Chapter 1 3ed 4th Draft - Home | College of Engineering ...web.engr.oregonstate.edu/~liujud/UDSS_3ed_Chp1draft.pdf · C I 1 1 hapte ntrod.1 SCOPE.2 THE S r 1 uctio ... d Retrofit

C I

1

1

Chapte

Introd

1.1 SCOPE

1.2 THE S

er 1

ductio

E A wide vardesign of Structural 2016, and this book associated

Thdetailed byDesign (A

In the AISC Sneeded foSpecificati

SPECIFICA

The ANSI/standard spand constrreader inteprovide deGuide 15,Specificatieditions ofAISC SpecSpecificati

CuAISC Spec

n

riety of desigsteel structurSteel Buildinreferred to asspecifically fwith bridges

he book addry the SpecificaSD). Both meaddition to th

Steel Construr structural sion. Througho

ATION

/AISC 360-16pecifications ruction of strerested in theetailed guidan, AISC Rehaions. This Def the Specificacifications 19ions for Strucurrent design cification. In

gns can be chares for buildinngs, publisheds the Specificfocus on the d

and industriaresses the coation: Load aethods are dishe Specificatiction Manualsteel design, out this book,

6 Specificatiopublished by

ructural steel e historical ance on the hiabilitation ansign Guide pation. The se923–2010, whctural Steel Bu

is carried ouaddition to th

Phot

aracterized asngs as governd by the Am

cation in this design of steeal structures, ioncepts and and Resistancescussed later iion, the priml. This referendesign table

, this is referre

n for Structuy the Americabuildings. T

aspects of theistorical strucnd Retrofit Grovides outlin

econd resourchich containsuildings produut under the he detailed p

to Courtesy Mi

s structural stned by the A

merican Institubook. The arel building stif addressed adesign critere Factor Desiin this chapte

mary referencence handbookes to simplifyed to as the M

ural Steel Buian Institute o

The first editioese specificatctural steel stGuide: A Rene compariso

ce is found ons a searchabluced from 19provisions pu

provisions, the

One World ichael Mahesh,

teel design. ThANSI/AISC 36ute of Steel Creas of applictructures. Theat all, is kept ria for the twign (LRFD) aer. e for this bookk contains tab

fy actual desiManual.

ildings is the of Steel Conson was publitions, AISC htandards. Theeference for

ons of the pron the AISC wle compendiu923 through 2ublished in the Specificatio

Trade Center, , Port Authority

his book deal60-16 SpecificConstruction (ation given the treatment orelatively briewo design apand Allowable

k is the 15th bles of the baign, and the

latest in a lostruction for tished in 1923has two resoue first is AIS

Historic Shovisions in theweb site, wwwum of all of 2010. he 2016 edition contains U

New York ty of NYNJ

ls with the cation for (AISC) in hroughout

of subjects ef. pproaches e Strength

edition of asic values

complete

ng line of the design 3. For the urces that

SC Design hapes and e different w.aisc.org, the AISC

ion of the User Notes

2 Chapter 1 Introduction

and a detailed Commentary that provides insights into the source and application of the provisions. The reader interested in additional background on the provisions discussed in this book is encouraged to investigate the materials cited in the appropriate sections of the Commentary. The Specification contains 14 chapters and 8 appendices. To provide a concise guide to the use of the Specification, a brief description is given here.

Chapter A: General Provision. This chapter provides the scope of the Specification and summarizes all referenced specifications, codes, and standards. It also provides the requirements for materials to be used in structural steel design and the design documents necessary to communicate that design. Chapter B: Design Requirements. This chapter gives the general requirements for analysis and design that are applicable throughout the entire Specification. It provides the charging language needed for application of the subsequent chapters. Chapter C: Design for Stability. This chapter, along with Appendix 7, addresses the requirements for the design of structures to ensure stability. It details those factors that must be taken into consideration in any analysis and design. Chapter D: Design of Members for Tension. This chapter applies to the design of members subjected to axial tension resulting from forces acting through the centroidal axis. Chapter E: Design of Members for Compression. This chapter addresses members subjected to axial compression resulting from forces applied at the centroidal axis. Chapter F: Design of Members for Flexure. This chapter applies to members loaded in a plane parallel to a principal axis that passes through the shear center or is restrained against twisting. This is referred to as simple bending about one axis. Chapter G: Design of Members for Shear. This chapter addresses webs of singly or doubly symmetric members subject to shear in the plane of the web. It also addresses other shapes such as single angles and hollow structural sections. Chapter H: Design of Members for Combined Forces and Torsion. This chapter addresses design of members subject to an axial force in combination with flexure about one or both axes, with or without torsion. It also applies to members subjected to torsion only. Chapter I: Design of Composite Members. This chapter addresses the design of members composed of steel shapes and concrete working together as a member. It addresses compression, flexure, and combined forces. Chapter J: Design of Connections. This chapter addresses the design of connections, including the connecting elements, the connectors, and the connected portions of members. Chapter K: Additional Requirements for HSS and Box Section Connections. This chapter addresses requirements in addition to those given in Chapter J for the design of connections to hollow structural sections and built-up box sections of uniform thickness and connections between HSS and box members. Chapter L: Design for Serviceability. This chapter summarizes the performance requirements for the design of a serviceable structure.

Introduction Chapter 1 3

Chapter M: Fabrication and Erection. This chapter addresses the requirements for shop drawings, fabrication, shop painting, and erection. Chapter N: Quality Control and Quality Assurance. This chapter addresses the requirements for ensuring quality of the constructed project. Appendix 1: Design by Advanced Analysis. The body of the Specification addresses design based on an elastic analysis. This appendix addresses design by alternative methods generally referred to as advanced methods. It includes the classical plastic design method and design by direct modeling of imperfections. Appendix 2: Design for Ponding. This appendix provides methods for determining whether a roof system has sufficient strength and stiffness to resist the influence of water collecting on the surface and forming a pond. Appendix 3: Fatigue. This appendix provides requirements for addressing the influence of high cycle loading on members and connections that could lead to cracking and progressive failure. For most building structures, fatigue is not an issue of concern. Appendix 4: Structural Design for Fire Conditions. This appendix provides the criteria for evaluation of structural steel subjected to fire conditions, including (1) the prescriptive approach provided for in the model building code and most commonly used in current practice and (2) the engineered approach. Appendix 5: Evaluation of Existing Structures. This appendix provides guidance on the determination of the strength and stiffness of existing structures by load tests or a combination of tests and analysis. Appendix 6: Member Stability Bracing. This appendix details the criteria for ensuring that column, beam and beam-column bracing has sufficient strength and stiffness to meet the requirements for member bracing assumed in the provisions of the Specification for design of those members. Appendix 7: Alternative Methods of Design for Stability. This appendix, along with Chapter C, provides methods of designing structures to ensure stability. Two alternative methods are provided here, including the method most commonly used in past practice. Appendix 8: Approximate Second-Order Analysis. This appendix provides a method for obtaining second-order effects by an amplified first-order analysis. The provisions are limited to structures supporting load primarily through vertical columns.

Each chapter of this book will identify those chapters of the Specification that are

pertinent to that chapter. The reader is encouraged to become familiar with the organization of the Specification.

1.3 THE MANUAL

The AISC Steel Construction Manual, 15th edition, is the latest in a series of manuals published to assist the building industry in designing safe and economical steel building structures. The first edition was published in 1928 and the ninth edition in 1989. These manuals addressed design by the allowable stress method. In 1986 the first edition of the load and resistance factor design


method manual was published, with the third edition published in 1999. The next in this unbroken string of manuals published in support of steel design and construction was the first manual to unify these two design methods and was published in 2005 as the 13th edition. The current edition of the Manual is the 15th. Students who purchase the Manual through the AISC Student Discount Program also have an opportunity to apply for a free AISC Student Membership at the same time. Students are encouraged to become AISC Student Members in order to take full advantage of all free member benefits.

