UnifiedDesignofSteel Structures 4thEd.
LouisF.Geschwindner
FormerVicePresident ofEngineeringandResearch AmericanInstituteofSteelConstruction;ProfessorEmeritus ofArchitecturalEngineering ThePennsylvania State University;andSeniorConsultant ProvidenceEngineering
JudyLiu
Professor School ofCiviland Construction Engineering.Oregon State University
CharlesJ.Carter
President AmericanInstitute of SteelConstruction
Dedication
This book is dedicated to our mentors teachers and former students each of whom taught us something and in the process contributed to who we are today.
Copyright C first and second editions 2008 2012 John Wiley & Sons Inc. third edition 2017 be reproduced stored in a retrieval system or transmitted in any form or by any means fourth edition 2023 Louis F. Geschwindner. All rights reserved. No part of this publication mayelectronic mechanical photocopying recording scanning or otherwise except as permittedunder Sections 107 or 108 of the 1976 United States Copyright Act without prior writtenpermission of the copyright holder.
Anyone making use of the information presented in this publication assumes all liability arisingfrom such use.
Cover photo: University of Rhode Island Fascitelli Center for Advanced Engineering Photo courtesy of Ballinger Philadelphia PA O Albert Vecerka/Esto
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Preface
INTENDED AUDIENCE
This book presents the basics of design of steel building structures and is based on the2022 unified specification ANSI/AISC 360-22 Specification for Structural Steel Buildings. It is intended primarily as a text for a first course in steel design for civil andarchitectural engincers. Such a course usually occurs in the third or fourth year of ancngineering program. The book can also be used in a second building-oriented course in steel design depending on the coverage in the first course. In addition to its use as aprovisions of the latest specification or to convert their practice from any of the old specifications to the new specification. Users are expected to have a firm knowledge ofstatics and strength of materials and have casy access to the AISC Steel Construction Manual 16" Edition.
UNIFIEDASDANDLRFD
A preferred approach to the design of steel structures had been elusive over a 20-yearperiod from 1986 to 2005.In 1986 the American Institute of Steel Construction (AISC) issued its first Load and Resistance Factor Design (LRFD) Specification for StructuralSteel Buildings. This specification came after almost 50 years of publication of anAllowable Stress Design (ASD) specification. Unfortunately LRFD was accepted by the academic munity but not by the professional engineering munity. AlthoughAISC revised the format of the ASD specification in 1989 it had not updated itsprovisions for over 25 years. This use of two specifications was seen as an undesirable situation by the professions related to the building industry and in 2001 AISC began thefirst unified specification bining the provisions of both the LRFD and ASD development of a bined ASD and LRFD specification. In 2005 AISC published itsspecifications into a single standard for the design of steel building structures. Thatspecification ANSI/AISC 360-05 Specification for Structural Steel Buildings reflected a major change in philosophy by AISC one that made the use of ASD and LRFD equallyacceptable as approaches for the design of steel buildings. That has been the case for thepast 17 years. Now the newest specification ANSI/AISC 360-22 Specification for Structural Steel Buildings continues that philosophy of equal status for design by ASDand LRFD.
The reader familiar with past editions of the separate ASD and LRFDhow these two diverse design philosophies could be effectively bined into one specification. This is a reasonable question to ask. The primary answer is that the 2005approach with a new ASD and a new LRFD that used the best of both previous specification was not a bination of the old ASD and LRFD provisions. It was a newapproaches. The first unified specification took a different approach. It was based on theunderstanding that the strength of an element or structure called the nominal strength in
the specification can be determined independent of the design philosophy. Once that nominal strength is determined the available strength for ASD or LRFD is determined asa function of that nominal strength. Thus the available strength of the element is always based on the same behavior and no inconsistency in behavior results from the use of ASDor LRFD. This important aspect of the unified specification is further explained inChapter 1.
The 2022 Specijfication shows AISC's mitment to maintaining both designapproaches. With only one strength equation for both methods any updates are appliedcqually to the ASD and LRFD provisions. The designer continues to be given the opportunity to apply engineering judgment in deciding what approach to use for design for Structural Steel Buildings the standard that guides the design of steel building either by ASD or LRFD. AISC has also issued ANSI/AISC 341-22 Seismic Provisionsstructures to resist seismic loads.
