Unified design of steel structures

460 Pages·2008·29.008 MB·English

by  Geschwindner, Louis F.

Unified Design
of Steel Structures
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WILLIAM J. PESCE PETER BOOTH WILEY
PRESIDENT AND CHIEF' EXECUTIVE OFFICER CHAIRMAN CF' THE BOARD
Unified Design
of Steel Structures
Louis F. Geschwindner
Vice President of Engineering and Research
American Institute of Steel Construction
and
Professor Emeritus ofA rchitectural Engineering
The Pennsylvania State University
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Preface
INTENDED AUDIENCE
This book presents the design of steel building structures based on the 2005 unified spec
ification, ANSI/AISC 360-05 Specification for Structural Steel Buildings. It is intended
primarily as a text for a first course in steel design for civil and architectural engineers.
Such a course usually occurs in the third or fourth year of an engineering 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 an undergraduate text, it provides a good
review for practicing engineers looking to learn the provisions of the unified specification
and to convert their practice from either of the old specifications to the new specification.
Users are expected to have a firm knowledge of statics and strength of materials and have
easy access to the AISC Steel Construction Manual, 13th Edition.
UNIFIED ASD AND LRFD
A preferred approach to the design of steel structures has been elusive over the last
20 years. In 1986, the American Institute of Steel Construction (AISC) issued its first
Load and Resistance Factor Design (LRFD) Specification for Structural Steel Buildings.
This specification came after almost 50 years of publication of an Allowable Stress Design
(ASD) specification. Unfortunately, LRFD was accepted by the academic community but
not by the professional engineering community. Although AISC revised the format of the
ASD specification in 1989, it had not updated its provisions for over 25 years. This use
of two specifications was seen as an undesirable situation by the professions and in 2001
AISC began the development of a combined ASD and LRFD specification. In 2005, AISC
published its first unified specification, combining the provisions of both the LRFD and
ASD specifications into a single standard for the design of steel building structures. This
new specification, ANSI/AISC 360-05 Specification for Structural Steel Buildings, reflects
a major change in philosophy by AISC, one that makes the use of ASD and LRFD equally
acceptable approaches for the design of steel buildings.
The reader familiar with past editions of the ASD and LRFD specifications will un
doubtedly question how these two diverse design philosophies can be effectively combined
into one specification. This is a reasonable question to ask. The primary answer is that this
specification is not a combination of the old ASD and LRFD provisions. It is a new approach
with a new ASD that uses the same strength equations as the new LRFD. A combination
of the old ASD provisions with the old LRFD provisions could lead, in some cases, to a
design wherein an element is treated as behaving elastically for ASD design and plastically
for LRFD design. The unified specification takes a different approach. It is based on the
understanding 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 as a 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 ASD or LRFD. This
important aspect of the unified specification is further explained in Chapter 1.
v
vi Preface
CHANGES IN BUILDING LOADS
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. The Amer
ican Society of Civil Engineers (ASCE) is continually revising ASCE-7 Minimum Design
Loads for Buildings and Other Structures, its standard for building loads. The International
Code Council (ICC) has issued its International Building Code (IBC), and the National
Fire Protection Association (NFPA) has issued its model building code (NFPA 5000). The
major changes brought about by these new standards are the inclusion of requirements
for consideration of seismic loading, which now applies to almost the entire country. In
response to the expansion of the requirements for seismic design, AISC issued ANSI/ AISC
341-05 Seismic Provisions for Structural Steel Buildings, a standard to guide the design of
steel building structures to resist seismic loads. For the calculation of loads within this text,
ASCE 7-05 provisions are used. For any actual design, the designer must use the loadings
established by the governing building code. The AISC seismic provisions are discussed in
Chapter 13.
UNITS
ANSI/A ISC 360-05 is, as much as possible, a unitless specification. In those rare instances
where equations could not be written in a unitless form, two equations are given, one in
U.S. customary units and one in SI units. The Manual presents all of its material in U.S.
customary units. The construction industry in this country has not adopted SI units in any
visible way, and it is not clear that they will in the foreseeable future. Thus, this book uses
only U.S. customary units.
TOPICAL ORGANIZATION
Chapters 1 through 3 present the general material applicable to all steel structures. This
is followed in Chapters 4 through 9 with a presentation of member design. Chapters 10
through 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 which all
steel design is based. It shows how these principles are incorporated into both LRFD and
ASD approaches. Chapter 2 introduces the development of load factors, resistance factors,
and safety factors. It discusses load combinations and compares the calculation of required