As is the case for the Specification, AISC has two resources to assist in addressing the historic aspects of steel design and construction. The first is, again, AISC Design Guide 15, AISC Rehabilitation and Retrofit Guide: A Reference for Historic Shapes and Specifications. This Guide provides properties of beam and column sections as old as the wrought iron shapes produced as early as 1873. The second resource is the electronic AISC Shapes Database. This database is available through the AISC web site www.aisc.org. It is a searchable database with properties for all shapes produced since 1873, consistent with the printed data in Design Guide 15. Access to the electronic shapes database is free to AISC members.

The Manual is presented in 17 parts as follows: Part 1: Dimensions and Properties Part 2: General Design Considerations Part 3: Design of Flexural Members Part 4: Design of Compression Members Part 5: Design of Tension Members Part 6: Design of Members Subject to Combined Forces Part 7: Design Considerations for Bolts Part 8: Design Considerations for Welds Part 9: Design of Connecting Elements Part 10: Design of Simple Shear Connections Part 11: Design of Partially Restrained Moment Connections Part 12: Design of Fully Restrained Moment Connections Part 13: Design of Bracing Connections and Truss Connections Part 14: Design of Beam Bearing Plates, Column Base Plates, Anchor Rods, and Column Splices Part 15: Design of Hanger Connections, Bracket Plates, and Crane-Rail Connections Part 16: Specifications and Codes Part 17: Miscellaneous Data and Mathematical Information


Each chapter of this book identifies those parts of the Manual that will be used with the material to be addressed. In many instances, the user will need to look in several parts of the Manual to fully understand the topics or solve the problems presented.

1.4 AISC WEB SITE RESOURCES

Another primary resource is the AISC web site, where there is information that is free to all visitors and additional electronic resources that are free to members only. Students will find a great deal of useful information on the AISC publications web site, www.aisc.org/epubs. The primary resources include electronic versions of the Specification, the Shapes Database, the Steel Construction Manual References, and the Steel Construction Manual Design Examples. The Specification, as described in Section 1.2 and the historic Shapes Database, as mentioned in Section 1.3, are available free to all through the web site. The 15th edition Steel Construction Manual Shapes Database is also available free to all. The AISC web site also includes an extensive array of journal and proceedings papers. All of the references cited in the Commentary and the Manual, for which AISC owns the copyright, are accessible under Steel Construction Manual Resources; Interactive Reference List.

Probably the most valuable aspect of the AISC web site for readers of this book is the complete set of the 15th edition Steel Construction Manual Design Examples. These examples are presented in four sections.

Section I: Examples Based on the AISC Specification. This section contains examples demonstrating the use of the specific provisions of the Specification, organized by Specification chapter. Section II: Examples Based on the AISC Steel Construction Manual. This section contains examples of connection design using the Specification and the tables found in the Manual. Section III: System Design Examples. This section contains examples associated with the design of a specific building and the application of the system-wide requirements. Section IV: Additional Resources. This section provides design tables for higher-strength steels than published in the printed Manual.

Although the topics covered in this book are supported by calculated example problems,

the reader might find the electronic Steel Construction Manual Design Examples helpful for further understanding of some of the specific provisions or design aids described in the book. In addition, some of the Design Examples go beyond the coverage in this book and provide additional useful information regarding typical design or detailing. The reader is encouraged to investigate what the AISC web site has to offer through both free and member only publications.

1.5 PRINCIPLES OF STRUCTURAL DESIGN

From the time an owner determines a need to build a building, through the development of conceptual and detailed plans, to completion and occupancy, a building project is a multi-faceted task that involves many professionals. The owner and the financial analysis team evaluate the basic economic criteria for the building. The architects and engineers form the design team and prepare the initial proposals for the building, demonstrating how the users’ needs will be met. This teamwork continues through the final planning and design stages, where the design drawings, specifications, and contract documents are readied for the construction phase. During this process, input may also be provided by the individuals who will transform the plans into a


real-life structure. The steel detailer, fabricator and erector all have a role in that process, and add their respective expertise to make the design constructible. Thus, those responsible for the construction phase of the project often help improve the design by taking into account the actual on-site requirements for efficient construction.

Once a project is completed and turned over to the owner, the work of the design teams is normally over. The operation and maintenance of the building, although major factors in the life of the structure, are not usually within the scope of the designer’s responsibilities, except when significant changes in building use are anticipated. In such cases, a design team should verify that the proposed changes can be accommodated.

The basic goals of the design team can be summarized by the words safety, function, and economy. The building must be safe for its occupants and all others who may come in contact with it. It must neither fail locally nor overall, nor exhibit behavioral characteristics that test the confidence of rational human beings. To help achieve that level of safety, building codes and design specifications are published that outline the minimum criteria that any structure must meet.

The building must also serve its owner in the best possible way to ensure that the functional criteria are met. Although structural safety and integrity are of paramount importance, a building that does not serve its intended purpose will not have met the goals of the owner.

Last, but not least, the design, construction, and long-term use of the building should be economical. The degree of financial success of any structure will depend on a wide range of factors. Some are established prior to the work of the design team, whereas others are determined after the building is in operation. Nevertheless, the final design should, within all reasonable constraints, produce the lowest combined short- and long-term expenditures.

The AISC Specification follows the same principles. The mission of the AISC Committee on Specifications is to “develop the practice-oriented specification for structural steel buildings that provide for life safety, economical building systems, predictable behavior and response, and efficient use.” Thus, this book emphasizes the practical orientation of this Specification.

1.6 PARTS OF THE STEEL STRUCTURE

All structures incorporate some or all of the following basic types of structural components:

1. Tension members 2. Compression members 3. Bending members 4. Combined force members 5. Connections The first four items represent structural members. The fifth, connections, represents the

contact regions between the structural members, which ensure that all components work together as a structure.

Detailed evaluations of the strength, behavior, and design criteria for these members are presented in the following chapters:

Tension members: Chapter 4 Compression members: Chapter 5

Be Co Co Th

elements a(steel and the design structural discussion

Testeel joistsmezzaninemany builFigures 1.1

In structures, compressiothe assump

Thare pins, wHowever, be introduexposed toperfectly s

Thand shear c

Coand chordstructures.

Figure 1.1Photo court

ending memb

ombined force

onnections:

he strength aare covered inconcrete worof steel strucsteel and the

n of the types ension membes; as diagonae floors, and plding types, a1 and 1.2 illusthe idealizedreversals of

on. Some memption that theyhe idealized twhich prevenin an actual s

uced to the teno some form straight, or if the primary loconsidered asompression md members in

Figure 1.3 sh

1 Use of Tentesy Ruby + As

bers:

e members:

and behavior n Chapters 8 arking togetherctures for earte various shapof loads and lers are typicalal members ipedestrian waas well as to strate typical d case, tensionf the overall lmbers will bey will carry tetension membnt any momestructure, the nsion membeof transversethe axial load

oad effect in ts secondary efmembers are an trusses andhows a typica

nsion Membessociates

Chapters

Chapter

Chapters

of structuraland 13, and thr) constructiothquake loadipes commonlload combinally found as wn structural b

alkways. Theysupport platapplications n members trload may chae designed forension only. ber is analyzeent or shear type of conn

er. This is alse load. Momed is not appliethe tension mffects.

also known asd joists and al use of struc

ers in a Truss

s 6 and 7

8

s 10, 11, and

l frames comhe special conon are presenting is presently used are dations is preseweb and chorbracing systey are also usetforms for meof tension meransmit conceange the tensr this action;

ed with the aforce from

nection normaso the case wents will also ed along the cmember is a c

s columns, stras vertical

ctural compres

Introd

12

mposed of a nsiderations tted in Chapteted in Chaptediscussed in Cented in Chaprd members inems; and as d as sag rods echanical equembers in actuentric tensile sion member others will ha

assumption thbeing transm

ally dictates twhen the tensi

be introduceentroidal axis

concentric axi

ruts, or posts.members in ssion membe

duction Cha

combinationthat apply to cer 9. An introder 13. The proChapter 3, an

pter 2. n trusses and hangers for bfor purlins an

uipment and ual structuresforces only. force from t

ave been desi

hat its end comitted to the that some benion member ied if the elems of the membial force, with

