Eurocode - Building Designer Handbook

CSC Fastrak ™

Structural steelwork analysis and design

.cscworld.com/fastrak

HANDBOOK EUROCODE BUILDING DESIGNER

Building Designer - Eurocodes Handbook page 2

CSC’s Offices Worldwide

CSC (UK) Ltd Yeadon House New Street Pudsey Leeds, UK LS28 8AQ Tel: (44) 113 239 3000 Fax: (44) 113 236 0546 Email: [email protected] [email protected] CSC Inc 500 North Michigan Avenue, Suite 300, Chicago, IL 60611, USA Tel: 877 710 2053 Fax 312 321 6489 Email: [email protected] [email protected]

CSC WORLD (Malaysia) Sdn Bhd Suite B-12-5, Block B, Level 12, North Point Offices, Mid Valley City, No.1, Medan Syed Putra Utara, 59200 Kuala Lumpur, Malaysia Tel: (60) 3 2287 5970 Fax: (60) 3 2287 4950 Email: [email protected] [email protected]

Civil & Structural Computing (Asia) Pte Ltd 3 Raffles Place #07-01 Bharat Building Singapore 048617 Tel: (65) 6258 3700 Fax: (65) 6258 3721 Email: [email protected] [email protected]

Civil & Structural Computing Pty Ltd Level 3, 349 Coronation Drive Milton QLD 4064 Australia Tel: 1300 882 393 Fax: +61 (07) 3378 5557 Email: [email protected] [email protected]

Friday 5 August 2011 – 12:00

Disclaimer

Disclaimer

page 3

CSC (UK) Ltd does not accept any liability whatsoever for loss or damage arising from any errors which might be contained in the documentation, text or operation of the programs supplied. It shall be the responsibility of the customer (and not CSC) •

to check the documentation, text and operation of the programs supplied,

•

to ensure that the person operating the programs or supervising their operation is suitably qualified and experienced,

•

to ensure that program operation is carried out in accordance with the user manuals,

at all times paying due regard to the specification and scope of the programs and to the CSC Software Licence Agreement.

Proprietary Rights

CSC (UK) Ltd, hereinafter referred to as the OWNER, retains all proprietary rights with respect to this program package, consisting of all handbooks, drills, programs recorded on CD and all related materials. This program package has been provided pursuant to an agreement containing restrictions on its use. This publication is also protected by copyright law. No part of this publication may be copied or distributed, transmitted, transcribed, stored in a retrieval system, or translated into any human or computer language, in any form or by any means, electronic, mechanical, magnetic, manual or otherwise, or disclosed to third parties without the express written permission of the OWNER. This confidentiality of the proprietary information and trade secrets of the OWNER shall be construed in accordance with and enforced under the laws of the United Kingdom. Fastrak documentation: © CSC (UK) Ltd 2011 All rights reserved.

Trademarks

Fastrak software: © CSC (UK) Ltd 2011 All rights reserved.

Fastrak™ is a trademark of CSC (UK) Ltd TEDDS® is a registered trademark of CSC (UK) Ltd Orion™ is a trademark of CSC (UK) Ltd The CSC logo is a trademark of CSC (UK) Ltd HOOPS™ is a trademark of Tech Soft 3D Autodesk and Revit are registered trademarks or trademarks of Autodesk, Inc., in the USA and/or other countries. Microsoft and Windows are either trademarks or registered trademarks of Microsoft Corporation in the United States and/or other countries. Acrobat® Reader Copyright © 1987-2011 Adobe Systems Incorporated. All rights reserved. Adobe and Acrobat are trademarks of Adobe Systems Incorporated which may be registered in certain jurisdictions. All other trademarks acknowledged.

page 4

Table of Contents

Building Designer Handbook - Eurocodes Chapter 1

Introduction

.

.

.

.

.

.

.

.

.

7

Chapter 2

Construction Methods and Member Types .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

8 8 10 10 11 11 13 14 15 15 16 17 17 17 20 23

.

Simple Construction

.

.

.

.

.

.

. . . Continuous Construction . . Member Beams and Member Columns . General Beams . . . . . General Columns . . . . Moment Framing and Gravity Loads . Backspan Beams . . . . General Points to Note . . . Additional Member Types . . . Trusses and Truss Members . . . Diaphragm Braces . . . . Shear Walls . . . . . Bearing Walls . . . . . Composite or Simple Beam?. Composite Beam Design .

Chapter 3

Sway Resistance .

Chapter 4

Diaphragm Modeling

. . Using Bracing . . . Using Steel Moment Frames . Using Other Moment Frames . Using Shear Walls . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

27 27 28 28 28

. . .Single diaphragm . Slab items defined . No diaphragm . . Taking slabs out of a diaphragm . Semi-Rigid Diaphragms . . Flexible Diaphragms . . . Storey Shears . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

30 30 31 31 32 32 34 34 34

Member End Releases and Member Orientation .

. . . .

. . . . .

. . . . . .

. . . . . .

. . . . . .

Load Cases and Load Combinations Nationally Determined Parameters (NDP’s) gamma factors . . . . . psi factors . . . . . . Gravity Load Cases . . . . Self Weight . . . . .


. . . . . . . . . . . . . . .

. . . . .

Moment Releases . Axial Releases . . Torsional Releases . Release from a Diaphragm Member Orientations . Supports and Base Fixity

Chapter 6

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . .

Rigid Diaphragms.

Chapter 5

. . . .

.

. . . . . .

. . . . . .

. . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

35 35 37 38 38 39 40

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

41 41 41 42 43 43

Table of Contents

page 5

. . . . . . . . . . . . . . . Lateral Load Cases . . . . . . . . Wind Loads . . . . . . . . . Combinations . . . . . . . . . The Construction Stage Combination . . . . . Manually Defined Combinations . . . . . . Equivalent Horizontal Forces (EHF) . . . . . Apply Imposed Load Reductions . . . . . The Combinations Wizard . . . . . . . Classifying Combinations and Setting the Critical Combinations Gravity Combinations . . . . . . . Lateral Combinations . . . . . . . Seismic Combinations . . . . . . . Setting the Critical Combinations . . . . . Imposed and Roof Imposed Loads Snow and Snow Drift Loads . Perimeter Loads . . .

Chapter 7

Analysis And Design Procedures .

Chapter 8

Building Effective Models

. Definitions. . . . . . . Building Validation . . . . . Overview of the Analysis and Design Process . Set Auto Design Mode . . . . Analysis Options . . . . . First-order or Second-order Analysis? . . Curved Beams . . . . . . Torsion Factors . . . . . Cracked Sections . . . . . Design Options . . . . . . Design Codes . . . . . . Design Control . . . . . Force Limits - Members . . . . Force Limits - Connections. . . . Element Pre-sizing . . . . . Portal Pre-sizing . . . . . Composite . . . . . . EHF Forces . . . . . . Initial Review of Analysis Results . . . Maximum Nodal Deflections . . . Sway Sensitivity . . . . . Loading Summary . . . . . Review of Selected Sections . . . Review Analysis Results . . . . Review Centre of Mass / Rigidity . . . Reviewing Storey Shear . . . . 3D Analysis Effects . . . . . Continuous Beam Example . . . Braces Carry Gravity Loads Example . . Refining Member Designs . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

44 44 45 45 45 46 46 46 47 47 48 50 51 51 51 51

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

52 52 53 53 55 55 56 57 57 57 58 58 58 58 58 59 59 59 59 60 60 60 61 62 62 62 63 63 64 67 69

. . . . . Place grid lines accurately . . . . . . Save time by using Attributes effectively . . . Use ‘Simple’ beams and columns where possible . . Use Perimeter Loading for edge beams where applicable Is it a Floor? . . . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

70 70 70 71 71 71

page 6

Table of Contents

Set the appropriate level of Diaphragm Action. Set the appropriate level of deflection checks . Building Size and Orientation. . . . Switch off irrelevant load combinations . . Design simple construction for gravity loads only Staged modelling and design. . . . Check the model analysis results . . .

Chapter 9

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

72 72 72 73 73 74 74

. . . . Imperfection for analysis of bracing systems - Clause 5.3.3 .

. . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . Imperfections for global analysis of frames - Clause 5.3.2 (6) . Torsional sway effects - Clause 5.3.2 (10) . . . . . Deflection checks . . . . . . . . . Absolute and Relative Deflections . . . . . . Deflections in Composite Beams (and Beams with Web Openings) . Foundation loads . . . . . . . . . . Vertical cross bracing . . . . . . . . . Foundation shear and vertical load . . . . . . Column axial load . . . . . . . . . Imposed Load Reductions . . . . . . . . Imposed Load Reductions applied to Brace Forces used in Column Design . Equivalent Horizontal Force Load Calculations. . . . . . Loads used in EHF load calculations . . . . . . . Gravity loads carried by Braces not accounted for in EHF load calculations . Axial load in discontinuous columns used twice in EHF load calculations . Consequences of changing Design Codes within an Existing Project . . BS models swapped to Eurocodes and designed . . . . . Eurocode models swapped to British Standards and designed . . .

. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

75 75 76 76 76 76 77 77 77 77 78 79 79 79 79 79 80 80 80 81 81 81 82

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

83 83 84 85 86 87 88 90 92

Assumptions and Limitations . Analysis Types Analysis Results Imperfections

Chapter 10


. . .

Sign Conventions

. . .

. . .

. . .

. . . .

. . . .

. . . . . . . . . . .

. . . . . . . . Object Orientation . . . . . . . . Beams (Simple, Composite and General) and Truss member (chord) Braces and Truss member (internal) . . . . . Columns (Simple and General) . . . . . . Shear Walls . . . . . . . . . . Foundations/Bases - Foundation Forces . . . . . Foundations/Bases - Base Reactions . . . . . Nodal Deflections . . . . . . . . .

. . . . . . . . .

. . . . . . . . .

Chapter 1 : Introduction

Building Designer - Eurocodes page 7

Building Designer Handbook - Eurocodes

Chapter 1

Introduction This handbook provides a general overview of Fastrak Building Designer in the context of design to Eurocodes. The applicable construction methods and member types are described, and the analysis/design procedures explained. In addition, guidance is provided on effective modelling with tips, and examples to help you to make the most of the software. A brief description of the contents follows: Construction Methods and Member Types — (Chapter 2) discusses the use of simple and continuous construction and describes the various member types available. Sway Resistance — (Chapter 3) describes the various means of providing lateral resistance. Diaphragm Modeling — (Chapter 4) describes the different types of diaphragm modeling available for transferring horizontal loads to the lateral load resisting system. Member End Releases and Member Orientation — (Chapter 5) describes end releases, axial releases, member orientation, supports and practical considerations. Load Cases and Load Combinations — (Chapter 6) describes the different load case and load combinations types. Note The Wind Wizard used for automatic loadcase generation is fully described in the ”EC1 1-4 Wind Modeller Handbook”. Analysis And Design Procedures — (Chapter 7) provides an overview of the steps required to analyse and design your building and describes the various analysis and design options. Note The member design procedures are fully described in the ”Eurocode Member Design Handbook” Building Effective Models — (Chapter 8) hints and tips for creating a model that quickly and efficiently yields results. Assumptions and Limitations — (Chapter 9) these are fully described here. Sign Conventions — (Chapter 10) conventions used in reporting the results.


Chapter 2

Chapter 2 : Construction Methods and Member Types

Construction Methods and Member Types To maximise your construction options Fastrak Building Designer provides a range of member types. Major topics

• • • • • • • • • •

Simple Construction Continuous Construction Composite Beam Design Member Beams and Member Columns General Beams Backspan Beams Trusses and Truss Members Diaphragm Braces Shear Walls Bearing Walls

Simple Construction For simple construction

The most effective design for a multi storey structure is still likely to be simple1 beams and columns with bracing to resist the lateral forces. Fastrak Building Designer will happily design moment frames or continuous beams automatically within a model, BUT, the design of these elements is much more comprehensive (and hence takes longer). For this reason you should only use such elements when your model specifically requires them. Note

If Fastrak Building Designer gives warnings about braces on Simple Beams, or intermediate floor levels on Simple Columns the answer is not necessarily to make the affected elements into General Beams/Columns. Look at the modelling and talk to CSC support if you are not sure of the route that you want to take.

Analytical Properties (simple construction)

Beams Both simple and composite beams are automatically configured with (and restricted to) pinned connections.

Footnotes 1. Pin type connections – thus in this context a Composite Beam is ‘simple’.



Columns When considering stability you should be aware that: columns set by you to be ‘Simple’ - the program automatically inserts pins just above every floor level, apart from at the base level. The insertion of these pins ensures that all lateral load is transferred to the lateral load resisting system. for columns set by you to be ‘Gravity Only Design’ - the program also automatically inserts pins just above every floor level, apart from at the base level. Note however that pins do not get inserted where the columns are connected to a braced bay. The insertion of these pins ensures that all lateral load is transferred to the lateral load resisting system. An example is shown in the figure below.

Added release My and Mz Note

If a column is switched so that it is not Gravity Only Design, the program automatically removes any pins within it (unless it has been marked as a Simple column).

Braces Braces are automatically configured with (and restricted to) pinned connections. Torsional releases can be applied and the brace can have an axial end release at one end to prevent vertical load being carried by the brace. Design Properties (simple construction) It is best to establish the default design properties (restraint assumptions, sections for study, and such like) by setting up appropriate default attributes. For information about working with attributes refer to - Fastrak Building Designer Help \ Working with Attributes The Eurocode theory and assumptions applicable to Fastrak’s Simple Beam, Composite Beam, general column and brace design modules is given in the Eurocode Member Design Handbook.



Composite or Simple Beam? Composite beam design is not a linear process, and some beams are simply not suitable for design as composite beams. You should take care when selecting beams for composite design, and set appropriate design attributes. The benefits of composite design are well known, however many beams are not suitable for composite design, including: • beams with no slab,

• very short beams, • beams with significant eccentric load (for example a beam supporting a column close to the support),

• beams with decking arrangements that will not allow effective composite action. In short you should be diligent about the use of composite beams. Exercise care when determining which beams are appropriate for composite design, if in doubt design all beams as simple beams first and simply select those beams that you wish to be composite at a second pass.

