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HANDBOOK Fastrak CSC Structural steelwork analysis and design .cscworld.com/fastrak

AISC Specification - Building Designer Handbook

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Page 1: AISC Specification - Building Designer Handbook

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Page 2: AISC Specification - Building Designer Handbook

Friday 16 November 2012 – 11:25

AISC Specification - Building Designer page 2

CSC Inc500 North Michigan Avenue, Suite 300,

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Page 3: AISC Specification - Building Designer Handbook

Disclaimer page 3

Disclaimer CSC Inc. 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.

ProprietaryRights

CSC Inc, 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: Fastrak software: © CSC Inc. 2012 © CSC Inc. 2012All rights reserved. All rights reserved.

Trademarks Fastrak™ is a trademark of CSC Inc.TEDDS® is a registered trademark of CSC Inc.Orion™ is a trademark of CSC Inc.CSC Inc.The CSC logo is a trademark of CSC Inc.

HOOPS™ is a trademark of CSC Inc.

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 © 1987-2012 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: AISC Specification - Building Designer Handbook

page 4 Table of Contents

AISC Specification - Building Designer Handbook

Chapter 1 Introduction . . . . . . . . . . . . . . . 7

Chapter 2 Construction Methods and Member Types . . . . . . . . . 8‘Simple’ Construction . . . . . . . . . . . . . . 8

Composite or simple beam?. . . . . . . . . . . . . 9Composite Beam Design . . . . . . . . . . . . . 10

Continuous Construction . . . . . . . . . . . . . 11Member Beams and Member Columns . . . . . . . . . . . 11General Beams . . . . . . . . . . . . . . . 13General Columns . . . . . . . . . . . . . . 14

Moment Framing and Gravity Loads . . . . . . . . . . . 15Backspan Beams . . . . . . . . . . . . . . 15General Points to Note . . . . . . . . . . . . . 16

Additional Member Types . . . . . . . . . . . . . 17Trusses and Truss Members . . . . . . . . . . . . . 17Steel Joists . . . . . . . . . . . . . . . 17Diaphragm Braces . . . . . . . . . . . . . . 18Shear Walls . . . . . . . . . . . . . . . 21Bearing Walls . . . . . . . . . . . . . . . 24

Chapter 3 Stability Design . . . . . . . . . . . . . . . 28Using Bracing . . . . . . . . . . . . . . . 28Using Steel Moment Frames . . . . . . . . . . . . . 29Using Other Moment Frames . . . . . . . . . . . . . 29Using Shear Walls . . . . . . . . . . . . . . . 29Seismic Frames . . . . . . . . . . . . . . . 31

Chapter 4 Diaphragm Modeling . . . . . . . . . . . . . 32Rigid Diaphragms. . . . . . . . . . . . . . . 32

Single diaphragm . . . . . . . . . . . . . . 33Slab items defined . . . . . . . . . . . . . . 33No diaphragm . . . . . . . . . . . . . . . 34Taking slabs out of a diaphragm . . . . . . . . . . . . 34

Semi-Rigid Diaphragms . . . . . . . . . . . . . 35Flexible Diaphragms . . . . . . . . . . . . . . 35Story Shears . . . . . . . . . . . . . . . . 36

Chapter 5 Member End Releases, Member Orientation and Supports . . . . . 37Moment Releases . . . . . . . . . . . . . . 37Axial Releases . . . . . . . . . . . . . . . 39Torsional Releases . . . . . . . . . . . . . . 40Release from a Diaphragm . . . . . . . . . . . . . 40Member Orientations . . . . . . . . . . . . . . 41Supports and Base Fixity . . . . . . . . . . . . . 42

Chapter 6 Load Cases and Load Combinations . . . . . . . . . . 43Gravity Load Cases . . . . . . . . . . . . . . 43

Self Weight . . . . . . . . . . . . . . . 43Live and Roof Live Loads . . . . . . . . . . . . . 44

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Table of Contents page 5

Perimeter Loads . . . . . . . . . . . . . . 44Lateral Load Cases . . . . . . . . . . . . . . 45

Wind Loads . . . . . . . . . . . . . . . 45Notional Loads . . . . . . . . . . . . . . 46

Seismic Load Cases . . . . . . . . . . . . . . 46Combinations . . . . . . . . . . . . . . . 47

Construction Stage Combination . . . . . . . . . . . 47The Combinations Wizard . . . . . . . . . . . . . 48

Classifying Combinations and Setting the Critical Combinations . . . . . . 50Gravity Combinations . . . . . . . . . . . . . 50Lateral Combinations . . . . . . . . . . . . . 51Seismic Combinations . . . . . . . . . . . . . 51Setting the Critical Combinations . . . . . . . . . . . 51

Chapter 7 Analysis And Design Procedures . . . . . . . . . . . 52Definitions. . . . . . . . . . . . . . . . 52Building Validation . . . . . . . . . . . . . . 53Overview of the Analysis and Design Process . . . . . . . . . . 53

Set Auto Design Mode . . . . . . . . . . . . . 56Analysis Options . . . . . . . . . . . . . . 57

First-order or Second-order Analysis? . . . . . . . . . . . 57Stability Coefficient Tolerance . . . . . . . . . . . . 57Reduced Stiffness Factor . . . . . . . . . . . . . 58Curved Beams . . . . . . . . . . . . . . . 58Torsion Factors . . . . . . . . . . . . . . 58Cracked Sections . . . . . . . . . . . . . . 58

Design Options . . . . . . . . . . . . . . . 59Design Codes . . . . . . . . . . . . . . . 59Design Control . . . . . . . . . . . . . . 59Force Limits - Members . . . . . . . . . . . . . 59Element Pre-sizing . . . . . . . . . . . . . . 59Live Load Reductions . . . . . . . . . . . . . 60Composite . . . . . . . . . . . . . . . 60Steel Joists . . . . . . . . . . . . . . . 60

Initial Review of Analysis Results . . . . . . . . . . . . 61Maximum Nodal Deflections . . . . . . . . . . . . 61Stability Coefficients. . . . . . . . . . . . . . 61Seismic Drift . . . . . . . . . . . . . . . 62Loading Summary . . . . . . . . . . . . . . 62Review of Selected Sections . . . . . . . . . . . . 62Review Analysis Results . . . . . . . . . . . . . 62Reviewing Stability Design . . . . . . . . . . . . 63Reviewing Story Shear . . . . . . . . . . . . . 64

3D Analysis Effects . . . . . . . . . . . . . . 64Continuous Beam Example . . . . . . . . . . . . 64Braces Carry Gravity Loads Example . . . . . . . . . . . 66

Refining Member Designs . . . . . . . . . . . . . 70

Chapter 8 Building Effective Models . . . . . . . . . . . . 71Place grid lines accurately . . . . . . . . . . . . . 71Save time by using Attributes effectively . . . . . . . . . . 71Use simple construction where possible . . . . . . . . . . 72Use Perimeter Loading for edge beams where applicable . . . . . . . 72Is it a Floor? . . . . . . . . . . . . . . . 72Set the appropriate level of Diaphragm Action . . . . . . . . . 73

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page 6 Table of Contents

Set the appropriate level of deflection checks . . . . . . . . . . 73Switch off irrelevant load combinations . . . . . . . . . . . 73Building Size and Orientation. . . . . . . . . . . . . 73Design simple construction for gravity loads only . . . . . . . . . 74Prevent out of plane instability . . . . . . . . . . . . 75Check the model analysis results . . . . . . . . . . . . 75Staged modeling and design . . . . . . . . . . . . . 76