. They are usall types of

rs.

apter 1 7

n of these composite duction to operties of nd a brief

open-web balconies, nd girts in pipelines.

s. In certain tension to gned with

onnections member.

nding may is directly

ment is not ber. h bending

ed as web f building

8

8 Chapter 1 Introductio

Figure 1.2 Th

strength isconditionsHistoricallof this idea

Thrarely resesurroundinlikely to bconditionscolumn, as

Thcompressiv

Begirts. Althapplication

Figure 1.3

on

2 Use of Ten

he idealized s heavily infl. The basic coly, design rulealized comprehe basic coluemble perfecng structure trbe connected produce bens distinct fromhe primary lve force accoending membhough all of n within a bui

3 Use of Co

nsion Membe

compression luenced by tholumn is theres for compreession membeumn is practict pins; the aransmits its lto the colum

nding effects im the idealizeload effect impanied by t

bers are knowf these are bilding:

lumns in a Bu

ers as Hanger

member carhe distance brefore definedession membeer. ically nonexiaxial load is load to the m

mn in such a in the membed column. in the pinnedthe secondarywn as beamsbending mem

uilding Frame

rs

rries only a between the sd as an axiallyers have been

istent in realnormally no

member; and way that mo

er, making it a

d-end columy effects of bes, girders, joimbers, each

e

concentric, csupports, as wy loaded mem

n based on the

l structures. Rot concentricbeams and s

oments are ina combined f

mn is therefoending and shists, spandrelname implie

compressive well as by thmber with pine behavior an

Realistic endc, due to theimilar compo

ntroduced. Alforce member

ore a concenhear. ls, purlins, lines a certain

force. Its he support nned ends. d strength

d supports e way the onents are ll of these r or beam-

ntric axial

ntels, and structural

1.

2.

3.

4.

5.

Fig

idealized bsupports.

Thlongitudinshear force

Ththis structuthe presencombined

Figure 1.4

Beams, giroften consiare usually may generaor joist. Ho

The bendinknown as beams and

Bending musually refe

Lintels are carrying thinto that arwindows.

Girts are usurface to twall.

gure 1.4 shobeam is show

he basic bendal centroidal e. Axial forcehe most commural element nce of both force membe

4 Building S

ders, and joisdered as the msupported by

ally be considowever, variat

ng members thspandrels or girders becau

members in roerred to as pu

bending meme weight of trea. They typ

sed in exteriothe exterior c

ows beams awn in the fig

ding memberaxis of the m

es and torsionmon combineis simultaneobending and

er, this case is

Structure Sho

sts form part members that

y columns. Jodered a highetions to this b

hat form the pspandrel bea

use the load c

oof systems urlins.

mbers that spthe wall abovpically are se

or wall systemolumns. They

nd girders ingure as a mem

r carries tranmember. The may occur as

ed force memously subjected axial tensios not as critica

wing Bending

of common t are directly ists are beam

er-order bendibasic scheme a

perimeter of aams. Their domes primari

that span be

pan across thve the openingeen spanning

ms. They trany may also as

n an actual smber with a

nsverse loads e primary loas secondary e

mber is knowned to bendingon representsal or common

g Members

Introd

floor systemssupported by

ms with fairly cing member care common.

a floor or roodesign may bily from one s

etween other

he top of openg as well as aacross the o

nsfer the latessist in suppo

structure unduniform load

that act in aad effects areeffects. n as a beam-g and axial cs a potential n as the beam-

duction Cha

s. The beamsy girders, whiclose spacingcompared wit

f plan in a bue different frside of the me

bending mem

nings in wallany other loa

openings for d

eral load fromorting the wei

der constructid supported o

a plane contae bending mo

-column, impcompression.

loading cas-column load

apter 1 9

s are most ich in turn g. A girder th a beam

uilding are rom other ember.

mbers are

ls, usually ad brought doors and

m the wall ight of the

ion. The on simple

aining the oment and

plying that Although

se for the ding case.

1

10 Chapter 11 Introduct

Figure 1.5Subjected

Fig

columns aconnectionbending mmembers oportal fram

Thshear forceas a speciathe basic bconsideratiapplied to

Beconsideredcolumn beratio of apThe beampractical d

Cotogether. W

ion

5 Schematicto Axial Load

gure 1.5a is aare joined witns, and the l

moments. Thisof the gable f

me shown in Fhe beam-colues, and bendinal case, repre

bending membions that musbeam-column

ecause of thed primary. Hecomes high, pplied momen

m-column is adesign approaconnections arWhether they

c Representatids and Bendin

a schematic ith rigid conneloading pattes is a typical aframe shown Figure 1.5c. mn may be reng moments aesenting a beber may be thst be accountens. e generalized owever, whecolumn beha

nt to moment an element iches are normre the collectconnect the a

ion of Steel Fng Moments

illustration ofections. Becaern, the vertiapplication ofin Figure 1.5

egarded as thact simultaneoam-column w

hought of as aed for in the

nature of then the ratio oavior will ovstrength is h

in which a vmally based ontion of elemeaxially loaded

Frames in Wh

f a multi-storause of the gecal membersf practical bea5b and the ve

he general struously. Thus, t

with no moma beam-columdesign of bot

he combined of axial load vershadow othhigh, beam bevariety of din interaction ents that joind members in

hich the Vertic

ry steel frameeometric confs are subjectam-columns; ertical compo

uctural elemethe basic colu

ments or transmn with no axth columns an

force elemento axial loa

her influenceehavior will oifferent forceequations.

n the membea truss or the

cal Members

e where the bfiguration, theed to axial lother exampl

onents of a sin

ent, where axumn may be tverse loads.

xial load. Thernd beams mu

nt, all load ed strength in

es. Similarly, outweigh othe types intera

ers of a steele beams and c

Are

beams and e types of loads and les are the ngle-story

ial forces, thought of Similarly, refore, the ust also be

effects are n a beam-

when the er effects. act. Thus,

l structure columns

1

1

1.7 TYPES

1.7.1 Bear

Figure 1.6Copyright © of a multi-unit, consi

Thbolts and function oand momeconnection

S OF STEE

It is difficuavailable tform the tloads. This

Stwith otherstage for th

ring Wall Co

Bearing wwarehouseconcrete bthe floor o

6 Building C© American In

-story frame, stent with the

he fasteners uwelds. The lof the specificents. Figure 1ns are present

EL STRUCT

ult to classifyo the designetotal structures combinationeel-framed bustructural m

he application

onstruction

wall constructes, shopping cblock masonryr roof. The fl

Connections nstitute of Steel

connections me assumptionsused in strucoad effects th connection t1.6 illustratested in Chapter

TURES

y steel structuer. The elemene of a buildin of membersuildings comaterials. A fen of structural

tion is primacenters, officey walls, on wexural memb

l Construction.

must ensure ths made in the ctural steel cohat the varioutype being cos a variety ofrs 10, 11, and

ures into neants of the struing, which ms is usually re

me in a wide vw examples al design prese

arily used fore buildings, a

which are placers are usuall

. Reprinted wit

hat the structudesign.

onnections tous elements o

onsidered. Thef connectionsd 12.

at categories, ucture, as defi

must safely anferred to as thvariety of shaare given in tented in subse

r one- or twand schools. ced the ends ly hot-rolled s

Introdu

th permission.

ural members

oday are almof the connecey include alls. The idealiz

due to the wfined in Sectiond economicahe framing syapes and sizethe followingequent chapte

wo-story buildThis system of the flexurastructural stee

uction Chap

All rights reser

s function tog

most entirely lction must rel of the possized represent

wide variety oon 1.6, are comally carry all

ystem. es and in comg paragraphs, ers.

dings, such anormally useal members sel shapes, alo

pter 1 11

rved

gether as a

limited to esist are a ible forces tations for

of systems mbined to l imposed

mbinations to set the

as storage es brick or supporting ne or in

1

1

12 Chapter 1

1.7.2 Beam

1 Introduct

combinatiobuilding is

m-and-Colu

Beam-andsuitable fothan two sstories. Cogirders, anlends itselsame size.