Composite Beam Design Composite design of beams is a complex procedure when done rigorously. We assume that you are familiar with the concepts of composite design before you use Fastrak Building Designer. Fastrak Building Designer’s composite design routines can automatically choose the optimum stud layout and automatically select an appropriate layout of transverse reinforcement to resist longitudinal shear. Thus the design of any composite beam may have a range of possible solutions. Example

A typical 9 m composite spine beam can be shown to be acceptable: • with studs at 190 mm cross-centres and a 457x191x67 UB, • with studs at 200mm cross-centres and a 457x191x74 UB, which of these solutions is better is up to you.

While it can sometimes be useful to optimise a design, you might well take the view that you would prefer to control the stud spacing and other critical design issues rather than allow the software to choose a different layout for every beam. Please consider the following when you set up the attributes for a Composite Beam. It is important to realise that you can define attributes that may make the design of composite beams impossible – for example if you set the stud spacing on a spine beam to 300 mm, but this does not provide the minimum amount of shear interaction then the selection of a suitable beam size is not possible. For a description of the Eurocode design methods, theory and assumptions applied to Composite Beams in Fastrak refer to the Eurocode Member Design Handbook.



Continuous Construction For continuous steel construction

Fastrak Building Designer allows you to model members which are more complex than pin ended beams and simple columns. There are currently four member construction types that you can use: • Member Beams – these can be any section in any material but cannot be checked or designed by Fastrak Building Designer – refer to ”Member Beams and Member Columns”.

• Member Columns – these can be any section in any material but cannot be checked or designed by Fastrak Building Designer – refer to ”Member Beams and Member Columns”.

• General Beams – these are restricted to steel sections. In the first Eurocode version of Fastrak Building Designer these cannot yet be checked or designed – refer to ”General Beams”.

• General Columns – these are restricted to steel sections but such columns can then be designed by Fastrak Building Designer – refer to ”General Columns”.

Member Beams and Member Columns For construction in other materials including concrete and timber

A member can be almost anything. The view above shows Member Beams and Member Columns being used to form concrete framing to support part of the steel structure. In addition concrete shear walls are shown which provide lateral stability and support various beams. (Refer to ”Sway Resistance” on page 27 for more notes on the alternative methods of providing lateral stability). The procedure for defining Member Beams and Member Columns is identical to the procedure for defining other beams and columns – you set up the default attributes and then create members by clicking between any two points. The two main topics that require some thought are given below.



Material/Section Properties

Fastrak Building Designer has default values for various materials. To use a material that is not listed choose Other and you will then be able to enter the properties directly.

Section properties can be calculated automatically for rectangular sections by entering the breadth and depth and clicking Calc. Props. The example above shows the section properties calculated for a concrete beam section. When analysing a concrete structure in isolation, for the purposes of establishing design forces you should use consistent properties for all members. So long as everything is proportionately correct, then the design forces will be correct. However, for the purposes of deflection estimation and in any model that mixes steel/ concrete/other materials, more attention needs to be paid to defining the correct properties. For concrete elements this means considering: • adjusting the gross section properties to allow for cracking, Note

In the above dialog, if you define ‘b’ ‘and ‘d’ then click Calc. Properties Fastrak Building Designer calculates the gross section properties of a simple rectangular section for you. You can make adjustments to the calculated values to allow for cracking and/or to allow for irregular shapes, etc.)



• adjusting the value of E (Young’s Modulus) to allow for load duration. Note

When you select a concrete grade an average short term value of E is indicated for guidance. You must always define the value of E to be used for analysis.

Analytical Properties (End Releases) This is common to Member Beams, Member Columns, General Beams, and General Columns, refer to ”Member End Releases and Member Orientation” on page 35.

General Beams Note

In the first Eurocode release of Fastrak Building Designer, General Beams can be modelled but can not yet be checked or designed.

General Beams are, in a sense, a more constrained subset of Member Beams: • You still have all the geometrical freedom to define the member at almost any angle/ orientation,

• General Beams are constrained to be a steel section, Creating General Beams You can create General Beams in the same way as any simple- or composite-beam. Simply create a new beam attribute set and set the Construction Type on the Design tab to General. Any new beam you create using this attribute set will be a General Beam. Note You can set the end releases as part of the attribute set (the default setting is pinned). You can create General Beams in several other ways: 1. While creating any beam (regardless of the current default attribute set), you can hold down the control key to indicate a series of points that define a continuous beam. Since Simple Beams and Composite Beams are never continuous this procedure will always convert the beam to make it a continuous General Beam. Note Continuous General Beams do not need to be co-linear, provided the web remains in a common vertical plane. 2. If you click on two Simple Beams with the Split/Join tool active Fastrak Building Designer converts these to a continuous General Beam. 3. If you insert points in Simple Beams by using the Modify tool and then move those points to create a non co-linear beam, then Fastrak Building Designer converts the beam into a continuous General Beam. Analytical Properties (End Releases) This is common to all Member Beams, Member Columns, General Beams, and General Columns, refer to ”Member End Releases and Member Orientation” on page 35. Design Properties As with simple and composite beams it is best to establish the default design properties (lateral bracing assumptions, sections for study, etc.) by setting up appropriate default attributes. If you have not set up the attributes you wanted you could of course edit the properties of any General Beam.



General Columns General Columns are, in a sense, a more constrained subset of Member Columns: • You still have all the geometrical freedom to define the member at almost any angle/ orientation,

• General Columns are constrained to be a steel section, • The advantage is that Fastrak Building Designer designs these steel sections automatically. • They can be designed for gravity and lateral loads, but if they are set for Gravity Only Design, they are designed for gravity combinations only. Note

When using simple construction you should set the columns as ‘Simple’.

• General Columns can be designated part of a Moment Frame. Since columns in a moment frame require a greater inertia than would otherwise be the case, General Columns use a different orderfile containing sections more suited to resist bending. Creating General Columns Simply create a new column attribute set with the desired properties. Any new column you create using this attribute set will be a General Column. While working in the 3D structure view you can also create columns by clicking on start and end points. While creating any column in this way you can hold down the control key to indicate a series of points that define a continuous column. Analytical Properties (End Releases) This is common to all Member Beams, Member Columns, General Beams, and General Columns - refer to ”Member End Releases and Member Orientation”on page 35. General Columns not set to be either Gravity Only Design or Simple are initially fixed at each floor level. You are able to introduce pins at any floor level as you then require. However if you set General Columns to be Gravity Only Design, they are treated similarly to General Columns set to be Simple, i.e. the program automatically inserts pins just above every floor level, apart from at the base level. Note however that pins do not get inserted where the columns are connected to a braced bay. Should a General Column be switched so that it is no longer Gravity Only Design, the program automatically removes any pins within it. Design Properties As with beams it is best to establish the default design properties (restraint assumptions, sections for study, and such like) by setting up appropriate default attributes. If you have not set up the attributes you wanted you could of course edit the properties of any General Column on an individual basis. For a description of the Eurocode design methods applied to General Columns in Fastrak refer to the Eurocode Member Design Handbook.



Moment Framing and Gravity Loads Backspan Beams We expect that (generally) for gravity loads you will only introduce moment framing locally and selectively. We anticipate that one of the most common requirements for this usage will be to define Backspan Beams.

The above view shows several simple examples of backspan beams at the first floor level. Note This model is for illustration purposes only - building layout is at your discretion. Consider the left-hand-side you will see a cantilever slab area. Some of the cantilevers are on column lines, however, others extend from the side of a supporting beam and so rely on a backspan beam to restrict rotation. Along the front elevation the column line steps back between foundation and first floor levels and so the columns from first floor to roof are supported on the ends of cantilevers with backspans. This can all be achieved very effectively by using General Beam design. However you may find that the general points noted below still apply.



General Points to Note Pattern Loading If you are creating continuous beams you should consider the possibility that pattern load cases could be critical. Fastrak Building Designer will NOT automatically create pattern load cases for continuous beams. However, you can create more load cases containing the appropriate loads and then create more combinations to cover the pattern load cases. Pattern loading is also applicable to the backspan beam, it will not affect the cantilever, but if the cantilever load is reduced/removed the sagging moments in the backspan will increase. Hence, if the cantilever moment is significant the pattern load cases are likely to be more important to fully investigate. Continuity

Care is required where the beam depth varies to either side of a beam or column web and it is desired for the supported beams to be continuous through the support. If the difference in flange levels is significant the load path through the two moment connections may not be clear or feasible. Undesirable local forces may be set up in the supporting member. Transfer Beams / Levels Levels where the columns that support the floors above are discontinuous are known as transfer levels and the beams/cantilevers that support these discontinuous columns tend to be known as transfer beams. The screen shot at the start of this chapter showed transfer backspan beams. You can design various transfer beam configurations within Fastrak Building Designer. Fastrak Building Designer considers imposed load reductions during the design of columns. However, these reductions are applied during the member design phase, the building analysis is always based on all loads applied simultaneously. By default therefore transfer beams will always be designed for the full (un-reduced) loading in the supported columns. If you regard this as over-conservative, then you can optimise the design of the transfer beams interactively.



Additional Member Types Plus the following additional member types

Fastrak Building Designer allows you to easily modelthe following more complex systems which might comprise multiple analysis members: • Trusses – Truss members can be formed in any material. They will attract loads and participate in the 3D structural analysis, elements of the truss can be checked (but not designed) provided they are defined in steel. – refer to ”Trusses and Truss Members”below.

• Diaphragm Braces – these are used to model flexible or semi-rigid diaphragms, serving to transfer lateral loads to the lateral load resisting systems in the 3D structural analysis. As they are not ‘real’ members they are neither checked or designed – refer to ”Diaphragm Braces”.

• Shear Walls – these are restricted to concrete or “Other” materials but cannot be checked or designed by Fastrak Building Designer – refer to ”Shear Walls” on page 20.

• Bearing Walls – these are subjected to gravity (vertical) loads only. They are typically built from masonry or timber but cannot be checked or designed by the software – refer to ”Bearing Walls”.

Trusses and Truss Members In the current version of Fastrak Building Designer you can define truss members in your model and then check their adequacy. Truss members will attract loads and participate in the 3D structural analysis, but elements of the truss can only be checked (but not designed). Fastrak Building Designer has a Truss Wizard to help you define many different types of truss.

Diaphragm Braces Diaphragm braces are not ‘real’ members - they are used in order to model those floors or roofs which can not be considered to be rigid due to their type of construction - they are usually termed ”Flexible Diaphragms” or ”Semi-Rigid Diaphragms”. Whether the resulting diaphragm is considered flexible or semi-rigid can be controlled by careful definition of the diaphragm brace propeties.



Creating Diaphragm Braces You create diaphragm braces in a similar way to simple braces. In the Brace attribute set you should specify ‘Diaphragm’ on the Design tab and then enter the required values for elastic modulus and area on the Size tab. Whilst they are applied singly, you are advised to always create a pair of cross diaphragm braces within a panel. Mostly these should connect between columns such that each column is restrained by at least two diaphragm braces at each floor level as shown in the plan below. .

In pitched roofs it would be advisable to tie the apex of roof members back to the vertical columns supporting the roof. .



You have to decide the layout of braces around openings. For small openings, the braces could cross the opening and join the surrounding columns - as shown for the smaller (lower) of the two openings in the figure below. For larger openings, they could be laid out around the opening to provide a triangulated framework connecting the nodes of the trimming steel. Two ways in which you might want to do this are shown below. .

Analytical Properties Diaphragm braces are released at both ends in the rotational y- and z-directions and fixed at both ends in the rotational x-direction. The only properties required are an area (A) and elastic modulus (E). Default properties are given in the attributes set, however as there is no 'correct' answer for how stiff a semi-rigid diaphragm should be, you are entirely responsible for determining appropriate values. Typically they need to be very slender members with low stiffness. To assist in this determination consideration should be given to the proportion of the horizontal loading that is resisted by each of the frames in the lateral load resisting system. This is achieved after analysis by considering the Storey Shear results. See ”Reviewing Storey Shear” for more details. Diaphragm braces have zero self weight. Since they are not ‘real’ members they are: • not designed,

• not listed, • not exported to 3D cad programs e.g. Revit. However they are exported to analysis programs eg S-Frame.



Shear Walls Shear walls are typically used to provide resistance to lateral loads and support other members. The following limitations apply to their use:

• • • •

Vertical walls only Rectangular walls only Concrete or “Other” materials only The shear walls will not be designed

Creating Shear Walls To create Shear Walls you should first create an appropriate Shear Wall attribute set. The attribute set consists of the wall’s material, thickness and analysis properties. If the material is specified as concrete, you should select the concrete grade. The program will then display a typical short term E value for the grade chosen. You will then need to decide on an appropriate value of E to be used in the analysis, taking into account factors such as creep, cracking and shrinkage. If the material is specified as “Other” you will also be required to specify an appropriate E to be used in the analysis. You can then create the wall itself from any of the 3D or 2D views: 1. While working in the 3D structure view or a frame view you can create a shear wall by clicking on start and end points at the base of the wall, followed by a third point which can be located anywhere in the floor at the top of the wall. The wall will extend vertically upwards between the start and end point. To the height defined by the third point. 2. While working in a 2D floor view you can create a shear wall by clicking on start and end points. You then select the construction levels at which the wall starts and ends. Analytical Properties A “mid-pier” idealisation is used for Shear Walls, this consists of: • Two horizontal elements at the bottom of the wall running between the two set out points and the mid point.

• Two horizontal elements at the top of the wall running between the two set out points and the mid point.

• Further pairs of horizontal elements for any intermediate construction level that is designated as a floor.

• Vertical elements joining the mid-points at the top and bottom of the wall and any intermediate floor levels.

• A fully fixed support is added at the midpoint of the wall baseline, unless it is being supported by one or more columns, another shear wall, or a transfer beam.



Consider the core wall arrangement shown below:

The “mid-pier” analytical model for this can best be reviewed graphically by showing the release state of the model (pick Select/Show/Alter State, and then pick Releases from the dialog).

If openings have been added to the wall the mid pier model will be modified accordingly. Additional vertical elements are introduced to the sides of the opening and a coupling beam introduced above. Addition of openings will reduce the strength, stiffness and self weight of the wall. Once a shear wall has been defined, extensions can be added to the wall ends. These do not increase its strength or stiffness, but the self weight would be increased. The strength and stiffness introduced to your structure will depend on the wall thickness and also the E value used in the analysis. Care should be taken to ensure that the E value used is realistic.