CHANGESINBUILDINGLOADS
In addition to the provisions for steel design issued by AISC structural engineering has seen many changes in the area of loads for which buildings must be designed. TheAmerican Society of Civil Engineers (ASCE) is continually revising ASCE 7 MinimumDesign Loads and Associated Criteria for Buildings and Other Structures its standard for building loads. The International Code Council (ICC) continues to issue itsInternational Building Code (IBC) on a thre-year cycle and it in turm is adopted by alocal jurisdiction on the schedule chosen by that jurisdiction. The IBC adopts ASCE 7 ANSI/AISC 360 ANSI/AISC 341 and other structural standards. For the calculation ofloads within this text ASCE 7-22 provisions are used. For any actual design the designer must use the loadings established by the goverming building code. ANSI/AISC 341 isdiscussed in Chapter 13.
UNITS
instances where cquations could not be written in a unitless form two cquations are ANSI/AISC 360-22 is as much as possible a unitless specification. In those raregiven one in U.S. customary units and one in SI units. The Marual presents all of its material in U.S. customary units. The construction industry in this country has notadopted SI units in any visible way and it is not clear that they will in the foreseeablefuture. Thus this book uses only U.S. customary units.
TOPICALORGANIZATION
followed in Chapters 4 through 9 with a presentation of member design. Chapters 10through 12 discuss connections and Chapter 13 provides an introduction to seismic design.
In Chapter 1 the text addresses the principles of limit states design upon whichand ASD approaches and shows how reliability varies with live-to-dead load ratio for all steel design is based. It shows how these principles are incorporated into both LRFD
both approaches. Chapter 1 also provides a description of ANSI/AISC 360-22 the Steel Construction Mamual and the AISC web site. These are the major source documents forall that is presented in this book. Chapter 2 introduces the development of load factors resistance factors and safety factors. It discusses load binations and pares the o s e go LRFD and the resulting system reliability is also addressed. Chapter 3 discusses steel as a structural material. It describes the availability of steel in a variety of shapes and thegrades of steel available for construction.
Once thefoundation for steel design is established the various member tpes areconsidered. Tension members are addressed in Chapter 4 pression members inChapter 5 and bending members in Chapter 6. Chapter 7 covers plate girders which aresimply bending members made from individual plates. Chapter 8 treats members subjected to bined axial load and bending as well as design of bracing. Chapter 9deals with posite members that is members posed of both steel and concrete working together to provide the available strength. Each of these chapters begins with adiscussion of that particular member type and how it is used in buildings. This isfollowed by a discussion of the specification provisions and the bchavior from which those provisions have been derived. The LRFD and ASD design philosophies of the 2022Specification are used throughout. Design examples that use the specification provisionsdirectly are provided along with examples using the variety of design aids available in the AISC Steel Construction Manual. All examples that have an LRFD and ASD ponentare provided for both approaches. Throughout this book ASD examples or portions of cxamples that address the ASD approach are presented with shaded background for caseof identification.
design. Chapter 10 introduces the variety of potential connection types and discusses the The member-oriented chapters are followed by chapters addressing connectionstrength of bolts welds and connecting elements. Chapter 11 addresses simpleconnections. This includes simple beam shear connections light bracing connections and direct bearing connections. Chapter 12 deals with moment-resisting connections. As withfirst. This is followed by the application of the provisions to simple shear connections the member-oriented chapters the basic principles of limit states design are developedand beam-to-column moment connections through extensive examples in both LRFD andASD.
The text concludes in Chapter 13 with an introduction to steel systems forseismic force resistance. It discusses the variety of structural framing systems availableand approved for inclusion in the seismic force resisting system.
NEWTOTHISEDITION
This fourth edition is based on ANSI/AISC 360-22 Specification for Structural Steeldeveloping the specification was charged by its chair to develop a new specification that Buildings the 2022 edition of the AISC specification. The mittee responsible forwould show minimal changes. Thus changes in the specification upon which this book isbased are limited to those areas where the mittee felt important knowledge had been gained since the 2016 edition. As a result the changes in this book related to thespecification provisions are also somewhat limited. Other changes have bcen