Chapter 9 Assumptions and Limitations . . . . . . . . . . . . 77Analysis Types . . . . . . . . . . . . . . . 77Analysis Results . . . . . . . . . . . . . . . 78Deflection checks . . . . . . . . . . . . . . . 78

Absolute and Relative Deflections . . . . . . . . . . . . 78Deflections in Composite Beams . . . . . . . . . . . . 79

Foundation loads . . . . . . . . . . . . . . . 80Vertical cross bracing . . . . . . . . . . . . . . 80

Foundation shear and vertical load . . . . . . . . . . . 80Column axial load . . . . . . . . . . . . . . 80

Live Load Reductions . . . . . . . . . . . . . . 80Notional Load Calculations . . . . . . . . . . . . . 81

Loads used in Notional Load calculations . . . . . . . . . . 81Gravity loads carried by braces not accounted for in Notional Load calculations . . . . 81Axial load in discontinuous columns used twice in Notional Load calculations . . . . 82

Chapter 10 Sign Conventions . . . . . . . . . . . . . . 83Object Orientation . . . . . . . . . . . . . . 83Beams (Simple, Composite and General) and Truss member (chord) . . . . . . 84Braces and Truss member (internal) . . . . . . . . . . . 85Columns . . . . . . . . . . . . . . . . 86Shear Walls . . . . . . . . . . . . . . . . 87Foundations/Bases - Foundation Forces . . . . . . . . . . . 88Foundations/Bases - Base Reactions . . . . . . . . . . . 90Nodal Deflections . . . . . . . . . . . . . . . 92

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Chapter 1 : Introduction AISC Specification - Building Designer page 7

AISC Specification - Building Designer Handbook

Chapter 1 Introduction

This handbook provides an overview of Fastrak Building Designer in the context of design to the AISC Specification. The applicable construction methods and member types are described, and the analysis/design procedures explained. In addition, guidance is provided on effective modeling 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.

Stability Design — (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, Member Orientation and Supports — (Chapter 5)describes the various end releases, member orientation and supports.

Load Cases and Load Combinations — (Chapter 6)describes the different load case and load combinations types.

Note The Wind Wizard and the Seismic Wizard used for automatic loadcase generation are fully described in the ASCE7-05 Wind Wizard Handbook and the ASCE7-05 Seismic Wizard Handbook respectively.

Analysis And Design Procedures — (Chapter 7)provides an overview of the steps required to analyze and design your building and describes the various analysis and design options.

Note The member design procedures are fully described in the AISC Specification - 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.

Page 8: AISC Specification - Building Designer Handbook

AISC Specification - Building Designer page 8 Chapter 2 : Construction Methods and Member Types

Chapter 2 Construction Methods and Member Types

To maximize your construction options Fastrak Building Designer provides a range of construction methods and member types.

Major topics • ‘Simple’ Construction• Continuous Construction• Composite Beam Design• Member Beams and Member Columns• General Beams• General Columns• Backspan Beams• Trusses and Truss Members• Steel Joists• Diaphragm Braces• Shear Walls• Bearing Walls

‘Simple’ ConstructionFor ‘simple’ construction (e.g. composite beams with pinned ends) The most effective design for a multi-story structure is likely to be simple1 beams and general

columns (designed for gravity only) with simple braces 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, the answer is not necessarily to make the affected elements into general beams. Look at the modeling and talk to CSC support if you are not sure of the route that you want to take.

‘Simple’ construction implies certain types of modeling and certain specific design rules (both inclusions and exclusions). We assume that you are familiar with these.

Footnotes1. Pin type connections – thus in this context a composite beam is ‘simple’.

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Chapter 2 : Construction Methods and Member Types AISC Specification - Building Designer page 9

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

When considering stability you should be aware that for those general columns set by you to be Gravity Only Design, the program automatically inserts pins just above every floor level. Note however that pins do not get inserted at the base level, or where the general 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.

Note If a general column is switched so that it is not Gravity Only Design, the program automatically removes any pins within it.

Design Properties (simple construction)It is best to establish the default design properties (lateral bracing assumptions, sections for study, etc.) by setting up appropriate default attributes. For information about working with attributes refer to - Building Designer Help \ Working with Attributes

Composite or simple beam?Composite beam design is not a linear process, and some beams are 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 then select those beams that you wish to be composite at a second pass.

Added release My and Mz

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AISC Specification - Building Designer page 10 Chapter 2 : Construction Methods and Member Types

Composite Beam DesignComposite design of beams is a complex procedure when carried out rigorously. We assume that you are familiar with the concepts of composite design before you use the software.

Fastrak Building Designer’s composite design routines can automatically choose the optimum stud layout and as such the design of any composite beam may have a range of possible solutions.

Example A typical 30 ft composite spine beam can be shown to be acceptable:• with studs at 7.5 in centers and a W 18x50, • with studs at 8 in centers and a W 18x55, which of these solutions is better is up to you.

While it can sometimes be useful to optimize 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 realize that you can define attributes that may make the design of composite beams impossible – for example by setting the stud spacing on a girder to 12 in, this does not provide the minimum amount of shear interaction, then the selection of a suitable beam size is not possible.

Note For more information refer to the Composite Beam chapter in the AISC Specification Member Design Handbook

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Chapter 2 : Construction Methods and Member Types AISC Specification - Building Designer page 11

Continuous ConstructionFor continuous steel construction (e.g. moment frames) Fastrak Building Designer allows you to model members which are more complex than pin

ended beams and columns. There are currently four member construction types that you can use:

• “Member Beams and Member Columns” – these can be any section in any material but cannot be checked or designed by the software.

• “General Beams” – these are restricted to steel sections but such beams can then be designed by the software.

• “General Columns” – these are restricted to steel sections but such columns can then be designed by the software.

All of the above members can carry both axial load and moment, the distinction being:• Member Beams and General Beams are ‘beam-columns’ typically dominated by moments

but with significant axial force and forces present in other axes excluding torsion. • Member Columns and General Columns are ‘column-beams’ dominated by axial but with

significant moments.

Member Beams and Member ColumnsFor 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 this example additional support and lateral stability is also being provided by concrete shear walls, these are a separate construction type - see “Shear Walls”. Also refer to “Stability Design” for 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.

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AISC Specification - Building Designer page 12 Chapter 2 : Construction Methods and Member Types

Section/Material PropertiesFastrak 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 property dialog for a concrete beam section.

Note that for the purposes of deflection estimation and in any model that mixes steel/concrete/other materials, 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. You should also bear in mind that global adjustments are automatically made to the inertias of all concrete beams, columns and shear walls during analysis to allow for cracking based on the values you specify on the Cracked Sections page of the Analysis Options dialog, see “Cracked Sections” on page 58.

• 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, Member Orientation and Supports”.

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General BeamsGeneral 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, • The advantage is that Fastrak Building Designer designs these steel sections automatically.• They can be designed for gravity and lateral loads.• They can be designated part of a Moment Frame. Such beams then adopt different initial

sizing criteria in an attempt to ensure auto-design of general beams in such frames have reasonable stiffness during the analysis/design process - see “Element Pre-sizing”

Creating General BeamsYou 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, Member Orientation and Supports”.

Design PropertiesAs 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 the attributes you require have not been set up, you could of course edit the properties of any General Beam directly.

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AISC Specification - Building Designer page 14 Chapter 2 : Construction Methods and Member Types

General ColumnsGeneral 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 for simple construction they should

be set for Gravity Only Design, in which case they are designed for gravity combinations only.

• 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 ColumnsSimply 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, Member Orientation and Supports”.

Design PropertiesAs with 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 Column on an individual basis.