Note

The alignment (Left, Centre, or Right) of the shear wall is for cosmetic purposes only and does not affect its analytical properties.

Note

Shear Walls do not act as a medium via which loads calculated by the Simple Wind Loading generator and Wind Wizard are applied to your structure. If this is required an additional Wind Wall panel would have to created in the same location as the shear wall.

Transfer Shear Walls A shear wall may be partially or fully supported by a beam or truss member, but only if the supporting member has concrete or ‘Other’ material properties and it’s model type for shear wall modelling is set to Top Edge Beam.



Bearing Walls Bearing walls are used to provide resistance to vertical loads but not lateral loads and to support certain other member types. Limitations The following limitations apply to their use: • Vertical walls only

• • • •

Wall is rectangular with a horizontal top. Analysis model considers vertical (gravity) load only. Design is not included. Members can only be defined onto the top of a bearing wall at grid intersection points, wall column positions and at a user defined distance along the wall.

• However, the following members can not be supported by bearing walls - columns, beams with moment connections and braces.

• A shear wall cannot be supported on a bearing wall but a bearing wall can be supported on a shear wall.

• Beam members cannot be continuous over a wall (in the first release). Please note that the program will allow beams to connect to the top of the wall at any slope or diagonal angle except • Horizontal along the top and parallel with the length of the bearing wall The supported end of a sloping beam will have reaction components in both vertical and horizontal planes, the horizontal component is ignored by the bearing wall. Features

• A bearing wall item can be defined across vertical steel but the wall panel will 'split' at the steel position - see figure below.



• A bearing wall item can be defined over the top of a beam but cannot have a beam within or on top of the definition extents - see figure below. This is because a beam cannot be supported in the plane of a bearing wall.

• Bearing wall items must be rectangular with their vertices at grid intersection points but do not have to coincide with steel members - see figure below.

• Bearing wall items can be defined across floor levels but will be split at each floor level. • Bearing walls can be connected to other bearing walls at ends or anywhere in their length and do not have to be orthogonal. Creating Bearing Walls To create Bearing Walls you should first create an appropriate Bearing Wall attribute set. The attribute set consists of the wall’s material, thickness and self weight. The wall material is simply an identifying name - e.g. concrete, block, masonry.



You can then create the wall itself from any of the 3D or 2D views: 1. While working in the 3D structure view or a frame view you can create a bearing wall by clicking on start and end points at the base of the wall, followed by a third point which can be located anywhere in the floor at the top of the wall. The wall will extend vertically upwards between the start and end point. To the height defined by the third point. 2. While working in a 2D floor view you can create a bearing wall by clicking on start and end points. You then select the construction levels at which the wall starts and ends. Analytical Properties Bearing walls are modelled using a series of vertical column members, 'wall columns', and horizontal beam members, 'wall beams', as indicated in the diagram below. The beams have pinned ends and are placed at the top of the wall spanning between the columns. The next panel above is pinned to the one below and similarly the lower end of a column is pinned to a supporting beam. At the lowest level the column is 'fixed' to a pinned support.

Members supported by the wall either (fortuitously) bear directly on one of the wall columns or on one of the wall beams at the head of the wall. All wall columns and wall beams in an individual panel are given properties automatically by Fastrak, based on the width of the panel with which they are associated.



For bearing walls that are defined between other vertical column members e.g. General Columns, the wall columns at the edge of the panel are omitted and the associated wall beam is connected to the General Column (for example) and the adjacent wall column - see figure below.

Irrespective of whether the wall spans between other vertical column members or not - any load applied to the wall beam at the edge of the panel is shared between the end column and the first internal column. This can result in some load being ‘lost’ directly into the supports. Load transfer in the bearing wall model is not the same as it would be in for example, a masonry wall. A point load applied at the top of a masonry wall would result in a distributed load on any beam supporting the masonry wall, whereas in a bearing wall the supporting beam would be subjected to a pair of point loads, (or possibly even a single point load if the applied load coincides exactly with a wall column location). Self weight of the bearing wall is concentrated in the wall beams so seismic weight is concentrated at the top of the wall and not split between the floor above and below.

Chapter 3 : Sway Resistance

Chapter 3


Sway Resistance A simple overview of some of the alternative ways in which you can provide sway resistance in Fastrak Building Designer. Major topics

• • • •

Using Bracing Using Steel Moment Frames Using Other Moment Frames Using Shear Walls

Using Bracing This is the most traditional approach and well positioned and proportioned bracing is undoubtedly the best method of providing sway resistance.

Fastrak Building Designer allows all sorts of bracing configurations including:

• tension only bracing, • K bracing, and • inverted V bracing (as shown in the screen shot above). Note

For V and inverted V bracing the analytical model may need to include sliding connections so that the beam is not supported by the bracing under gravity loads. For further information on how to do this see ”Add bracing” in the Quick Start Guide.

Note

Care must be taken if modelling vertical cross bracings, see ”Vertical cross bracing” in the Assumptions and Limitations chapter for more details.



Using Steel Moment Frames If you have to provide stability using moment frames, then you can do so within the software using General Beams and General Columns. In such circumstances it is highly likely that you will want to consider the advantages and disadvantages of introducing some level of base fixity. The base fixity options are noted in Supports and Base Fixity”Supports and Base Fixity”on page 40. Where General Beams and General Columns are used in this way, you should designate them as being part of a moment resisting frame. This will ensure that the initial sizes assigned in the analysis/design process are reasonable. The designation can be carried out graphically by using the Moment Frames feature located on the Building tab in the Show/Alter State dialog. Alternatively the designation can be set by editing the properties of each member.

Using Other Moment Frames You are able to create moment frames using any material and section by using Member Beams and Member Columns. It is not necessary to designate such members as being Moment Frames as they only participate in the analysis and are not designed. If you attempt to provide stability using other materials and framing, then you need to pay particular attention to the definition of appropriate section and material properties. This was touched upon in ”Member Beams and Member Columns” on page 11.

Using Shear Walls

Stability for the very simple frame shown above is provided by shear walls. In this view the walls are rendered as if they are large solid panels, however the modelling idealisation being used is actually a “mid-pier” vertical beam element with a fixed base, and rigid cantilever arms extended out at each floor level to support any attached beams or slabs.



Swapping to the axis stick view shown below and switching off the beams and columns the idealisation becomes more apparent.

For further information refer to the article Shear Wall Analysis - New Modelling, Same Answers - The Structural Engineer, 1st February 2005, Vol. 83 No.3, page 20 which is available on the CSC web site - select Services, Technical Papers. This modelling idealisation of shear walls with beam elements is traditionally well accepted. The points made in ”Using Other Moment Frames” on page 28 regarding section and material properties are of course important. In recent years we have seen a trend towards Finite Element modelling of shear walls. This can be accomplished by exporting the Fastrak Building Designer model to general analysis software such as CSC S-Frame, editing it to remove the General Beams and then meshing up the wall panels. While this appears to be a more detailed approach that has advantages such as the ability to deal with irregular openings in wall panels, there are disadvantages. For instance, you do not escape from the need to consider making the appropriate adjustments to gross section and material properties as touched upon in ”Member Beams and Member Columns” on page 11. But, it can be done...


Chapter 4

Chapter 4 : Diaphragm Modeling

Diaphragm Modeling In a typical building lateral load resistance is provided at a few discrete points and it is assumed that applied lateral loads will be distributed to the ‘lateral load resisting system’ (LLRS) either by floor diaphragm action or by a bracing system. Thick concrete floors provide adequate diaphragm action to distribute these loads. These diaphragms are usually assumed to be 'rigid'. However, floors and roofs of different construction can also be used to transmit the horizontal loads to the LLRS but are considered not to be 'rigid', instead they are classed as either ‘semi-rigid’ or ‘flexible’. All three types of diaphragm can be modelled in Fastrak Building Designer. Major topics

• • • •

Rigid Diaphragms Semi-Rigid Diaphragms Flexible Diaphragms Storey Shears

Rigid Diaphragms A rigid diaphragm will maintain exact relative positioning of all nodes that it constrains, i.e. the distance between any two nodes constrained by a diaphragm will never change, therefore no axial load will develop in any member that lies in the plane of a diaphragm between any two constrained nodes. You can however elect to remove General Beam, Member Beam and truss chord nodes from the diaphragm, allowing axial forces to develop within those members. Any asymmetry in the stiffness of the LLRS can produce 'twist' in the diaphragm - also referred to as torsion effects. Out of plane effects are usually minimized or eliminated. Note

Nodes at support positions, (either column or supplementary), are automatically excluded from all diaphragms.

In Fastrak Building Designer, at each floor level there are 3 options for rigid diaphragm modeling: • Single diaphragm (Default Setting)

• Slab items defined • No diaphragm It is also possible to switch diaphragm action off for one or more individual slabs within a floor by Taking slabs out of a diaphragm. General Beams, Member Beam and Truss Chords can be taken out of a diaphragm in order to allow axial forces to develop within those members - see ”Release from a Diaphragm”



.Single diaphragm

This option switches diaphragm action on for an entire floor. Note that completely isolated areas of the floor are constrained by the same diaphragm. Hence, in the example above, if lateral load is applied to the left hand block it will be resisted by the combined bending of both blocks. The blocks can not move independently at the level of the diaphragm. This would produce incorrect results. Note

The extents of a diaphragm are best reviewed graphically (pick Select/Show/Alter State, and then pick Show Diaphragm from the dialog). Each independent diaphragm is shown in a different colour.

Slab items defined

Using this option discrete diaphragms are created for each area of interconnected slabs. If this option were applied to the example from the previous section a more realistic model would be created. Two separate diaphragms would exist at each floor level above the podium. As a consequence lateral load applied to the left hand block is not resisted by the right hand block. Each can move independently.



No diaphragm

This option switches diaphragm action off for an entire floor.

Taking slabs out of a diaphragm

It is possible to switch diaphragm action off for one or more individual slabs within a floor. This is only possible if the diaphragm has been defined using the Slab items defined option. To demonstrate this, the example from the previous section is modified to include a link bridge between the blocks. Initially, by using the Slab items defined option, a single diaphragm is created at the level of the bridge. This constrains all the floor nodes within both blocks at the level of the bridge, so that at this level the blocks can not move independently. Providing the linking slab is substantial this may be considered to be appropriate. However, if the link becomes more slender, a point will be reached where this is no longer the case.



By using the Alter Diaphragm function in the Show/Alter State dialog, diaphragm action can be switched off for the slab within the link bridge.

Because the remaining areas of slab at this level are no longer considered interconnected, two discrete diaphragms are formed and the blocks act independently.



Semi-Rigid Diaphragms A semi-rigid diaphragm cannot be assumed to be rigid. It can deform in plane (beam bending) and is influenced by the distribution of the stiffness of the ‘lateral load resisting system’ (LLRS). Consequently, there can be 'twist' and the distribution of the horizontal loads is a complex interaction of the stiffness of the diaphragm and the LLRS.

• In Fastrak Building Designer, semi-rigid diaphragms are modelled by introducing ”Diaphragm Braces”. within the plane of the floor.

Flexible Diaphragms The accepted definition of a ‘flexible’ diaphragm refers to the behaviour that allows for some deformation in plane of the diaphragm (similar to beam bending) but without the 'twist' that can occur in rigid diaphragms. As such the distribution of the lateral loads is not influenced by the distribution of the stiffness of the LLRS. A flexible diaphragm can be considered as a discrete form of semi-rigid diaphragm. Floors constructed from timber decking or thin sheets of profiled steel which, importantly, deform at the joints between sheets might be considered as flexible diaphragms.

• In Fastrak Building Designer, flexible diaphragms are modelled in the same way as semi-rigid diaphragms, by introducing ”Diaphragm Braces”. within the plane of the floor.

Storey Shears When modelling semi-rigid and flexible diaphragms, the designer cannot be sure of the 'correct' value to enter for the elastic modulus and area of the diaphragm braces. One way in which he can make this judgement is by consideration of the proportion of the horizontal loading that is resisted by each of the frames at each level in the lateral load resisting system, LLRS. This is achieved after analysis by considering the Storey Shear results. See ”Reviewing Storey Shear” for more details.

Chapter 5 : Member End Releases and Member Orientation

Chapter 5


Member End Releases and Member Orientation Once you start using Member Beams, Member Columns, General Beams, General Columns and Shear Walls, you are no longer dealing with a simple model where all the beams have pinned ends and only resist major axis moments. The design forces established in frames with moment connections are all distributed according to relative member stiffnesses. Therefore, in addition to ensuring that the member properties are correct, you need to review and take control over member end releases and member orientations. Note An important double check on all of this is to spend some time reviewing the analysis results. You may want to read ”Initial Review of Analysis Results” on page 60 for some notes/tips on this. Major topics

• • • • • •

Moment Releases Axial Releases Torsional Releases Release from a Diaphragm Member Orientations Supports and Base Fixity

Moment Releases

Member end moment releases are best reviewed graphically by showing the release state of the model (pick Edit/Show/Alter State, and then pick Moment Releases from the Analysis tab of the dialog). • Moment releases are indicated by an arrow with a double arrowhead.

• The releases for all supports, General Beams, General Columns, Member Beams and Member Columns are shown.

• The releases for Simple Beams and Composite Beams are not shown purely to limit screen clutter – they are always released for major and minor axis bending.



In the view above the front-left-elevation is created with General Beams and General Columns to form a moment resisting frame. The downward arrows at the nodes at the end of most of the beams therefore indicate that the beams are pinned in the minor axis moment direction. While this view is active you can click on node positions to select them (one or several at a time) and edit the releases via the Properties pane. Note You can only select end nodes for the currently active member type. Releases for inactive member types are shown in grey. When setting/changing moment releases for General Beams the options available include: Free — Used to indicate the free end of a cantilever. (Not really needed analytically, but needed to set effective lengths more appropriately.) Simple Connection — The connection is pinned for both major axis (My) and minor axis (Mz) bending. Moment Connection — The connection is fixed for major axis (My) bending but remains pinned for minor axis (Mz) bending. Fully Fixed — The connection is fixed for both major axis (My) and minor axis (Mz) bending. Continuous — This setting is automatically applied when a continuous beam is created and effectively creates a non-editable fully fixed connection between the spans of the continuous member. The connection can only be edited by splitting the beam. When setting/changing moment releases for Member Beams the options are slightly different: Free — Used to indicate the free end of a cantilever. Pinned — This is the same as the Simple Connection noted above, the connection is pinned for both major axis and minor axis bending. Pinned About Local y — This setting creates a pinned connection for major axis bending but the connection remains fixed for minor axis bending. (x is along the member, y is the major cross section axis and z is the minor cross section axis.) Pinned About Local z — This is the same as the Moment Connection noted above for General Beams, the connection is fixed for major axis bending but remains pinned for minor axis bending. Fully Fixed — This is the same as the Fixed Connection noted above, the connection is fixed for both major axis and minor axis bending.



