre formed with square edge

Figure 7.9 Mitre with square edge

BOND-BREAKERS Tilt up construction involves casting concrete elements on a previously cast concrete slab. The success of the technique depends very largely on being able to lift one hardened concrete element off another without damaging either of them. To achieve this clean separation, a bond-breaker is required.

7-4


A bond-breaker is a chemical compound that is applied to the hardened casting surface before placing the fresh concrete for a tilt-up panel. The compound is designed to prevent the fresh concrete from sticking to the hardened concrete and it must be chosen with care and applied correctly to achieve a successful panel lift.

of the work, and the consequential risks of imperfect performance of the bond-breaker. The selection process should take the following factors into consideration:

Effect of surface The quality of the casting surface is very important. A sound, dense, smooth concrete surface is essential. This can be achieved only by using quality concrete, well finished and cured. If there is any doubt about the quality of this surface, advice on bond-breaker type and application rate should be sought from the manufacturer. Proper curing of the casting surface will help to achieve a dense surface with low absorption, which will enhance the performance of a bond-breaker. Many of the bond-breakers currently available will also act as curing compounds to promote these desirable surface characteristics.

•

Is the material produced by a reliable manufacturer?

•

Is it a dependable bond-breaker? Application should not be so critical that slight errors will result in panels sticking.

•

Is it a good curing agent? Poor curing can leave a surface that is weak, porous and difficult to work with.

•

If not, is the bond-breaker compatible with the curing compound used?

•

Can it withstand rain showers? Rain and heat adversely affect the performance of some bondbreakers.

•

Is it durable enough to survive delays in casting the panels? Some products will oxidise sufficiently to require replacement in a few days.

•

Does the compound dry rapidly? Slow drying materials can cause delays or collect dirt while still tacky.

•

Can the panels be painted? Some compounds may be transferred to the panels and must be removed by mechanical means before painting is possible; others may take a long time to oxidise.

•

Will the panels look clean? Some compounds may leave stains that would be unsightly on an unpainted surface.

•

Can a floor treatment be applied to the casting surface? Some compounds must be physically removed before a floor hardener can be applied; some are incompatible with many floor tile adhesives.

•

Is it economical? This assessment should include all the above considerations, not just cost per litre.

Bond-breaker types Film-forming bond-breakers are compounds that form a waterproof barrier coat on the concrete surface. Consequently most of them also function as curing compounds if applied to concrete immediately after finishing. A second bond-breaking coat must still be applied later. This approach avoids the potential problems of incompatibility between curing compound and bond-breaker. Resin-based film-forming bond-breakers are designed so that the film will oxidise and break down over a period of time, depending on exposure to weather and sunlight. Wax-based film-forming bondbreakers do not break down in this way and tend to leave a residue which can interfere with applied finishes such as paint, floor sealers and adhesives. Non-film-forming bond-breakers can be either reactive or non-reactive types. Reactive bondbreakers work by combining with alkalis in the casting surface to produce a soapy layer, which prevents bonding. Non-reactive bond-breakers function as waterproofers. They do not react chemically with the casting surface, but block its pores and repel fresh concrete paste, thus preventing a bond. Non-film-forming bond-breakers generally do not function as effective curing compounds.

Application Application of the bond-breaker is one of the most important jobs on a tilt-up site. For best results, the compound must be applied evenly, at the correct rate per square metre and must cover the surface totally. Whether application is by spray, brush or roller, it is a wise precaution to ensure that operatives know what they are doing, why they are doing it, how important it is and how to check that they are doing it correctly. When applying a bond-breaker, it is essential that the manufacturer’s instructions are followed carefully. The following points are offered as a general guide.

Selection Selection of one of these types of bond-breakers for a particular project is an important process and should be undertaken with care. It should be noted that cost per litre is not a good basis for comparison, as the true cost of a bond-breaker must take account of the application rate, relative ease of use, cost of removal of any residue, cost of any disruption to other aspects

•

7-5

Make each application of bond-breaker in two light coats, applied at right angles to each other. Most compounds contain a fugitive dye as a visual check on coverage.


•

If the first application is being used for curing, apply it immediately after final floating. The bond-breaker layer will be applied after formwork is positioned.

•

Before applying the compound, check that the surface is clean and free of dust and debris.

•

Apply the compound evenly, paying particular attention to panel edges. Do not leave bubbles of bond-breaker on the surface.

•

Do not walk on the casting surface until the bond-breaker coating is dry.

•

If there is a delay before casting, or if some areas look questionable, check the integrity of the bond-breaker by sprinkling a few drops of water on it. If the water does not bead up, recoating may be required.

•

Do not place concrete onto bond-breakers that are wet with rain or heavy dew. The surface should be dried and inspected first.

•

Exposed wood surfaces for forming may perform differently from slab concrete.

REINFORCEMENT Fixing the reinforcement Before any panel reinforcement is placed in position, the bond-breaker must be applied to the casting slab. Edge formwork must also be erected, checked and fastened in place. Reinforcement must be securely fixed within the formwork so that it is not displaced by people walking over it during the placing of the concrete. Laps between sheets of fabric should be tied and the reinforcement held in the correct location by purposemade chairs of suitable material such as concrete, plastic or plastic-coated wire. Care should be taken to ensure that chairs and cover-blocks are suitable for the proposed surface finish and treatment for visible faces. Extra reinforcement such as edge bars and corner bars must be tied into position to ensure that it cannot move during concreting. All reinforcement can be pre-assembled into mats and placed in the panels complete, provided the mats can be lifted. This technique minimises the likelihood of damage to the bond-breaker from people walking on it.

Summary - bond-breakers

•

Ensuring adequate bond-breaking is one of the most important aspects of tilt-up construction.

•

Bond-breakers from reputable manufacturers will generally perform well if used in accordance with their instructions.

•

If doubt exists regarding application rate, compatibility, procedures, etc. the manufacturer should be consulted.

•

If no other information is available, compatibility problems and procedures can be checked by casting small trial panels and lifting them.

•

For best results, site personnel should be aware of the function, properties and limitations of the bond-breaker and the importance of applying it correctly.

Finally, the whole panel must be checked to ensure that all reinforcement is of the correct size and in the right place. Particular attention should be paid to ensuring that the specified cover is obtained at edges. In sandwich panels, the reinforcement for the structural inner leaf is broadly similar to that described above except that special chairs are needed to support the reinforcement without punching into the insulation and hence altering cover and location. In addition, a lightweight mesh is installed in the outer leaf, the minimum thickness of which is normally determined by the reinforcement sizes and cover requirements for durability at any rebates. It should be noted that a thin layer of tightly adhering rust on the reinforcement is not a problem and can give a better bond to concrete than a clean, bright bar.

Checklist

Further information on bond-breakers and their effect on painting may be found in Helpful hints on tilt-up (1) construction, No. 3 from the Tilt-up Concrete Association.

When checking a panel prior to pouring concrete, the following points should be considered:

PANEL FINISHES Procedure The whole casting procedure, its location, sequence, lifting timing requirements, etc. can be influenced by the choice of finish to be used (see Chapter 9). Guidance on methods of production to achieve different finishes is outside the scope of this publication, and reference should be made to other (2, 3 , 4 , 5) specialist publications .

7-6

• •

Have the right bar sizes been used?

• •

Are the laps correct?

•

Are the bars for each face properly separated from each other?

•

Are the inserts correctly positioned and securely tied in place and has any necessary extra local reinforcement been correctly positioned?

Do the bar locations and spacings conform to the drawings? Is the reinforcement properly spaced and tied to hold it in the correct position?


•

Do the lifting inserts have the correct orientation? And has any extra reinforcement at the inserts been correctly located?

•

Is the bond-breaker still effective? (Use the water-drop test.)

•

Is the slab surface free of dust, tie-wire and other debris?

key the dovetail end into the concrete. Installers then lightly tread the insulation sheets down onto the concrete to expell any remaining air. Either immediately, or more typically for larger tilt-up panels, after first set, the reinforcement for the top inner wall is placed and the concrete cast in the usual manner. Metal anchor and pins - Various systems are available in stainless steel. These generally consist of three elements: centrally placed cylindrical anchors; remote anchors which are set at an angle so that they are flexible in one axis and yet stiff in the other to carry shear forces; and pins which are flexible in two axes but rigid along their long axis to accommodate forces due to temperature and moisture, and to resist imposed loadings (see Chapter 5).

Only when all the reinforcement and inserts have been properly assembled and checked should the concrete be placed in the panels.

EMBEDMENTS Pick-up points and brace attachments Fixings for pick-up points and brace attachments should be accurately and securely positioned as indicated on the panel setting-out drawing for each panel (see Chapter 3, Figure 3.3). All fixings must be installed in accordance with the manufacturer’s requirements. They should also be properly tied to the reinforcing bars to prevent dislodgement during the concreting of the panels. The reinforcement will also ensure a ductile, rather than brittle, failure due to overload.

With these systems, the anchors must be attached to the reinforcement cage before casting the bottom layer of concrete. Reinforcement dowels are inserted through the fittings to lie in the plane of the wall and wired in place. Pins are simply clipped onto the reinforcement. The system therefore slightly impedes concreting and finishing operations. The insulation sheet is generally installed after the concrete has set, by cutting holes for anchors and by pushing the sheet over the pins.

A check should be made that all fixings, particularly those for lifting, are correctly located prior to placing the concrete. Failure to do so may require additional fixings to be installed at a later stage, which will take time and can disrupt the erection programme.

CONCRETING, FINISHING AND CURING THE PANELS (6, 7)

Normal good practice should be applied to tilt-up panel construction. However some specific points for tilt-up are worth noting. As craneage is not required on site until the panels are erected, placing by skip may not be economical. Careful planning may allow sufficient access for delivery by mixer truck chute, or pumping may be an option for larger sites. To ensure a consistent colour for visible faces the same mix should be used for all the panels.

Weld-plates and other connections Welded plate connections, steel fixings for beams and other items should also be accurately and securely positioned. The reinforcement used to tie back these fittings can often be used to secure them to main reinforcing bars

SANDWICH INSULATION

Good compaction is essential and use of a vibrating screed is good practice. Extra care is needed around fixings, at corners and edges, and at areas of reinforcement congestion. Poker vibrators may be used for thicker sections, taking care not to damage the visible face. To help with this, some contractors have developed waffle pokers or other methods such as the use of a rope sling to keep the poker axis horizontal. No final finishing should be attempted until all bleed water has disappeared. Finishing will vary depending on the required internal surface finish specified and the whether stack-casting is to be used.

Installation of insulation and ties Panel ties or anchors connect the two leaves and allow the outer leaf to be supported off the inner without restricting movements due to changes in temperature and moisture. Proprietary tie systems normally aim to minimise cold bridging. Chapter 5 considers sandwich panel types and design, while this section covers installation. There are two generic systems generally available and installation depends on both the material of the anchor or pins and their geometry.

In hot, dry conditions, the top surface of the concrete should be protected against rapid drying by shielding the surface from winds, shading from the sun and timing the placement to avoid the worst conditions. Spraying curing compounds onto the surface will also help control evaporation from the surface and reduce the risk of plastic cracking, which could cause a significant reduction in the tensile capacity of a tiltup panel.Tilt-up panels should be cured properly to

Fibre composite connector pins - These are generally the easiest to install. The system consists solely of stubby cylindrical ties which are inserted into the wet concrete of the bottom (outer) layer through pre-drilled holes in tightly butted sheets of insulation previously laid on top. Ties are inserted through the holes into the concrete and twisted 90° to

7-7

Tilt-up designofand construction Construction panels

ensure that the full potential concrete strength is developed As with bond-breakers, two further points need to be stressed. Firstly, to be effective, the curing compound needs to be properly applied to give uniform and complete coverage to the concrete surface. This application should take place just when the sheen of surface moisture has disappeared but while the concrete is still damp. Secondly, the compatibility of the chosen compound with the bond-breaker and its effect on subsequent surface treatments needs to be evaluated. Wax emulsion will, for example, impair the bond of future surface coatings. There are many parallels between panel and floor slab construction. Further relevant guidance on construction is given in Chapter 4 under the heading ‘Floor slab design and construction’.

REFERENCES 1. Tilt-up Concrete Association. Helpful hints on tilt-up construction, No. 3. Tilt-up Concrete Association, Iowa, USA, 1993. 2 pp 2 Monks, W. Visual concrete: design and production. British Cement Association, Wexham Springs (now Crowthorne), 1988. 47.101. 28 pp. 3 Monks, W. Textured and profiled concrete finishes. Cement & Concrete Association (now British Cement Association), Wexham Springs (now Crowthorne), 1986. 47.107. 12 pp 4 Monks, W. Exposed aggregate concrete finishes. Cement & Concrete Association (now British Cement Association), Wexham Springs (now Crowthorne), 1985. 47.108. 16 pp. 5 Monks, W. Tooled concrete finishes. Cement & Concrete Association (now British Cement Association), Wexham Springs (now Crowthorne), 1985. 47.109. 8 pp. 6. British Cement Association. Concrete on site. Set of eleven booklets. British Cement Association, Crowthorne, 1993. 45.200. 7. Ready-mixed Concrete Bureau. The essential ingredient - Site practice. Ready-mixed Concrete Bureau, Crowthorne, 1994. 22 pp. 97.341.

7-8


8

PANEL ERECTION

This chapter covers all practical panel erection issues and activities. These are given in chronological order, from determination of adequate panel concrete strength and preparations for lifting through to actual lifting, and finally bracing, release and panel base grouting.

•

PANEL STRENGTH Before a panel may be lifted it must be confirmed that the concrete has reached the required design strength. This can be determined by use of temperature-matched cubes cured alongside the panels or assessed, where some previous experience exists, from time/temperature records, as indicated in Reference 6 and 7 in Chapter 3. Evidence of adequate strength avoids premature lifting, which may cause cracked panels or actual panel failure, as the concrete will be less resistant to flexural stresses. Similarly, failure of the lifting inserts may also occur as the surrounding concrete also governs their strength.