Foaffords addwith the us


ion

on with open s shown in Fig

mn Constru

-column consor large-area btories but maolumns are pnd joists, whicf to economyAn example

or multi-storyditional savinse of composi

8 Beam-and-tesy Douglas S

web steel joigure 1.7.

uction

struction is thbuildings suchay have a largplaced accordch are used foy in fabricatioof this type o

y buildings, tngs. Further aite columns a

-Column BuilSteel Fabricatin

sts or cold-fo

he most commh as schools a

ge number of ding to a regor the floor anon and erectioof structure is the use of cadvances can and other elem

lding ng Corporation

FiguPhoto

ormed steel sh

monly used syand shoppingspans. It is al

gular, repetitind roof systeon, because mshown in Fig

composite stebe expected

ments of mixe

ure 1.7 Bearo courtesy Dou

hapes. An exa

ystem for steeg centers, whilso suitable foious grid tha

ems. The regumost of the mgure 1.8 eel and concras designers

ed constructio

ring Wall Buiuglas Steel Fab

ample of a bea

el structures toich often haveor buildings wat supports thularity of the members will

rete flexural become mor

on systems.

lding bricating Corpo

aring wall

oday. It is e no more with many he beams, floor plan be of the

members re familiar

oration


Be

to resist latagainst thethrough th

Figure 1.1moment-rewall and cunbraced b

A

which is o

9 Braced Beatesy Douglas S

eam-and-coluteral loads. Ae action of late bending stif

10 Idealizedesistant frameore-braced bubents.

frame withooften afforded

am-and ColumSteel Fabricatin

umn structuresA frame in wh

eral loads, suffness of the o

d Illustration oe; (b) truss-bruilding; (e) flo

out member-ed by having

mn Building ng Corporation

s rely on eitheich all connec

uch as wind anoverall frame

of Several Tyaced frame; (oor plan of bu

end restraint the elements

er their connections are mond earthquakee.

ypes of Beam-(c) core-braceuilding with a

needs a seps along one o

Introdu

ections or a seoment resistanes, and overal

-and-Columned frame; (d) a combination

arate lateral or more of th

uction Chap

eparate bracinnt provides rell structural st

n Framed Strufloor plan of n of braced an

load resistinhe column lin

pter 1 13

ng system esistance tability,

uctures(a) shear nd

ng system, nes act as

1

1

14 Chapter 1

1.7.3 Long

1 Introduct

braced fratruss, whicschemes inas a bracedsection of bracing syof the nece

Cohave been other. Of c

Figframed stru

g-Span Cons

This type vertical loone-way trin Figure 1

LoIn such cathe distanc

Mspan constsubstantial


ion

mes, as seen ch is designenvolve shear wd core systemf the core. Thystem for the bessary serviceombinations odesigned as m

course, such agure 1.10 shuctures.

struction

of constructad-carrying erusses, two-w1.11. Arches oong-span consases the buildce from the exany designertruction systel distances, of

11 Long-Spantesy Douglas S

in Figure 1.ed to take thewalls and rein

m and can be hhe core servebuilding, it sees, including eof these typesmoment resisa choice recoghows an idea

tion encompaelements, suc

way space trusor cables coulstruction is al

ding may be axterior wall tors would alsoems. Dependiften with exce

n Structure Steel Fabricatin

9. One of thee loads imponforced concrhighly efficienes a dual purerves as the velevators, stais of constructtant in one di

gnizes the threalized represe

asses steel-frh as covered sses, or plate ld also be uselso used in bua core- or otho the core. o characterizeing on the geellent econom

ng Corporation

e most commosed by wind rete cores. Thnt because of rpose in this vertical conduircases, electrtion are also cirection of theee-dimensionentation of s

ramed structud arenas. The

and box girded, although tuildings that rherwise brace

e single-story eometry of th

my.

mon types of d and seismiche latter type mf the rigidity o

case: In adduit in the comricity, and othcommon. Fore building andnal nature of tseveral types

ures with lonlong distanc

ders. A long-sthey are not crequire large,ed structure, w

rigid frameshe frame, suc

bracing is thc action. Othemay also be rof the box-shadition to prov

mpleted structuher utilities. r example, frad as truss brathe structure.s of beam-an

ng spans betes may be sp

span structureonsidered her, column-freewhere the lon

s as examplesch structures

he vertical er bracing referred to aped cross viding the ure for all

ames may aced in the

nd-column

tween the panned by e is shown re. e interiors. ng span is

s of long-can span

1 1.7.4 Highh-Rise Const

High-rise cin Figure structures addition, omulti-story

Pasystem in hthe analysirise buildicrucial to t

Soquantified are presenwhen a streccentricitwind loadconsiderinwill be ma

Frresistance.quite succeChicago, IYork City,Center in Cthe Willis

truction

construction 1.12. The lwarrant treat

over the past y frame desigarticular care high-rise conis of lower-risngs but havethe proper desome of these

through a not in all structructure is dis

ty of the columds, the resultng the secondaking a seriouraming system Thus, attempessful. This tIllinois, shown, shown in FiChicago, showTower in Chi

refers to mularge heights ting them ind40 years sev

gn, such as themust be exe

struction. It isse structures,

e significantlysign of the hieffects may b

ormal, linearlytures, they masplaced lateramn loads. Whting effects m-order effects

us and perhapsms for high-ripts at makingtube may be in in Figure 1gure 1.13b; awn in Figure icago (former

lti-story buildand unique

dependently fveral designere super compoercised in thes not just a mbecause man

y less impactgh-rise structbe referred toy elastic analyay be more sially, additionhen added to tmay be signs. A designers unconservatise buildings

g the perimetein the form o.13a, or a fram

a solid wall tu1.13c; or sev

rly known as

dings of signiproblems en

from typical rs have develosite column

e choice and matter of extrany effects playt on frames oture. as second-orysis of the fraignificant in

nal moment ithe moments nificantly larr who does notive error. reflect the in

er of a buildinof a truss, asme, as in the

ube with cutouveral interconthe Sears Tow

FigurStructPhoto Corpo

Introdu

ificant heightncountered ibeam-and-co

loped a numband the steel design of the

apolating fromy a major roleof smaller he

rder effects, bame. Althoughigh-rise struis induced inand shears pr

rger than thoot incorporat

ncreased impng act as a unwith the Johnformer Worlduts for windo

nnected or bunwer), shown i

re 1.12 Highture courtesy Doug

oration

uction Chap

; an examplein the designolumn constrber of new coplate shear w

e lateral loadm the principle in the desigight. These e

because they gh second-orductures. Forn a column droduced by grose computedte both types

portance of lanit or tube havn Hancock Bd Trade Cent

ows, as used indled tubes, sin Figure 1.13

h-Rise Buildin

glas Steel Fabr

pter 1 15

e is shown n of such ruction. In oncepts in wall. d resisting les used in gn of high-effects are

cannot be der effects r example, due to the ravity and d without of effects

ateral load ve proven uilding in

ter in New n the Aon such as in 3d.

ng

ricating

1

1

16 Chapter 1

1.7.5 Gabl

1 Introduct

Figure 1.1(c) the AonPhoto (b) co

le-Frame Co

Many desiThese struengineeredof frames fand similais seen in F

ion

13 High-Risn Center; (d) ourtesy Leslie

onstruction

igners incluductures lend d building indfor storage wr types of struFigure 1.14.