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Chapter 2 : Construction Methods and Member Types AISC Specification - Building Designer page 15

Moment Framing and Gravity Loads

Backspan BeamsWe 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.

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AISC Specification - Building Designer page 16 Chapter 2 : Construction Methods and Member Types

General Points to Note

Pattern LoadingIf 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 negative 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.

ContinuityCare 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 / LevelsLevels 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 live 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 optimize the design of the transfer beams interactively.

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Chapter 2 : Construction Methods and Member Types AISC Specification - Building Designer page 17

Additional Member TypesPlus the following additional member types Fastrak Building Designer allows you to easily model more complex systems which might

comprise multiple analysis members. The following additional member types and systems are available:

• “Trusses and Truss Members” – 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 (i.e. not yet be designed) provided they are defined in steel.

• “Steel Joists” – Steel joists are simply supported secondary steel members. They will attract loads and participate in the 3D structural analysis, they can be checked or designed.

• “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.

• “Shear Walls” – these provide resistance to lateral loads in the analysis model. Concrete shear walls can be checked or designed, (shear walls in other materials are analyzed but are not checked/designed).

• “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.

Trusses and Truss MembersIn the current version of the software you can define truss members in your model and these 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.

Steel JoistsSteel joists (or bar joists) - are simply supported secondary members - which do not support any other members. They only support loaded areas.

• Steel joists can be defined with ends at differing levels.• They can not support any other member.• Slab and roof loads are supported by steel joists and loads are distributed to them.• Steel joists are aligned to the analysis wire by their top chord.

Standard typesSteel joists are constrained to standard types specified by the Steel Joist Institute. They are standardized in terms of span, depth and load carrying capacity. There are four standard types of steel joist available in Fastrak Building Designer.

• K series joists -open web, parallel chord steel joists - depths 8" to 30" with spans up to 60ft. • KCS series joists - K series adapted and specially designed for constant moment/shear

along length (position of point loads become irrelevant). • LH series joists - long span joists - depths 18" to 48" for clear spans up to 96ft. • DLH series joists - deep long span joists - depths 52" to 72" for clear spans up to 144ft.

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Special Joists“SP” suffixes can be added to K, LH and DLH Series joists. Special Joists can handle 'non-uniform' loading situations. They will attract loads and participate in the 3D structural analysis, but they can not be checked or designed. Load diagrams for the relevant joist can be output to forward to the fabricator for designing.

Joist GirdersThese are provided as an option to support steel joists. They will attract loads and participate in the 3D structural analysis, but they can not be checked or designed.

Creating Steel JoistsYou can create steel joists 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 Steel Joist. Any new beam you create using this attribute set will be a Steel Joist.

Analytical PropertiesSteel joists must be simply supported and cantilever ends can not be defined. They cannot be released axially.

Only Joist Girders and SP joists are able to support members along their length.

The inertia and area values are taken directly from the Steel Joist Institute tables.

Diaphragm BracesDiaphragm 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 properties.

Creating Diaphragm BracesYou 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.

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While 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..

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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 opening framing. Two ways in which you might want to do this are shown below..

Analytical PropertiesDiaphragm 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 Story Shear results. See “Reviewing Story Shear” for more details.

Diaphragm braces have zero self weight.

Since they are not ‘real’ members they:• do not interact with any of the members that they "cross" including one another - i.e. they

only interact at their ends,• are not designed,• are not listed,• are not exported to 3D cad programs e.g. Revit.

However they are exported to analysis programs e.g. S-Frame.

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Shear WallsShear 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• Only “Concrete” shear walls are checked/designed

Creating Shear WallsTo 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 another material is specified you will also be required to specify an appropriate E to be used in the analysis.

Note The shear wall inertia is adjusted during analysis to allow for cracking based on the values you specify on the Cracked Sections page of the Analysis Options dialog, see “Cracked Sections” on page 58.

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” idealization 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 support (which is fully fixed in the plane of the wall and pinned out of plane) 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.

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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.

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Note The alignment (Left, Center, 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.

Design Properties The design method for concrete shear walls is described in the Shear Walls section of the AISC Specification - Member Design Handbook.

Transfer Shear WallsA 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 modeling is set to Top Edge Beam.

Using Shear Walls to Provide StabilityThis topic is covered in the Stability Design section, refer to “Using Shear Walls”.

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Bearing WallsBearing walls are used to provide resistance to vertical loads but not lateral loads and to support certain other member types.

LimitationsThe 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 wallThe 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.

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• 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 WallsTo 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.

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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 modeled 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.

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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.

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Chapter 3 Stability Design

This chapter provides a simple overview of some of the alternative ways in which you can meet stability design requirements in Fastrak Building Designer.

Major topics • Using Bracing• Using Steel Moment Frames• Using Other Moment Frames• Using Shear Walls• Seismic Frames

Using BracingThis is the most traditional approach and well positioned and proportioned bracing is undoubtedly the best method of providing stability.

Fastrak Building Designer allows all sorts of bracing configurations including: • tension only bracing,• cross bracing,• K bracing, • V 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 Great care must be taken if modeling vertical cross bracings, see “Vertical cross bracing” in the Assumptions and Limitations chapter for more details.

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Using Steel Moment FramesIf 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”.

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 FramesYou 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”.

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 modeling idealization 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.

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Swapping to the axis stick view shown above and switching off the simple beams and columns the idealization becomes more apparent.

For further information refer to the following article which was published in the UK: • 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 modeling idealization of shear walls with beam elements is traditionally well accepted. The points made in “Using Other Moment Frames” regarding section and material properties are of course important.

In recent years we have seen a trend towards Finite Element modeling 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”. But, it can be done...

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Seismic FramesIf you want to specify specific frames as a Seismic Load Resisting System, note that only “Gravity and Lateral” General Beams, General Columns and braces can be included. However, only some of the Gravity and Lateral members may be part of Seismic Load Resisting Systems, not all.

At a frame/member level, R is considered in the X and Y direction and the following frame types are available:

• Special Moment Frame (SMF)• Intermediate Moment Frame (IMF) • Ordinary Moment Frame (OMF) • Special Concentrically Braced Frames (SCBF) • Ordinary Concentrically Braced Frames (OCBF)

The designation of frame type is done graphically by using the Seismic X or Seismic Y feature located on the Building tab in the Show/Alter State dialog.

The following frame types are beyond scope:• Eccentrically Braced Frames (EBF) • Special Truss Moment Frame (STMF) • Buckling Restrained Braced Frames (BRBF) • Composite Special Concentrically Braced Frames (C-SCBF) • Composite Ordinary Braced Frames (C-OBF) • Composite Eccentrically Braced Frames (C-EBF)

Seismic Frames are only checked at the Full Design stage.

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Chapter 4 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 modeled in Fastrak Building Designer.

Major topics • Rigid Diaphragms• Semi-Rigid Diaphragms• Flexible Diaphragms• Story Shears

Rigid DiaphragmsA 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 .

• 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

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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 towers can not move independently at the level of the diaphragm. Having one diaphragm across both blocks 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 color.

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.

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No diaphragm

This option switches diaphragm action off for an entire floor. Obviously, in this case an alternative means of transferring lateral loads i.e. a bracing system, would have to be provided.

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.

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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 DiaphragmsA 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 modeled by introducing “Diaphragm Braces”. within the plane of the floor.

Flexible DiaphragmsThe accepted definition of a ‘flexible’ diaphragm refers to the behavior 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.

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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 modeled in the same way as semi-rigid diaphragms, by introducing “Diaphragm Braces”. within the plane of the floor.

Story ShearsWhen modeling 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 Story Shear results. See “Reviewing Story Shear” for more details.