Missing, damaged or misaligned lifting inserts should be corrected by attaching new, possibly chemical, fixings. These should be approved by those responsible for this aspect of the design. Expansion anchors should however, be avoided. Replacement bracing inserts and fixing inserts should be installed while the panel is still on the casting bed.

ERECTION SEQUENCE It is important to ensure that the panels are erected in accordance with the specified sequence (see Chapter 3). Generally, panels should be erected progressively from one end of the building. The sequence of erection should, whenever possible, be designed to avoid multiple handling of panels. Generally, because of the increased risk of cracking, the designer should be notified before any lifted panels are lowered back for storing flat. In such circumstances the designer must determine the support system. However, panels should preferably be stored in the vertical position using temporary props, but great care is needed to ensure their safety in the temporary state. In some cases where continued activity is required on the panels (for example, cleaning of brick faces), A-frames may be needed in place of conventional bracing. In essence, the panels should be stored only in a position approved by the designer.

A slightly longer curing period may be required for stack-cast panels, as the last and youngest panel in the stack must be lifted first. Speed of construction may dictate that special curing or a stronger mix may be justified, but this needs to be addressed at the planning stage.

PREPARATIONS FOR ERECTION Before starting the lifting sequence, full consultation between the contractor and the crane driver will avoid misunderstanding and delays. The following actions need to be carried out before starting to erect the panels:

•

Check that the panel levelling pads have been correctly located.

• •

Clear the site for crane access and mobility.

•

Confirm procedures in respect to overhead powerlines.

•

Check that the concrete in the panels has attained the specified strength for lifting.

•

Check that the inserts are correctly located, and look for any signs of lack of compaction around them.

•

Clean out all lifting, bracing and fixing insert recesses.

•

Remove any concrete fins caused by grout seepage before lifting (this is easier and safer than when erected).

•

Preferably fix roof-framing members (running along panels) to panels before erection.

Preferably attached top fixing of bracing to the panel before lifting.

CRANES

Provide sufficient room for crane outriggers and panel bracing.

Assuming adequate preparations for lifting, the capacity of the mobile crane, the experience of the erection gang and safe rigging determine the speed, efficiency and safety of panel erection. The crane should be of adequate size to easily handle the largest panel on the job. Factors such as the height of the panel, the amount of movement of the crane, and the position of the crane relative to the panel must be considered. The crane should be able to erect panels without sliding or jerking them upward and with a minimum number of moves. The lifting position of the crane will vary, depending on whether the panel is cast inside face up or outside face up, and whether the crane must work from inside the building or outside. The safest, quickest, and least expensive method is to lift the inside face of the

8-1


Figure 8.1 Rigged panel being lifted

panel from inside the building. But it might be necessary to cast the panel outside face up to achieve a particular finish or shape on the outside. The crane may have to work outside because of the lack of unobstructed interior space. Stack-casting of panels may overcome the lack of interior space.

Although the number of lifting inserts cast-in determines the amount of rigging, the actual rigging configuration used may vary. It is usual and far better for the rigging configurations to be designed to be self-equalising (Figure 8.1). To facilitate this equalisation, the rigging system often includes the use of slings running through sheaves (pulleys) on spreader or lifting beams.

A crane’s rated maximum capacity refers to its capacity at a minimum radius and often bears little relation to its actual capacity to lift large tilt-up panels. The selection of crane size should be made at the planning stage considering working radius and boom extension required (see Chapter 3). Before erection, a check should be made that the crane’s load scale is operating correctly.

The crane rigging cables must also be of sufficient length. Due to the extra loads imposed by their geometry, short cables may result in cracked panels or overloaded inserts. Cable of the largest sensible diameter is recommended to minimise tension stretch. Although thinner cables may be of sufficient strength, their ‘springiness’ may increase impact loading.

RIGGING THE PANELS The design for lifting (see Chapter 6) determines the number and position of lifting fixings. Lifting or rigging configurations are then determined so as to minimise the load on the inserts. Most manufacturers/ suppliers of fixings can provide guidance on rigging configurations and may also provide rigging hardware. However, in most cases a specialist supplier will supply the rigging hardware. The following comments are adapted from the typical guidance given in (1,2,) manufacturers’ brochures .

Cables should be kept vertical and any side pulls should only be executed following prior consultation with the hardware supplier. To avoid costly delays, rigging changes should be kept to a minimum. It may be more economical to add a few inexpensive inserts on the lighter panels for consistency than to constantly change rigging. Where possible, three rows of inserts should be avoided due to the complex rigging configurations required, but this is a design issue that must be addressed prior to the erection phase. 8-2

Panel erection

Some variations are possible on site, such as converting, say, a two-high rigging to four-high without removing existing rigging. However, this will require consultations with the hardware supplier.

spreader beam to connect each crane hook to a common lift point. Although this method is sometimes used it is preferable to use a single larger capacity crane where available. Blind lifting - A reverse lift or blind lift is one where the crane operator is unable to see the upper face of the panel when the panel is lifted. This method is used occasionally but should be avoided whenever possible. However, a reverse lift is sometimes necessary, when, for example, the panels are erected from outside the building. It may also be unavoidable when erecting the last panel.

STRONGBACKS Panels that are oddly shaped, elongated, or with large or awkwardly located openings are often strengthened for lifting by adding strongbacks (Figure 8.2). This may be in addition to extra reinforcement in the panel. The designer will normally have taken into account the size, shape and weight of the panel and whether strongbacks are to be used for lifting purposes. Potential clashes between strongbacks, roof /floor support angles, props, and the rigging should be resolved at the planning stage. Any changes to the specified strongback system should be referred to the designer.

The main danger is that the panel leans towards the crane and additional precautions may need to be taken. If a blind lift is going to occur it is important that this aspect is discussed at an early stage with the prospective crane company. Top lift - This method is normally only used for small panels typically not more than 4.5 m high. In this case the lift inserts are positioned in the top of the panel rather than on the face, and the panel will usually require the provision of reinforcement for lifting, which may be more than is required for inservice loadings.

LIFTING METHODS A single crane generally erects tilt-up panels with the panel rigged for a typical multi-point lift. However, there are other methods that can be employed for different panel shapes and erection requirements.

This is not efficient in panel design but leaves an unmarked face and the panel hangs plumb. This method is also commonly used for factory-produced panels.

Tandem lift - Sometimes for very large panels (typically in excess of 60 tonnes) two cranes are used in tandem. This will normally require the use of a

Figure 8.2 Panel strengthened by strongbacks for lifting

8-3


braced timber packings can be used at the rotating edge to prevent such damage.

True vertical lift - The erection of panels on a boundary against an existing wall can often be difficult. Face-lifted panels will always hang slightly off vertical by 3º to 5º. Top-lifted panels will hang vertical. However this lifting configuration is not economical for panels over about 4.5 metres high. One solution is to use an extra set of inserts in the top edge for face-lifted panels. The load can be transferred to these using a second crane or the panel could be temporarily propped off vertical and re-lifted off the top edge. Alternatively, offset lifting brackets can be used, so can trigger mechanisms, which lock the lifting ropes against the top of the panel when the panel is nearly vertical. These operations need to be planned beforehand and should be carried out only by an experienced crane operator.

Wedges can be used to overcome suction or a chemical bond due to poor bond-breaker application. The wedges should be hammered in line with the lifting inserts to minimise stress on the panel.

LIFTING SEQUENCE An experienced lifting crew will usually comprise two riggers, a foreman, carpenters and labourers together with a crane operator. The sequence of events required to erect a tilt-up, as outlined by (3) (4) Hughes and others , is as follows: 1. The crane operator lowers the spreader bar with rigging attached, and the riggers connect the cables to the pick-up inserts.

Walking a panel

2. The riggers straighten cable tangles to prevent snagging whilst the carpenters or labourers check that the braces will hang loose as the panel is lifted.

This is a system used in some countries where the crane lifts the panel and carries the panel to within setting distance of its position in the wall. This is often referred to as ‘walking’ the panel. This method is often used for one of the corner panels, which may need to be cast some distance away from the corner, requiring it to be carried (walked) to its final position. In the UK ‘walking’ of panels is normally carried out only with crawler cranes.

THE LIFT

3. At the foreman’s signal, the crane operator lifts, tightening the cables and pulling the panel to break the suction bond. Generally, wedges will be used to free the panel when the lifting force as shown on the crane’s load indicator reaches a value of 20% in excess of the calculated value under self-weight. The panel will lift slowly, pivoting about the lower edge resting on the floor slab. Lifting continues until the panel is near vertical and off the floor. Crane operators should apply a smooth and even lifting pressure to allow air to penetrate the interface and break the suction force. Water around the perimeter of the panel may seal the edge and prevent penetration of air. If, after heavy rain, water is lying against a panel, lifting should not be attempted since suction forces will be substantially increased. The water should be swept away from panel edges. (Sections of timber can be placed under the panel to support it if it has to be lowered back down to the horizontal. This also helps to prevent scratching of the floor slab if the panel slides during subsequent lifting. However, horizontal storing and double handling can cause problems (see ‘Erection sequence’ on page 8-1)).

When all the checks have been carried out and the panel is fully rigged it is then ready to be lifted.

4. The panel is then swung gently into position by the crane.

Lifting should be carried out so that the panel rotates about the bottom edge. Appropriate joint detailing can hide any damage to this edge. Bottom-edge chamfers are normally used to reduce this risk. Care should be taken to avoid sliding or dragging panels across the finished floor because of the risk of damage to panel, casting slab, personnel and equipment. With stack-cast panels, more care is needed to prevent the panels sliding off the stack and damaging the face of the panel beneath. Raised and

5. The panel is then lowered so that it rests onto prelevelled pads with the cables still taut. If necessary, additional shims, typically of tough PVC, may be used to level the panel. The erection crew use leverage (pry) bars and wedges to move the panel into the final position on its grid lines. To provide spaces for leverage bars, some contractors create small formwork notches, say 25 x 100 mm, in the bottom edge of the panels when casting.

While travelling, the panel should be tethered in place to the crane. Very high loads can be exerted onto the slab by this method.

The walk-out panel The crane may be able to place most of the panels from inside the building, but at least one of the panels must be positioned after the crane exits. This last panel is often called the ‘walk-out’ or ‘closure’ panel, and must be set as a blind lift with the crane outside the building. It is also not uncommon to mark the top face of this panel ‘walk out panel’, just to ensure it is not positioned with the crane still inside the building. It may also be set vertical in a temporary position next to the opening, reducing the crane reach at final erection.

8-4

Panel erection

6. When the panel rests securely on the pads, the braces are extended and holes are drilled in the floor slab for their attachment. With the crane still holding the panel, the braces are adjusted (using their integral turnbuckles) until the panel is approximately plumb.

Panel sticks to the floor slab This may be caused by suction created by water under the panel and may require the use of wedges as discussed earlier. Failure of the bond-breaker might result in some floor slab clinging to the panel (or vice-versa) when lifted, requiring it to be chipped off and the floor and panel made good. This problem, however, can be eliminated by proper attention to the type and application of the bond-breaker.

7. After the braces are secure and the panel is plumb, the crane slackens the cables and the lifting hardware is disconnected. If using ‘quick release’ fixings, sometimes called ‘ground release’ fixings, release is achieved by simply tugging a lanyard cable.

Panel does not hang correctly This is caused either by miscalculation of the proper location of lift points, or by physical failure to place them correctly as designed. If the panel cannot be manhandled into position, it can be lowered and additional bolted-on lift inserts attached to better balance the panel. In some instances this can be corrected by using special offset lifting adapters.

8. The crew gathers tools and work proceeds to the next panel, moving the crane if necessary. The elapsed time to lift and position a typical panel is between 15 to 30 minutes, depending on size. Lintel panels or L-shaped panels and other panels that require strongbacks may take a little longer. It may be seen from this that about 100 m or so of walling can be erected in a single day, thus demonstrating the speed of construction of tilt-up panels.

POSITIONING AND LEVELLING THE PANELS The rigging arrangements are designed so that as the panel rises it gradually rotates until it is in a near upright position at lifting. From here the crane jibs out and lowers the panel to its required position onto previously levelled pads located under each end of the panel.

SAFETY PRECAUTIONS The general safety precautions to be applied during the lift are given in more detail in Chapter 11, but some of the more important items in respect to the actual lift are included here for additional emphasis.

•

No personnel should pass beneath a nonvertical panel, under any circumstances.

•

All personnel should be at a safe distance from the panel when lifting the panel from horizontal to vertical.

The accurate erection of the first few panels is critical. Extra time spent in plumbing these in both directions and establishing the correct line will repay itself in quicker erection of succeeding panels. Extra time should also be allowed on the first panels for the erection team to become familiar with the procedure.

•

When tail ropes are used to control the swing of the panel, personnel should work clear of the panel.

Panels must be moved smoothly at all times to avoid shock loading which may induce cracking or possibly damage the crane.

•

Personnel should work clear of the panel edges as the panel may slew sideways.

Adjustments and tolerances

•

Tail ropes should never be wrapped around the hands or other parts of the body during the lift.

•

Whenever possible, panels should be lifted with the working gear facing the crane.

•

No attempt should be made to lift panels in strong winds.

It is of the utmost importance that the specified panel and joint tolerances are realistic. Once established they must be maintained. In general, panel variations lead to a growth in overall wall length. Depending on their size, joint details may be used to absorb these variations either progressively at each joint or collectively at one location, eg. at an oversail corner or doorway.

•

Braces, including knee braces when specified, should be connected before releasing the lifting gear.

If tilt-up panels are being used in conjunction with in-situ construction, then the tolerances for tilt-up panels should not be used to absorb the construction errors of the in-situ work.