(a)

(c) se Buildings (the Willis ToE. Robertson A

de the single-themselves

dustry has capwarehouses, in

uctures. An ex

(a) the John Hower. Associates, RL

-story frame particularly

pitalized on thndustrial buildxample of a p

(b)

(d)

Hancock Cent

LLP

as part of thwell to full

he use of thisdings, temporpre-engineere

ter; (b) the W

he long-span ly welded cs system throurary and permed metal build

World Trade Ce

construction construction. ugh fine-tune

manent office ding with a ga

enter;

category. The pre-

ed designs buildings,

able frame

1 1.8 DESIG


GN PHILOS

A successfdesign loais economithe primarlabor and impossibletask of thepublished consideratiby the desbusiness cl

Toloads that termed thethe materistrength or

In corresponddegree to wfor accomp

Alexact magnthe structustandards those magn

Asthe behavigiven by twhole. Faito have ocfailure is t

14 Pre-engintesy Metal Buil

SOPHIES

ful structural ds without ovical to build ary concern of

materials we to design a e designer is by the Amerions that predsigner will dlimate. o perform a swill be exert

e load effect oial and the sr capacity of t

its simplest ding connectiwhich this is plishing this glthough past nitude of the

ural elements, and, althoughnitudes. Loads is the case fior and strengthe magnitudeilure may eithccurred if defthe result of

neered Metal lding Manufac

design resulverstressing aand operate fof an owner, swill vary fromstructure thatto produce arican Institutdict structuraldictate the ec

structural dested on each eor the requirehapes that cothe element.form, structu

ons, so that thaccomplished

goal have beeexperience mloads that withis is usuall

h the values ds, load factorfor loading, sgth of structure of the loadher occur as tflections, for a lack of str

Building cturers Associa

lts in a structany componenor its intendedafety must bem one geogt is equally eca safe and serte of Steel Cl behavior anonomy of a

ign, it is necelement throued strength. Itompose these

ural design ihe strength ofd is often termen used over tmight seem toill be appliedly not the casgiven are spers, and load cosignificant unral members.that causes t

the physical cinstance, exc

rength (colla

ation

ture that is sants, does not d life span. Ae the primary

graphic locaticonomical in rviceable stru

Construction and material re

particular so

cessary to quaughout the lifet is also necese elements. T

is the determf the structuremed the margthe years. o indicate thad to the structe. Design loaecific, signifiombinations

ncertainty is a The true indthe failure ofcollapse of paceed certain p

apse) or stiffn

Introdu

afe for its oct deform or vAlthough econy concern of ion to anothall locations.

ucture, designare based on sponse. The u

olution in a p

antify the caufe of the strucssary to accoThis is referr

mination of me is greater thgin of safety.

at the structurture, and the ads are providicant uncertaiare discussed

associated witdication of loaf a componenart of the builpredetermineness (deflecti

uction Chap

ccupants, can ibrate excessnomy may apthe engineer

her, making . Because the

n criteria suchtechnical m

use of these pparticular loc

uses and effecture. This is unt for the bered to as the

member sizes han the load e

Numerous ap

ral designer kexact strength

ded by many inty is associ

d in Chapter 2th the determad-carrying cnt or the strulding, or be ced values. Whion), these ph

pter 1 17

carry the ively, and pear to be . Costs of it almost

e foremost h as those

models and provisions cation and

ects of the generally

ehavior of e nominal

and their effect. The pproaches

knows the h of all of codes and iated with

2. mination of capacity is cture as a

considered hether the henomena


reflect the limits of acceptable behavior of the structure. Based on these criteria, the structure is said to have reached a specific limit state. A strength failure is termed an ultimate limit state, whereas a failure to meet operational requirements, such as deflection, is termed a serviceability limit state.

Regardless of the approach to the design problem, the goal of the designer is to ensure that the load on the structure and its resulting load effect, such as bending moment, shear force, and axial force, in all cases are sufficiently below each of the applicable limit states. This ensures that the structure meets the required level of safety or reliability.

Three approaches to the design of steel structures are permitted by the AISC Specification:

1. Allowable strength design (ASD) 2. Load and resistance factor design (LRFD) 3. Design by inelastic analysis

The design approaches represent alternative ways of formulating the same problem, and all have the same goal. All three are based on the nominal strength of the element or structure. The nominal strength, most generally expressed as Rn, is determined in exactly the same way, from the exact same equations, whether used in ASD or LRFD. Some formulations of design by inelastic analysis, such as plastic design, also use these same nominal strength equations whereas other approaches to inelastic design model in detail every aspect of the structural behavior and do not rely on the equations provided through the Specification. The use of a single nominal strength expression for both ASD and LRFD permits the unification of these two design approaches. It will become clear throughout this book how this approach has simplified steel design for those who have struggled in the past with comparing the two available philosophies. The following sections describe these three design approaches, any one of which is an acceptable approach to structural steel design according to the AISC Specification.

1.9 FUNDAMENTALS OF ALLOWABLE STRENGTH DESIGN (ASD)

Prior to 2005, allowable strength design was referred to as allowable stress design. It is the oldest approach to structural design in use today and has been the foundation of AISC Specifications since the original provisions of 1923. Allowable stress design was based on the assumption that under actual load, stresses in all members and elements would remain elastic. To meet this requirement, a safety factor was established for each potential stress-producing state. Although historically ASD was thought of as a stress-based design approach, the allowable strength was always obtained by using the proper combination of the allowable stress and the corresponding section property, such as area or elastic section modulus.

The current allowable strength design approach is based on the concept that the required strength of a component is not to exceed a certain permitted or allowable strength under normal in-service conditions. The required strength is determined on the basis of specific ASD load combinations and an elastic analysis of the structure. The allowable strength incorporates a factor of safety, Ω, and uses the nominal strength of the element under consideration. This strength could be presented in the form of a stress if the appropriate section property is used. As a result of doing this, the resulting stresses will most likely again be within the elastic range, although this is not a preset requirement of the Specification.

The magnitude of the factor of safety and the resulting allowable strength depend on the particular governing limit state against which the design must produce a certain margin of safety.


Safety factors are obtained from the Specification. This requirement for ASD is provided in Section B3.2 of the Specification as

na

RR ≤Ω

(AISC B3-2)

which can be stated as

Nominal StrengthRequired Strength (ASD) Allowable StrengthSafety Factor

≤ =

The governing strength depends on the type of structural element and the limit states

being considered. Any single element can have multiple limit states that must be assessed. The safety factor specified for each limit state is a function of material behavior and the limit state being considered. Thus, it is possible for each limit state to have its own unique safety factor. For example, the limit state of yielding of a tension member is given by

n y gP F A=

where Fy is the steel yield strength and Ag is the gross area of the member. The safety factor is

1.67.Ω= Thus, for steel with a yield strength of 50 ksi, the allowable strength is

50.030

1.67gn

gAP A= =

Ω

Design by ASD requires that the allowable stress load combinations of the building code

be used. Loads and load combinations are discussed in detail in Chapter 2.

1.10 FUNDAMENTALS OF LOAD AND RESISTANCE FACTOR DESIGN (LRFD)

Load and resistance factor design explicitly incorporates the effects of the random variability of both strength and load. Because the method includes the effects of these random variations and formulates the safety criteria on that basis, it is expected that a more uniform level of reliability, and thus safety, for the structure and all of its components will be attained.

LRFD is based on the concept that the required strength of a component under LRFD load combinations is not to exceed the design strength. The required strength is obtained by increasing the load magnitude by load factors that account for load variability and load combinations. The design strength is obtained by reducing the nominal strength by a resistance factor that accounts for the many variables that impact the determination of member strength. Load factors for LRFD are obtained from the building codes for strength design and will be discussed in Chapter 2. As for ASD safety factors, the resistance factors are obtained from the Specification.

The basic LRFD provision is provided in Section B3.1 of the Specification as

u nR R≤ φ (AISC B3-1)

which can be stated as

Required Strength (LRFD) Resistance Factor Nominal Strength=Design Strength≤ ×


Again considering the limit state of yielding of a tension member, n y gP F A=

and the resistance factor is 0.90φ = . For steel with a yield strength of 50 ksi, the design strength is

0.90(50) 45n g gP A Aφ = =

LRFD has been a part of the AISC Specifications since it was first issued in 1986.