Continuous — This setting is automatically applied when a continuous beam is created and effectively creates an non-editable fully fixed connection between the spans of the continuous member. The connection can only be edited by splitting the beam. Note

Note

If the sign conventions seem confusing the very best way to review what you are doing is to show the graphical representation of the member releases as discussed at the start of this section. You can also edit the intermediate connections on columns.

Axial Releases

Pick Edit/Show/Alter State, and then pick Axial Releases from the Analysis tab to graphically review the axial release state of the model. • The axial releases for all General Beams, General Columns, Member Beams, Member Columns and truss chords are shown and can be edited.

• The axial releases for V Braces are shown, but can not be edited. • The axial releases for Simple Beams, Composite Beams, Braces, Truss internals and sides are not shown purely to limit screen clutter – they are always released axially.

• General Beams and Member Beams can be released axially at either end, but not both. • If a beam is continuous it can only be released axially at one or other of it’s extreme ends. • Columns and Member Columns can only be released axially at the top end. While this view is active clicking on a node will toggle its state between Fixed and Released. Note

You can only select end nodes for the currently active member type.



Torsional Releases

Pick Edit/Show/Alter State, and then pick Torsional Releases from the Analysis tab to graphically review the torsional release state of the model. • The default torsional release state is Fixed.

• You are prevented from releasing both ends of the same member in torsion. • Clicking on a node will toggle its state between Fixed and Released. Note

You can only select end nodes for the currently active member type.

Release from a Diaphragm A diaphragm will maintain exact relative positioning of all nodes that it constrains, i.e. the distance between any two nodes constrained by a diaphragm will never change, therefore no axial load will develop in any member that lies in the plane of a diaphragm between any two constrained nodes. You can however elect to remove General Beam, Member Beam and Truss Chord nodes from the diaphragm, allowing axial forces to develop within those members. For example, consider the braced tower shown below, in which a lateral load has been applied at first floor level:



If a diaphragm has been activated at that level, then by default none of the lateral load will be transferred into the General Beam. To generate axial force in the beam you are required to edit the beam properties and exclude one or both of the beam ends from the diaphragm.

Because in the above example the load is being applied to end 1 of the beam, this is the end that requires to be excluded. If only end 2 were excluded the load would remain in the diaphragm. Note

Nodes at support positions, (either column or supplementary), are automatically excluded from all diaphragms.

Member Orientations All member orientations are reflected in the graphical views as appropriate. By default beams of all types are placed with their major axis horizontal (in the global XY plane). For the vast majority of beams this will be the required orientation. By default vertical columns are placed with their major axis in the global XZ plane. Clearly, at best, these default column orientations are only likely to be correct around 50% of the time. You can control the orientation of each newly inserted member by setting the appropriate member orientation (under the Alignment tab) of the current attribute set. You can edit the orientation of one or more selected members simultaneously by adjusting the appropriate details in the Property pane. An exception to the above is the case of inclined Columns – as you define these Fastrak Building Designer calculates the orientation angle automatically so that the web is vertical. You can not edit this angle. Caution

Since foundation shears and moments are reported relative to each column’s local axis system the automatic calculation of the member orientation for inclined Columns can initially be a little confusing.



Supports and Base Fixity A few points are worth noting on this topic: 1. The view in ”Moment Releases” also shows that you can view and edit support releases when viewing member releases graphically. 2. Supports are automatically created as you create any columns, by default these supports are always released (pinned). 3. For General and Member Columns you can select the support and adjust the base fixity between different fixity settings: a) Pinned – the default setting. b) Nominally Pinned – where a rotational spring stiffness (10 or 20% of column stiffness) is automatically calculated and applied. c) Nominally Fixed – where a rotational spring stiffness of 100% column stiffness is automatically calculated and applied. d) Fixed – a fully fixed support. e) In cases b and c above you can also specify a user-defined base fixity. Note

A nominally fixed support is not the same as a fully fixed support, a nominally fixed support will rotate according to the spring stiffness and this will affect deflections. If you have a genuinely fixed support you are indicating that no rotation will occur at the support.

The above options may be of particular interest where you want to achieve overall stability by frame action as opposed to diagonally braced panels. Overall you should find that, by default, members tend to be pinned in Fastrak Building Designer, so if you are editing releases you will generally be adding fixity. This is the opposite of the way in which most analysis packages work (where everything is initially fixed and releases have to be added) but we consider this to be a more conservative and realistic approach.

Chapter 6 : Load Cases and Load Combinations

Chapter 6


Load Cases and Load Combinations Loads are applied to the model in loadcases categorised by type, (Dead, Imposed, Wind, etc.) Once the loadcases have been created, combinations are then generated for design. Various ‘Wizards’ are provided to assist in this process. Major topics

• • • • •

Nationally Determined Parameters (NDP’s) Gravity Load Cases Lateral Load Cases Combinations Classifying Combinations and Setting the Critical Combinations

Nationally Determined Parameters (NDP’s) The Eurocode has differing NDP’s for the Eurocode (Base) and for each of Eurocode (UK), Eurocode (Irish) etc. These are defined in the relevant country's National Annex. Gamma () factors and psi () factors for each National Annex are listed below:

gamma factors Factor

EC Base value

UK Value

Irish Value

EQU combs Gj,sup

1.10

1.10

1.10

Gj,inf

0.9

0.9

0.9

Q (fav)

1.5

1.5

1.5

Gj,sup

1.35

1.35

1.35

Gj,inf

1.0

1.0

1.0

Q (fav)

1.5

1.5

1.5



0.85

0.925

0.85

Gj,sup

1.0

1.0

1.0

Gj,inf

1.0

1.0

1.0

Q

1.3

1.3

1.3

STR combs

GEO combs



psi factors Factor

EC Base value

UK Value

Irish Value

0

1

2

0

1

2

0

1

2

Category A - imposed domestic/residential

0.7

0.5

0.3

0.7

0.5

0.3

0.7

0.5

0.3

Category B - imposed office

0.7

0.5

0.3

0.7

0.5

0.3

0.7

0.5

0.3

Category C - imposed congregation

0.7

0.7

0.6

0.7

0.7

0.6

0.7

0.7

0.6

Category D- imposed shopping

0.7

0.7

0.6

0.7

0.7

0.6

0.7

0.7

0.6

Category E- imposed storage

1.0

0.9

0.8

1.0

0.9

0.8

1.0

0.9

0.8

Category H- imposed roofs

0

0

0

0.7

0

0

0.6

0

0

Snow Loads < 1000m

0.5

0.2

0

0.5

0.2

0

0.5

0.2

0

Wind Loads

0.6

0.2

0

0.5

0.2

0

0.6

0.2

0



Gravity Load Cases Gravity loadcases can be created for: • self weights,

• • • • •

dead, snow, snow drift, imposed, and roof imposed loads

Self weight loads can all be determined automatically. However other gravity load cases have to be applied manually as you build the structure.

Self Weight Self weight - excluding slabs loadcase Fastrak Building Designer automatically calculates the self weight of the structural beams/ columns for you. The Self weight - excluding slabs loadcase is pre-defined for this purpose. It can not be edited and by default it is added to each new load combination.

Self weight of concrete slabs Fastrak Building Designer expects the wet and dry weight of concrete slab to be defined in separate loadcases. This is required to ensure that members are designed for the correct loads at construction stage and post construction stage. These loadcases are not pre-defined. However, two loadcase types are reserved to assist in their creation: Slab Wet — select this loadcase type to define the wet weight of concrete at construction stage. Slab Dry — .select this loadcase type to define the dry weight of concrete, post construction stage. Fastrak Building Designer can automatically calculate the above weights for you taking into account the slab thickness, the shape of the deck profile and wet/dry concrete densities. It does not explicitly take account of the weight of any reinforcement but will include the weight of decking. Simply click the Automatic Loading check box when you create each loadcase. When calculated in this way you can’t add extra loads of your own into the loadcase. If you normally make an allowance for ponding in your slab weight calculations, Fastrak Building Designer can also do this for you. When specifying the slab Attributes - you will find two ways to add an allowance for ponding (on the Floor Construction tab). These are: • as a value, by specifying the average increased thickness of slab

• or, as a percentage of total volume. Using either of these methods the additional load is added as a uniform load over the whole area of slab.



Imposed and Roof Imposed Loads Imposed Load Reductions Reductions can be applied to imposed loads to take account of the unlikelihood of the whole building being loaded with its full design imposed load. Reductions can not however be applied to roof imposed loads. Note If the imposed load is considered as an accompanying action (i.e. a  factor is applied to the imposed load case in a combination) then as stated in the Base Eurocode Cl3.3.2, the imposed load reduction can not be applied at the same time - see ”Apply Imposed Load Reductions”. Although the code allows for imposed load reductions to be applied to floors (beams), Fastrak Building Designer does not implement this. Only the imposed loads on columns are reduced. If a level is not set to be a floor then no reductions are accounted for at that level and it will not be counted as a floor in determining the amount of reduction to make. (See ”Is it a Floor?”on page 71). Note A floor that has loads that do not qualify for imposed load reduction does not count in the storey count. (Unlike the approach for the BS 5950 design code.) The method used for determining the reductions is dependant on the National Annex: • In the Base Eurocode a formula is given in Clause 6.3.1.2(11), this is also used if the Irish National Annex is selected.

• In the UK, the NA permits an alternative method of reduction using NA 2.6. Definition of psi factors for imposed load cases Imposed loads are categorised as follows: • Category A - domestic/residential

• • • •

Category B - office Category C - congregation Category D - shopping Category E - storage

The default values of 0, 1 and 2 vary depending on the category selected and also with the National Annex being worked to. The values can be edited if required. Definition of psi factors for roof imposed load cases Roof imposed loads are not categorised so the default values of 0, 1 and 2 only vary depending on the National Annex being worked to. Again, the values can be edited if required.

Snow and Snow Drift Loads Definition of psi factors for snow and snow drift load cases The default values of 0, 1 and 2 can vary depending on the National Annex being worked to. The values can be edited if required. Note Snow drift loads are considered to be accidental load cases and are combined in the Accidental combinations.



Perimeter Loads Provided you have a gravity loadcase selected, (other than one of the self weight cases mentioned above), you will be able to access the Create Perimeter Load... command from the Loading menu. This will generate a uniform load for you around the entire building perimeter.

Lateral Load Cases Lateral loadcases can be created for ”Wind Loads” as detailed below.”Equivalent Horizontal Forces (EHF)” are also lateral loadcases, however these are only accessible when creating load combinations.

Wind Loads The EC1-4 Wind Wizard The Wind Wizard is run to create a series of static forces that are combined with other actions due to dead and imposed loads in accordance with BS EN 1990. The following assumptions/limitations exist:• The shape of the building meets the limitations allowed for in the code.

• It must be a rigid structure. • The structure must be either enclosed or partially enclosed. • Parapets and roof overhangs are not explicitly dealt with. For further information on the wind loading capabilities of Fastrak Building Designer refer to the EC1 1-4 Wind Modeller Handbook. Simple Wind Loading If use of the Wind Wizard is not appropriate for your structure there is a facility to load walls with a stepped horizontal pressure load - this facility is referred to as Simple Wind Loading. Simple wind loads are created in a similar way to other loadcases. Within the Loadcases dialog, provided the load type is set to Wind, an extra ‘Wind’ loading tab becomes available. The Generate... button on this tab is then used to create the stepped pressure. Alternatively, provided a loadcase of type wind is currently selected, the same Simple Wind Loading... functionality can be accessed from the Loading menu. To apply the wind to the building you must create a series of walls. The Simple Wind loading strikes all outward facing walls which can be seen in the wind direction defined. If the wind strikes an inward facing wall then it passes through the wall and does not load the structure. The simple way to verify in which direction your wall surface faces is to use Show/Alter State in the structure view. Definition of psi factors for wind load cases The default values of 0, 1 and 2 vary depending on the National Annex being worked to. The values can be edited if required. .



Combinations Once your load cases have been generated as required, you then combine them into load combinations. If The Construction Stage Combination is required, click the Add Construct. button. Additional combinations can either be created manually, by clicking Add... - or with the assistance of The Combinations Wizard, by clicking Generate...

The Construction Stage Combination If you have created a Slab Wet loadcase you are required to generate a Construction Stage load combination so that it is considered in the design process. Other loadcases can be included in this combination, however loadcases of type: Slab Dry and Wind are specifically excluded. If you add/remove a load case type from this combination - the factors are defaulted as follows: • 'Self weight' - default Strength factor = 1.35, default Service factor = 1.0

• 'Slab Wet' - default Strength factor = 1.35, default Service factor = 1.0 • 'Dead' - default Strength factor = 1.35, default Service factor = 1.0 • 'Imposed'- default Strength factor = 1.5, default Service factor = 1.0 The Construction Stage load combination is then used specifically in the design of any composite beams within the model. Note The Slab Wet loadcase can not be included in any other combination.