LIFTING PROBLEMS

The levelling pads should be compressed fibre sheet, in-situ concrete or PVC shims of adequate strength to carry the loads. Steel shims should not be used since they can corrode, and can have too much frictional resistance, which can result in diagonal cracks near the ends of the panel as the panel shrinks. The tendency to crack can also be avoided by providing additional horizontal reinforcing at the bottom of the panel and continuous support from grouting under the panels.

Tilt-up construction has been developed and refined over the years and nearly every lifting problem can be avoided by proper planning. The following problems can occur, but can be avoided by paying proper attention to them during the design and construction phase. Further details may be found in Reference 3.

8-5


Concrete levelling pads should be the full thickness of the panel and at least 500 mm long. PVC shims should be at least 150 mm long. The length of the pads is dependent on the panel bearing stresses. The PVC shims have a high compressive strength 2 (typically 55 N/mm ), sufficient to resist the weight of even the largest panels. Shims are available in a range of thicknesses (typically between 1.5 and 6.5 mm) and should be limited to a maximum combined thickness of 25 mm.

type being considered. A minimum of two per panel should be generally employed. The braces are attached to the panel normally by fixings incorporated during its fabrication. The foot of the brace is usually attached to the floor by special anchor bolts inserted into the holes drilled into the floor slab (Figure 8.5). Sometimes the braces may need to be strengthened by additional knee bracing (Figure 8.6)

BRACING

Tilt-up panel (height = h) Prop fixed to wall panel

Because a tilt-up panel is not a completed wall immediately upon lifting, temporary support in the form of bracing provides the necessary protection against the forces of wind (Figure 8.3). Bracing gives the contractor time to turn isolated panels into one unified structure.

Fx=0.75Fw

h 2

Fw Wind force

A number of variables will effect the amount of force each brace each must resist and includes wind velocity, surface area of the panel, the presence or absence of openings, the dimensions of the panel and the angle of each brace relative to the panel.

2h 3

h 2 5

4

Prop fixed down to slab or deadman

3

Figure 8.4 shows a typical example of a brace set-up from which the required propping force may be determined.

0.5h Prop force Fb = 1.25Fw

The number of braces is then found by dividing this force by the safe working load for the particular brace

Figure 8.4 Typical bracing set-up

Figure 8.3 Example of braced panels

8-6

Panel erection

Figure 8.5 Connecting brace to floor slab

•

Whenever possible, affix the bracing to the panel before lifting.

•

During the lifting process, the braces should not hang freely below the base level of the panel. This may be achieved by the use of adjustable brace lengths or by the use of tail-ropes.

•

If bracing inserts are on the opposite face of the panel to the lifting inserts, tilt panel just past vertical in order to install the bracing.

•

When attaching the braces after the panel has been positioned cannot be avoided, the panels should be held firmly, safely and just past vertical by the crane whilst the braces are installed with the use of a ladder.

•

Adjustable braces should have stops on the threads to prevent over-extension.

•

Generally, a minimum of two braces should be used for each panel.

•

Whenever possible, avoid using normal expansion anchors as the bracing inserts in the floor. Instead, use special proprietary products.

•

Bracing bolts should be checked at regular intervals.

RELEASE OF PANEL The panel must not be released from the crane until the braces attached to the panel have been installed (including any knee, end or cross bracing) and the panel is approximately plumb. Some adjustment of the braces may be made after release (by turning the threaded portion), but the panel should be within about 100 mm of plumb before release.

Longitudinal brace

Knee brace attachment Main brace

Before being released from the crane the panel must also be level so that the vertical joint between panels will be of a uniform width.

Knee brace

FINAL GROUTING Figure 8.6 Strengthening main brace with a knee brace

The gap between the bottom of the panel and the footing should be grouted or dry packed to transfer the load to the footings. This will normally need to be done before roof or other members are installed.

On occasions, for example, when there is no floor slab or where it is required to brace the panel externally to the structure, the foot of the brace may be attached to a deadman cast into the ground, typically a large auger drilled hole or pit filled with concrete (see Figure 8.4). The weight of the deadman is typically required to be at least 80% of the maximum applied brace load.

Braces - general conditions for use

REFERENCES 1. The Burke Group. Burke on tilt-up. Burke, San Mateo, USA, 1984. 54 pp.

(1, 2)

•

Panels must be braced in accordance with the specification.

•

All braces should be marked with their maximum safe working load. In the case of adjustable braces, the safe working load, both zero extension and at maximum extension, should be marked.

2. Dayton Superior. Tilt-up construction handbook . Miamisburg, USA, 1985. 55 pp 3. Brooks, H. The tilt-up design and construction manual. HBA Publication, New Port Beach, USA, 1994. 370 pp. 4th Edition. 4. Cement and Concrete Association of New Zealand. Tilt-up technical manual. C&CA New Zealand, Porirua, 1990. TM 34. 32 pp.

8-7


8-8


9

NON-STRUCTURAL CONSIDERATIONS

This chapter contains a loose grouping of what are mostly non-structural design issues. Thermal design considers air penetration, thermal insulation and thermal mass effects and includes the performance of sandwich panels. Following a brief discussion of sound insulation the main range of finishes available is presented. Paints, their specification and application are then examined in more detail since they are used on the majority of tiltup panels. Finally, the main options for panel joints are considered, together with their advantages and disadvantages. insulation values are meaningless where air can flow through the myriad joints of some systems or even between the leaves of built-up systems. On the other hand, tilt-up is routinely used for buildings requiring controlled atmospheres because its large monolithic panels and widely spaced accessible joints are ideal for reducing air penetration. These qualities are readily exploitable for most buildings and may be considered in the overall thermal design strategy.

THERMAL DESIGN OF A BUILDING In contrast to many metal cladding systems, tilt-up construction provides a high-mass alternative, consisting of monolithic panels that are normally structural. This offers consequential benefits in air tightness, insulation, and thermal mass. There are basically three aspects requiring consideration in the thermal design of structures:

•

An evaluation must be made of the desired indoor environmental conditions.

•

An assumption of the typical weather conditions that must be taken into account when developing the best design to suit specific requirements.

•

Design should ensure that the procedures and physical properties of different structural materials are utilised effectively to ensure the best possible control of living and working environment.

Thermal insulation Thermal options for tilt-up wall panels - Tilt-up panels achieve good levels of thermal insulation, either as solid panels with internally/externally applied insulation, or as sandwich panels where the insulation is incorporated between the inner and outer leaves. The advantage of sandwich panels is that the insulation is enclosed by robust protective concrete layers during their production and this simplifies site construction by reducing second fix work.

The main factors determining the thermal response of a building are: the heat gains or losses through the various structural elements including the walls, windows, roofs and floors; the internal heat loads; and the rate of ventilation. The structural heat gains or losses are dependent on certain properties of the elements concerned.

The U-value for these elements can be easily adjusted to comply with the recommended values given in Approved Document L (2) to the Building Regulations or with the more stringent requirements that would be required, for example, for cold stores. Thus, use of this type of building element is compatible with moves to decrease energy use.

Insulated tilt-up concrete wall panels can provide the level of insulation necessary to meet the Building Regulations, and give thermal mass and reduced air penetration. Thus the physical properties combine to give highly efficient thermal building design. In the USA, the thermal mass of sandwich tilt-up panels has been successfully exploited in warehouses to reduce temperature fluctuations by up to 8°C. Prison authorities in Arizona and California have specified similar construction to maintain temperatures at 19 to 21°C – an established factor in reducing prisoner (1) disturbances .

The thermal transmittance of insulated tilt-up panels is affected by the type of insulation and, to a degree, by any connectors used in their fabrication. However, the calculated difference in the averaged U-value of a sandwich panel with stainless steel as opposed to composite ties is not more than a few percent when considering a typical building. The exact difference will depend on the extent and thermal efficiency of the installed steel tie system. Indeed, the effect on the overall energy consumption of the building is likely to be minimal as that will be largely influenced by heat losses through roof and floor and by air changes. In very critical situations, however, such as cold o stores operating at -20 C or so, the composite ties may become more beneficial as they virtually eliminate thermal bridging.

Air penetration In the UK, it is only recently that attention has been drawn to the need not just for adequate insulation and thermal mass, but also for airtight structures. International studies have shown that Britain lags behind in achieving the latter, and that theoretical

As with any other building element, moisture absorption by the insulation can also influence the

9-1


thermal behaviour of both insulated solid panels and insulated sandwich panels. Water has a higher thermal conductivity than air. Thus, if the insulating layer absorbs water, its thermal performance will be affected. It normally becomes an issue only in the cases of external insulation and then only when inappropriate insulation is used. Loss of insulation is not normally a problem with tilt-up since panels are normally internally insulated or are of sandwich construction, which is protected by dense concrete.

wall ties. However, the U-value indicated later in this chapter for sandwich panels with steel and composite ties allows for a typical arrangement of ties and connections in order to give an example of their calculated effect.

Likewise, interstitial condensation is not a problem as the concrete, ties and insulation are resistant to the effects of water under most normal building conditions

Insulation materials - The materials should: not be combustible or, in a fire, produce appreciable smoke or noxious and toxic fumes; be inherently proofed against rotting, mould, fungal growth and attack by vermin; not give rise to objectionable odours at temperatures at which they are to be used; not cause a known hazard to health, either in use or upon removal. A low thermal conductivity should also be obtained through the entire working temperature range.

In order to obtain an expression for the overall heat flow through the wall generally, it will normally be acceptable to determine an equivalent single resistance for the parallel configurations.

Calculation of thermal transmittance, U-value, for a building element - For a solid element composed of n different layers in thermal contact with each other, the U-value can be calculated using the series parallel method below:

The forms of insulation used for tilt-up construction, particularly 1 1 1 U = = × when used in sandwich panels, Σ R  d 1  A d 2 d n 1 1 commonly meet all the above  + +  + + ... +  hi Ai ho Ao  A1 × λ1 A2 × λ 2 An × λ n  requirements. Also, being protected by concrete, the panels retain their insulation and do not present a problem in terms of a fire hazard as can be Where 1/hi and 1/ho are respectively the surface the case with some other forms of construction (see resistances of the inner and outer surfaces (ie. Ai and Chapter 5, ‘Fire growth and spread’ on page 5-18). Ao respectively) to heat flow; λ1 ...λn and d 1 .. d n are respectively the thermal conductivities and Thermal performance of sandwich panels thicknesses of n successive layers of the different Thermal resistance of the inner and outer surfaces of materials comprising the element; A1...... An are the the structure should be calculated, taking into account cross-sections of elements through which heat flow the conditions of exposure, position of the member takes place; and A is the total cross section area of the and its emissivity (3). building element. Other factors being fixed, the heat The use of internal/external or sandwich insulation flow across an element is proportional to its U-value. enables tilt-up panels to be easily designed to meet Thermal bridging - Thermal bridges are formed by the provisions of the Building Regulations. materials with relatively low resistance to heat flow Knowing the thermal conductivity of different such as nails, metallic fasteners and concrete passing components of the wall, thicknesses of the layers and through or penetrating an otherwise well-insulated the surface resistance, an overall U-value can be element. (3) calculated using the series parallel method as In sandwich panel construction, connectors pass described earlier. Generally the insulation layer will through the layer of thermal insulation that is placed extend to the perimeter of the panel. Where it does to restrict the heat flow across the element and thus not cover the entire panel and solid concrete is may conduct heat from one side of the insulator to framed around it, this reduced performance should be the other. calculated. The effect on the thermal transmittance (U-value) of Tilt-up sandwich panels can be more energy efficient such an element depends on the total area and than those types of construction where the interior thermal conductivity of the materials used in the wall is merely covered by insulation or where air fabrication of the connectors. leaks in cavities between thin wall layers reduce the As indicated under ‘Concrete sandwich panels (steel energy efficiency significantly. There is growing ties)’, on page 9-3, thermal bridging by large recognition that circulation or loss of air in such components or connections can be a problem, but the cavities can increase the heat flow through the bridging caused by the ties used in an insulated tiltbuilding element and seriously reduce the effective up panel may not be that significant. thermal insulation. (2)

Approved Document L indicates that account needs to be taken of the effect of thermal bridging by use of the series parallel method, but this does not extend to

Traditional sandwich panels have two layers of concrete that are connected together by ties (typically

9-2

Non-structural considerations

steel or composite) and separated with a layer of insulation. Ultimately, the choice between steel and composite connectors will depend on a combination of cost, thermal and mechanical performance, design constraints, and efficiency of construction.

6500 Panel ties at 1200 mm over wall

250 mm

Concrete sandwich panels (steel ties) - Steel ties are used to connect the two layers while passing through the insulation (Figures 9.1 and 9.2). Both concrete and steel have very low thermal resistance in comparison with building insulation materials (150 mm of concrete has an R-value, thermal resistance, of 0.09 m2 K/W, nearly 1/8 of the thermal resistance of 25 mm of polystyrene foam which is equal to 0.7 m 2 K/W). The steel ties will form thermal bridges but their effect may not be that significant, as shown by the following example.

7500

The U-value of a typical plain sandwich panel with stainless steel ties ( λ = 16 W/m K) having an 125 mm inner concrete leaf, 75 mm thickness of insulation (λ = 12.1 W/m K) and a 75 mm outer concrete leaf is computed to be 0.371 W/m 2 K. This value is well within the Building Regulations minimum requirement of 0.45 W/m 2 K and demonstrates the good insulation achievable with tiltup construction.

150 mm

Connector pins Flat anchor Sleeve connector anchor

In the absence of any ties, the U-value for the same wall and insulation layer would be 0.36 W/m 2K. This shows that the effect of the ties is small and increases the heat flow by only a few percent when compared with a hypothetical panel without ties .