1.11 INELASTIC DESIGN

The Specification permits a wide variety of formulations for the inelastic analysis of steel structures through the use of Appendix 1. Any inelastic analysis method will require that the structure and its elements be modeled in sufficient detail to account for all types of behavior. An analysis of this type must be able to track the structure’s behavior from the unloaded condition through every load increment to complete structural failure. The only inelastic design approach that will be discussed in this book is plastic design (PD).

Plastic design is an approach that has been available as an optional method for steel design since 1961, when it was introduced as Part 2 of the then current Specification. The limiting condition for the structure and its members is attainment of the load that would cause the structure to collapse, usually called the ultimate strength or the plastic collapse load. For an individual structural member this means that its plastic moment capacity has been reached. In most cases, due to the ductility of the material and the member, the ultimate strength of the entire structure will not have been reached at this stage. The less stressed members can take additional load until a sufficient number of members have exhausted their individual capacities so that no further redistribution or load sharing is possible. At the point where the structure can take no additional load, the structure is said to have collapsed. This load magnitude is called the collapse load and is associated with a particular collapse mechanism.

The collapse load for plastic design is the service load times a certain load factor. The limit state for a structure that is designed according to the principles of plastic design is therefore the attainment of a mechanism. For this to occur, all of the structural members must be able to develop the yield stress in all fibers at the most highly loaded locations.

There is a fine line of distinction between the load factor of PD and the safety factor of ASD. The former is the ratio between the plastic collapse load and the service or specified load for the structure as a whole, whereas the latter is an empirically developed, experience-based term that represents the relationship between the elastic strength of the elements of the structure and the various limiting conditions for those components. Although numerically close, the load factor of plastic design and the factor of safety of allowable stress design are not the same parameter.

1.12 STRUCTURAL SAFETY AND INTEGRITY

The preceding discussions of design philosophies indicate that although the basic goal of any design process is to ensure that the end product is a safe and reliable structure, the ways in which this is achieved may vary substantially.

In the past, the primary goal for safety was to provide an adequate margin against the consequences of overload. Load factor design and its offshoots were developed to take these considerations into account. In real life, however, many other factors also play a role. These include, but are not limited to the following:

1. Variations of material strength

2. 3. 4. 5. 6. 7.

These factcomponenthe fact tha

Thload variasolution, thand resistaevaluated own specifThis methfailure wilthis probab

In parameterssafety varithought ofmore unifoelement inHowever, varies undrather, theothers.

FoR, are assuby the beresistance mean valufull represe

Variati

Accura

Influen

Presenc

Lack of

Variati

tors consider nts. An even gat different tyhus, a methodability will bherefore, is toance factor dand the resufic factor in e

hod recognizell actually ocbility. No spe

ASD, the vs. They are luies with eachf as LRFD wform level of n an LRFD dea detailed an

der different loe goal is to s

or the developumed to each ll-shaped curis greater tha

ues, Qm and Rentation of th

ons of cross-s

acy of method

nce of workma

ce and variati

f member stra

ons of locatio

only some ofgreater sourceypes of load hd of design thbe burdened o deal with sadesign, wherlting safety m

each combinates that there icur. Howeve

ecific level of variabilities oumped togethh strength lim

with a single lreliability th

esign will be nalysis of relioad combinatsimply have a

pment of LRFhave a variabrves in Figur

an the load effRm, it would bhe data shows

sectional size

d of analysis

anship in sho

ion of residua

aightness

ons of load ap

f the sources e of variation

have different hat does not awith unaccou

afety as a probe the probabmargins detertion, and eachis always a fr, this methoprobability o

of load and her through thmit state but load factor. L

han ASD desthe same, regability under tions. In ASDa safe structu

FD, load effebility that canre 1.15. Strufect, R > Q. I

be relatively ean area wher

e and shape

op and field

al stresses

pplication poi

of variation on is the loadin

variational chattempt to incunted-for soubabilistic con

bilistic characrmined statisth material limfinite, though

od does not atof failure is gi

strength are he use of a si

does not vaLRFD designigns. That isgardless of ththe LRFD p

D there is no aure, although

ect (member fn be describeductures can bf it were appreasy to ensurere the two cur

Introdu

ints

of the strengtng, which is fharacteristics

corporate the urces of uncncept. This is cteristics of tically. Each

mit state is alsh very small, attempt to attaiven or implie

not treated ingle factor oary with loadns are generals, the probabihe type of loarovisions shoattempt to att

h some eleme

force), Q, andd by the normbe consideredropriate to coe a structure’rves overlap.

uction Chap

th of a structufurther comp

s. effects of str

certainty. Thethe foundatioload and strload type is

so given its owchance that

ach a specificed by the Spec

explicitly asof safety. Thed source. ASlly expected ility of failurd or load com

ows that reliatain uniform rents will be s

d resistance (mal distributiod safe as lon

oncentrate sols safety. How

pter 1 21

ure and its licated by

rength and e realistic on of load rength are s given its wn factor. structural

c value to cification. s separate e factor of D can be to have a

re of each mbination. ability still reliability; safer than

(strength), ons shown ng as the ely on the wever, the

2


Figure 1.1

Figure 1.1 This area occurrenceThe smalle

Anand load ecases wheris positiveleft of the value, (R –shown in Fof (R – Q)

A presented amore usefuof the resireliability and load emeans and

Thcarried outfactors, anwhereas re

Figure 1.1

ion

15 Probabili

16 Probabili

represents caes of failure. er the region onother approaffect. Figure re (R – Q) < 0, the structureorigin represe– Q)m, must bFigure 1.16 as. third represe

as ln(R/Q). Tul in the derivistance and lindex β. Unfeffect compo

d standard devhe statistical at by the appro

nd safety factoesistance facto

17 Probabili

ity Distributio

ity Distributio

ases where thSafety of theof overlap is, ach to presen1.16 shows th

0, the structure is considereents the probabe maintaineds βσ(R–Q), whe

entation of thThe logarithmi

vation of the load effect dafortunately, wonents. Thus, viations. analyses requopriate specifors have beenors and safety

ity Distributio

on, R and Q

on, (R – Q)

he load effecte structure is the lower the

nting the datahe same data re is said to haed safe. In thiability of failud at an approere β is the rel

he data is shic form of thefactors requi

ata, the probwe know the

we must rel

uired to establfication commn established. y factors for e

on, ln(R/Q)

t exceeds thea function o

e probability oa is to look a

as Figure 1.1ave failed, anis presentationure. To limit

opriate distancliability index

hown in Figue data is a weired in LRFD

bability of faiactual distribly on other c

lish an appromittees, and tLoad factors

each limit stat

e resistance af the size of of failure.

at the differen15 but presennd for all casen of the data,that probabilce from the ox and σ(R–Q) is

ure 1.17. In ell-conditioneD. If we knowilure can be butions for recharacteristic

opriate level othe resulting ls are presentedte are given in

and therefore this region o

nce between nts it as (R – Qes where this d, the shaded aity of failure,origin. This ds the standard

this case, thed representatw the exact di

directly relatelatively few s of the data

of reliability hload factors, d in the buildn the Specific

identifies of overlap.

resistance Q). For all difference area to the , the mean distance is d deviation

he data is tion and is istribution ted to the resistance

a, such as

have been resistance

ding codes cation.