Manually Defined Combinations As you build up combinations manually, the combination factors are automatically adjusted as load cases are added and removed from the combination. If you add/remove a load case type from a combination - the factors are defaulted as follows: • 'Self weight' - default Strength factor = 1.35, default Service factor = 1.0

• • • • •

'Slab Dry' - default Strength factor = 1.35, default Service factor = 1.0 'Dead' - default Strength factor = 1.35, default Service factor = 1.0 'Imposed'- default Strength factor = 1.5, default Service factor = 1.0 'Roof Imposed'- default Strength factor = 1.05, default Service factor = 1.0 With an Imposed load case

• 'Wind' - default Strength factor = 0.75, default Service factor = 0.5 • 'Snow' - default Strength factor = 0.75, default Service factor = 0.5 • With No Imposed load case • 'Wind' - default Strength factor = 1.5, default Service factor = 1.0 • With Wind load case • 'Snow' - default Strength factor = 0.75, default Service factor = 0.5 • With no Wind load case • 'Snow' - default Strength factor = 1.5, default Service factor = 1.0



Equivalent Horizontal Forces (EHF) EHF’s are used to represent frame imperfections. The Eurocode requires they are applied to all combinations. (Lateral wind combinations therefore should also have EHF’s applied). Note

Fastrak Building Designer allows you to set up combinations without EHF’s to save time during the initial Gravity Sizing, however you should ensure that they are subsequently taken into account at the Full Design stage.

EHF’s are automatically derived from the factored load cases within the current combination. They are applied in the analysis as a horizontal force at each beam column intersection with a magnitude of 0.5% of the vertical load in the column at the column/beam intersection. They can be applied to your combinations in each of four directions as follows: • EHF X+

• EHF X• EHF Y+ • EHF YNote

If required EHF's can be applied in both X and Y within a load combination and factored appropriately to represent an application at an angle to the X/Y axes,

Caution

See Equivalent Horizontal Force Load Calculations”Equivalent Horizontal Force Load Calculations” in the Assumptions and Limitations section of this document for important information about which loads are taken into account in the EHF calculations.

Apply Imposed Load Reductions All imposed load cases can be set to have ‘imposed load reductions’ calculated. However, reductions can not be applied if the imposed load case is an ‘accompanying action’ within the combination (i.e. if 0 has been applied). If you define load combinations manually it is therefore your responsibility to check the Apply Imp. Reductions box if required when the combination is defined. If you use the combinations wizard to automatically generate your load combinations, the imposed load reductions will only be applied to those combinations where 0 is not used.



The Combinations Wizard Accessed via the Generate... button, this automatically sets up combinations for both strength and serviceability. Combination Wizard - Initial Parameters At the start of the wizard, you need to define certain parameters so that the correct combinations are generated - these are described below:

Combination for design of structural members (STR) You can chose between: • Table A1.2(B) - Eq 6.10, or

• Table A1.2(B) - Eq 6.10,a&b Eq 6.10 is always equal to or more conservative than either 6.10a or 6.10b. The most economic combination of 6.10a or b will depend on if permanent actions are greater than 4.5 times the variable actions (except for storage loads).

Include GEO combinations - Table A1.2(C) - Eq 6.10 The Eurocode version of Fastrak Building Designer does not currently design foundations, however if you require the foundation design forces reporting you should check this option.

Include Accidental combinations - Table A2.5 Eq 6.11a&b If you have defined an accidental load type such as Snow drift you should check this option for the correct load combinations to be generated. Note

The Combinations Wizard refers to the relevant National Annex when determining the factors to apply in the above combinations, as they may vary from the Base Eurocode values.

Type of Structure for Default Generated Combinations You can chose between: • Multi-storey building, or

• Portal frame building The default generated combinations that result are likely to be the most onerous for a typical structure of that type. Combination Wizard - Combinations The second page of the wizard lists the combinations applicable (with appropriate factors) for the selections made on the first page. Any factors in bold will be multiplied by the relevant psi factors for that load case. The type of structure chosen on the previous page affects which combinations default to being generated.



The combination names are automatically generated as per the table below:

No.

BS EN 1990 State and Eqn

Type

Load Combination

1

Str – 6.10

Gravity

Str1 - Gj,supD+QI+ QRI

2

“

“

Str2 - Gj,supD+Q0I+QS

3

“

Lateral (EHF)

Str3.n - Gj,supD+QI+QRI+EHF

4

“

“

Str4.n - Gj,supD+QI+Q0S+EHF

5

“

“

Str5.n - Gj,supD+Q0I+QS+EHF

6

“

Lateral (Wind+EHF)

Str6.n - Gj,supD+QI+Q0S+Q0W+EHF

7

“

“

Str7.n - Gj,supD+Q0I+QS+Q0W+EHF

8

“

“

Str8.n - Gj,supD+Q0I+Q0S+QW+EHF

9

“

Uplift (Wind+EHF)

Str9.n - Gj,infD+QW+EHF

1

Str – 6.10a&b

Gravity

Str1 - Gj,supD+Q0I+ Q0RI

2

“

“

Str2 - Gj,supD+Q0I+Q0S

3

“

“

Str3 - Gj,supD+QI+QRI

4

“

“

Str4 - Gj,supD+Q0I+QS

5

“

Lateral (EHF)

Str5.n - Gj,supD+Q0I+Q0RI+EHF

6

“

“

Str6.n - Gj,supD+Q0I+Q0S+EHF

7

“

“

Str7.n - Gj,supD+QI+QRI+EHF

8

“

“

Str8.n - Gj,supD+QI+Q0S+EHF

9

“

“

Str9.n - Gj,supD+Q0I+QS+EHF

10

“

Lateral (Wind+EHF)

Str10.n - Gj,supD+Q0I+Q0S+Q0W+EHF

11

“

“

Str11.n - Gj,supD+QI+Q0S+Q0W+EHF

12

“

“

Str12.n - Gj,supD+Q0I+QS+Q0W+EHF

13

“

“

Str13.n - Gj,supD+Q0I+Q0S+QW+EHF

14

“

Uplift

Str14.n - Gj,infD+QW+EHF

1

Geo - 6.10

Lateral (EHF)

Geo1.n - Gj,supD+QI+QRI+EHF

2

“

“

Geo2.n - Gj,supD+QI+Q0S+EHF

3

“

“

Geo3.n - Gj,supD+Q0I+QS+EHF


No.


BS EN 1990 State and Eqn

Type

Load Combination

4

“

Lateral (Wind+EHF)

Geo4.n - Gj,supD+QI+Q0W+Q0S+EHF

5

“

“

Geo5.n - Gj,supD+Q0I+QS+Q0W+EHF

6

“

“

Geo6.n - Gj,supD+Q0I+Q0S+QW+EHF

7

“

Uplift (Wind+EHF)

Geo7.n - Gj,infD+Q,1W+EHF

1

Acc 6.11

Lateral (EHF)

Acc1.n - G+Ad+1I+EHF

2

“

Lateral (Wind+EHF)

Acc2.n - G+Ad+2I+1W+EHF

Combination Wizard - Service Factors The next page of the wizard indicates which combinations are to be checked for serviceability and the factors applied. Combination Wizard - NL’s The last page of the wizard is used to set up the ‘NL’s or notional loads (for example the equivalent horizontal forces). You can specify EHF’s and factors in each of four directions. For each direction selected a separate EHF combination will be generated. Click Finish to see the list of generated combinations.

Classifying Combinations and Setting the Critical Combinations Having created your combinations you classify them as either Gravity Combinations, Lateral Combinations, or Seismic Combinations and also indicate whether they are to be checked for strength or service conditions, or both. Note If generated via the Combinations Wizard they are classified for you automatically. You also have the option to make any of the combinations inactive. At the same time you should nominate which are to be the critical combinations for the automatic sizing process. (Gravity Sizing and Lateral Sizing) - see ”Setting the Critical Combinations” Note

For details of Gravity Sizing, Lateral Sizing and Full Design see ”Overview of the Analysis and Design Process”in the next chapter.



Gravity Combinations These combinations are used for Gravity Sizing. (They are not used for Lateral Sizing.) All members in the structure are automatically sized (or checked) for the gravity combinations during the gravity sizing process. In addition, all members in the structure are checked for the gravity combinations during the Full Design process.

Lateral Combinations These combinations are used for Lateral Sizing. (They are not used for Gravity Sizing.) All members which have not been set as Gravity Only are sized (or checked) for the lateral combinations during the gravity sizing process. In addition, all members which have not been set as Gravity Only are checked for the lateral combinations during the Full Design process.

Seismic Combinations Note

Although included in this documentation, these are only available for use in regions where seismic design is required.

These combinations are not considered in either the Gravity Sizing or Lateral Sizing. All members are checked for the seismic combinations during the Full Design process.

Setting the Critical Combinations You are required to identify at least one critical gravity combination and one lateral combination. The critical lateral combination could contain notional, or wind loads (not seismic). Up to four lateral combinations can be selected, typically one of each sign (i.e. +X,+Y,-X,-Y) in which case they will be acting at 90 degrees to each other. The purpose of nominating critical combinations is to reduce the time taken to perform the Gravity Sizing and Lateral Sizing processes. It is down to your judgement as the designer to identify the most critical combinations. Given that this choice may not be clear or may be made incorrectly, there is the potential for sections to fail under other design combinations. However, this situation will be detected because Fastrak Building Designer always requires you to perform a Full Design, (which is a full analysis/check design process for all active load combinations) before the building can be given a 'valid overall design' status. Note

For details of Gravity Sizing, Lateral Sizing and Full Design see ”Overview of the Analysis and Design Process”in the next chapter.


Chapter 7

Chapter 7 : Analysis And Design Procedures

Analysis And Design Procedures This chapter provides an overview of the analysis and design process, and describes the various options involved. Suggested techniques for reviewing the answers are also given. Major topics

• • • • • • • •

Definitions Building Validation Overview of the Analysis and Design Process Analysis Options Design Options Initial Review of Analysis Results 3D Analysis Effects Refining Member Designs

Definitions Some definitions of words and phrases used in the remainder of the chapter are given below:

Alpha-crit (cr ) the factor by which the design loading would have to be increased to cause instability in a global mode. cr is determined from a sway stability analysis and used to determine the sway sensitivity of the structure.

First-order analysis linear elastic analysis that takes no account of the effect on the forces due to deformations of the structure.

Second-order (P-Delta) analysis analysis that takes into account the effect on the forces due to deformations of the structure. Either: • by using the Amplified Forces Method, or

• by a rigorous method using a two step iterative approach. Amplified Forces Method using this method, second order sway effects due to vertical loads are calculated by amplifying horizontal loads as per Clause 5.2.2 (5)B.

Gravity members gravity members resist only vertical loading. They are designed for gravity combinations only. Simple Beams, Simple Columns and Composite Beams are typically gravity members. However other members can also be set to ‘Gravity Only’

Lateral members lateral members resist vertical (gravity) and horizontal (lateral) loading. General Beams, General Columns and Braces are lateral members. They are designed for gravity and lateral combinations.



Equivalent Horizontal Forces these are used for two purposes: • for the calculation of cr

• to represent frame imperfections EHF’s are calculated as 0.5% of vertical Dead and Imposed loads. These are sometimes referred to as Notional Loads.

Building Validation Validation is a check on your structure which you must perform before you can analyse and design it. Validation checks all elements in your structure for a wide range of conditions. If any condition is not satisfied then Fastrak Building Designer tells you. Two types of validation message can be displayed.

Errors Error messages prevent the analysis from continuing until appropriate corrective action is taken.

Warnings Although warning messages do not prevent the analysis process from continuing, it is very important that these messages are reviewed to decide whether any action is warranted. Note

For assistance in understanding and resolving error and warning messages please refer to the Analysis section of the Fastrak Building Designer Help

Overview of the Analysis and Design Process Every design member in your model will be set into one of two possible modes: Check Design mode — you assign your desired section size to the member and the program then determines if the section is sufficient. Automatic Design mode — you select the desired section type for the member and the program then automatically determines a suitable size of the chosen section type. For those members set in Automatic Design Mode a two stage sizing process is performed.

Gravity Sizing A first order analysis is performed and then the members are designed for those gravity combinations that you have nominated ‘critical’. Note

During the initial Gravity Sizing, to save time you can choose ‘critical’ combinations that do not include EHF’s. However you should ensure that EHF’s are subsequently taken into account at the Full Design stage.

The analysis is carried out using ‘guessed’ section sizes and the subsequent design will result in sections that are either smaller or larger than these. Hence there may be some slight discrepancy between the original analysis results and those that would have been obtained if the final sections had been used. (A second run of the gravity sizing would remove any discrepancy.)



You might now choose to make adjustments to the model or select an alternative critical gravity combination. On completion of the gravity sizing process all members will be set into Check Design mode. At this point it is possible that the lateral members are under-sized (having been designed for the critical gravity combinations only) so it is recommended that you reset them to Automatic Design mode (See ”Set Auto Design Mode” on page 55) before moving on to the next step in the automatic design process which is Lateral Sizing. Alternatively you can proceed straight to the Full Design.

Lateral Sizing The analysis type you have selected in Analysis Options is performed and then the members are designed for the lateral combinations that you have nominated ‘critical’. The critical combinations may include gravity or lateral (notional loads or Wind) loadcases. You can choose up to four lateral combinations - essentially allowing selection of one in each principal wind or EHF direction. An auto-design is carried out for the lateral members based on the results of this analysis. The gravity members have already been auto-designed for the gravity combinations during the Gravity Sizing stage and so these are not considered. Potentially the gravity members could be marginally affected by changes in section size elsewhere in the structure and by second-order effects (if appropriate) but if this is the case it will be picked up in the Full Design stage. If any section sizes change the analysis is then re-performed with the new section sizes, followed by a check design for the ‘critical’ combinations. On successful completion of the above sizing processes a suitable section is assigned to each member automatically. Each member is then set to Check Design Mode. At this stage you can review the results for the section sizes that have been assigned and if necessary replace sections you don’t like with your preferred alternatives. You can also re-run the sizing process with alternative critical lateral combinations if required. You can then move on to the Full Design.



Full Design When every member in the model is set to Check Design Mode, a final check must be performed for all members for every active load combination, based on up-to-date analysis results. This is required before you proceed to output the calculations. The members to be considered in the full design process are specified via the Design Control page of the Design Options. By default all members are selected. The full design process is as follows: • A first-order analysis of all unfactored loadcases is carried to establish Serviceability Limit State requirements such as deflections.

• EHF’s are determined for every active combination in which they have been included. • Having established the EHF’s, their contributions to frame deflections are determined using a first-order analysis. crvalues are also established.

• The analysis type that you have set in Analysis Options is then performed for all active combinations to establish design forces.

• All members are checked for the appropriate design requirements. Gravity members are checked for gravity combinations, lateral members are checked for all combinations. Only active combinations are checked.