Figure 9.1 Typical concrete sandwich panel with steel ties

Internal concrete leaf

It is assumed in this example that the sleeve connector is not filled with concrete (creating a thermal bridge) and that there is no perimeter bridging. If such bridging does occur the effect should be allowed for as it can be significant Figure 9.1 shows a typical concrete sandwich panel with steel ties, and Figure 9.2 shows a typical cross section of a similar panel.

d1

External concrete leaf

d2

d3

Concrete sandwich panels (composite ties) - A sandwich panel system (Figures 9.3 and 9.4) using composite ties is marketed by Thermomass Building Systems. It is a specific type of concrete sandwich panel often used in tilt-up construction and provides a wall with a high degree of thermal performance.

The panels incorporate prefabricated Styrofoam (EPS) insulation. The thermal conductivity of this type of insulation is known to be low and around 0.03 W/m K, and it has a closed cell structure, which restricts the uptake of moisture.

Insulation

Figure 9.2 Typical cross section of a concrete sandwich panel with steel ties

This building system also reduces the heat loss and gains through building elements through the thermal bridging effect as fibre composite materials are used in the fabrication of connectors (PC-connectors). These materials have a much lower thermal conductivity than steel connectors.

steel ties, the U-value reduces to about 0.361 W/m 2, virtually identical to a panel without ties. From this example, the U-value calculated for a panel with PCconnectors is better than for steel ties, but not appreciably so. However, since the actual U-value will depend on the number of ties and connectors and their installation, it is best assessed individually for

Taking a sandwich panel of the same dimensions as previously but with composite connectors rather than

9-3


each panel configuration. The use of composite ties may, however, become more beneficial where there is a need for extreme insulation as might, for example, be the case with cold stores. Also localised cold spots due to thermal bridging, which can occur with metal ties, are eliminated.

Solid insulated panels - The alternative to insulated sandwich panels is internally or externally insulated solid panels. These can provide similar levels of insulation to sandwich panels but normally have the insulation applied after the erection of the panels as part of the other building works. This insulation can be positioned part internal and part external (as in the case of the Glenrothes project shown in Chapter 2). The position relative to the concrete panel will radically affect the usable thermal mass. In these cases the external insulation can be protected by an earth berm, which also provides some insulation and allows the internal insulation to be raised above the circulation area thus eliminating damage due to traffic within the building. The amount of insulation for this form of tilt-up panel can be readily obtained (2) from Approved Document L to the Building Regulations.

6500 Panel ties at 400 mm over wall

250 mm

Thermal mass

7500

Thermal mass is the property that enables building materials to absorb, store and later release significant amounts of heat or coolth. Structures constructed from concrete and masonry have a unique energysaving advantage because of their inherent thermal mass. Smaller peak internal temperatures compared with those of the exterior environment, stability of inside temperatures, and the time lag between the occurrence of peak internal and external temperatures are the favourable thermal characteristics observed in energy-efficient high mass construction.

150 mm

Composite connectors

By reducing the amplitude of the internal temperature fluctuations, thermal mass reduces the cooling loads and makes the environment more comfortable. Delay of peak temperatures in the internal environment, known as thermal lag, shifts the peak cooling loads to cooler times of the day when any air-conditioning equipment installed can work more efficiently.

Figure 9.3 Typical concrete sandwich panel with composite ties

Internal concrete leaf

d1

External concrete leaf

d2

A number of energy-efficient commercial developments in the UK have demonstrated clearly the effectiveness of concrete’s passive cooling effect and thermal inertia in controlling the thermal (4) environment within a building . The calculation of heat and energy balance with consideration of thermal mass and thermal capacity (5) are set out in prEN 832 and the CIBS Building (6) energy code .

d3

When the insulation layer is located between the two concrete layers or externally - allowing a concrete surface to be in direct contact with the internal space – significant thermal capacity is presented to release and store energy, and limit temperature swings in the interior of the building. The impact of thermal mass is more significant in reducing the cooling loads than the heating loads.

Insulation

Tilt-up sandwich walls - Insulated tilt-up sandwich panels are designed to meet all the criteria required for energy efficiency as set by the standards. The

Figure 9.4 Typical cross section of a concrete sandwich panel with composite ties

9-4


exposed internal concrete leaf (generally the structural leaf) can store and release significant thermal energy due to its thermal capacity. Therefore, it can help prevent wide temperature swings in the interior of the buildings and produce a more comfortable living or working environment.

At the frequencies controlled by mass, the Sound Reduction Index increases at a rate of about 4 dB for each doubling of mass, which for a particular material means a doubling of thickness. However, this is a significant change since it relates to a log scale, and doubling the mass or thickness effectively reduces the sound energy level by half.

Since the insulation layer is located in between the two concrete layers it allows the concrete mass to be in direct contact with the internal space. Therefore, this type of construction is more effective in moderating the indoor temperatures compared with a wall which is lined by an insulation material on its internal surface.

Single panel solid concrete wall - The sound insulating performance of a plain concrete panel can be approximated using the mass law, assuming that it is well sealed at the joints and edges and that bypass routes are adequately blocked. According to Building Regulations, a 190 mm thick wall of in-situ concrete 3 (minimum density of 2200 kg/m ) can provide reasonable sound resistance of 52 dB. The use of a plaster coating is optional.

In the USA, building thermal performance Standard ASHRAE.90.1 (7) allows designers to recognise the beneficial effects of high thermal mass by calculating an effective U-value incorporating a correction for fabric energy storage. A sandwich panel system supplier has calculated effective U-values, modified for dynamic effects, for a range of constructions and locations in the USA (8). Depending on the local climate, typical effective U-values are improved by a factor of between 1.3 and 3.0 over steady-state values.

Sandwich panels - The typical sound resisting performance of a double or multi-layer construction is different from and more complex than that of a single wall.

However, mass law could be applied to estimate the sound insulation of tilt-up sandwich panels in frequency regions where resonance and coincidence do not occur. It is recommended that only the combined thickness of the two concrete layers is considered in the calculation.

SOUND INSULATION The general subject of sound insulation is so complex that in-depth study is outside the scope of this design manual. It is, however, possible to outline the principal objectives of sound insulation and to give general guidance on the sound performance properties of concrete tilt-up panels.

Ideally ties should not be rigid, and should be kept to a minimum in sandwich panels to minimise the formation of sound bridges. When sound insulation is critical it is recommended that tests for different frequency regions are undertaken to give a better understanding of the sound insulating behaviour of tilt-up concrete sandwich panels.

There is a need for separating walls between dwellings, and indeed walls between individual rooms of all types of buildings, to posses adequate sound insulation. It can also be beneficial, and in some instances will be necessary, to provide adequate sound insulation to external walls in order to reduce sound entering or exiting a room or building.

In respect to airborne sound, concrete tilt-up walls can be provide excellent levels of sound insulation, typically in the order of 52 dB or better. This can be compared with normal metal cladding systems, which, due to their low mass, often only provide less than 30 dB sound reduction without resorting to specialist designs for which the cost may be uneconomic. This is a very significant difference and is a major benefit offered by tilt-up construction.

Sound insulation in buildings may be provided to resist the transmission of both airborne and structural or impact sound. Resistance of a wall to airborne sound transmission mainly relies on its weight, stiffness and the degree of isolation between the leaves in the case of multilayer panels. Concrete can provide such airborne sound insulation and provide excellent control of impact sound when used in conjunction with a floating/resilient surface layer.

PANEL FINISHES Surface treatments and finishes A major factor in the acceptance and increased use of tilt-up construction in Australia, the USA, and New Zealand in recent years has been the improved appearance of the structures. A wide variety of aesthetic effects has been created using treatments and finishes, most of which are simple to achieve (Table 9.1). In addition to the advent of specialised paints and coatings at reasonable cost, much of the improvement is due to the sensitive detailing of panels.

Mass law One way to assess the performance of an element to resist airborne sound is by use of the mass law, which relates the sound reduction index to the mass of the wall. The mass law should, however, only be used to give an approximate guide to the insulation obtainable. In practice the insulation obtained is always a few dB less than the theoretical maximum.

Thoughtful use of grooves, textures and colour can break down the scale of a large flat wall to make it

9-5


more appealing, and can highlight particular areas or features. Although several of these finishes can be achieved on the face-up side of a panel at casting, it is normal to apply them to the face-down surface in order to ensure consistent quality finishes, to avoid lifting fittings in the visible outer face, and to allow stack-casting.

Grooves and relief The creation of grooves in tilt-up panels at the time of casting is one of the easiest ways of providing visual interest to otherwise large flat areas of concrete. If the width of the groove is chosen to match a standard paintbrush or roller, it becomes a simple matter to apply a coloured coating in the groove that will contrast with the colour of the panel.

Table 9.1 Types of finishes used for tilt-up panels Type

Variations

Coloured concrete

Cement/aggregates

Fairfaced concrete

Cement/aggregates

Exposed aggregate /sand bed

Cement/aggregates

Such grooves are easily formed in face-down casting by fixing strips of sealed timber, or expanded polystyrene or polyethylene to the casting bed. The sides of the grooves should be tapered by a minimum of 15º to permit easy removal of the groove former and to avoid sharp corners, which may chip. Groove formers may be stuck to the floor slab using gunapplied mastic. Close attention to correct setting out and location is essential to ensure high quality finishes.

Acid etched Formliners

Elastomeric 1 (Polyurethane, silicone rubber)

A similar technique using sheet material such as plywood or expanded polystyrene is possible for simulating windows, forming architectural features and providing relief on an otherwise flat surface. It is possible to use shaped formers to produce indented letters or logos to give the building a company identity.

2

Rigid plastic (GRP, ABS, PVC) Coatings

Paints Textured

Trompe l’oeil

Paint, texture and shape

Feature strips

Shape and position

Brick

Slips

In all cases, the depth of the groove or shape should be less that 20 mm. Deeper indents could have a significant impact on durability and structural strength, as the reduced section may become the critical one for design for strength and durability. Well thought out detailing can avoid reduced sections at critical locations such as horizontal bands at the mid-height of panels.

Cropped bricks Brick outer leaf Openings/pierced

Size, shape and location

1.

Generally multiple factory use only, due to high cost.

2.

Typically single-use, lower cost.

Formliners are an alternative method for producing a series of grooves to give a ribbed finish. They are available in a variety of patterns. They can be made of rubber, plastics, timber, or metal decking and are laid on the casting bed to form a profiled or textured surface against which to cast the concrete. In general, selective and sensitive use of formlined texture/relief in relatively small areas is more effective than larger areas, depending on the fineness of the relief produced. When using formliners, particular care must be taken to choose an appropriate bond-breaker.

Procedure The whole casting procedure, its location, sequence, lifting timing requirements, etc. can be influenced by the choice of finish to be used. Although it is possible to give a special finish to both faces of a tilt-up panel, this is not recommended. The internal face is normally given a plain smooth finish suitable for painting or leaving as formed.

Exposed aggregate

It is important to appreciate the economics and production implications of certain finishes. For instance, the use of expensive elastomeric formliners for large areas of relief will dictate sequential casting involving repeated use of the same formwork, more suited to factory precasting than site casting.

Exposed aggregate finishes may be formed on either face-up or face-down surfaces, but quite different techniques are required. For face-up work, the traditional water-washing approach that is used for paving is appropriate. Chemical retarders are often used to create exposed aggregate finishes on panels cast face-down. Care must then be taken when placing the concrete not to abrade the retarders. Protect the casting surface during the initial placement of concrete by deflecting the pour with shovels, a wood baffle or similar equipment. Subsequent wet concrete should be

A similar situation may occur for some brick finishes that involve jigs for locating the brick units. However, cheaper single-use formliners, especially for smaller areas, may be economical for site tilt-up involving panels cast simultaneously.

9-6


placed onto previously placed material and then spread with rakes or other hand devices.

Brick finish A variety of methods is available to provide or simulate brick finishes to tilt-up panels. The Scott Brick System from the USA supplies brick slips with dovetail keys on their concealed face, which fit into a snap-together spacing system that simulates both stretcher and stack bonds. The sacrificial spacing system replicates a lightly recessed jointing effect and the visible face of the brick slips is protected by wax against grout staining.

Sand embedment is the normal technique used for face-down casting. The selected larger aggregate or stone is spread in a single layer over a thin bed of sand and tamped into place. While pouring the concrete care must be taken not to displace any aggregate from the sand bed. On lifting the panel, the sand is brushed off to leave the embedded aggregate showing on the surface. This latter technique can also be used to create patterns or designs, either by using selected coloured aggregates or by choosing pieces of flat stone to form a mosaic or inlay. It is also common practice to delineate different areas of exposed aggregate by means of grooves or recessed plain bands.

Another method successfully employed in the UK uses bricks cropped in half along their long axis, through holes formed during normal manufacture. However, the bricks are thicker than slips and require a spacing system in the mould used for their casting, which must be sequential, so compromising fast tiltup construction.

Using a suitable concrete mix, an effective exposed aggregate appearance can also be achieved by sandblasting, grit-blasting or acid-etching the finished panel.

A further alternative is for tilt-up to be used as the loadbearing inner leaf of the building. This allows a brick outer to be laid later, off the critical path, and tied to the concrete panel across the cavity formed.

Surface treatments Surface treatments are one of the easiest and most versatile ways of improving the appearance of smooth-finished surfaces and can vary from simple acrylic paints or high-build elastomeric coating systems to chemically-bonded finishes or stains guaranteed for up to 25 years. The wide range of colours available makes it possible to choose an attractive colour scheme, whether it is for decoration or to give corporate identity to a building. Surface treatments can be applied to the total surface or used to highlight parts of it; they can be easily reinstated after damage and may be changed to give a new image following a change in ownership or tenancy.