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Chapter 5 Member End Releases, Member Orientation and Supports

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 61 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 Releases from the dialog). 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.) Moment releases are indicated by a double arrowhead.

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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 select node positions (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.

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 (Mx) and minor axis (My) bending.

Moment Connection — The connection is fixed for major axis (Mx) bending but remains pinned for minor axis (My) bending.

Fully Fixed — The connection is fixed for both major axis (Mx) and minor axis (My) 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 — For member beams the focus is on the analysis model and so the usual analytical sign conventions are applied. X is along the member, Y is the major cross section axis and Z is the minor cross section axis. Hence this setting creates a pinned connection for major axis bending but the connection remains fixed for minor axis bending. These conflicting sign conventions are a universal issue when moving between analysis and design codes.

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. Due to the conflicting sign conventions noted above the minor axis is referred to as Z rather than Y.

Fully Fixed — This is the same as the Fixed Connection noted above, the connection is fixed for both major axis and minor axis bending.

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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 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.

Note You can also edit the intermediate connections on General 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. • General 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.

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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 DiaphragmA 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:

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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 OrientationsAll member orientations are reflected in the graphical views as appropriate.

By default the software places beams of all types with their major axis horizontal (in the global XY plane). For the vast majority of beams this will be the required orientation.

By default the software places vertical columns 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 General 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 general columns can initially be a little confusing.

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Supports and Base FixityA 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. Fastrak Building Designer automatically creates supports 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.

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Chapter 6 Load Cases and Load Combinations

Loads are applied to the model in loadcases categorized 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 the process.

Major topics • Gravity Load Cases• Lateral Load Cases• Seismic Load Cases• Combinations• Classifying Combinations and Setting the Critical Combinations

Gravity Load CasesGravity loadcases can be created for:

• self weights,• dead, • snow, • live, and• roof live loads

Self weight loads can all be determined automatically. However other dead and live 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 slabsFastrak 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 for composite beams. 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.

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Fastrak Building Designer can automatically calculate the above weights for you taking into account the slab thickness, the shape of the steel 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 concrete over pour (ponding) in your slab weight calculations, the software can also do this for you. When specifying the slab Attributes - you will find two ways to add this allowance 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 extra load is added as a uniform load over the whole slab.

Live and Roof Live LoadsRoof live loads can not be applied to slabs, they can only be applied to roof elements. Therefore, in models where slabs exist at the roof level it is necessary to overlay a roof on top of the slabs in order to define the roof live load.

Reductions in live and roof live load can be applied to take account of the unlikelihood of the whole building being loaded with its full design live load. The reduction is calculated based on total floor area supported by a design member (beam or column). Roof live and live load types each have their own reductions applied in accordance with either Section 4.8 and 4.9 of ASCE 7-05, or Section 4.7 and 4.8 of ASCE 7-10 as appropriate.

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. (See “Is it a Floor?” on page 72).

Note The load reduction only applies to horizontal simple and composite beams, and vertical general columns. For beams the strength check will take account of these reductions, however deflections are conservatively calculated ignoring them.

Note When assessing the live load reduction it is always assumed that any cantilever slab area associated with a beam or column is small and hence the following KLL factors are always used. Columns - 4, Beams - 2 and cantilver beams 1 as per table 4.2 in ASCE7-05&-10.

Perimeter LoadsProvided 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.

For further information on applying any of the gravity loads refer to the Quick Start Guide

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Lateral Load CasesLateral loadcases can be created for:

• Wind Loads, and• Notional Loads

Wind Loads

The ASCE7-05&-10 Wind Modeller

Fastrak Building Designer has a fully functional Wind Modeller which assesses wind loading on your building structure via a choice of methods:

If designing to ASCE 7-05 you can choose between:• Rigid Buildings of All Heights (Clause 6.5.12.2.1)• Low-Rise Buildings (Clause 6.5.12.2.2)

If designing to ASCE 7-10 you can choose between:• Directional Procedure Part 1 - Rigid Buildings of All Heights (Chapter 27)• Envelope Procedure Part 1 - Low-Rise Buildings (Chapter 28)

The Wind Wizard is run to create a series of static forces that are combined with other actions due to dead and live loads in accordance with Section 2.3.2 of ASCE 7-05 or ASCE7-10.

Note Access to the Wind Wizard is prevented until at least one wall or roof have been defined.

The following assumptions/limitations exist:• The structure is an enclosed building.• The wind is being calculated to apply to the Main Wind Force Resisting System (MWFRS).• It must be a rigid structure.• The structure must be either enclosed or partially enclosed. • Parapets and roof overhangs are not explicitly dealt with.

Note For further information refer to the ASCE7-05 Wind Wizard Handbook.

Simple Wind LoadingIf 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 Loadcase 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.

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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, look at Show/Alter State in the structure view.

Notional LoadsFastrak Building Designer allows you to build notional loads automatically into your combinations as follows:

• Notional load X+• Notional load X-• Notional load Y+• Notional load Y-

The magnitude of the notional load is calculated as 0.0031 of the gravity component in each combination.

There are specific requirements in the AISC Specification about when and in which combinations the notional loads should be included. Although you have complete control on the use of notional loads, a warning will be issued in some cases if you do not include them correctly. Note that it is possible to include two notional loads in the same combination provided they are not acting in opposing directions. For example (X+ Y+) or (X+ Y-) are both acceptable, but (X+ X-) is not.

For further information on when notional loads must be included see “Stability Coefficient”

Caution See Notional Load Calculations in the Assumptions and Limitations section of this document for important information about which loads are taken into account in these calculations.

Seismic Load CasesIn a structure where the seismic requirements must be met, Fastrak Building Designer will use the Equivalent Lateral Force (ELF) method. Where the seismic requirements are particularly severe then the ELF method cannot be used and in this case the building is Beyond Scope.

Given that the ELF method is valid then the Seismic Wizard is run to create a series of static forces that are combined with other actions due to dead and live loads in accordance with Section 2.3.2 of ASCE 7-05&-10.

Note Access to the Seismic Wizard is prevented until at least one dead and one live loadcase have been defined.

Seismic load cases can only be created by the Seismic Wizard, it is not possible to define these loadcases manually.

The base load cases created act in each direction +/- X/Y and depending on the Seismic Design Category may also act with a +/- eccentricity. Footnotes1. The value of 0.003 is used and not 0.002 in order to avoid calculating b as per equation A-7-2 of the 2005 AISC

Specification, or equation C2-2a of the 2010 AISC Specification.

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For further information on the seismic design capabilities of Fastrak Building Designer refer to the ASCE7-05 Seismic Wizard Handbook.

CombinationsOnce your load cases have been generated as required, you then combine them into load combinations. If a 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...

Construction Stage CombinationIf you have created a Slab Wet loadcase you are required to generate a Construction Stage load combination so that it can be considered in the design process. Other loadcases can also be included in this combination, however loadcases of type: Slab Dry; Wind and Seismic are specifically excluded.

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.

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The Combinations WizardDepending on which code the design is being carried out to, (ASD or LRFD), then the relevant combinations to LRFD 1-7 (including for both strength and serviceability) or ASD 1-8 (including for both strength and serviceability) are set up automatically.