Since the load combinations and resistance and safety factors have been established, the

reliability can be determined for specific design situations. The reliability index, β, is given in the Specification Commentary as

( )

2 2

ln m m

R Q

R Q

V Vβ =

+ (AISC C-B3-2)

where Rm is the mean resistance, Qm is the mean load effect, as discussed earlier, and VR and VQ are the coefficients of variation of resistance and load effect respectively. Design according to LRFD is given by

u nR R≤ φ (AISC B3-1) where the required strength, Ru is another term for the load effect, Q, Rn is the nominal strength and ϕ is the resistance factor. The reliability of design is determined when the required strength is exactly equal to the available strength. Thus, Equation B3-1 can be rewritten as nQ R= φ . The load effect will depend on the load combination being considered. Thus, for the LRFD live load plus dead load combination, written in terms of the live-to-dead load ratio, L/D,

( )( )1.2 1.6 1.2 1.6 nQ D L L D D R= + = + = φ (1) From Ravindra and Galambos1 the mean resistance is given by

m n m m mR R M F P= (2)

and the coefficient of variation of the resistance is given by

2 2 2R m F PV V V V= + + (3)

Mm is the mean of the ratio of the actual yield stress to the specified yield stress and VM is

the coefficient of variation; Fm is the mean of the ratio of the actual section property to the Manual value and VF is the coefficient of variation; and Pm is the mean of the ratio of the test specimen strength to the predicted strength using the Specification equations and the actual material and geometric properties and VP is the coefficient of variation.

Solving Equation 1 for Rn and substituting into Equation 2 yields

( )( )1.2 1.6m m m m

L D DR M F P

+=

φ (4)

Rearranging Equation 2 yields

mm m m

n

RM F PR

= (5)

1 Ravindra, M.K. and Galambos, T.V. (1978), “Load and Resistance Factor Design for Steel,” Journal of the Structural

Division, American Society of Civil Engineers, Vol. 104, No. ST9, September, pp. 1,337–1,353.


Thus, combining Equations 4 and 5 gives

( )( )1.2 1.6 mm

n

RR D L DR

⎛ ⎞= + ⎜ ⎟φ⎝ ⎠

(6)

From Ravindra and Galambos the mean load effect for dead load plus live load is

m m mQ D L= + (7)

which can, with some manipulation, be rewritten as

( )( )( )m m m m mQ D L D D L L L D D= + = + (8) They also give the coefficient of variation of the load effect which can be written as a

function of the live-to-dead load ratio as

( )( ) ( )( )( )2 2m D m L

Qm

D D V L L L D VV

Q

+= (9)

If Equations 6 and 8 are substituted into Equation C-B3-2 the reliability index, β, will be

given in terms of the live-to-dead load ratio, L D , and the resistance factor, ϕ. Thus,

( )( ) ( )( )2 2

1.2 1.61 ln m

n m mR Q

L DRR D D L L L DV V

⎡ ⎤⎛ ⎞+β = ⎢ ⎥⎜ ⎟⎜ ⎟φ ++ ⎢ ⎥⎝ ⎠⎣ ⎦

(10)

For LRFD, the live plus dead load combinations are 1.4D and (1.2D + 1.6L). The

effective dead load factor as a function of the live-to-dead-load ratio can be taken as

( )LRFD

1.4max

1.2 1.6i

iL D

⎡ ⎤γ = ⎢ ⎥+⎣ ⎦

(11)

and the mean load effect dead load multiplier as

( )( )im m m i

Q D D L L L D= + (12) Thus, Equation 10 can be generalized to address other LRFD load combinations as

follows: ( )

( ) ( )( )LRFD

2 2 2 2

1.2 1.61 1ln ln i

ii

m m

n m m n mR Q R Q

L DR RR D D L L L D R QV V V V

⎡ ⎤⎛ ⎞ ⎡ ⎤+ ⎛ ⎞γβ = =⎢ ⎥⎜ ⎟ ⎢ ⎥⎜ ⎟⎜ ⎟φ + φ+ +⎢ ⎥ ⎝ ⎠⎣ ⎦⎝ ⎠⎣ ⎦

(13)

where LRFDiγ is the effective LRFD load factor for the load combination under consideration, imQis the mean load effect multiplier for that load combination, and

iQV is the coefficient of variation of the load effect, all as a function of the varying load ratio as indicated by the subscript i.


To convert Equation 13 for use with ASD load combinations, LRFDiγ is replaced by ASDiγand nRφ is replaced by nR Ω . Thus, Equation 13 becomes

( )ASD

2 2

1 ln i

ii

m

n mR Q

RR QV V

⎡ ⎤⎛ ⎞γβ = ⎢ ⎥⎜ ⎟Ω⎢ ⎥+ ⎝ ⎠⎣ ⎦

(14)

Based on extensive studies for A992 steel (Bartlett et al.2) and the original work for the

development of the 1986 AISC Specification by Ravindra and Galambos, the following values can be used:

Mm = 1.055; VM = 0.058 Fm = 1.00; VF = 0.05 Pm = 1.02; VP = 0.06

Thus, ( )( )( )

( ) ( ) ( )2 2 2

1.055 1.00 1.02 1.076

0.058 0.05 0.06 0.097

m n n

R

R R R

V

= =

= + + =

Based on Galambos et al.3 (1982), the ratio of mean to code specified dead and live loads

can be taken as, for dead load Dm/D = 1.05; VD = 0.10

and for live load

Lm/L = 1.00; VL = 0.25

The values for imQ and imV will be functions of the live-to-dead load ratio. The reliability index, β, based on Equations 13 and 14, for a live-to-dead load ratio from

1.0 to 5.7 is presented in Figure 1.18 for a compact wide-flange beam under uniform moment for both LRFD and ASD. The figure is based on the load combination of live load plus dead load and statistical variations consistent with those used in the development of the Specification. It is seen that the reliability of design by LRFD is somewhat more uniform for this condition than design by ASD and that at a live to dead load ratio of approximately 3, the two approaches yield the same reliability. The higher the reliability index is, the safer the structure. Regardless of the numerical value of β, any structure that meets the requirements of the Specification will be sufficiently safe. A more detailed discussion of the statistical basis of steel design is available in Load and Resistance Factor Design of Steel Structures.4 Since the introduction of the 2005 AISC Specification, design by ASD and LRFD have essentially been equivalent and differ only by the effect of load combinations.

2 Bartlett, R.M., Dexter, R.J., Graeser, M.D., Jelinek, J.J., Schmidt, B.J. and Galambos, T.V. (2003), “Updating Standard

Shape Material Properties Database for Design and Reliability,” Engineering Journal, American Institute of Steel Construction, Vol. 40, No. 1, pp. 2–14.

3 Galambos, T.V., Ellingwood, B., MacGregor, J.G. and Cornell, C.A. (1982), “Probability-Based Load Criteria: Assessment of Current Design Practice,” Journal of the Structural Division, American Society of Civil Engineers, Vol. 108, No. ST5, May, pp. 959–977.

4 Geschwindner, L. F., Disque, R. O., and Bjorhovde, R. Load and Resistance Factor Design of Steel Structures. Englewood Cliffs, NJ: Prentice Hall, 1994.


Figure 1.18 Reliability Index vs Live-to-Dead Load Ratio for Compact Simply Supported Wide-Flange Beams with Uniform Moment

General structural integrity requires a continuous load path to the ground for resisting all

gravity and lateral loads that might be applied to the structure. With the introduction of the 2016 AISC Specification, provisions that address structural integrity beyond these general requirements have been introduced. The requirements in Section B3.9 are beyond normal strength requirements and are intended to improve the connectivity of the structure and thus the performance of the structure under undefined extraordinary events. These requirements apply only to a small set of structures where additional structural integrity is mandated.

1.13 LIMIT STATES

Regardless of the design approach, ASD or LRFD, or the period in history of the design’s execution, 1923 or 2018, all design is based on the ability of a structure or its elements to resist load. This ability is directly related to how an element carries that load and how it might be expected to fail, which is referred to as the element’s limit state. Each structural element can have multiple limit states, and the designer is required to determine which of these limit states will actually limit the structure’s strength.

There are two types of limit states to be considered: strength limit states and serviceability limit states. Strength limit states are those limiting conditions that, if exceeded, will lead to collapse of the structure or a portion of the structure, or to such serious deformations that the structure can no longer be expected to resist the applied load. Strength limit states are identified by the Specification, and guidance is provided for determination of the nominal strength, Rn, the safety factor, Ω, and the resistance factor, φ. Examples of the more common strength limit states found in the Specification are yielding, rupture, and buckling.