Combinations

Proprietary high-build surface treatments are available to produce a wide variety of textured finishes from fine to coarse. They can be sprayed, brushed, rolled, combed or trowelled onto the concrete and can be coloured as desired. Such coatings have the advantage of masking minor imperfections in the concrete surface, but will not conceal major blemishes.

The USA-based L. M. Scofield Company suggests that at least one month prior to placing any concrete that will be textured, a sample panel having adequate horizontal and vertical dimensions should be made for approval on the job-site using the contemplated materials, mix designs, pour rates, and construction techniques including timing for stripping of the formwork or liner.

For any surface treatment to be successful, the surface must be properly prepared to receive it. In tilt-up construction it is also advisable to check what effect the bond-breaker/curing compound may have on any subsequent treatment. Some materials are incompatible and will impair coating adhesion unless physically removed. Pressure washing may be sufficient to do this once the chemicals have started to break-up with UV exposure. Further information on surface treatments is given later in this chapter in ‘Painting tilt-up panels’.

The TCA also recommend test panels as an effective means of setting a standard for variations in colour or surface finishes. It is thought that a representative test tilt-up panel could include:

The various finishes possible for tilt-up construction need not be considered in isolation. Some stunning effects can be achieved by combining two or more finishes on the same panel.

Mock-ups and test panels For many tilt-up buildings it is usual to erect a small test panel for all parties concerned to agree upon standards, particularly for finishes. Mock-ups such as these allow alterations before construction and easy reference during construction.

• • • • • •

Trompe l’oeil

Range of aggregate size and colour Range of admixture/grout colour Edges and reveals Formliner joints Lifting/bracing component holes Degree of abrasive blasting.

Examples of panel finishes

Careful use of rebates and/or paints can give apparent relief, shading and form to flat panels.

Examples of a range of panel finishes are shown in Figure 9.5 on the next page.

9-7


Figure 9.5 Exales of panel finishes

Figure 9.5 Examples of panel finishes

9-8


of the selection process given in the above reference is shown in Figure 9.6

Achieving successful finishes Although it is a relatively simple matter to achieve a wide variety of different finishes for tilt-up construction, attention paid to the following points will maximise the potential for success:

•

Ensure the casting bed is free from physical blemishes that will be reflected in the panel surface.

•

Use grooves or reveals to break up large panel areas. This reduces scale, provides visual interest and creates smaller defined areas for easier coating application.

•

•

•

Main reason for treatment

Properties required

Select coating textures which are appropriate for the visual effect required and the condition of the panel concrete at casting. Areas that will be subject to close scrutiny probably justify more elaborate textures.

Preferred route, but not possible Specific type of treatment

Ensure that the curing compound and bondbreaker will not interfere with the application of the final coating. Follow the recommendations of the coating manufacturer with regard to surface preparation and the method of application.

Service conditions Application conditions Assessment Assessment of test data and case histories Health and safety (Cost)

Potentially suitable types of treatment

Preferred route, but not possible Proprietary products Same generic type

Conduct trials to ensure that the chosen finishes produce the desired effect on the concrete surfaces produced on the job-site from the proposed floor slab finish.

Proprietary products Different generic types

Figure 9.6 Outline of the selection process for a surface treatment

The range of surface treatments appropriate for tilt-up construction is very wide, covering the full spectrum of cost and appeal. The more expensive techniques are not always applied to a whole project: it is quite common to use these eye-catching treatments only at the front of the building, or for small areas, applying an alternative finish to the remainder. In this way, the effect on overall cost is small, although the improvement in appearance is considerable.

Paint materials Surface treatment classification by generic material type is complex (Figure 9.7, page 9.10) and although certain generic performance characteristics can be ascribed, the degree to which they are achieved depends critically on the particular formulation. Therefore, it is unwise to substitute a specific product for another of the same generic type for cost or other reason without careful consideration. A further useful subdivision of treatments based on film thickness yields the following two categories:

PAINTING TILT-UP PANELS Painting is the most common method used to finish tilt-up panels, its primary function being to enhance the appearance of the structure, through properties such as colour, texture, cleanability, opacity and mould resistance. This section therefore examines surface treatments in more detail.

Selection of paints

•

Coatings which depend on the formation of a continuous surface film to shield the surface.

•

Materials which impregnate the concrete but do not depend on the formation of a significant surface film. These may be either: i)

There are many different surface treatments on the market with differing benefits and weaknesses, which must be appreciated when selecting material appropriate to a specific situation. It is beyond the scope of this publication to give more than brief pointers to help in the consideration of manufacturers’ recommendations for selection of appropriate paints, preparation and application methods.

penetrants which line the concrete pores and prevent ingress of liquids by absorption or capillary action or

ii) sealers which block the pores with minimal surface film or affect on appearance. Table 9.2, on page 9-10, gives a simplified overview of the common applications of the main types of surface treatment relating to appearance. It is presented only as an initial guide to contribute to informed discussions with suppliers of specific treatments, and should be supplemented by test results, case histories and predictions of the service conditions of the actual tilt-up panels.

(9)

The Concrete Society’s guide provides more detailed guidance on the selection and application of paints, referred to as ‘surface treatments’. An outline 9-9


Inorganic

Cementitious (usually polymermodified)

Organo-metallic Siliconates Stearates

Silicates Silicofluorides

Thermosetting

Two-pack

Solvented Epoxies Polyurethanes

Miscellaneous Bituminous Oleo-resinous and drying oils Alkyds*

One-pack Moisture-curing polyurethanes (may be solvented)

Water-borne Epoxies

Organic

Organo-silicon Silanes Siloxanes Silicones

Thermoplastics and synthetic rubbers

Solvented Acrylic Vinyls Acrylated rubber Chlorinated rubber Polyurethane (one-pack)**

Solvent-free Epoxies Polyurethanes (Meth)acrylates Unsaturated polyesters Vinyl esters

Water-borne Acrylic Vinyls Acrylated rubber Styrene-butadiene

* Including urethane modified materials which may be referred to as polyurethanes.

** Non-reactive - solid or high viscosity polymers in solution.

Figure 9.7 A classification of surface treatments (Taywood Engineering Ltd)

Table 9.2 Use of generic treatments to enhance or maintain appearance Inorganic

Hybrid

Organic r e l a e s g n i l l i f e r o P

g n i t t e s o m r e h T

3

3

3

3

3

3

3

r e b c i b t s u a r l c p i o t e m r h t e n h y T S

s u o e n a l l e c s i M

s u o i t i t n e m e C

d e s a b e t a c i l i S

Mould, dirt staining

3

3

Uniformity after repair

3

3

Anti-graffiti

3

3

(3)

(3)

(3)

3

(3)

(3)

(3)

3

3

3

3

3

3

3

Use

Colour, texture, reflectance

e t a e n t o a c r i e i l t S S 3

e e n n o t e a c a x i o l i l i l i S S S 3

Comments

Some products only

This table is for initial guidance only. It is prescriptive and should not be used directly for the final selection or specification of a surface treatment

Whole life cycle performance of the paint is an important consideration in its selection. Poor performance is likely if basic procedures necessary to produce painted panels are ignored.

Achieving a successful paint-job Some of the factors that can influence the quality of a painted finish include: concrete cure, surface dryness, moisture content, temperature, use of bond-breakers and curing compounds, paint type, paint application method, or interaction of more than one of the above

9-10


items. Material from the TCA datasheet on the painting of tilt-up panels (Reference 10) is summarised below, amended for UK practice. It presents some of the basic techniques used by successful tilt-up contractors to improve consistency and quality of painted finishes. Bond-breaker - Some of the bond-breaker applied to the floor slab to prevent sticking can be transferred to the down-side face of the tilt-up panel and must be removed prior to painting the panel. To aid removal, bond-breakers formulated for tilt-up are normally designed to degrade under the UV component of daylight.

It is advisable to check with the manufacturer of the bond-breaker and paint to make sure the materials are compatible and, if there is any doubt as to the compatibility, do a test application. Note that the test paint sample may need to dry for some days before adhesion testing. CIRIA Technical Note 139 (11) and (12) draft European Standard pr EN 1542 give further details of testing.

3.

The use of a washer having an oscillating tip with a pressure of at least 300 psi is recommended.

5.

Clean all joints in addition to the panel faces. The same factors that prevent paint from adhering to the panel will also prevent joint sealants from adhering.

6.

In the USA, many sealants are painted over, requiring checks with the sealant manufacturer regarding compatibility, curing time and other factors affecting paint adhesion. Generally such joints should cure for at least 10 days prior to painting.

Application methods include spraying, rolling and brushing, and are specific to the treatment under consideration. Advice about methods is given in (16) BS 6150 . The choice of paint system and its method of application will be affected by the method of access (eg. mobile platform) and health and safety issues.

Service life Experience in the USA is of paints typically lasting some seven to 12 years before needing repainting, primarily due to fading. During the research of this publication, several European paint manufacturers have come forward with products with claimed special benefits for tilt-up. For instance Andura claim special properties to cope with moisture trapped in new concrete, and some Keim products carry guaranteed service lives of up to 25 years (see Chapter 13). It is not feasable to provide a full list of manufacturers and products, and specifiers are encouraged to contact a variety of sources for comparison of properties, application methods, service lives, and costs.

WEATHER RESISTANCE OF PANELS AND JOINTS

Surface preparation - Surface preparation is the key to a successful paint job. It must be done consistently and thoroughly with no exceptions. All bondbreakers or form release agents, oils, dust, mould and mildew must be removed from the panel. TCA tips include: 1. Wait a minimum of 30 days after erection before cleaning the panels. Clean panels prior to any patching so that the repair will adhere to the base. Clean again if necessary.

The most economical and effective method for tilt-up panels is the use of a power washer. Bond-breaker and paint manufacturers typically have recommendations regarding the removal of bond-breakers, indicating whether power washing, detergent washing, etc. is required. (13) Further advice is also given in BS 6270 , (14) (15) ASTM D4258 ; and ASTM D4259 .

The concrete should change colour after surface drying from a mottled grey to a more consistent light grey.

Application

Panel curing and drying - The typical tilt-up panel is erected between seven and 14 days after pouring. Leaving the panel in its horizontal position for longer periods increases the strength of the concrete but does not allow excess moisture to escape from the face of the panel in contact with the slab. Moisture levels in concrete can sometimes give rise to problems with application and performance. Some paint formulations are more tolerant of application to damp surfaces and are more vapour permeable and thus less likely to suffer bond failure from vapour trapped behind the coating. However, even these will perform better if applied to dry concrete. In view of this, the manufacturers’ recommendations should be sought on applicability to tilt-up panels which, as with all new concrete, will contain some moisture even when surface-dry.

2.

4.

General The high quality concrete used in the production of tilt-up wall panels results in an exterior surface that is durable, resistant to freeze-thaw effects, and resistant to water penetration. Concrete is typically specified to have a minimum 2 compressive strength of 35 N/mm and can be airentrained to give enhanced durability or where subject to the effects of de-icing salts. For this reason, special sealers and protective coatings are not (17) generally needed . Further guidance on weathering of buildings may be found in Reference 18. The following section gives guidance on the appearance and weatherproofing of joints and on joint width and sealants. Further information and illustrations of joints are given in Chapter 3.

9-11


joint widths, it is preferable to allow cumulative tolerances to be absorbed at corners or opening (see Chapter 3).

Joints The importance of joint detailing in respect of the cost, appearance and performance of a tilt-up building cannot be over-emphasised. Joint details must be compatible with the structural design assumptions, the erection procedures, the fixing details and the construction tolerances. When this is done, the expected service life of a sealant can be up to 20 years, and even more under favourable conditions (19) . The main aspects of joint design which need to be considered are given below.

The selection of the sealant for joints is complex and involves the consideration of a number of factors, eg. expected movement, type of sealant, width-to-depth ratio of sealant. A full discussion of all factors is outside the scope of this manual, but detailed evaluations can be found in BS 6213 (19) and BS 6093 (20) (21) . Overseas publications include the ACI guide (22, 23) and the BRANZ publications .

Appearance - The number of joints should be kept to a minimum. If a small-panel appearance is desired then this can be achieved by the use of false joints (grooves) in the panel surface. It is usually desirable to express the joints, not to try to hide them. The use of a recess or a dark band of paint on either side will help mask any variation in the width of a joint. It will also minimise the effects of any variable weathering at the joint line. In certain circumstances, for example with heavily ribbed panels, it may be possible to conceal the joints in the overall texture of the wall.

The following general points for sealants should be noted:

•

Wide joints lower the strain due to volume movements and are to be preferred.

•

The preferred width-to-depth ratio varies with different sealant types. For an elastic sealant this ratio is 2:1. Adopting the correct geometry helps to minimise the stresses developed.

•

Sealants should be bonded only on the two side faces.

Bevels at the edges of panels are desirable as they reduce the vulnerability to damage during handling and mask the effects of construction tolerances. Corners of tilt-up buildings can be designed in a number of ways. Oversail joints, for example, can be used to alleviate build-up of construction tolerances where it is acceptable to show a panel edge on one façade (its prominence will depend on the design details used on the face of the panels). Mitred joints allow a uniform surface treatment of both walls, but they do impose greater restrictions on construction and erection tolerances. (Details of these joints are given in Chapter 10.)

•

Backup rods, which do not bond to the sealant, are available to control the depth and profile of the sealant.

•

To enable a good bond to be made with the sealant, the concrete faces at the joint should be dense, smooth, clean and dry.

•

The compatibility of the form-release agent and any curing compound with the adhesion of the chosen sealants should be checked.

•

The extension and compression capacities of mastic sealants will be inadequate for most tilt-up structures.

Weather-tightness and maintenance - Joints between wall panels will usually need to be weathertight. Face-sealed joints or gasket joints are usually preferred between panels, although more expensive open-drained joints can be used for very exposed situations.

•

The effect of ageing and exposure on the sealant must be considered

•

Most tilt-up buildings are not tall and therefore access to the joints for maintenance of repair may not be difficult or costly.