Set Auto Design Mode After each run of Gravity Sizing every member is set to Check Design Mode. Typically you will want to reset some of the members to Auto Design Mode before re-running the Gravity Sizing, or moving on to the Lateral Sizing. This can be achieved by picking Set Auto Design Mode from the Design menu. Members can either be reset by their element type, or if required, only those members affected by the lateral sizing can be reset. Alternatively, you can use the Auto Design item (located on the Design tab of the Show/Alter State dialog) to click, (or box around) members to switch them between Auto and Check Design Modes.

Analysis Options Under the Design menu there is an Analysis Options… setting which shows the dialog below.



First-order or Second-order Analysis? Unless cr is greater than 10 (in which case second-order effects can be ignored), it is essential that your final design utilizes one of the second-order analysis approaches. During the initial sizing process you may however choose to run a first-order analysis. Proceeding in this way you can obtain sections and an overall building performance with which you are satisfied, before switching to P- analysis. The following approach to setting the analysis type is suggested: 1. On the Analysis Options dialog, initially set the analysis type to First-order analysis. 2. Perform Gravity Sizing using first-order analysis in order to obtain a figure for cr 3. Refine the design if necessary until cr is greater than 3.0. (This makes the structure suitable for a final design using the amplified forces approach, which is favoured over the rigorous two step iterative approach). 4. When cr is greater than 3.0 change the analysis type to Second-order analysis -Amp. forces method. 5. Apply lateral loads to the structure and perform Lateral Sizing. 6. With the analysis type still set to Second-order analysis - Amp. forces method perform Full Design. Note If rigorous second-order analysis (i.e. P-Delta - two step iterative) is used during the initial sizing process, you may find that it can be more sensitive to parts of your model that lack stiffness. Our strong recommendation is that you use the Second-order analysis - Amp. forces method where possible/applicable - but be aware that EC3 classes certain structures outside the scope of this method (see ”Validity of Amp. Forces Method” below). For such structures, a similar approach to that shown above is suggested, however, you would need to refine the design during gravity sizing (step 3 above) until cr is at least greater than 2.0, and then use the Second-order analysis - two step iterative approach for the lateral sizing and full design (steps 4-6 above). Validity of Amp. Forces Method Clause 5.2.2 (6)B lists limitations on the applicability of the Amp. Forces method. It is therefore your responsibility when selecting this method to ensure all of the following:• all storeys have a similar distribution of vertical load

• all storeys have a similar distribution of horizontal load • all storeys have a similar distribution of frame stiffness with respect to the applied storey shear forces Also according to clause 5.2.1 (4)B limitation: • roof slope shallow - not steeper than 1:2 (26 degs)

• axial compression in beams or rafters (i.e. General Beams) Ncr / Ned <= 11.1 This last condition is automatically checked by the program and a warning is given against any affected General Beams.



Curved Beams If you have defined curved beams in your structure, then the Curved Beams page is applicable. This allows you to tell Fastrak Building Designer into how many equivalent straight sections it is to split the curved member. Curved members are modelled in analysis as a number of straight members. You control the minimum number of such elements by the value that you set for Minimum number of segments on the Curved Beams page. We advise that to avoid local effects then the change in angle between any two straight elements round the curve should be about 2.5°. Fastrak Building Designer adjusts the loading around the curve to model it as accurately as possible on the straight elements. This introduces small errors in the applied loading versus the reactions in the design tree. If you find that the loading on curved members does not have a check against it (that is that the applied loading to reactions tolerance is exceeded) then the likely reason is that the curved member is not split finely enough to give an accurate solution. In this case you need to increase the Minimum number of segments round the curve.

Torsion Factors The Torsion Factors page is provided to permit you to adjust the torsional stiffness for individual element types. The default is that the full torsional stiffness will be utilized for all member types for all analyses. If the resulting torsion in any member exceeds the torsion force limit then a warning is given in the design results. You could then decide to relieve this by reducing the torsion factor and re-analysing the model. The forces would then be carried by other means.

Cracked Sections The Cracked Sections page is provided to permit you to define the cracked section properties of concrete members. For analysis of the structure for all analyses and all load combinations, the concrete sections beams, columns and walls will use the cracked section properties defined on this page.



Design Options Under the Design menu there is a Design Options… setting which shows the dialog below.

Design Codes You can switch between design codes from this page. Note that if you change between codes, all design combinations will need to be recreated.

Design Control This page of the dialog provides various control options. The Perform check of fields provide a way to speed up the design process when you want to only modify a particular part of the design of your structure. For instance if you have designed all the floors in your model, and are satisfied with the resulting beams, but you want to work with the columns, you can remove the check against the types of member with which you are satisfied, and Fastrak Building Designer will ignore these during the design process. These options only affect the checking process. If a particular element needs to be designed, then this will happen irrespective of the settings you make here.

Force Limits - Members A full 3D analysis may expose small forces that are normally ignored in the design of members. The options for ignore forces below simply provide you with a way of setting negligible/nominal force levels with which you are comfortable. When the small forces from the 3D analysis are below the specified threshold levels they are ignored so that design can proceed automatically. If the forces are above these limits, then you will be warned during the design process but the forces will still be ignored.

Force Limits - Connections Similar to above, the options for ignore forces below simply provide you with a way of setting negligible/nominal force levels with which you are comfortable. When the small forces from the 3D analysis are below the specified threshold levels they are ignored so that connection design can proceed automatically. If the forces are above these limits, then you will be warned during the design process but the forces will still be ignored.



Element Pre-sizing For second-order analysis it is useful to have the initial section sizes bearing some resemblance to those that are eventually chosen. For this reason length/depth and L/ryy ratios are employed to prevent the initial sections from being under-sized. Note The program defaults are considered reasonable, however, they are not fine tuned to any particular structure type. For example, if you generally work on seven storey braced frames you may prefer to set different pre-sizing limits to somebody predominantly working on three storey moment frames.

Portal Pre-sizing Portal frames are analysed but not designed in Fastrak Building Designer, therefore it is important that they are assigned sensible section sizes for the building analysis results to be realistic. In Check Design Mode you should ensure suitable section sizes are used. In Automatic Design Mode, sections are selected based on the following pre-sizing criteria. • For rafters - the maximum slenderness ratio out of plane, L/ryy should not exceed 240. (Where L is the developed length of the rafter from the sharp end of the haunch to the apex. Which for mansard is the combined pair of rafters, for flat topped is measured to the centre line of the mid rafter, and for mono measured to the mid point of the rafter).

• For columns - the maximum slenderness ratio out of plane, L/ryy should not exceed 120. (Where L is the height of the column from the base to underside of haunch). Although the default pre-sizing values can be edited, you are advised not to change them as they have been specifically chosen to prevent the initial sections from being under-sized. Note Although not displayed on screen, an additional criteria is applied to the above slenderness ratios. For rafters the slenderness ratio must be less than the limit specified by you (default 240) and 25L whichever is the greater. For columns it must be less than the limit specified by you (default 120) and 20L

Composite The calculation of the effective width is only carried out for Composite Beams if they lie within the tolerance on rectilinearity set here. The default tolerance is 15 degrees; at greater angles you will be prompted to enter the effective width manually.

EHF Forces On this tab, the basic frame imperfection factor, 0 (1/200) has reduction factors, h and m applied in the X and Y directions to determine the actual frame imperfection factors, X and Y, as per Clause 5.3.2 (3). Where; • The height of structure is used to calculate the reduction factor, h. (Note that this would only have an effect for structure heights between 4 and 9m high).

• The Number of columns in the X direction is used to calculate the reduction factor, mX. • The Number of columns in the Y direction is used to calculate the reduction factor, mY. The X and Y factors are then applied in the design combinations to determine the magnitude of the EHF forces.



Initial Review of Analysis Results

Before spending time looking at detailed design results it is always worth reviewing things at a broad-brush overview level. The most basic checks are made obvious within the workspace summaries shown above.

Maximum Nodal Deflections Maximum deflections are identified and noted – if any of these are clearly much too high then there may be mechanisms developing or the defined structure is simply not capable of dealing with the loads being applied to it. If extreme deflections are being reported then the analysis results may be suspect. If there are any issues of this nature, then you should investigate these before spending (potentially wasted) time looking at detailed design results.

Sway Sensitivity Once again, before spending too much time reviewing detailed designs, it is also advisable to review the sway results and decide on the approach you will take to design for sway. It is conceivable that any changes you make in order to deal with sway could affect the designs of many elements. The sway sensitivity of the structure is evaluated by reviewing the cr factor, which is calculated for all column stacks in both the X and Y directions. The worst column stack result in each direction (lowest cr) is reported in the workspace summary.



Non Sway Sensitive Frames (cr > 10) Provided cr > 10 in both directions the frame is classified as Non Sway. Second- order effects are small enough to ignore and a first-order analysis is acceptable. Sway Sensitive Frames (3 < cr < 10) If cr lies in the range between 3 and 10, the frame is classified as Sway Sensitive. Secondorder effects can not be ignored and as a consequence a first-order analysis is not acceptable. For this classification Fastrak Building Designer provides two methods of solution: 1. Amplified forces method - A multiplication factor is derived and is then applied to all forces which induce this sway effect. This method may not be valid for all structure types. 2. Two step iterative approach - A rigorous second-order analysis. Extremely Sway Sensitive Frames (2 < cr < 3) If cr lies in the range between 2 and 3, the frame can be classified as Extremely Sway Sensitive. Second- order effects can not be ignored and a full second order analysis (the two step iterative approach) should be adopted - whilst the analysis/design process in Fastrak Building Designer can deal with this, a better strategy would be to ‘improve’ the lateral stiffness of the structure. If cr is less than 2 the structure is unsuitable as it does not have sufficient lateral stiffness. A message is displayed to this effect in the Project Workspace. Note

For further advice regarding the most suitable method of solution see ”Analysis Options”

Loading Summary This is simply a mathematical double check – does the sum of applied loads equal the sum of the base reactions? If there is a discrepancy identified by this comparison (but the maximum deflections noted above seem reasonable) you will need to consider if loading in the model has been applied correctly - e.g. have you applied a line load that extends beyond the floor slab area?



Review of Selected Sections It is always worth spending a little time reviewing the results to see if they are in line with expectations. Where you have moment framing this is even more essential. A quick review does not need to look at the design detail, simply look for items such as: • do the typical beam and column sizes look reasonable?

• • • • • • •

where you expect to see a large beam or column have you got one? are there large beams or columns where you did not expect them? where you expect similar sizes have you got similar sizes? where you expect symmetry is there symmetry? do you have zero moment at pin connections? do you have non-zero moment at moment connections? have you limited the use of composite beams to situations where composite beams are practical in reality?

Review Analysis Results It is always worthwhile taking time to review the analysis results for your entire model as this gives you important information on how your structure is working. You can view deflected shape diagrams, axial load, shear force, bending moment and foundation load diagrams for all member types in your structure, or limit the views to just those particular member types of particular interest. Note The model can also be exported to S-Frame where all the same sorts of results for the static analysis can be reviewed and there is also access to more advanced analysis options, e.g. Buckling Analysis (analytical assessment of cr), Vibration and Response Spectrum Analysis, etc.

Review Centre of Mass / Rigidity To assist your assessment of how the structure will behave under lateral load, graphical feedback is provided on the floor centre of mass and the floor centre of rigidity, both in the floor view and the structure view. This information is given by load case and combination. The various possible gravity and wind load cases in X and Y directions with their eccentricities are reported back graphically by showing the load applied to each floor as a single force together with its point of application. Centre of Mass For any given load case or combination, all gravity loads (self weight, slab dry, imposed, etc.) applied to a given floor have a centre of action (or centre of mass), a point about which the loads would balance if a pinned support were positioned at this location in plan. To review graphically the centres of mass within the model pick Select/Show/Alter State, and then pick Centre of Mass/Rigidity from the Analysis tab of the dialog.



Centre of Rigidity Any given floor has a centre of rigidity or bending stiffness based on the stiffness of the structure that supports it (i.e. the columns, walls etc. below). Due to the complex nature of assessing the stiffness of such varied structural systems, the centre of rigidity is only an approximation. To review graphically the centres of rigidity within the model pick Select/Show/Alter State, and then pick Centre of Mass/Rigidity from the Analysis tab of the dialog.

Reviewing Storey Shear To assist your assessment of the performance of any semi-rigid/flexible diaphragms, graphical feedback is provided for the storey shear at each floor level - storey shear being that proportion of the total horizontal load that is resisted at each storey. This information is given by load case and combination and is reported back graphically as a series of horizontal loads just below each floor level in X and Y directions. By making the appropriate selection in View Options you can choose to display just the X or Y direction storey shear, or both. To review storey shear click the Storey Shear icon on the Output Graphics toolbar. To prevent the display of irrelevant results, storey shears less than a specified minimum value are not displayed. The default is 1kN, but this can be adjusted in Design Options if required. For buildings with floors that are not readily identifiable or are sloping - the storey shears may be inaccurate.

3D Analysis Effects Traditional design approaches tend to involve idealisation and simplification of the analysis model. Very often this would have meant simplification of the structure into discrete 2D planes, which could be analysed either by hand or in a simple 2D analysis. Engineers working with 3D analysis packages sometimes encounter unexpected results, which only make sense after some careful consideration. For the purposes of this document we are calling these 3D Analysis Effects. Since Fastrak Building Designer allows you to model moment frames and uses a full 3D analysis to generate design forces we anticipate that you may encounter these sorts of effects. The following two subsections illustrate two simple examples.



Continuous Beam Example

The above model is not intended to be highly realistic, it does however illustrate a 3D Analysis Effect quite clearly. Continuous beams (spanning 6 m then 9 m then 4.5 m) run from right to left of this floor area. These are supported on simple steel beams spanning front to back which are in turn supported by columns. There are 3 internal lines of continuous beams which all receive the same loading. Designing by hand most engineers would probably consider that the analysis of a single 2D continuous beam line with pinned supports (as below) would be an adequate idealisation.

A more accurate analysis would attempt to model the spring effect at each of the supports – that is the supports are not completely fixed against vertical translation.



This spring effect is inherently modelled in a full 3D analysis and the results after analysis and design in Fastrak Building Designer are shown below.