LRFD Strength Combinations (ASCE7-05)

The following are the basic load combinations according to ASCE7-05 (2.3.2):-1. 1.4 (Dead)

2. 1.2(Dead) + 1.6(Live) +0.5(Roof live or Snow)

3. 1.2(Dead) + 1.6(Roof live or Snow) + (Live or 0.8Wind)

4. 1.2(Dead) + 1.6(Wind) + Live + 0.5(Roof live or Snow)

5. 1.2(Dead) + 1.0(Earthquake) + Live + 0.2(Snow)

6. 0.9(Dead) + 1.6(Wind)

7. 0.9(Dead) + 1.0(Earthquake)

LRFD Strength Combinations (ASCE7-10)

The following are the basic load combinations according to ASCE7-10 (2.3.2):-1. 1.4 (Dead)

2. 1.2(Dead) + 1.6(Live) +0.5(Roof live or Snow)

3. 1.2(Dead) + 1.6(Roof live or Snow) + (Live or 0.5Wind)

4. 1.2(Dead) + 1.0(Wind) + Live + 0.5(Roof live or Snow)

5. 1.2(Dead) + 1.0(Earthquake) + Live + 0.2(Snow)

6. 0.9(Dead) + 1.0(Wind)

7. 0.9(Dead) + 1.0(Earthquake)

ASD Strength Combinations (ASCE7-05)

The following are the basic load combinations according to ASCE7-05 (2.4.1):-1. 1.0 (Dead)

2. 1.0(Dead) + 1.0(Live)

3. 1.0(Dead) + 1.0(Roof live or Snow)

4. 1.0(Dead) + 0.75(Live) + 0.75(Roof live or Snow)

5. 1.0(Dead) + (1.0(Wind) or 0.7(Earthquake))

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6. 1.0(Dead) + 0.75(Wind or 0.7(Earthquake)) + 0.75 Live + 0.75 (Roof Live or Snow)

7. 0.6(Dead) + 1.0(Wind)

8. 0.6(Dead) + 0.7(Earthquake)

ASD Strength Combinations (ASCE7-10)

The following are the basic load combinations according to ASCE7-10 (2.4.1):-1. 1.0 (Dead)

2. 1.0(Dead) + 1.0(Live)

3. 1.0(Dead) + 1.0(Roof live or Snow)

4. 1.0(Dead) + 0.75(Live) + 0.75(Roof live or Snow)

5. 1.0(Dead) + (0.6(Wind) or 0.7(Earthquake))

6. a. 1.0(Dead) + 0.45(Wind) + 0.75(Live ) + 0.75 (Roof Live or Snow)b. 1.0(Dead) + 0.525(Earthquake) + 0.75(Live) + 0.75 (Snow)

7. 0.6(Dead) + 0.6(Wind)

8. 0.6(Dead) + 0.7(Earthquake)

LRFD Service Combinations (ASCE7-05)

The following are the basic service load combinations:-

Short term effects

1. 1.0(Dead) + 1.0(Live) +1.0(Roof live)

2. 1.0(Dead) + 1.0(Live) + 0.5(Snow)

Drift effects

1. 1.0(Dead) + 0.7(Wind) + 0.5(Live) + 0.5(Roof live or Snow)

LRFD Service Combinations (ASCE7-10)

The following are the basic service load combinations:-

Short term effects

1. 1.0(Dead) + 1.0(Live) +1.0(Roof live)

2. 1.0(Dead) + 1.0(Live) + 0.5(Snow)

Drift effects

1. 1.0(Dead) + 0.7(Wind) + 0.5(Live) + 0.5(Roof live or Snow)

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ASD Service Combinations (ASCE7-05)

The following are the basic service load combinations:-

Short term effects

1. 1.0(Dead) + 1.0(Live)

2. 1.0(Dead) +1.0(Roof live or Snow)

3. 1.0(Dead) + 0.75(Live) +0.75(Roof live or Snow)

Drift effects

1. 1.0(Dead) + 1.0(Wind)

2. 1.0(Dead) + 0.75(Live) + 0.75(Wind) + 0.75(Roof live or Snow)

ASD Service Combinations (ASCE7-10)

The following are the basic service load combinations:-

Short term effects

1. 1.0(Dead) + 1.0(Live)

2. 1.0(Dead) +1.0(Roof live or Snow)

3. 1.0(Dead) + 0.75(Live) +0.75(Roof live or Snow)

Drift effects

1. 1.0(Dead) + 0.6(Wind)

2. 1.0(Dead) + 0.75(Live) + 0.45(Wind) + 0.75(Roof live or Snow)

Classifying Combinations and Setting the Critical CombinationsHaving created your combinations you classify them as either gravity, lateral, or seismic and also indicate whether they are to be checked for strength, or service conditions, or both.

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.)

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

Gravity CombinationsThese 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.

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In addition, all members in the structure are checked for the gravity combinations during the Full Design process.

Lateral CombinationsThese 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 CombinationsThese combinations are not considered in either the Gravity Sizing or Lateral Sizing.

All members which have not been set as Gravity Only 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.

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Chapter 7 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

DefinitionsSome definitions of words and phrases that are used in the remainder of the chapter are given below:

Automatic Design mode you select the desired order file for the member and the program then automatically determines the most suitable section from the list.

Check Design mode you assign your desired section size to the member and the program then determines if the section is sufficient.

Direct Analysis Method (DAM) a rigorous second order analysis. that allows for both P-(big)Delta effects and P-(little)delta effects eliminating the need for calculating the effective buckling length (K factor). - see Appendix 7 of the 2005 AISC Specification, or Chapter C of the 2010 AISC Specification.

Drift the absolute horizontal deflection of a column or the relative deflection of two floors within a building when it is usually called “interstory drift”.

First-order analysis a standard linear elastic static analysis.

Gravity members gravity members resist only vertical loading. They are designed for gravity and seismic combinations only. Simple beams and composite beams are typically gravity members. However, columns and other members can also be set to ‘Gravity Only’.

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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, lateral and seismic combinations.

Live load live loads on roofs and floors are treated separately, (Snow is different again).

Live load reductions reduction in live load to take account of the unlikelihood of the whole building being loaded with its design load. Based on total floor area supported by a design member (beam or column). Roof Live and Live load types each have their own reduction.

Notional loads used to allow for an assumed out-of-plumbness and as a minimum level of lateral load. See Appendix 7 of the 2005 AISC Specification or Chapter C of the 2010 AISC Specification for more information.

P-Delta analysis analysis that allows for the presence of second-order effects - referred to in this documentation as Second-order analysis.

Stability Coefficient, (2/1) the ratio of second-order drift to first-order drift, after modifying the first-order drift for the effects of reduced stiffness - see 7.3(2) of the 2005 AISC Specification, or C2.1(2) of the 2010 AISC Specification for more information.

Building ValidationValidation is a check on your structure which you must perform before you can analyze 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/design process from continuing until appropriate corrective action is taken.

Warnings Although warning messages do not prevent the analysis/design 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:

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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 (e.g. ‘W’, ‘M’, ‘S’.) 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’. 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”) 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 As part of the lateral sizing process the notional loads are calculated as 0.0031 of the gravity component in each combination.

A second order analysis is performed and then the members are designed for the lateral combinations that you have nominated ‘critical’. The critical combinations may include gravity, lateral (notional loads or Wind) but not seismic loadcases. You can choose up to four lateral combinations - essentially allowing selection of one in each principal wind or notional load 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 but if this is the case it will be picked up in the Final Check 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.

Footnotes1. The value of 0.003 is used and not 0.002 in order to avoid calculating b as per equation A-7-2 of the 2005 AISC

Specification, or equation C2-2a of the 2010 AISC Specification.

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Stability AnalysisA stability analysis is always executed during the lateral Sizing in order to determine the first and second-order story drifts. From these the ‘Stability Coefficient’, (2/1) can be calculated. This is the ratio of second-order drift to first-order drift, after modifying the first-order drift for the effects of reduced stiffness - see 7.3(2) of the 2005 AISC Specification, or C2.1(2) of the 2010 AISC Specification for more information. If the ratio is greater than 1.71 then notional loads should be applied in all combinations. When the Stability Coefficient exceeds 1.71 the design of the column is given a warning status - use Show/Alter State to view. It is your responsibility to ensure that all combinations include notional loads.