•

Where long life is required a high performance material (eg. uv resistant) is desirable.

•

If the sealant is to be painted it must be compatible with the paint system.

Illustrations of joints are given in Chapter 3, Figure 3.11. The advantages and disadvantages of the four types of joint are summarised in Table 9.3. At a corner joint the situation is different. The movement of the joint will include some shearing as well as tension and compression, so the criteria for the selection of the joint-sealing material will be different.

REFERENCES 1. Glass, J. Evaluation of tilt-up construction in relation to selected UK building types. Post Graduate Research School, School of Architecture, Oxford Brookes University, UK. 1997.

Joint width and sealants - Joints must be able to accommodate rotation and the variations in width caused by construction and erection practices. They must also allow the panels to move relative to each other as the environment changes, eg. changes in temperature or humidity.

2. HMSO. The Building Regulations 1991, Approved Document L. HMSO, London, 1994. 74 pp. 3. Chartered Institute of Building Services . Thermal properties of building structures. CIBSE, London, 1980. Guide A3. 58 pp.

The two simple recommendations for weather joints between panels are: (1) they work best in the range 12 to 25 mm, and (2) in order to maintain specified 9-12


Table 9.3 Comparison of weather-tight joints Joint type

Face-sealed joint

Advantages

Disadvantages

Simple edge profile (no grooves necessary). Completed joints easy to inspect.

Effectiveness of seal totally dependent on continued adhesion and performance of sealant. Access necessary to front face of panel after erection. In order to ensure good adhesion surface of concrete needs to be clean, smooth, dense and dry. Sealant exposed to deteriorating influences of weather (UV light etc.)

Open-drained joint

Basic sealing mechanism dependent on geometry not on adhesion. Will tolerate larger construction variations and subsequent movement. Installation during wet weather possible.

Requires more complex edge formwork. Profiled edge can be prone to damage during construction. Installation of baffle can be difficult. Drumming of baffle caused by wind may be objectionable.

Rear air seal protected from UV light and weather. Gasket joint

Simple edge profile. Quick to install. Completed joints easy to inspect

Precompressed impregnated filler strips

Simple edge profile.

Maximum construction width tolerance is about ± 4 mm, requiring special consideration during design and construction. Access necessary to front face of panel after erection.

Quick to install. Completed joints easy to inspect. Will tolerate larger construction variations and subsequent movement. Installation during wet weather possible.

Sealant exposed to deteriorating influences of weather (UV light etc.) Installation is low-temperature sensitive. Synthetic impregnate may be necessary for longest life.

Does not need wide joints to cater for large movements. Seal dependant on precompression not just adhesion.

4. Flynn, M, O’Neill, B. and Shaw, G. Project profile - Powergen headquarters. RCC, Crowthorne, 1996. 97.361. 13 pp.

design of new buildings and services: (a) Heated and naturally ventilated buildings. CIBSE, London, 1981.

5. British Standards Institute. prEN 832. Thermal performance of buildings - Calculation of energy use for heating - Residential buildings. CEN, August 1992.

7. The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). Standard 90.2. 1989. 8. CTC. Thermomass manual. Geographical considerations for mass factor effect in concrete insulated sandwich panels. Composite Technologies Corporation, Ames, Iowa, 1996.

6. Chartered Institute of Building Services Engineers. Building energy code, Part 2, Calculation of energy demands and targets for

9-13

Tilt-up design and construction Non-structural considerations

9. Concrete Society. Guide to surface treatments for protection and enhancement of concrete. Concrete Society, Slough, 1997. Technical Report No.50. 87 pp. 10. Tilt-up Concrete Association. Tilt tips No.3, Painting tilt-up panels. TCA, Ames, USA, 1993. 2 pp. 11. CIRIA. Standard tests for repair materials and coatings of concrete, Part 1, Pull-off tests. CIRIA, London, 1993. 41 pp. Technical Note 139 12. British Standards Institution . Draft European Standard pr EN 1542. BSI, Milton Keynes. 13. British Standards Institution . BS 6270. Code of Practice for cleaning and surface repair of buildings, Part 2. BSI, Milton Keynes, 1985. 20 pp. 14. American Society for Testing of Materials . D4258, Standard practice for surface cleaning of concrete for coating. ASTM, Philadelphia, USA, 1992. 15. American Society for Testing of Materials. D4259, Standard practice for abrading concrete. ASTM, Philadelphia, USA, 1992. 16. British Standards Institution . BS 6150:1991 Code of practice for painting buildings. BSI, Milton Keynes, 1991. 132 pp. 17. Portland Cement Association. Precast concrete loadbearing wall panels. Building system report . PCA, Skokie. 12 pp. 18. Hawes, F. Appearance matters 6 - The weathering of concrete buildings. Cement & Concrete Association (now British Cement Association), Wexham Springs (now Crowthorne), 1986. 47.106. 48 pp. 19. British Standards Institute BS 6213. Guide to the selection of joint sealants. Milton Kenyes, BSI, 1982 (1992). 32pp. 20. British Standards Institute BS 6093. Code of practice for design of joints and jointing in building construction. Milton Keynes, BSI, 1993. 48 pp. 21. American Concrete Institute . CI 504R-77. Guide to joint sealants for concrete structures. ACI manual of concrete practice Part 5 ACI, Detroit, 1990. 22. Building Research Association of New Zealand. Sealed joints in external claddings: 1 Joint design. BRANZ, Judgeford, 1984. Building Information Bulletin 238. 23. Building Research Association of New Zealand. Sealed joints in external claddings: 2 Sealants. BRANZ, Judgeford, 1984. Building Information Bulletin 238.

9-14


10

CONNECTION DETAILS

This chapter examines the need for connections in structures incorporating tilt-up panels, the requirements on their performance, and their conceptual design. It then reviews connection types and presents typical details which designers may wish to develop or adapt to suit the particular demands of individual projects. However, it should be noted that the diagrams are illustrative only, and do not show all the detail of construction. contractor may, however, provide some input into the design process.

GENERAL CRITERIA Tilt-up panels often fulfil a multitude of functions as part of the structural system of a building. In addition to serving as the external cladding (non loadbearing panel), it is common for wall panels to carry vertical and horizontal loadings (loadbearing panel). The connections must therefore be designed to allow panels to transfer the various forces from the roof and floor members into the foundations.

When tilt-up is precast off site, the design architect and structural engineer provide information in the contract documents to identify the materials, locations, and the magnitude and type of loading for all connections. The precast producer then designs and details the panels and connections. The shop drawings and structural calculations are then submitted to the architect and structural engineer for approval.

Connections must also be capable of providing a degree of ductility for relief of temperature and shrinkage stresses, for seismic energy absorption, and to allow safe predictable behaviour under fire loading. Finally, it is important that cosmetic cracking of concrete around connections is minimised, particularly at service load level.

Because each project has a unique set of design problems it is important that the architect discusses connection ideas with the structural engineer and (5) precast concrete producer early in the design process . Connections should be protected from fire to the same degree as the component they support. Connections should also:

When designing connections, strength and serviceability criteria must be met. Details that are not properly considered in design may result in costly construction delays or unsafe structures. BS 8110 (1) part 1: section 5 and Reference 2 provide information on design and detailing of precast construction but they do not relate specifically to tilt-up.

• • • •

Be easy to install Have adequate ductility and flexibility Have provision for on-site adjustment Be repetitive and accommodate tolerances.

Although general in nature, the above conceptual design considerations are important since the connections need not only to be structurally adequate but must also ensure efficient construction and enable (4) the take up of tolerances .

More detailed information directly relevant to tilt-up may be found in References 3, 4, and 5 which give an overview of important design criteria and materials that should be considered in connections for tilt-up construction. The following section highlights some of the common requirements for connections used on tilt-up, and discusses the types available. The information and details given are for general guidance only and the designer must assess their suitability and adapt as necessary for the particular project under consideration.

Load path - Each structure with all its elements and connections should be considered as a single structural system. Each connection is not an isolated element but is part of an integrated system. An applied external load (including that caused by volume changes) is distributed by load paths in the structural building system to the support foundations. Load paths induce internal forces between elements of the system. In an effort to simplify the connections, an efficient design considers the number and magnitude of internal forces within a structural system.

CONNECTION DESIGN Conceptual design When tilt-up is site cast, the architect and structural engineer together will be responsible for determining the materials, locations, and the magnitude and type of loading for all connections. They will also produce the designs and details of the panels and connections that are given to the tilt-up contractor. The tilt-up

Failure modes - The designer should be aware of the potential modes of failure in each connection. Sufficient redundancy should be provided to eliminate the potential for a progressive collapse. Failure mechanisms are often obvious and easy to

10-1


define. Failure modes that are difficult to determine should be assessed by testing.

TYPES Rigid connections can be subject to unanticipated stresses due to volume changes, and may fail. An alternative to a rigid connection is one that relieves stress by allowing movement to occur. Flexibility can be attained in various ways: bearing pads supporting structural members can offer stress relief and low friction materials allow a member to slip, thus accommodating movement.

Connections that subject concrete to tensile forces can result in brittle failure modes. Unlike a ductile failure, a brittle failure is usually sudden and without warning. If a ductile connection (such as by yielding of reinforcement after cone failure of the concrete) cannot be provided to cater for the ultimate loading, the engineer should increase the safety factor of the connection.

Connections can be made flexible through the use of slotted holes in bolted connections (Figure 10.1 (a)). The bolt is tightened sufficiently to hold the member but still allow it to move with little restraint. If the connection is bolted tight against the end of the slot, movement is restricted: this can be avoided by using friction-grip bolts.

Design loads - Some design loads are obvious such as vertical imposed and dead loads and lateral loads due to wind, soil pressure and seismic events. In connection design, less apparent loads such as temporary erection loads and volume changes must also be considered.

Over-stiff connections can introduce unwanted restraints. The amount of fixity of a connection influences the load paths, which in turn affect other elements of the structural system. Therefore, the design must consider connections as an integral part of the structure.

Alternatively, movement can be accommodated through elastic and, ultimately, plastic flexibility of the connection (Figure 10.1 (b)).

Connections are often designed with the intention of resisting only one type of loading. An example is a connection that has a large tensile capacity but little shear capacity to accommodate movement due to volume changes.

Angle with oversize holes bolted to castin inserts

Ductility of connections - Ductile connections are those that exhibit an ability to withstand deformation and load beyond the initial yield. It is desirable to design connections to behave in a ductile manner so they can support loads if unexpected forces occur and large deformations develop.

Angle welded to channel webs only

Channel fixed at middle-third of panels to minimise restraint

(a) Slotted angle connection

(b) Flexing connection

Restraint to volume change - Shrinkage from drying, changes in temperature, and creep cause movements in wall panels. Where possible, it is advisable to design connections that will accommodate all likely volume changes.

Figure 10.1 Corner connections to permit movement

Shrinkage occurs due to drying of the concrete. If immersed in water after drying the concrete absorbs water and expands but it does not return to its original volume. Concrete also expands or contracts as the ambient temperature increases or decreases. Differential volumetric effects can also induce outof-plane bending that may also need to be addressed.

• • • •

Connections used in tilt-up construction can be broadly categorised into four main groups: Cast-in-place connections Steel plates with welded studs Embedded inserts Drilled-in inserts.

Cast-in-place connections - Cast-in-place connections are made by casting infill sections between the erected panel components with overlapping reinforcement projecting from the ends of the panel (see Figure 10.15 later in this chapter). The result is a very strong but often expensive solution for connecting the panel elements. Cast-inplace connections tend to be very rigid and therefore proper attention to thermal and shrinkage stress build-up must be given. Ductility after yielding can be attained but usually not without considerable concrete distress.

Durability - Durability refers to a material’s ability to maintain its strength and serviceability throughout its service life. Exposure of connections to weather may foster deterioration of the components and results in a subsequent reduction in strength. Proper protection is, therefore, essential. In climates where freeze-thaw cycles occur, concrete should have sufficient strength or be air entrained. Connections utilising wood may need to be treated, whilst exposed steel components must be given protective coatings (eg. galvanised). Alternatively, the use of stainless steel or non-ferrous materials may be necessary.

10-2

Connection details

Steel plates with welded studs - Steel plates with (6) welded studs are one of the most common tilt-up connections (Figure 10.2). Typically, an anchored embedded steel angle or plate is cast into the panel. The plate is either anchored by fully embedded reinforcement or it is provided with short studs crossed by panel reinforcement thus providing ductility to any failure cone of concrete. Subsequent connections are made by site welding to the exposed metal surfaces. These connections are sufficiently strong for most applications, are fast and inexpensive, and can be designed with reasonable ductility. However, the use of site welding requires careful specification and inspection.

(b) Ferrule inserts

Reinforcement usually detailed to pass through failure cone to ensure ductility

(c) Coil inserts

Figure 10.2 Steel plates with welded studs

Figure 10.3 Embedded inserts

Embedded and drilled-in inserts - Embedded inserts such as the ferrule or coil inserts (Figure 10.3), or drilled-in inserts such as the expansion anchor, allow bolted connections to be made directly. These eliminate the need for site welding, reduce the requirements for pre-planning, and provide a convenient means for correcting errors.

DETAILS Details for connecting structural components to tiltup panels are not easily standardised. Variations in the type of roof and floor systems, and designers’ own preferences, have resulted in a wide variety of connection types.

Embedded and drilled-in inserts are most often used for light loads or for fastening non-structural elements. They are the cheapest but least reliable. Both types of inserts should be avoided in seismic applications or where heavy vibrations occur, because of their poor cyclic loading characteristics.

Some of the common connection details are now reviewed to demonstrate the simplicity of the principles involved. Mastery of these simple concepts offers considerable design flexibility in developing or varying details to suit individual projects. 10-3


designed and detailed to carry vertical loads, transverse loads due to out-of-plane wind or seismic forces, and sometimes longitudinal shear forces.