Notice that different sections are chosen for the central continuous beam line on grid 3 when compared with the beam on either side of it. A first reaction to this sort of result might be to suspect that the design is wrong. However, a closer examination shows that the design is correct, and that it is correctly based on differing design forces. The results diagrams for the central continuous beam line are shown below.



Compare these with the equivalent diagrams for one of the adjacent beams shown below.

Note

The maximum sagging moment has reduced from 255.3 kNm to 245.0 kNm.

This sort of variation is enough to force the selection of a larger beam on the central beam line where the moments are higher. To see an alternative you might drive the design a different way. For example you might decide to use nothing larger than a 914 deep section, so you change the two cross beams on grids B and C to 914 305 UB 224 and put the 3 continuous beams back into design mode. This time the design converges quickly (because the conflict between the stiffness of the supporting beams and the stiffness of the continuous beams is removed). The result is shown below.

All member designs pass so this is an alternative and completely acceptable design.



Braces Carry Gravity Loads Example This is probably a simpler example of a 3D analysis effect, however it does initially seem to fly in the face of traditionally accepted design practice. In traditional hand calculations the load chase-down puts all gravity loads into the columns. Where columns are also part of a bracing system providing sway stability, brace and column loads for the sway case are assessed in isolation and are added to the column loads for column design checks as necessary. The possibility that braces carry gravity loads is never considered in this traditional hand calculation approach.

Consider the simple model shown above. It is braced on all four sides and in this example the initial sizes of the braces have been made very large to exacerbate the analytical effect. When the columns are designed, the resulting section sizes for the front face of the structure can be displayed in a 2D Frame View as shown below.

When reviewing these results you might wonder why the column at C which is part of a braced panel is smaller than the column at B which supports the same floor area.



A good way to investigate these sorts of effects is to display the design forces graphically within Fastrak Building Designer. Shown below is the same 2D Frame View but with the Axial Force display activated.

This view shows that the column at grid C which is part of the braced bay loses load to the brace at second floor level. The large brace forces are clear to see. If we change the brace section size to a smaller, more realistic section as shown below, then Fastrak Building Designer finds that the same section size is adequate for both columns.

We suggest that you keep brace sizes to a realistic minimum during building design. This realistic minimum is likely to be driven by sway considerations. For further information see How’s the structure working? in the extended Quick Start Guide.



Refining Member Designs You can export any designed member to its respective design package for refinement of the original design. In doing so you may decide to select a different section (larger or smaller), interactively. You can then return this amended section size to the main model (where you will have to re-analyse and check your model). Caution

A revised section may seem to work satisfactorily when designed in isolation, however, it is quite possible that it will fail when checked after re-analysis of the full model. It is also entirely possible that other members in your model may fail (or have more capacity in hand) since the distribution of forces will be affected by the different section size which you picked. In addition any second order effects that are considered in the full model will not be present when the section is designed in isolation.

For further information see Refining Member Designs in the Eurocode Member Design Handbook.


Chapter 8

Chapter 8 : Building Effective Models

Building Effective Models Fastrak Building Designer is a design based structural modeller. Please remember that Fastrak Building Designer is a modelling package, which dictates the design model and which creates analysis models to accomplish this design. It is important that you recognise that you must take ownership of the creation of the model and the results that the software gives. It is possible to create complete building designs quickly and easily with Fastrak Building Designer, however as it is a design based modeller you should take account of the following points1. Major topics

• • • • • • • • • • • •

Place grid lines accurately Save time by using Attributes effectively Use ‘Simple’ beams and columns where possible Use Perimeter Loading for edge beams where applicable Is it a Floor? Set the appropriate level of Diaphragm Action Set the appropriate level of deflection checks Building Size and Orientation Switch off irrelevant load combinations Design simple construction for gravity loads only Staged modelling and design Check the model analysis results

Place grid lines accurately You can define grid lines quickly and simply in Fastrak Building Designer. Alternatively you can import them into your model from a DXF file. If you are importing grid lines from DXF files, please ensure: • that the grid lines you are using are accurate,

• that the DXF file you are importing only contains grid lines. If you are in doubt we advise you to use Building Designer’s ability to import a DXF file and create a ghost image of the structure. You can then add your Building Designer grid lines on top of the ghosted DXF image.

Save time by using Attributes effectively It is important to realise that the attribute sets are used to set up defaults for the elements (beams, columns …) in your model. The attribute sets are not linked to the elements once they are created. Footnotes 1. This is particularly important if you are a new user, until you become familiar with Fastrak Building Designer.



You can quickly make major changes to your model, for example changing the grade of steel, quickly and easily, you need to change the appropriate attribute(s) and then apply these attribute(s) to the members. IMPORTANT — when you create any member it takes the current default attributes. The default setting for a Simple Beam (unless you change it) is that it is fully restrained. Please take care if creating beams that are not fully restrained. Once created a member is separated from the attribute set that it was created from and holds its own attributes. A new attribute set can however be applied very easily to an existing member if required.

Use ‘Simple1’ beams and columns where possible Fastrak Building Designer will happily design moment frames or continuous beams automatically within a model, but, the design of these elements is much more comprehensive (and hence takes longer). For this reason you should only use such elements when your model specifically requires them.

Use Perimeter Loading for edge beams where applicable Fastrak Building Designer applies floor loading, area loads, line loads and point loads to the slabs in your model and distributes them in the direction of span of the slab. If you wish to apply loading directly to a beam, (particularly if the beam supports a slab), then you should use element loads which apply the load directly to the beam without involving the slab. To aid in the application of load to edge beams Fastrak Building Designer has a Create Perimeter Load facility. You can access this from the Loading menu.

Is it a Floor? When you define construction levels you have a number of choices/settings to control.

Construction levels are simply levels that you need to identify in order to construct your model.

Footnotes 1. Pin type connections – thus in this context a composite beam is ‘simple’.



By setting a construction level to be a Floor you are indicating that it is a major level in the building. Floor levels are used to determine items such as your inter story height and positions from which column splices are laid out. If a level is not set to be a floor then no live load reductions will be accounted for in the beams at that level, or in the columns supporting that level. Once a level is set to be a floor you also have the option to activate Diaphragm action within it. There will certainly be a number of levels that are clearly floor levels, but there could be many others that are not. For example you create intermediate levels in order to define: • half landing levels and stairs,

• K Bracing – you require a construction level for the intermediate bracing connection points,

• steps in the building floor levels. Where you define a level which is clearly not a floor, then you should not check the floor box.

Set the appropriate level of Diaphragm Action You can switch diaphragm action on or off for a given floor, or select linked slabs to be a diaphragm. If you switch diaphragm action on for a complete floor, you must also then decide if this applies to the entire floor, or to part of the floor only. If the latter please set your diaphragm to “slab items defined”. For further information about the diaphragm options available refer to ”Diaphragm Modeling”on page 30.

Set the appropriate level of deflection checks Fastrak Building Designer provides very comprehensive deflection checks on all beams. You can set limits on the deflections for a variety of conditions (dead load only, imposed load only and/or total load). At the same time in order to allow for deflections with beams with significant web penetrations a sophisticated integration based deflection check is employed. You should take care when setting the range of deflection checks. You may consider the default deflection limits conservative for some buildings. Deflection and deflection checks are relative to the ends of the individual members. For cantilevers the supported end is treated as encastre when determining the relative deflection. See ”Deflection checks”on page 77.

Building Size and Orientation The automatic calculation of EHFs and sway checks are performed on the basis that we are checking a single building. Each portion of the building between expansion joints should be looked at separately for sway stability. When using Building Designer we assume that you are following this logical process. Note EHF’s and sway stability are calculated in the global X and Y direction, you should take care to input the model with this in mind.



Switch off irrelevant load combinations Where you are looking at design changes, for example to rationalise an area of floor, you can switch off all the irrelevant load combinations. For example if you are looking to redesign a series of composite floor beams, then it is likely that only the construction stage load combination and the dead + imposed load combination are relevant. This may allow the you to switch off all other load cases and concentrate on the gravity design issues.

Design simple construction for gravity loads only In order to speed the design process a distinction is made at two levels:• combinations - there are two types of combination

• Gravity combinations - those combinations consisting of gravity loads only (Self Weight, Dead, Slab Dry, Slab Wet, Imposed, Roof Imposed and Snow)

• Lateral combinations - those combinations which in addition to gravity load contain lateral loads due to EHFs or Wind.

• beams, columns, braces and truss members - these are by definition/can be set to be • Gravity Only - designed for gravity combinations • Lateral and Gravity (not Gravity Only) - designed for both combination types - gravity, and lateral Setting General Columns that do not help resist lateral loads to be designed for gravity loads only can significantly reduce the design time. General Beams and Braces are always designed for both gravity and lateral combinations. Engineering judgement will be required when identifying members as being 'gravity load only'. For example: • if an inclined braced member connects to a simple/composite beam, axial force in the brace (from both gravity and lateral loads) puts the beam into bending and therefore the beam should be designed for both gravity and lateral loads.

• potentially, simple beams in a sloping roof would also need to be designed for both gravity and lateral load Note

If a simple, or composite beam is flagged to be designed for both gravity and lateral combinations, only the component of the lateral load that acts in the plane of the strong axis of the member is considered. Any axial loads, or loads in the weak axis are ignored. A validation warning is provided if the ignored loads exceed a preset limit.



Staged modelling and design Our major piece of advice when you are modelling in Fastrak Building Designer is: DO NOT BUILD THE ENTIRE MODEL BEFORE YOU VALIDATE AND DESIGN IT. It is important that you build the model, validate and design it in a staged process, for example: • Validate and design ONE floor before copying it up the building - there are often many nuances to creating your model, in particular with composite design, and it is much more effective to resolve any issues once (before you copy the floor to other levels in your model) than it is to copy the floor to (say) ten other floor levels, and then address the (usually simple) issues on each copied floor (in this case ten times the work!).

• Resolve the gravity design before looking at the lateral design - pinned beams (such as the composite beams) will mostly be unaffected by any lateral load, and hence you may design the beams looking at gravity load only. This is done by selecting the critical gravity combination and using the Perform Gravity Sizing command.

• Resolve the building stability before applying all combinations - lateral load resisting systems can be sized by selecting the four most critical lateral load combinations and running the Perform Lateral Sizing command.

• Checking the entire structure - the entire structure can be checked by setting the relevant combinations to be active and running the Perform Full Design command. Using this ‘stepped’ process to carry out the design you should find the software provides detailed design very effectively.

Check the model analysis results Upon completion of the design process the Workspace presents: Model deflection results — these are not a pass/fail for the details of the model but simply an indication of the total defection of the model under all the differing loads applied. If the model suffers from excessive deflections then a warning will be shown. In this case the remaining results could be incorrect as the overall building analysis may be indicating that the building will collapse. This may be irrelevant if you are looking at a gravity design and are happy to ignore lateral load, but it could also be an indicator of Sway Sensitivity within the building. Model deflection results — these are not a pass/fail for the details of the model but simply an indication of the total defection of the model under all the differing loads applied. If the model suffers from excessive deflections then a warning will be shown. This could be an indicator of Sway Sensitivity - in extreme cases the overall building analysis may be indicating that the building will collapse. Sway Stability — Fastrak Building Designer will carry out a full sway stability analysis for the EHFs applied to the building. Significant failure, may mean the lack of overall stability such as the omission of diaphragm action or simply a loose piece of the structure not connected into the main bracing/diaphragm action. Use the deflection results to look at this. Load in versus load out — All loads are checked in and out of the model it is essential that you check these results.

Chapter 9 : Assumptions and Limitations

Chapter 9


Assumptions and Limitations This chapter decribes the assumptions and limitations that apply to Fastrak Building Designer. It is recommended that you familiarise yourself with these before using the software. Note

In addition to the limitations listed below it is very important that you are aware of the further limitations that apply in relation to wind load generation, seismic load generation and member design. You will find these limitations are fully described in the relevant sections of the EC1 1-4 Wind Modeller Handbook, and the Eurocode Member Design Handbook.

Major topics

• • • • • • • • •

Analysis Types Analysis Results Imperfections Deflection checks Foundation loads Vertical cross bracing Imposed Load Reductions Equivalent Horizontal Force Load Calculations Consequences of changing Design Codes within an Existing Project

Analysis Types Fastrak Building Designer utilizes two different analysis methods: first- and second-order analysis. First-order analysis — is a standard linear elastic static analysis in which any effect on forces due to changes in the geometry of the structure are ignored. Second-order analysis — is performed using either the Amplified Forces Method, or a rigorous method using a two step iterative approach. When using the rigorous second-order analysis method the accuracy of the result will increase with the number of nodes within members. Within a member the number of internal nodes in determined by the number of incoming members. Fastrak Building Designer adds at least one internal node into members if there are no incoming members except in the following circumstances:

• Simple Beams and Composite Beams that are part of a diaphragm1. • Braces and Truss internals, since these are designed for axial force only. In this way, the rigorous second-order analysis method in Fastrak Building Designer allows for:

Footnotes 1. Simple Beams and Composite Beams are assumed to have negligible axial load and are not designed for any second-order moments irrespective of whether they are part of a diaphragm or not. However, if the axial force exceeds a certain specified limit you are warned that the force might be significant.



• P- (P-big delta) effect resulting from gravity loads acting on the drift of the entire structure or a part of the structure

• P- (P-little delta) effect resulting from the line of action of the axial force in a member acting at an eccentricity due the deformed shape of the member. First-order analysis is used to determine the global deformations for drift calculations. It is also used to the determine serviceability limit state requirements, e.g. the relative deflections of members.

Analysis Results When using the rigorous second-order analysis method, the forces and moments for design are obtained directly from the design combination results of the analysis. This is because for this type of analysis, superposition no longer holds. Serviceability Limit State results and the design forces and moments from all other analysis types are obtained from the loadcase results. This is because for first-order analysis, and for second-order analysis using the Amp. Forces method, superposition does apply.

Imperfections Imperfection for analysis of bracing systems - Clause 5.3.3 Fastrak Building Designer does not explicitly deal with 'brace imperfections' to Clause 5.3.3 of EC3.