If the Stability Coefficient exceeds 2.85 the structure is unsuitable and using Show/Alter State will show that the particular column is ‘Beyond Scope’.

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.• The seismic loads are determined as individual nodal forces and moments from the

first-order analysis for every active seismic combination.• The notional loads are determined for every active combination in which they have been

included. • Having established the seismic and notional loads, their contributions to frame

deflections/first-order story drift are determined using a first-order analysis. • A second order analysis of all active combinations is then performed to establish design

forces and second-order story drift.

Note The DAM requires that the members in this analysis are given a reduced stiffness. This is implemented by reducing the elastic modulus, E-value, and the shear modulus, G-value, of the steel to model the stiffness reductions. These reductions in the moduli are made for all materials e.g. concrete and timber general members and the components of the shear wall models.

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The reduced stiffness is not used in the calculation of deflections and vibrations under serviceability limit state which are determined from the first-order analysis. Note also that the reduced stiffness does not apply to the calculation of design strengths e.g. in the flexural buckling calculations.

• The Stability Coefficient, (2/1) is determined and checked as in the Lateral Sizing stage.• For active seismic combinations the seismic story drift, is checked against the limiting

value, max

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

Set Auto Design ModeAfter 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.

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

First-order or Second-order Analysis?It is essential that your final design utilizes second-order analysis. However, second order (P-) analysis can be more sensitive to parts of your model that lack stiffness. For this reason there is also the option to run a first-order analysis to obtain sections and an overall building performance with which you are satisfied before switching to P- analysis.

Stability Coefficient Tolerance If very small deflections were to be used in the calculation of the Stability Coefficient. then potentially very high Stability Coefficients could erroneously be reported.

To prevent this, the Stability Coefficient Tolerance provides you with a means to control the value of deflection that can safely be ignored.

If the second order drift is less than the tolerance defined here (default stack height/10000), the Stability Coefficient is returned as 'N/A' with a note to say that the 'Drift is small enough to be ignored'.

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Reduced Stiffness Factor For correct design to the AISC Specification using the DAM, this should be set to 0.8. As an alternative to setting the analysis to first-order (see above) to explore the reason for the second order analysis failure, it is possible to alter this factor. If you set it to a value of say 10, this will stiffen both the Modulus of elasticity (E) and the shear modulus of elasticity (G) by a factor of 10 in the second order analysis. Although the results will not be able to be used for a valid design, it should now be possible to run the analysis to see which member might fail a design and hence be the cause of the analysis instability. This factor can then be reduced towards 0.8 for further investigation.

Note For further information on resolving analysis failures see the Analysis section of the Fastrak Building Designer Help

Curved BeamsIf you have defined curved beams in your structure, then the Curved Beams page is applicable. This allows you to tell Building Designer into how many equivalent straight sections it is to split the curved member. Curved members are modeled 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°. 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 FactorsThe 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-analyzing the model. The forces would then be carried by other means.

Cracked SectionsThe 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. The default values displayed on this page are taken from 10.11.1 of the 2005 version of the Building Code Requirements for Structural Concrete ACI 318.05.

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Design OptionsDesign Options… are located under the Design menu.

Design CodesYou can switch between design codes from this page. Note that if you change between LRFD and ASD, all design combinations will need to be recreated.

Design ControlThis 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 - MembersA full 3D analysis may expose small forces that are normally ignored in the design of members. The options for ignore forces below on the second page of the dialog 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.

Element Pre-sizingFor second-order analysis it is useful to have the initial section sizes bearing some resemblance to those that are eventually chosen. For this reason minimum length/depth and L/ryy ratios are employed to prevent the initial sections from being under-sized.

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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 story braced frames you may prefer to set different pre-sizing limits to somebody predominantly working on three story moment frames.

Live Load Reductions

Certain districts like Chicago impose an upper limit on the live load reduction calculation over and above ASCE7-05&-10. The defaults on this page are the ASCE7-05&-10 values but these can be reduced if required.

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.

Steel JoistsThe load settings on this page determine the suitability or otherwise of steel joists within the structure.

The gross moment of inertia reduction is also specified here, (default 15%).

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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 DeflectionsMaximum 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.

Stability CoefficientsOnce again, before spending too much time reviewing detailed designs, it is also advisable to review the stability coefficients and decide on the approach you will take to designing for stability. It is conceivable that any changes you make in order to deal with stability could affect the designs of many elements.

Methods of providing lateral stability are discussed in the chapter “Stability Design”.

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Seismic DriftShould your model be subject to seismic loading, once your have performed a Full Design, you will have Seismic drift results to review.

Before spending too much time reviewing the detail of your seismic designs, it is also advisable to review the seismic drift, and should this indicate a problem, decide on the approach you will take to alter the structure overall stability to resist seismic actions more effectively. These changes could alter the structure configuration and hence the designs of many elements.

Methods of providing lateral stability are discussed in the chapter “Stability Design”.

Loading SummaryThis 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 structure has been applied correctly.

Review of Selected SectionsIt is always worth spending a little time reviewing the results to see if they are in line with expectations. Where you have moment frames 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 ResultsIt 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.

In some cases this may also help you to identify issues with the analysis model.

Note The model can also be exported to various analysis programs including CSC S-Frame. Here all the same sorts of results for the static analysis can be reviewed and in most cases there is also access to more advanced analysis options, e.g. Buckling Analysis, Vibration and Response Spectrum Analysis.

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Reviewing Stability DesignTo assist your assessment of how the structure will behave under lateral load, graphical feedback is provided for the floor center of mass and for the floor center of rigidity, both in the floor view and the structure view. This information is given by load case and combination.

The various possible gravity, wind and seismic 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.

A stability analysis is performed as part of the lateral analysis, this will determine the first and second order story drifts and the corresponding stability coefficient, (2/1).

Center of MassFor any given load case or combination, all gravity loads (self weight, slab dry, live, etc.) applied to a given floor have a center of action (or center 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 centers of mass within the model pick Edit/Show/Alter State, and then pick Center of Mass/Rigidity from the Analysis dialog.

Center of RigidityAny given floor has a center of rigidity or bending stiffness based on the stiffness of the structure that supports it (i.e. the columns, walls etc. below).

To review graphically the centers of rigidity within the model pick Edit/Show/Alter State, and then pick Center of Mass/Rigidity from the dialog. Due to the complex nature of assessing the stiffness of such varied structural systems, the center of rigidity is only an approximation.

Stability CoefficientAs part of the initial gravity sizing, a first-order analysis is performed. From this the deflections in global X and Y directions at all nodes are determined. The maximum drifts obtained are displayed on the Design tab of the Project Workspace.

During the lateral sizing, a second-order (P-Delta) analysis is performed providing you have selected this off the Analysis Options. Second-order deflections for the critical lateral combinations are determined.

The stability coefficient, (2/1) is the ratio of second-order drift to first-order drift (after modifying the first-order drift for the effects of reduced stiffness). If the ratio is greater than 1.71 then notional loads should be applied in all combinations. Above this value a normal building structure will be quite flexible and additional lateral load resistance should be included if possible. In addition the accuracy of the analysis results are model dependant. A warning to this effect is given.

If the stability coefficient, (2/1) exceeds 2.85 the structure is unsuitable and a message is displayed to this effect in the Design Results.

If you have selected a first-order analysis from the Analysis Options, an approximation to second-order deflections is used and the limits are then 1.5 and 2.5.