Main roof and floor connections Main roof and floor structural elements are attached to tilt-up panels by a variety of connection details that transfer forces to provide stability and, in many cases, give immediate support to the element during construction. The connections used for tilt-up follow the usual principles adopted for precast concrete, but some have been specially developed for this form of construction.

The pocketed connection has the added benefit of reducing eccentricity of load. The steel member is commonly site welded to the angle seat. An alternative to this connection is a flat steel plate (6) with stud anchor or reinforcement tie embedded in the concrete (Figure 10.4 (b)). The angle seat is usually welded on before by hand but can also be attached after the panel is erected.

Seat for steel truss - This is often provided by a pocket recessed in the plane of the panel with an anchored angle seat (Figure 10.4 (a)). It is commonly

Both of the above examples avoid projections above the surface of the panel to allow for easy screeding and finishing, or for stack-casting one panel on top of another. Seat for steel beam - Recessed pockets (similar to Figure 10.4 (a)) are also sometimes used for beam connections when the vertical reaction is of a light or moderate nature.

Joist pocket formed with block-out Continuous chord angle welded to truss

For heavy loads, a corbel or full height pier should be considered in order to provide sufficient concrete depth to install confining ties (Figure 10.5). Alternatively, a large flush plate with embedded anchors may be used with an angle seat welded on before or after erection, similar to that shown in Figure 10.4 (b).

Cast-in angle seat with truss connected by weld or bolts

Reinforcement welded to angle seat

Chord angle

(a) Pocketed connection for steel truss

Continuous chord angle welded to joists

Angle seat with attached tie bar

Angle seat with joist connected by weld or bolts

Reinforced section (confining ties)

Support pier cast with panel

Embedded plate with stud anchors

(b) Truss supported by angle welded to embedded plate

Figure 10.4 Connections for steel trusses

Figure 10.5 Seat angle on pier for heavily loaded beam

10-4

Connection details

Support for timber joist - Timber roofs and floors have been used for a number of tilt-up projects, although this may not be common in the UK.

A system popular for timber roof construction on tiltup is to use timber joists supported on a timber wall plate (Figure 10.6).

Reinforcement from panel cast into topping or concreted into cores

The wall plate is connected with bolts, cast into the panel, or drilled in before erection. This is generally sufficient for vertical loads but is considered to be inadequate for transverse loads, and it is recommended that additional transverse steel strap ties are installed to prevent separation of the roof or floor deck from the tilt-up panel.

Precast concrete hollow-core units

Bearing pad where required

(a) Continuous top ledge to support precast concrete floor units Steel strap cast in panel and fixed to joists Decking

Reinforcement from panel concreted into cores or cast into topping as above

Timber joist

Timber plate bolted to panel

Precast concrete hollow-core floor units Bearing pad where required

(b) Continuous corbel to support precast concrete floor units

Figure 10.6 Timber wall plate connection

Figure 10.7 Support for precast concrete hollowcore units

Support for precast concrete hollow-core units Hollow precast concrete floor or roof slabs can be supported by a ledge on a tilt-up panel (Figure 10.7 (a)), on a continuous corbel (Figure 10.7 (b)) or by an angle (as shown in Figure 10.4 (b)). However, panels with corbels are more difficult to produce. The use of a ledge is common where storey-height panels are used or at roof level. But at intermediate floors in a multi-story panel it is more common to use a steel angle (Figure 5.1, Chapter 5) due to the limitations of construction.

Support for precast concrete double-tee beams Support for a double-tee beam may be provided by pocketed connections (Figure 10.8 (a)) or a continuous horizontal corbel (Figure 10.8 (b)). In both cases the units would normally be supported on neoprene pads so as to allow for some rotational movement when the beam is loaded.

Ties can be provided by embedded panel anchors and site welding or by reinforcement cast into a concrete topping.

The slabs are sometimes supported on a neoprene strip to even out the bearing stresses. Lateral reinforcing ties may be detailed to be cast into some of the cores or into the structural topping when used.

The limitations and alternative support as indicated for hollow-core units are also applicable to doubletee beams.

10-5


perimeter is provided by reinforcement within the panels and ties between the panels or by the use of a perimeter angle, which often serves as support for the roof or floor as shown in Figure 10.4(b) above.

Welded connection between plates in panel and beam

Steel angle connections - With this method, sitewelded connections are made between the roof or floor diaphragm to the continuous angle. The chord angle will in turn typically be welded to cast-in anchor plates or fastened to machine bolts embedded in the concrete (Figure 10.9). This connection will also carry small vertical loads. Where bolted connections are used the steel angle plate is provided with slotted holes to allow the panel to shrink without restraint from the bolts. The angle is also often attached to the panels away from its edges (say at third points) to further reduce panel restraint (Figure 10.10).

Precast concrete double-tee units

Neoprene bearing pad

Angle attached by cast-in bolts or welded to cast-in plates

(a) Continuous top ledge to support precast concrete double-tee units

Welded connection between steel decking and panel angle

Welded connection between plates in panel and beam

(a) Edge connection to steel decking

Bar welded between angle and cast-in panel plate

Precast concrete double-tee units

(b) Edge connection to double-tee unit Neoprene bearing pad Plate welded between studded panel plate and plate cast-in panel

Reinforced corbel

(b) Continuous corbel to support precast concrete double-tee units

Figure 10.8 Double-tee support to wall panel

(c) Edge connection to hollow-core unit

Roof or floor diaphragm connections Figure 10.9 Connections between roof/floor diaphragm and perimeter support angle

Diaphragm action of a floor or the roof may serve to support the walls and transmit lateral wind, soil, or earthquake forces back into shear walls and foundations. The roof or floor is stiffened or braced to perform as a diaphragm (a large deep horizontal beam with the deck acting as the web and its (3) perimeter acting as the flange) . Continuity of the

Perimeter reinforcing steel connection - This detail (Figure 10.11) is popular with timber roof and floor systems. The timber wall plate (or steel channel) transmits vertical and longitudinal loads into the panel. The reinforcing bar is cast into the panel with

10-6

Connection details

For most buildings it will be necessary for certain panels to resist overturning forces (Figure 10.12 (b)) due to wind or other specified loadings (eg. due to earthquakes). When this is necessary, as in the case of high, narrow panels in earthquake zones, the panels should be connected in pairs, or at most, in groups of three. When this is done, additional reinforcement may be required in the panels in order to minimise the effect of increased shrinkage stresses due to this restraint.

Angle plate or floor/roof connections attached to panel at intermediate positions to minimise restraint (shown at 1/3 positions)

Shear force from roof diaphragm

Figure 10.10 Attachment of connections to reduce restraint (a) Panels not connected Cylindrical cardboard sleeve encasing top chord bar at both ends of panel L/3

Bars spliced together at ends of panel

L L/3

Shear force from roof diaphragm L/3

Discontinuous wall plate attached to panel via slotted holes

(b) Panels connected in pairs

Figure 10.12 Panel shear wall stability

The shear wall connection of this type should have a high static strength with good ductility under cyclic (3) loading . Since it is used primarily for transmitting shear forces, it should be recessed below the surface of the panel and anchored with splayed deformed bars (Figure 10.13). Split pipe connectors as shown in Figure 10.13 can be used with several of the other details shown in this Section to provide restraint but with the ability to offer some give to movements. The embedded angle (Figure 10.14) with stud anchors can have adequate static strength but is poor in cyclic loading, and cracks may occur around the connector as a result of volumetric panel movement. Therefore, it is generally only used to tie together or attach panels of short length. For very large shear forces, a cast-in-place stitch may be used such as illustrated in Figure 10.15. Again, because of the effect of restraint, it needs to be used with discretion. It may also be necessary to tie other panels or columns, and examples are shown in Figure 10.16.

Figure 10.11 Reinforcing bar chord connection

a sleeve at the outer one-third ends to allow for panel expansion or contraction. A full-strength lap splice is made at panel joints by site welding. This reinforcement system can also be used with a roof or floor that is supported by a pocketed connection with intermittent fixings from the deck.

Panel-to-panel connections There are wide differences in opinion as to whether panels need be connected structurally to one another at their vertical joints. There are those who suggest that two or three welded connectors should be provided at each panel joint, particularly for buildings located in seismic zones. On the other hand, there is the view that unconnected panels enable expansion and contraction between panels to take place, thereby reducing the build up of stresses. It is believed that unconnected panels may also perform better in a large earthquake (due to structural damping) (3). An assessment of these variations in opinion indicates that there is insufficient evidence to require arbitrary panel-to-panel connections, and that only those connections required for structural stability need be provided. This now seems to be the view of most tilt-up designers.

Panel-to-foundation connections In general, it is recommended that some kind of connection is made between the wall panel and the ground floor or foundation. This is most important in seismic zones but is also good practice in other situations since, occasionally, panels have been displaced by impact from equipment, eg. forklift (3) trucks .

10-7


Panel reinforcement

Splayed connector plate anchors

(a) Panel-to-panel detail Panel reinforcement extended into pilaster

Pilaster cast after panel erection

ELEVATION

Split pipe connector welded to anchor plates

Panel reinforcement

(b) Panel-to-pilaster detail

Figure 10.15 Cast-in-place connection

The connection can sometimes be made at slab level (Figure 10.17) or alternatively connections can be made both to the foundation and the concrete ground floor slab (Figure 10.18). The slab connection serves to transmit longitudinal and transverse loads and may take the form of a dowel projecting from the panel or a welded embedded anchor. A dowel, welded plate, or continuous longitudinal slot or upstand in the strip footing provides transverse restraint for the panel at this level and can provide a degree of moment restraint to the panel, which is discussed further in Chapter 7.

ENLARGED SECTION

Figure 10.13 Panel-to-panel connection

Embedded angle with studs

Plate welded across angles

Connections for sandwich panels Many of the forgoing details can in essence be used for sandwich panels that have the added advantage of built-in insulation and the ability to reduce temperature movements of the inner leaf. The use of two leaves also provides greater scope for variations in construction details as the outer leaf masks the inner panel allowing, for example, full-width pockets and fixings to be used. Another benefit is that it enables initial eccentricities of vertical loads to be minimised or even eliminated.

(a) Embedded angles Embedded plate with studs

Plate welded across plates

(b) Embedded plates Cast-in sockets

A range of typical connection details is shown in Figures 10.19 to 10.31, which are adapted from Reference 7.

Plate bolted to castin sockets

Connection details – further information Design of connections for tilt-up requires consideration of strength and serviceability of all materials that may be affected, whether they are concrete, steel or wood. For further requirements related to these materials, the appropriate Codes of Practice should be consulted. For data on the performance of the connection elements, refer to the manufacturer’s design data.

(c) Cast-in sockets

Figure 10.14 Embedded connections in panels

10-8

Connection details

Loose angle welded in place Recessed joint mortared up

Shims for initial support and levelling

Welded Reinforcement

Grout for long-term support

Welded plates

(a) Welded connection Make-up strip Mortared recess Main floor Cast-in threaded insert Continuous footing

(a) level site Tolerance hole for bolt through panel

(b) Bolted connection Loose angle bolted or welded to cast-in fixings in panel and column

Continuity strip or welded connection

Concrete column

Continuity strip reinforcement

Steel column Steel clip shown bolted to cast-in socket. (Alternatively weld to cast-in plate) Concrete support with dowel cast in to footing

(c) Bolted and welded connection (Shown as split column section)

Shims for initial support and levelling

Mortar filled

Grout Continuous footing

Concrete column

(b) Stepped site

Continuous rebates

(d) Slotted connection

Figure 10.16 Other panel connections

Figure 10.17 Simple panel-to-floor slab connections

10-9


Shims for initial support and levelling. (Grout carries panel load in service)

Dowel screwed into threaded coupler in panel

Grouted anchor dowel

Grout

(a) Dowelled - 1

(b) Dowelled - 2

Angle welded to cast-in plate in panel and bolted to foundation

Plate anchored into panel

Welded plate

Angle anchored into panel

(c) Embedded plate with angle

(d) Embedded fixings with welded plate

50 mm nominal grouted recess

(b) Recessed

Figure 10.18 Typical panel-to-floor slab connections

10-10

Connection details

Sealant and backing strip Additional ties at upper level

Mitered tolerance joint

Roof truss

Fixing welded or bolted to embedded anchor plates (but see Connection design on page 10-1) Thermal insulation

Angle ledge welded or bolted to embedded anchor plates

Inner concrete leaf Ties joining inner and outer leafs together

Figure 10.19 Mitred corner joint

Figure 10.21 Parapet with angle support

Outer leaf discontinuous at corner to allow for movement Sealant and backing

Fixing welded or bolted to embedded anchor plates (but see Connection design on page 10-1)

Direct bearing of roof truss (Welded or bolted to angle seat)

Figure 10.20 Butted corner joint

Figure 10.22 Roof with top support

10-11


Leveling shims (Joint grouted between shims along length of wall) Sealant and backing strip

Fixing welded or bolted to embedded anchor plates

Angle ledge welded or bolted to embedded anchor plates Insulation as necessary

Figure 10.23 Roof with angle support

Figure 10.25 Panel with simple welded connection to floor slab

Dowelled grouted joint

Roof deck and framing

Sealant and backing strip

Levelling shims (Joint grouted between shims along length of wall)

Roof steelwork attached to embedded anchor plates

Figure 10.24 Roof edge support

Figure 10.26 Panel with simple dowelled connection to floor slab

10-12

Connection details

Embedded continuity strip

Precast concrete hollow core units In-situ concrete

Foundation recessed to clamp panel Reinforcement welded to support angle and concreted into hollow-core unit Levelling shims (Joint grouted between shims along length of wall)

Angle welded or bolted to embedded anchor plates

Caulking strip

Figure 10.27 Panel tied to slab and restrained in foundation

Figure 10.29 Intermediate floor connection

Inner concrete leaf

Restraint block with dowel into foundation Fixing welded or bolted to anchor plates

Caulking strip

Timber subframe to window / door frame

Levelling shims (Joint grouted between shims along length of wall)

Figure 10.28 Retaining panel propped by slab and restrained by foundation

Figure 10.30 Insulated opening

10-13

Connection details

More detailed information on connections can be found in References 3, 4, 6, which show typical connection details used in tilt-up connections, and which also contain design guidance. References 2 and 8, although not strictly applicable to tilt-up, provide guidance on fixings for precast members and for non-loadbearing vertical elements. Due to local environmental conditions such as temperature, humidity, expansive soils or because of locally accepted details and practices, some of the details shown may not perform satisfactorily or prove economical in all situations. Therefore, each detail and the specific requirements for the connection should be studied thoroughly before any decision to adopt its use.