Imperfections for global analysis of frames - Clause 5.3.2 (6) When designing columns to EC3, you can, under certain circumstances, be given a Warning status on the ‘Strut Buckling’ page in the Results Viewer. This message relates to member imperfections and is issued if the calculated in-plane non-dimensional slenderness for a particular column exceeds the limit given in Clause 5.3.2(6). Fastrak does not automatically make specific allowance for local bow imperfections in the analysis and the ‘warning’ is to draw your attention to this fact. It is suggested that this clause is primarily aimed at ‘moment resisting frames’ that provide all or a substantial portion of the lateral load resisting system. Where locally in the structure moment connections are provided for example to reduce deflection, increase spanning capability of beams or where short cantilevers are connected, you might consider that this clause does not apply. You might consider this particularly justified when the lateral load resisting system is provided by an independent system of bracing and/or by shear walls. In such cases you can choose to ignore the warning or you can remove such members from the ‘Moment Frames’ in your building. (Uncheck the tick box ‘Moment Frame’ on the Design page of the Column Properties dialog.) In the latter case the warning will no longer be issued. Currently, if you choose to include member imperfections in the analysis, you must adjust the model yourself perhaps using the equivalent forces approach defined in 5.3.2 (7).



Torsional sway effects - Clause 5.3.2 (10) Fastrak Building Designer enables you to meet the requirements for initial (global) sway imperfections by automatically calculating the equivalent horizontal forces (EHF). You are able to include this in any design combination as positive or negative EHF in the global X or global Y directions. You can also apply a different proportion in the X and Y simultaneously. However, there is currently no facility to apply some EHF in the positive global X (or Y) simultaneously with others in the negative global X (or Y). Thus, it is not possible using the predetermined EHFs to satisfy Clause 5.3.2 (10). To do this you can calculate and apply your own set of nodal forces in the appropriate directions at the appropriate levels.

Deflection checks Absolute and Relative Deflections Fastrak Building Designer calculates both absolute and relative deflections. Relative deflections measure the internal displacement occurring within the length of the member and take no account of the support settlements or rotations, whereas absolute deflections are concerned with deflection of the structure as a whole. The difference between relative and absolute deflections is illustrated in the cantilever beam example below.

Relative Deflection

Absolute Deflection

Absolute deflections are given in the structure analysis results diagram. The deflections reported in the Project Workspace analysis results summary are also absolute deflections. Relative deflections are the ones used in the member design. They are given numerically in the design summary dialogs for individual members. Where possible they are also given in the individual analysis results diagrams.

Deflections in Composite Beams (and Beams with Web Openings) The structure analysis results diagram within Fastrak Building Designer shows global unfactored (see the load combination input where you can define this factor) deflections based upon the steel member size. It cannot account for the staged construction of a composite beam or other details such as web openings. As such the 3D graphical report should be used for building movement studies only, (though they may sometimes be useful in looking at ponding).



Deflections of composite beams, (or beams with openings) can only be taken from the calculations within the design summary for the individual member.

These deflections are always 'in span' - they account for the staged nature of the construction and design process (which cannot be considered within an elastic analysis), and web openings if they exist - they will consider pre camber of the beam and ponding - if you have allowed for this. Note

For further details about how the staged nature of construction is accounted for in the above calculations see Theory and Assumptions in the Composite Beam Engineer’s Handbook.

Note

For further details about how to apply pre-camber see Camber in the Composite Beam Engineer’s Handbook.

Note

For further details about how to make an allowance for ponding see Self weight of concrete slabs.

Foundation loads There can be some differences in the base load values between the Fastrak Building Designer summary table and the individual column designs. This occurs if there is bracing coming in at the column base – the brace loading at the base of the column is not handled in the standalone application – it is irrelevant to the column design. The foundation values in the Fastrak Building Designer summary table are correct for foundation design these should be used and not the standalone values.



Vertical cross bracing Foundation shear and vertical load When vertical cross bracings are modelled, only one member is considered active, irrespective of lateral load direction. This means that at the base, only one of the two foundations has the correct shear and vertical load. If the load direction is able to be reversed (and hence the other brace should be active) then you need to allow for the correct shear and vertical load in the corresponding foundation loads.

Column axial load When vertical cross bracings are modelled, only one member is considered active, irrespective of lateral load direction. The axial load in a column is only included into the column where the bracing is connected. This means that if the lateral load is able to reverse, one column of a braced pair will have some axial load that is not accounted for in the column design. The diagram below illustrates this. In the design model, the left-hand column will be designed without the axial compression that is actually present when the correct brace is active in tension only. Therefore great care must be taken when selecting active/inactive bracing members, and in cases where bracing loads are significant, additional checks on columns may be necessary. T

T T

C

C

Design Model

Actual Loads

Imposed Load Reductions Imposed Load Reductions applied to Brace Forces used in Column Design Where a brace connects to a column node, the brace force can optionally be set to be included in the eccentricity moment calculations for the column to which it connects. This is achieved by checking the appropriate box on the Releases tab of the Brace Properties dialog. When included, the vertical component of the brace force is combined with any beam end reactions which connect to the same face of the column. Consequently, if imposed load reductions have been applied, these brace forces used in the calculation of eccentricity moments will also be used in the calculation of imposed load reductions in the column design. This may result, correctly, in a small adjustment to the column design forces.



Equivalent Horizontal Force Load Calculations Loads used in EHF load calculations All EHFs are determined from the loads hitting a column at a floor - as a result column element loads (loads applied directly to columns by the user) and column self weight can not be included in the calculation of EHF forces.

Note

To apply loads at the top of a column so that they get included in the EHF calculations you should use nodal loads, as opposed to column element loads.

Gravity loads carried by Braces not accounted for in EHF load calculations Any gravity load carried between floors in a brace element is not picked up in the calculation of EHF forces - in a majority of structures this is not an issue as braces tend to carry very little gravity load. However just occasionally they can be used in a model where they do carry gravity load - see brace supporting cantilever beam/slab in the figure below. (actually in this case the user should use a General Beam and not a brace and then the calculation of EHF forces would be correct).

A quick look at the load case summaries in the design tree will advise the user of the relative size of the error in the EHF calculation: EHF = 0.005 x grav load.



Axial load in discontinuous columns used twice in EHF load calculations If a structure has a transfer beam carrying a column, or if wall mid pier models do not align vertically in a wall (due to openings etc.) then the EHFs for the axial load in the supported column are used twice in the EHF calc - once when they are applied to the supported column and a second time when picked up in the level of the transfer beam. This is actually conservative as too much EHF is applied to the structure. A quick look at the load case summaries in the design tree will advise the user of the relative size of the error in the EHF calculation: EHF = 0.005 x grav load.

Consequences of changing Design Codes within an Existing Project If you change design codes in mid project, certain default values are reset and some data will be lost. Details of the effects of swapping between British standards and Eurocodes are provided below: Note

It is strongly advised that you save a copy of the existing model prior to changing design codes.

BS models swapped to Eurocodes and designed The following items are lost and will need to be re-specified: • Wind load cases generated as part of the wind wizard are deleted.

• All factored combinations are deleted. The following items are changed: • Steel properties for density, E and G are set to be those for the Eurocodes.

• Concrete properties for density, E and G are set to be those for the Eurocodes. • The cracking factors for concrete sections are set to be 0.2 for beams and walls and 0.4 for columns.

• Simple Columns are set to be General Columns with a “simple” flag set. • Concrete filled columns - are no longer concrete filled. They are designed as steel columns and the self weight of concrete is no longer present (concrete filled columns not currently available for design to Eurocodes).

• Slabs with fibre reinforcement are set to have no fibres and no other reinforcement (fibres not currently available for design to Eurocodes).

• Slabs made from PC planks are set to profiled metal decking (PC planks not currently available for design to Eurocodes).

• All studs in composite beams become the default EC stud (19mm diameter, 100mm high and 450N/mm2 tensile strength). The following items are not designed to Eurocodes: • Beams with openings in webs (not in version 11) - can be taken to Fastrak Simple Beam or Fastrak Composite Beam and designed to the BS and returned.

• General beams (version 11) - can be taken to Fastrak General Beam and designed to the BS and returned.



• Westok beams (version 11) - can be exported to the Westok CellBeam software. • Although the internals of truss members are designed, the top and bottom chords are not (version 11).

• Gable posts (version 11). • Portal frames (version 11) - can be taken to Fastrak Portal Frame and designed to the BS and returned.

Eurocode models swapped to British Standards and designed The following items are lost and will need to be re-specified: • Wind load cases generated as part of the wind wizard are deleted.

• All factored combinations are deleted. The following items are changed: • Steel properties for density, E and G are set to be those for the British Standards.

• Concrete properties for density, E and G are set to be those for the British Standards. • The cracking factors for concrete sections set to be 1.0 for beams, columns and walls. • All studs in composite beams become the default BS stud (19mm diameter, 100mm high and 100kN). The following items are recovered from a previous BS design: • General Columns with a “simple” flag are set to be Simple Columns.

• Concrete filled columns are reset to be concrete filled. • Slabs with fibre reinforcement are reset to have fibre reinforcement. • Slabs made from PC planks are reset to PC planks.

Chapter 10 : Sign Conventions

Chapter 10


Sign Conventions This chapter describes those sign conventions applicable to Fastrak Building Designer. Major topics

• • • • • • • •

Object Orientation Beams (Simple, Composite and General) and Truss member (chord) Braces and Truss member (internal) Columns (Simple and General) Shear Walls Foundations/Bases - Foundation Forces Foundations/Bases - Base Reactions Nodal Deflections

Object Orientation Fastrak Building Designer takes account of an object’s orientation when displaying the analysis results. Therefore, to apply the sign convention correctly you need to know which is end 1 and which is end 2 for beams/walls and you also need to know which is Face A for columns. If you switch the option to show the Element Direction on, then Fastrak Building Designer shows an arrow on all beams, walls and columns. This arrow points from the start to the end of beams/walls and from the bottom to the top of columns along Face A. Note: Re the positive and negative depiction of moments

+M

+ moment shown as above

-M - moment shown as above

The arrow always shows the direction of moment. Arrow reversed for -ve moment.



Beams (Simple, Composite and General) and Truss member (chord) For loads applied as shown, the sign convention applicable to beams and truss member (chords) is as indicated.

+

This sign convention is applicable to:

+ End 2

+

z y

• Minor shear, moment • Axial

x End 1

2D Analysis Results • Major shear, moment

Applied loads

• Minor shear, moment • Deflection • Axial

+ -

+

z

Report/Export • Element Design

Major Axis Shear, Moment and Deflection

+ -

+

y

+

x

Minor Axis Shear, Moment and Deflection

+ x Axial Force (+ve compression)

Design Results • Major shear, moment

• Minor shear, moment • Deflection • Axial

+

x

3D Graphic • Major shear, moment



The beam end force sign convention as shown is applicable to:

End 2

-Mz y

-vx

End 1

z

+Mz x

-vx -My

+My

DXF • Beam end forces Report/Export • Beam end forces

+vz

+vy +vz Beam end forces (for applied loads shown on previous page)

Braces and Truss member (internal) For braces and truss internals, axial compression is positive and axial tension is negative. This convention applies to: 3D Graphic • Axial Design Results • Axial DXF • Bracing forces Applied loads

Report/Export • Bracing forces

+

Axial Force (Compression +ve, Tension -ve)



Columns (Simple and General) For loads applied as shown, the sign convention applicable to columns is as indicated. This sign convention is applicable to:

Face A

3D Graphic • Major shear, moment

• Minor shear, moment • Axial 2D Analysis Results • Deflections

• Major shear, moment • Minor shear, moment • Axial

Applied Loads

Design Results • Deflections

+

• Major shear, moment • Minor shear, moment • Axial

+

DXF • Column splice loads

-

Report/Export • Column splice loads

Major Axis Shear, Moment and Axial

Note: Major Moment is about the Major Axis Major Shear is in the plane of the Minor Axis. Note: Simple Columns only cater for moments due to load eccentricity.

+

Minor Axis Shear and Moment



Shear Walls For loads applied as shown, the sign convention applicable to shear walls is as indicated.

End 1 End 2

This sign convention is applicable to: 3D Graphic • Major shear, moment

• Minor shear, moment • Axial Report/Export • Shear wall forces Applied Loads

+

+

Major Axis Shear, Moment and Axial

+

Minor Axis Shear and Moment



Foundations/Bases - Foundation Forces Fastrak Building Designer shows foundation forces in the 3D graphics, these are the forces that act on the foundation. Note that elsewhere in the output, DXF and Excel export Fastrak Building Designer gives the base reactions. For loads applied as shown, the sign convention applicable to the 3D graphic for foundation forces is as follows: Columns (note aligned with the local axis system of the column) • Major shear, moment

• Minor shear, moment • Axial This sign convention is applicable to: • 3D graphic foundation forces

A

C

-Mminor

+Fmajor

-Mmajor +Fvert

+Fminor



Shear Walls (note aligned with the local axis system of the wall) • Major shear, moment

End 1


End 2

This sign convention is applicable to: • 3D graphic foundation forces

-Mminor

+Fmajor

-Mmajor +Fvert

+Fminor Supplementary supports (note aligned with the global axis system) • X shear, moment

• Y shear, moment • Axial

Z

Y

X +FX

-MX +MY +FY +Fvert

This sign convention is applicable to: • 3D graphic foundation forces



Foundations/Bases - Base Reactions Fastrak Building Designer shows base reactions in the reports, DXF output and in the export to Excel , these are the reactions from the foundation. Note that in the 3D graphics, Fastrak Building Designer gives the forces acting on the foundation. For loads applied as shown, the sign convention for these base reactions is as follows: Columns (note aligned with the local axis system of the column) • Major shear, moment


C

+Fminor

+Fmajor

A

+Fvert

-Mmajor

-Mminor

This sign convention is applicable to: DXF • base reactions Report/Export • base reactions



Shear Walls (note aligned with the local axis system of the wall) • Major shear, moment

End 1


End 2

This sign convention is applicable to: DXF • base reactions

+Fminor

Report/Export • base reactions

+Fvert

-Mmajor

-Mminor

+Fmajor

Supplementary supports (note aligned with the global axis system) • X shear, moment

• Y shear, moment • Axial

Z

Y

X This sign convention is applicable to: DXF • base reactions Report/Export • base reactions

+Fvert +FY

-MX +MY

+FX



Nodal Deflections +Y +X Z

-Z

Y X Global Axes

In the 3D graphic, nodal deflections relate to the global axis system as shown.

Eurocode - Building Designer Handbook

Recommend Documents