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Note that seismic design has its own, more severe, requirements. Hence, if there are seismic combinations specified then the building is declared unsuitable when > max - for more details see the ASCE7-05 Seismic Wizard Handbook.

Reviewing Story ShearTo assist your assessment of the performance of any semi-rigid/flexible diaphragms, graphical feedback is provided for the story shear at each floor level - story shear being that proportion of the total horizontal load that is resisted at each story.

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 story shear, or both.

To review story shear click the Story Shear icon on the Output Graphics toolbar.

To prevent the display of irrelevant results, story shears less than a specified minimum value are not displayed. The default is 0.1 kip, but this can be adjusted in Design Options if required.

For buildings with floors that are not readily identifiable or are sloping - the story shears may be inaccurate.

3D Analysis EffectsTraditional design approaches tend to involve idealization and simplification of the analysis model. Very often this would have meant simplification of the structure into discrete 2D planes, which could be analyzed 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 realistic, it does however illustrate a 3D Analysis Effect quite clearly.

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Continuous beams (spanning 20 ft then 30 ft then 15 ft) run from right to left of this floor area. These are supported on simple (pin-ended) steel beams spanning front to back which are in turn supported by the 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 shown below) would be an adequate idealization.

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 modeled 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 running horizontally 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.

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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 negative moment has reduced in the adjacent beam.

This sort of variation is enough to force the selection of a larger beam on the central beam line where the moments are higher.

Braces Carry Gravity Loads ExampleThis 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 stability, brace and column forces from the lateral load are assessed in isolation and are added to the column loads for column design checks as necessary.

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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 reviewing the results after design you might wonder why the column at C1 which is part of a braced panel is smaller than the column at B1 which supports the same floor area.

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You can review the analysis results for the individual columns within Fastrak Building Designer, where you can see the axial results as shown below. The column at grid intersection B1 is on the left and the column which is part of the braced bay at C1 is on the right. It shows that the column at C1 loses load to the brace at second floor level.

These sorts of effects can also be reviewed at a structure or a frame level. The view below shows the axial load results for the front elevation. To see this you would first have to create a Frame view for grid line 1.

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The large brace forces are clear to see. If we change the brace section size to a smaller, more realistic section, then Fastrak Building Designer finds that the same section size is adequate for both columns.

The effect still occurs but it is less significant in this instance.

When you define braces in Fastrak Building Designer you can either specify their section size and these are then checked for you, or you can allow the program to auto design them. As a closing point of guidance on this topic, we suggest that you keep brace sizes to a realistic minimum during building design. This realistic minimum is likely to be driven by stability considerations.

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Refining Member DesignsYou 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-analyze 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 AISC Specification Member Design Handbook.

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Chapter 8 Building Effective Models

Fastrak Building Designer is a design based structural modeller.

It is possible to create complete building designs quickly and easily with Fastrak Building Designer, however you should take account of the following points.

Note Please remember that Fastrak Building Designer is a modeling package, which dictates the design model and which creates analysis models to accomplish this design. It is important that you recognize that you must take ownership of the creation of the model and the results that the software gives.

Major topics • Place grid lines accurately• Save time by using Attributes effectively• Use simple construction 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• Switch off irrelevant load combinations• Building Size and Orientation• Design simple construction for gravity loads only• Prevent out of plane instability• Check the model analysis results• Staged modeling and design

Place grid lines accuratelyYou 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 Fastrak Building Designer’s ability to import a DXF file and create a ghost image of the structure. You can then add your grid lines on top of the ghosted DXF image.

Save time by using Attributes effectivelyIt is important to realize 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.

You can quickly make major changes to your model, for example changing the grade of steel, by changing the appropriate attribute(s) and then applying these attribute(s) to the members.

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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 braced continuously against lateral translation. Please take care if creating beams that are not braced continuously against lateral translation.

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 simple construction where possibleFastrak 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 applicableFastrak 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 that beam supports a slab), then you should use element loads which apply the load directly to the member 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.

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.

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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 ActionYou 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 “Reviewing Stability Design”.

Set the appropriate level of deflection checksFastrak 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, live load only and/or total load).

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 78.

Switch off irrelevant load combinationsWhere you are looking at design changes, for example to rationalize an area of floor, you can switch off irrelevant load combinations by making them inactive in the combinations dialog.

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 + live load combination are relevant. This may allow the you to switch off all other load cases and concentrate on the gravity design issues.

Building Size and OrientationThe automatic calculation of (NLs) and building stability checks are carried out on the basis that we are checking a single building. Effectively each portion of a structure between expansion joints should be looked at as a separate building for stability.

The Seismic wizard determines the equivalent lateral forces and then applies these load cases in combinations to the structure during analysis.

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Note NL’s, building stability and seismic loads are calculated in the global X and Y direction, you should take care to input the model with this in mind.

Design simple construction for gravity loads onlyIn order to speed the design process a distinction is made at two levels:-

• combinations - there are three types of combination• Gravity combinations - those combinations consisting of gravity loads only (Self

Weight, Dead, Slab Dry, Slab Wet, Live, Roof Live and Snow)• Lateral combinations - those combinations which in addition to gravity load contain

lateral loads due to Notional Loads or Wind.• Seismic combinations - those combinations consisting of gravity and/or lateral loads

as well as seismic load cases.• beams, columns, simple braces and truss members - these are by definition/can be set to

be • Gravity Only - designed for gravity combinations and seismic combinations• Lateral and Gravity (not Gravity Only) - designed for all combinations types - gravity,

lateral and seismic

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 simple 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 identified 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 warning is provided if the ignored loads exceed a preset limit.

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Prevent out of plane instabilityLong members in a model that have axial force in them can be unstable during second-order analysis because their individual elastic critical buckling load factor is lower than the elastic critical buckling load factor of the building as a whole and is less than 1.0. However, often such members, for example the rafters in a portal frame, are stable in design because there are many smaller members or sheeting, for example, that restrain the member in reality. They fail in the analysis because it is too resource intensive to model all the individual restraining members in the model which would also add unwanted clutter.

To prevent or to reduce the incidence of such failures during the analysis a multiplier can be applied to the minor axis inertia of these members which caters for the effect of the restraining members.

This multiplier can be applied to Simple Beams, Composite Beams, General Beams and Beam Members. It is defined on the Analysis tab of the Attributes set by checking the "Prevent out of plane instability" box and then entering a suitable value in the "Multiply minor axis inertia by" field.

Note This multiplier is applied to prevent unwanted behaviour in the analysis model. While the analysis results may be affected by this adjustment, there is no amplification of the minor axis inertia in the design of the member.

Check the model analysis resultsUpon completion of the design process the Workspace presents:

Model deflection results— these are not a pass/fail for 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 the remaining results could be incorrect as the overall building analysis may be indicating that the building is close to collapse.

This may be irrelevant if you are looking at a gravity design and are happy to ignore lateral load for the time being. However, it may also mean that the forces for which the members in the building are being designed could alter once the building is stabilized.

Building Stability — Fastrak Building Designer carries out a full Stability Coefficient analysis as part of the lateral analysis, post gravity sizing. Significant failure, as indicated by a high Stability Coefficient, (2/1) 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 system. Use the deflection results to look at this.

Seismic Stability — if seismic load combinations are included in the model, then Fastrak Building Designer will calculate the seismic stability coefficients for the building in X and Y. Should one of these exceed the maximum allowable limit then the building is not suitable to resist the seismic forces and should be reconsidered by the user.