Serviceability limit states are not as well defined as strength limit states. If a serviceability limit state is exceeded, it usually means that the structure has reached some performance level that someone would find objectionable. The Specification addresses design for serviceability in Chapter L and defines serviceability in Section L1 as “a state in which the function of a building, its appearance, maintainability, durability, and the comfort of its occupants are preserved under typical usage.” Chapter L lists deflections, drift, vibration, wind-induced motion, thermal expansion and contraction, and connection slip as items to be considered, although no specific limitations are given for any of these limit states.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 2 4 6

ASDLRFD

Live-to-Dead Load Ratio, L/D

Rel

iabi

lity

Inde

x, β


Strength and serviceability limit states will be addressed throughout this book as appropriate for the elements or systems being considered.

1.14 BUILDING CODES AND DESIGN SPECIFICATIONS

The design of building structures is regulated by a number of official, legal documents that are known commonly as building codes. These cover all aspects of the design, construction, and operation of buildings and are not limited to just the structural design aspects.

The model code currently in use in the United States is the ICC International Building Code. Model codes are published by private organizations and have been adopted, in whole or in part, by state and local governments as the legal requirements for buildings within their area of jurisdiction. In addition to the model codes, cities and other governmental entities have written their own local building codes. Unfortunately, since the adoption of a building code is in great part a political activity, the regulations in use across the country are not uniform. A new International Building Code is published every 3 years but not adopted as quickly as issued. Thus, building codes with effective dates from 2003 to 2015 are still in use. In addition, governmental bodies will often adopt a model code with local amendments. Because of the technical nature of the AISC Specification, local amendments normally do not affect those aspects of steel design but they often do modify the loading definitions and thus do ultimately affect steel design.

To the structural engineer, the most important sections of a building code deal with the loads that must be used in the design, and the requirements pertaining to the use of specific structural materials. The load magnitudes are normally taken from Minimum Design Loads for Buildings and Other Structures, a national standard published by the American Society of Civil Engineers (Structural Engineering Institute) as ASCE/SEI 7. The loads presented in ASCE/SEI 7 may be altered by the model code authority or the local building authority upon adoption, although this practice adds complexity for designers who may be called upon to design structures in numerous locations under different political entities. Throughout this book, ASCE/SEI 7 will be referred to simply as ASCE 7 as it is most commonly referred to in the profession.

The AISC Specification is incorporated into the model building code by reference. The Specification, therefore, becomes part of the code, and thus part of the legal requirements of any locality where the model code is adopted. Locally written building codes also exist and the AISC Specification is normally adopted within those codes by reference also. Through these adoptions the AISC Specification becomes the legally binding standard by which all structural steel buildings must be designed. However, regardless of the Specification rules, it is always the responsibility of the engineer to ensure that their structure can carry the intended loads safely, without endangering the occupants.

1.15 INTEGRATED DESIGN PROJECT

This section introduces a building to be used in subsequent chapters of this book as an integrated design project. It is a relatively open-ended design project in that only a limited set of design parameters are set at this point. Several options will be presented in subsequent chapters so that the project can be tailored at the desire of the instructor.

The building is a four-story office building with one story below grade. It is located in Downers Grove, Illinois, at approximately 42°N latitude and 88°W longitude. This is a 102,000 ft2 building with approximately 25,500 ft2 per above-grade floor. For the first three floors, the floor-to-floor height is 13 ft 6 in. For the top floor, the floor-to-roof height is 14 ft 6 in. The below-grade floor-to-floor height is 15 ft 6 in. The façade is a lightweight metal curtain wall that extends 2.0 ft above the roof surface, and there is a 6.0 ft screen wall around the middle bay at the roof to conceal mechanical equipment and roof access. All steel will receive spray-applied fireproofing as necessary.

2


Ba

with bay ssouth direcin Figures floor framresisting sypair of one

Figure 1.1

Figure 1.2

Much of toanalysis anmodel of th1.24 showremoved.

ion

ased on prelimsizes of 30.0 ction, as show1.20 through

ming plan showystem consise-bay chevron

19 Schematic

20 Representa

oday’s structund design softhe given prels the results o

minary discusft in the east

wn in Figure h 1.22. To acws an openints of a pair o

n braced fram

Plan for Integ

ative Second-

ural analysis atware. Figureiminary fram

of the same co

ssions with tht-west directio1.19. Repres

ccommodate ang bounded bof three-bay m

mes in the nort

grated Design

Floor Framin

and design is 1.23 shows a

ming system deomputer mod

he architecturon and 45.0 fentative floora two-story a

by column linmoment framth-south direc

n Problem

ng Plan

accomplishedan example oeveloped usin

del with the gr

ral design teamft, 30.0 ft, anr and roof fraatrium on thenes A, C, 4, ames in the easction.

d through thef a complete ng RAM Struravity-only str

m, the designnd 45.0 ft in taming plans a first floor, thand 5. The last-west direct

e use of integrthree-dimens

uctural Systemructural elem

n will start the north-are shown he second ateral load tion and a

rated sional m. Figure ments

Figure 1.2

Figure 1.2

Figure 1.2System

21 Representa

22 Representa

23 Three-Dim

ative First-, T

ative Roof Fra

mensional Com

hird-, and Fou

aming Plan

mputer Model

urth-Floor Fr

l of Complete

Introdu

raming Plan

e Structure fro

uction Chap

om RAM Stru

pter 1 29

uctural

3

1 1p 2tf 3p

4c

30 Chapter 1

1.16 PROB

1. Where coprovisions of

2. What resothe determinafound in a bui

3. Which cprovides infor

a. gede

b. dec. ded. de

ane. re

en

4. In the AIScan one find:

a. thb. dec. di

std. de

1 Introduct

Figure 1.2from RAM

LEMS

ould one finthe 1961 AIS

ource would ation of propilding built in

chapter of rmation about

eneral requiresign esign of memesign of connesign of memnd tension equirements fnsure stability

ISC Steel Con

he AISC Specesign consideimensions anteel shapes esign of comp

ion

24 Three-DimM Structural S

nd informatioSC Specificati

be most likelperties of a sn 1954?

the AISC t:

rements for

mbers for flexunections mbers for com

for design of y

nstruction M

cification erations for bond properties

pression mem

mensional ComSystem

on about theion?

ly to assist insteel member

Specification

analysis and

ure

mbined forces

f structures to

Manual, where

olts for structural

mbers

mputer Mode

e

n r

n

d

s

o

e

l

5. Lteam 6. Afive struceach 7. Ptypeident

8. Wcombof m 9. Lsystecons 10. Iprim 11. P

el Showing O

List and definm for the desig

All structuresbasic struct

ctural compoh.

Provide an es of constrtify specific b

a. Bearb. Beamc. Longd. Highe. Gabl

What type bined proper

materials to res

List and desems commostruction.

In designing mary concern o

Provide a sim

nly Lateral L

ne the three bgn of any buil

s are compotural types.

onents and pr

example of eruction. To buildings in y

ring wall m-and-columng-span h-rise e-frame

of structurrties of two osist the applie

cribe two tyonly used

a steel structof the design

mple definition

Load Resisting

basic goals oflding.

sed of some List these frovide an ex

each of the the extent

your own loca

n

ral system or more differed loads?

ypes of laterain beam-an

ture, what muengineer?

n of structura

g Systems

f a design

or all of five basic xample of

following possible,

ale.

uses the rent types

al bracing nd-column

ust be the

al design.


12. Describe the difference between a strength limit state of a structure and a serviceability limit state. 13. Give a description of both the LRFD and ASD design approaches. What is the fundamental difference between the methods? 14. Provide a brief description of plastic design (PD).

15. Identify three sources of variation in the strength of a structure and its components. 16. Provide three examples of strength limit states. 17. Provide three examples of serviceability limit states.

Documents

Chapter 1 3ed 4th Draft - Home | College of Engineering ...web.engr.oregonstate.edu/~liujud/UDSS_3ed_Chp1draft.pdf · C I 1 1 hapte ntrod.1 SCOPE.2 THE S r 1 uctio ... d Retrofit