REFERENCES 1. British Standards Institution . BS8110 Part 1: Structural Use of Concrete, 1985. 124 pp. 2. Institution of Structural Engineers . Structural joints in precast concrete: manual. ISE, London, 1987. 56 pp. 3. Weiler, G. Connections for tilt-up construction. Concrete International, Vol. 83, No 3, June 1986. pp 24-28. 4. Portland Cement Association. Connections for tilt-up wall construction. PCA, Skokie, USA, 1987. EB 110.OID. 39 pp. 5. Portland Cement Association. Precast concrete loadbearing wall panels. PCA, Skokie, USA, 1987. Building Systems Report. PA167.01B. 12 pp. 6 BRC Square Grip. Peikko, Fastening plate design manual. BRC Square Grip, Sutton-in Ashton, 1998. 7 Composite Technologies Corporation Inc. Thermomass building system. Design folder. CTC, Iowa, USA. 8. British Standards Institution . BS 8200. The design of non-loadbearing vertical enclosures. BSI, Milton Keynes, 1985. 76 pp.

10-14


11

SAFETY REQUIREMENTS

This chapter contains safety information drawn mainly from countries where tilt-up is well established. It may be used when considering the safety aspects of particular UK projects. based upon weight but also on how far the crane must reach and how far the crane may have to travel with a panel.

GENERAL Until recently, the main responsibility for safety during construction work has been with contractors, but with the introduction of the CDM Regulations in 1995, this is now shared by clients and designers, who are specifically targeted by many of the requirements of the Regulations. This chapter briefly discusses just some of the issues considered important to safety in those countries where tilt-up is in common use. This will help the above parties determine their responsibilities when considering the Regulations. Planning supervisors may also wish to consider the material when constructing their health and safety plans. By its very nature, much of the construction process in forming tilt-up panels is inherently safe. Up to the point of lifting, construction is at ground level with good access, and involves easily handled components with the minimal use of heavy plant. Side forms are lightweight and starter bars are often unnecessary. Concrete, the heaviest component, is often delivered by mixer truck chute or by pump. Finally, before lifting, all possible operations are performed on the horizontal panel to avoid unnecessary working at a height. For instance, bracing props and welded or bolted fittings are attached at this point. Due to the size and weight of the elements concerned, the lifting and subsequent temporary bracing phases prior to the structure being selfsupporting introduce particular demands on safety planning. The Tilt-up Concrete Association (TCA) has recently introduced a safety checklist for tilt-up concrete construction (1) which, together with other material, has been used to produce the list below. However, it may be necessary to make some modifications or additions to these guidelines in order to suit UK law and practice.

1. Prior to construction Hire an erection sub-contractor and crew experienced in the handling of tilt-up or precast panels.

•

Select a crane with a capacity capable of lifting the heaviest panel (including an allowance for suction – see Chapter 3) plus the weight of the rigging gear. Crane selection will not only be

The panel contractor should obtain from the erection sub-contractor documentation attesting to the crane’s certification, also a certificate of insurance.

•

Ensure that there is a proper sub-base under the floor slab. This will be the casting area as well as a working surface. The slab is only as good as the sub-base upon which it is placed.

•

Check the floor slab for adequate strength to support the crane if required.

•

Obtain a properly designed and detailed tilt-up package that is supported by a professional engineer.

•

Obtain a bracing manual showing braces appropriate for design wind loads.

•

Obtain approved shop drawings for each panel showing all pertinent information.

• •

Develop a panel casting and erection sequence.

•

Inspect the panel formwork for proper placing of reinforcing, inserts, embedded items and for dimensional accuracy.

Always test the bond-breaker prior to casting any panels. Verify that the bond-breaker is compatible with any curing or sealing compounds that may have been used on the floor slab.

2. Prior to erection day

SAFETY CHECKLIST

•

•

11-1

•

Perform a site inspection. Look for any underground hazards, overhead wires, rough terrain, or soft sub-grade on which the crane will travel. Make notes of any corrections that need to be made or any hazardous areas.

•

Rig the crane prior to the date on which erection is to start.

•

The panel contractor should verify that the crane is in good working condition.

•

Check that lifting inserts are properly located, strongbacks properly installed, and that the concrete has gained the required strength at lifting. This information should be recorded in the erection manual.

•

Install entrance and exit ramps for the crane to position itself onto the floor slab. Do not allow


•

the crane to exert its weight on the extreme edge of any portion of the slab.

•

• •

Provide the crane operator with weights of individual panels, predicted suction, and instructions on the lifting sequence.

Check to make sure all the floor slab blockouts are covered. If water gets under the slab, it could weaken the sub-grade and the crane may crack the slab.

4. During the lift

•

Itemise the equipment required for a proper and safe lift. Ensure that the tools and equipment are well maintained.

Provide a clean working area with debris and obstacles removed.

•

Do not lift panels when wind conditions would produce unsafe conditions during a lift.

•

No personnel should pass beneath a non-vertical panel, under any circumstances.

•

Personnel not involved with the panel lifting procedure should be clear of the lifting area.

Identify erection sub-contractor’s crew. A minimum crew should consist of the crane operator, rigger foreman, two journeyman riggers and welders if required.

•

Provide a clean working area with all debris and obstacles removed.

•

•

Locate proper shim points on the footing to prevent overloading the footing prior to grouting under the panels. The design engineer can help you with these locations.

If possible, fully extend outriggers and use cribbing to spread the outrigger loading. If outriggers cannot be fully extended, then the crane capacities must be reduced.

•

Inspect all rigging gear prior to loading the inserts. Rigging gear must be properly aligned and free of snags.

•

Make certain that the rigging configuration matches that shown in the erection manual.

•

Check to be sure that braces will not be trapped by the rigging once the panel is in its final position.

•

Be alert for panels that may be stuck to the casting surface. These may require releasing carefully with wedges or pry bars as loads to the lifting inserts may be twice that designed for, causing possible insert withdrawal.

• •

Hold safety meeting before any lifting starts.

•

Draw up an erection manual containing all necessary information for erection.

Ensure that each member of the crew understands their position and the responsibility that goes with it.

3. At the safety meeting

•

Create a safety checklist and have all relevant staff sign and check the list after the safety meeting has been conducted.

•

Instruct personnel never to place themselves under a panel while it is being tilted, on the blind side of the panel while the crane is travelling with it, or between the crane and the panel.

•

At a predetermined lifting force, carefully release the panel using pry bars and wedges.

•

•

If you must ‘walk’ a panel, be alert to all obstacles in the path of the crane and the crew.

At the site, do not allow horseplay or unnecessary talking.

•

•

Take extra precautions when lifting panels with special shapes or special rigging.

Instruct personnel to remain alert at all times and to look out for fellow workers.

•

•

Do not use any damaged or bent braces, lifting hardware or bolts.

While on the site, proper attire should be worn at all times (ie. hardhats, shoes, etc).

•

• •

Make certain that any strongbacks shown on the erection details are included on the panels.

Address all fall protection requirements.

•

Clearly define the function and responsibility of each person on the lifting crew.

•

Demonstrate the use of the lifting hardware, bracing hardware, and proper use of any tools and equipment that are to be used.

•

5. After the lift

Identify the rigging foreman. Ensure that the rigging foreman and the crane operator know all the hand signals that they will be using to communicate with each other. Instruct the other personnel that the only person that should signal the crane operator is the rigging foreman.

Instruct the construction gang never to reach their hands under a panel to adjust a shim or a bearing pad.

11-2

•

Be alert when plumbing panels to their final upright position. Make sure that the panel being plumbed does not strike another previously erected panel.

•

Support panels as close as possible to the vertical prior to attaching braces to the floor slab.

•

Never release the crane load if the bracing does not appear adequate.

•

If the bracing design calls for a support system of knee, lateral, end or cross bracing, it should be completely installed prior to releasing the crane load.

Safety requirements

•

• •

•

If the lateral and end bracing cannot be installed with the panel load still on the crane, then the completion of this bracing must not be further than one panel behind the lifting schedule.

Main contractor

All bracing should be installed on all erected panels at the end of the working day.

Placing reinforcement in floor

At the beginning and end of the working day, all brace inserts should be checked to ensure that they are tight and have not worked loose throughout the night or day. Check brace inserts daily.

Placing and compaction of concrete for floor

Maintain a daily torque log on brace insert tightening.

Panel erection sequence

Access for crane/trucks Preparation for floor Placing brace inserts in floor Curing compound for floor and application Panel casting sequence (building programme) Crane/panel weight ratio Safe working environment/procedures

•

If at all possible, grout under the erected panels prior to the end of the working day.

•

Do not remove any braces until all the structural connections are completed and the lateral resistive system is in place and completed. The structural engineer can help you determine if it is safe to remove any or all of the panel braces.

Bond-breaker type and application

Be careful when backfilling the infill strip of the ground slab so that you do not exert excessive pressure on the tilt-up panel.

Lifting and bracing insert type

•

Tilt-up sub-contractor* Curing compound for panels and application Design of panels for lifting Selection and use of strongbacks Positioning inserts in panel Positioning reinforcement in panels

(2)

Placing, compacting and curing concrete in panels

Part 1 of the Australian Standard covers safety requirements in some detail. It focuses particularly on lifting and bracing phases, with reference to design, material, fittings and construction aspects.

Bracing design Casting sequence Lifting procedures Rigging gear and lifting

INDIVIDUAL RESPONSIBILITIES

Erection sequence

Since efficient tilt-up construction requires a team approach, safety must involve the designer, contractor and lifting contractor. Each team member should have an agreed role in clearly defined procedures and with known responsibilities.

Crane position Safe working environment/procedures * This assumes the tilt-up contractor is responsible for both the production and erection of the panels.

(3)

The C&CA of Australia suggests the following breakdown of responsibilities, which has been modified to suit that more likely for the procurement route and particular needs of a UK project. This will vary, depending on the procurement method adopted. Some of these responsibilities are shown as being shared, but in such cases one of the team will need to assume overall responsibility. It will also be necessary to further sub-divide these responsibilities if separate parties carry out the construction and erection.

REFERENCES 1. Tilt-up Concrete Association. Safety checklist for tilt-up concrete construction. TCA, Mount Vernon, USA, 1996. 4 pp. 2. Standards Australia. AS 3850.1, 1990. Tilt-up concrete and precast concrete elements for use in buildings. Part 1: Safety requirements. SA, Sydney, Australia, 1990. 16 pp. 3. Cement and Concrete Association of Australia . Tilt-up technical manual. C&CA Australia, Sydney, 1990. 24 pp. (Amended to a series of data sheets 1997).

Structural designer Overall building stability Fire resistance of elements In-service design of panels Load design of floor/pavement Drawings and documentation for the contractor Panel size/shape/tolerance Panel fixings and their positions

11-3

11-4


12

SPECIFICATIONS

It is beyond the scope of this manual to produce a standard specification for tilt-up construction for the UK. (1) However, a new British national specification has been developed by BRE, Construct and the RCC for the construction of concrete frames in buildings. This may prove suitable for use on tilt-up projects with appropriate amendments permitted in Part 2 of the Specification. Alternatively, readers wishing to research this matter may refer to the Australian Standard, Tilt-up concrete and precast concrete elements for use in buildings, Part 2: (2) Guide to design, casting and erection of tilt-up panel .

REFERENCES 1. Building Research Establishment. National concrete frame specification for building construction. BRE, Garston, 1998. 60 pp. 2. Standards Australia. A 3850.2. Tilt-up concrete and precast concrete elements for use in buildings, Part 2: Guide to design, casting and erection of tilt-up panel. SA, Sydney, Australia, 1990. 28 pp.

12-1

12-2


13

SPECIALIST SUPPLIERS AND SERVICES

The following list of suppliers of products and services, particularly relevant to tilt-up, is not exhaustive. It is composed mainly of those individuals and companies who have come to light during the compilation of this manual, and inclusion or omission does not constitute endorsement or censure by the RCC. This list will be updated from time to time, and can be obtained from the Reinforced Concrete Council at Century House, Telford Avenue, Crowthorne, Berks RG45 6YS.

Name of organisation and address

Tilt-up expertise (see key)

FOR THIS EDITION THIS LIST HAS BEEN PRODUCED SEPARATELY

Key to codes 1 Tilt-up design experience 2 Tilt-up contracting experience 3 Tilt-up fittings supply a) Lifting b) Panel fixing c) Bracing d) Rigging e) Other 4 5 6 7 8 9 10 11 12

Tilt-up panel lifting design Tilt-up chemicals (curing agents, bond breakers, etc) Tilt-up components Specialist paints/treatment for concrete Tilt-up trade association/licensing Sandwich panel systems Overseas supplier UK sales/agency Architectural finishes a) Form liners b) Brick slips c) Other 13 Tilt-up manuals/publications

13-1

Telephone & fax number

Manual TiltUp

Recommend Documents