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Load in versus load out — all loads are checked in and out of the model, it is essential that you check these results. Any difference between the two is typically because:

• the analysis model is not particularly stable and the model has large deflections • you have applied loads to the structure that are not being fully dispersed by slabs and

members - for instance a patch load falls off the slab.

Staged modeling and designOur major piece of advice when you are modeling 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.

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Chapter 9 Assumptions and Limitations

This chapter decribes the assumptions and limitations that apply to Fastrak Building Designer. It is recommended that you familiarize 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 ASCE7-05 Wind Wizard Handbook, the ASCE7-05 Seismic Wizard Handbook and the AISC Specification - Member Design Handbook

Major topics • Analysis Types• Analysis Results• Deflection checks• Foundation loads• Vertical cross bracing• Live Load Reductions• Notional Load Calculations

Analysis TypesFastrak 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 a two step iterative method incorporating a geometric stress stiffness matrix. This more rigorous analysis was chosen over the amplified first-order method due to the possible limitations of the latter as discussed in the Commentary to the AISC Specification.

When using second-order analysis 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 second-order analysis in Fastrak Building Designer allows for:

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

Footnotes1. 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.

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• 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.

For design using ASD, prior to second order analysis all loads are factored up by 1.6 and before design all design forces are reduced by a factor of 1.6 as required by 2005 AISC Specification 7.3 (1), or 2010 AISC Specification C2.1(4).

First-order analysis is used to determine the global deformations for drift, story drift and seismic drift calculations. It is also used to determine the relative deflections of members. The ‘Stability Coefficient’ (2/1) uses results from both the second- and the first-order analyses.

Second-order analysis uses a reduced stiffness in accordance with Appendix 7 of the 2005 AISC Specification, or Chapter C of the 2010 AISC Specification. A value of 0.8 is applied to both the Elastic Modulus, E and Shear Modulus G of all materials and to any spring stiffness you may have specified at a support.

Analysis ResultsCombination results are always obtained from the second-order analysis, whereas loadcase results are obtained from the first-order analysis. Superposition does not hold.

e.g. for a combination the base reactions reported do not equate to the sum of loadcase base reactions x load factors.

Deflection checks

Absolute and Relative DeflectionsFastrak 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.

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 Deflection Absolute Deflection

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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 member analysis results diagrams.

Note For composite beams the relative deflections are not displayed graphically. See “Deflections in Composite Beams” for details.

Deflections in Composite Beams 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 camber. As such the 3D graphical report should be used for building movement studies only, (though they may sometimes be useful in looking at concrete over pour).

Deflections of composite beams 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) - they will consider pre camber of the beam and concrete over pour- 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 concrete over pour see “Self weight of concrete slabs”.

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Foundation loadsThere 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 loadWhen vertical cross bracings are modeled, 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 loadWhen vertical cross bracings are modeled, 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.

Live Load ReductionsNo live load reductions are applied to General Beams, sloping General Columns, truss members or braces.

T

T

C

T

C

Design Model Actual Loads

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Chapter 9 : Assumptions and Limitations AISC Specification - Building Designer page 81

Notional Load Calculations

Loads used in Notional Load calculations

All Notional Loads 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 Notional Load calculations.

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

Gravity loads carried by braces not accounted for in Notional Load calculations

Any gravity load carried between floors in a brace element is not picked up in the calculation of Notional Loads - 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 on the right in the figure below. (actually in this case the user should use a general beam and not a brace and then the calculation of Notional Loads 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 Notional Load calculation: Notional Load = 0.003 x gravity load.

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Axial load in discontinuous columns used twice in Notional 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 Notional Loads for the axial load in the supported column are used twice in the Notional Loads calculation - 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 conservative as too much Notional Load 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 Notional Load calculation: Notional Load = 0.003 x gravity load.

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Chapter 10 : Sign Conventions AISC Specification - Building Designer page 83

Chapter 10 Sign Conventions

This chapter describes the sign conventions as applicable to Fastrak Building Designer.

Major topics • Object Orientation• Beams (Simple, Composite and General) and Truss member (chord)• Braces and Truss member (internal)• Columns• 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

+ moment shown as above

- moment shown as above

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

+M

-M

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AISC Specification - Building Designer page 84 Chapter 10 : Sign Conventions

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:

3D Graphic• Major shear, moment• Minor shear, moment• Axial

2D Analysis Results• Major shear, moment• Minor shear, moment• Deflection• Axial

Design Results• Major shear, moment• Minor shear, moment• Deflection• Axial

Report/Export• Element Design

Major Axis Shear, Moment and Deflection

Minor Axis Shear, Moment and Deflection

Axial Force (+ve compression)

End 1

End 2

+

+

xy

z+

Applied loads

+x

z +

+ -

- -

+x

y+

+ -

- -

x

+

-

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Chapter 10 : Sign Conventions AISC Specification - Building Designer page 85

Braces and Truss member (internal)

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

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

DXF• Beam end forces

Report/Export• Beam end forces

-vx

+vz

+vy

-My

+Mz

End 2

+vz

+My

-Mz

End 1

xy

z-vx

Applied loads

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

Report/Export• Bracing forces

Axial Force (Compression +ve, Tension -ve)

+

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AISC Specification - Building Designer page 86 Chapter 10 : Sign Conventions

Columns

Applied Loads

For loads applied as shown, the sign convention applicable to columns is as indicated.

This sign convention is applicable to:

3D Graphic• Major shear, moment• Minor shear, moment• Axial

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

Design Results• Deflections• Major shear, moment• Minor shear, moment• Axial

DXF• Column splice loads

Report/Export• Column splice loads

Note: Major Moment is about the Major AxisMajor Shear is in the plane of the Minor Axis.

Note: Simple Columns only cater for moments due to load eccentricity.

Major Axis Shear, Moment and Axial

Minor Axis Shear and Moment

Face A

+

-

+

+

-

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Chapter 10 : Sign Conventions AISC Specification - Building Designer page 87

Shear Walls

Applied Loads

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

This sign convention is applicable to:

3D Graphic• Major shear, moment• Minor shear, moment• Axial

Report/Export• Shear wall forces

Major Axis Shear, Moment and Axial

Minor Axis Shear and Moment

End 2

End 1

+

-

+

+

-

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Foundations/Bases - Foundation ForcesFastrak 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 forcesA

C

-Mminor

-Mmajor

+Fminor

+Fmajor

+Fvert

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Chapter 10 : Sign Conventions AISC Specification - Building Designer page 89

Shear Walls (note aligned with the local axis system of the wall)

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

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

Supplementary supports (note aligned with the global axis system)

• X shear, moment• Y shear, moment• Axial

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

End 2

End 1

-Mminor

-Mmajor

+Fminor

+Fmajor

+Fvert

X

Y Z

-MX+FX

+MY

+Fvert

+FY

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AISC Specification - Building Designer page 90 Chapter 10 : Sign Conventions

Foundations/Bases - Base ReactionsFastrak 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• Minor shear, moment• Axial

This sign convention is applicable to:DXF

• base reactionsReport/Export

• base reactions

AC

-Mmajor

+Fminor

+Fmajor

+Fvert

-Mminor

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Shear Walls (note aligned with the local axis system of the wall)

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

This sign convention is applicable to:DXF

• base reactionsReport/Export

• base reactions

Supplementary supports (note aligned with the global axis system)

• X shear, moment• Y shear, moment• Axial

This sign convention is applicable to:DXF

• base reactionsReport/Export

• base reactions

End 2

End 1

-Mminor

-Mmajor+Fminor

+Fvert

+Fmajor

X

Y Z

-MX +FX

+MY

+Fvert

+FY

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Nodal Deflections

Global Axes

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

X

Y

Z

+X

+Y

-Z