strength for both LRFD and ASD. Chapter 3 discusses steel as a structural material. It
describes the availability of steel in a variety of shapes and the grades of steel available for
construction.
Once the foundation for steel design is established, the various member types are con
sidered. Tension members are addressed in Chapter 4, compression members in Chapter 5,
and bending members in Chapter 6. Chapter 7 covers plate girders, which are simply bend
ing members made from individual plates. Chapter 8 treats members subjected to combined
axial load and bending as well as design of bracing. Chapter 9 deals with composite mem
bers, that is, members composed of both steel and concrete working together to provide the
available strength. Each of these chapters begins with a discussion of that particular member
type and how it is used in buildings. This is followed by a discussion of the specification
provisions and the behavior from which those provisions have been derived. The LRFD
and ASD design philosophies of the 2005 specification are used throughout. Design exam
ples that use the specification provisions directly are provided along with examples using
Preface vii
the variety of design aids available in the AISC Steel Construction Manual. All examples
that have an LRFD and ASD component are provided for both approaches. Throughout this
book., ASO examples, or portions of examples that address the ASD approach, are presented
with shaded background for ease of identification.
The member-oriented chapters are followed by chapters addressing connection design.
Chapter I 0 introduces the variety of potential connection types and discusses the strength
of bolts, welds, and connecting elements. Chapter 11 addresses simple connections. This
includes simple beam shear connections and light bracing connections. Chapter 12 deals
with moment-resisting connections. As witb the member-oriented chapters, the basic prin
ciples of limit states design are developed first. This is followed by the application of the
provisions to simple shear connections and beam-to-column moment connections through
extensive examples in both LRFD and ASD.
The text concludes in Chapter 13 with an introduction to steel systems for seismic force
resistance. It discusses the variety of structural framing systems available and approved for
inclusion in the seismic force resisting system.
EXAMPLES AND HOMEWORK PROBLEMS IN LRFD AND ASD
The LRFD and ASD design philosophies of the 2005 specification are used throughout.
Design examples that use the specification provisions directly are provided along with ex
amples using the variety of design aids available in the AJSC Steel Construction Manual.
All examples that have an LRFD and ASD component are provided for both approaches.
Throughout this book, ASD examples, or portions of examples that address the ASD ap
proach, are presented with shaded background for ease of identification.
GOAL: Select a double-angle tension member for use as a web member in a truss and determine the
maximum area reduction that would be permined for holes and shear lag.
GIVEN: The member must carry an ASD required strength, P = 270 kips. Use equal leg angles of
0
A36stcel.
Step I: Detennine the minimum required gross area b~ on the limit state of yielding
=
A« ml•= 270/(36/1.67) 12.5 in.2
Step 2: Based on this minimum gross area, from Manual Table 1-15. select
~L6x6x9h6withA#= 12.9 in.2
Each chapter includes homework problems at the end of the chapter. These problems
are organized to follow the order of presentation of the material in the chapters. Several
problems are provided for each general subject. Problems are provided for both LRFD and
ASD solutions. There are also problems designed to show comparisons between ASD and
LRFD solutions. These problems show that in some instances one method might give a
more economical design, whereas in other instances the reverse is true.
viii Preface
WEBSITE
The following resources are available from the book website at www.wiley.com/
college/geschwindner. Visit the Student section of the website.
• Answers Selected homework problem answers are available on the student section
of the website.
• Errata We have reviewed the text to make sure that it is as error-free as possible.
However, if any errors are discovered, they will be listed on the book website as a
reference.
• If you encounter any errors as you are using the book, please send them directly to
the author ([email protected]) so we may include them on the website, and correct these
errors in future editions.
RESOURCES FOR INSTRUCTORS
All resources for instructors are available on the Instructor section of the website at
www.wiley.com/college/geschwindner.
The following resources are available only to instructors who adopt the text:
• Solutions Manual: Solutions for all homework problems in the text.
• Image Gallery of Text Figures
• Text Figures in PowerPoint format
Visit the Instructor section of the website at www.wiley.com/college/geschwindner to reg
ister and request access to these resources.
ACKNOWLEDGEMENTS
I would like to thank all of my former students for their interactions over the years and the
influence they had on the development of my approach to teaching. In particular I would like
to thank Chris Crilly and Andy Kauffman for their assistance in reviewing the manuscript,
checking calculations, and assistance with the figures. I would like to thank Charles Carter
of AISC, a former student and valued colleague, for his authorship of Chapter 13. A special
note of thanks is due Larry Kruth of Douglas Steel Fabricating Corporation for his review
and assistance with figures in Chapters 10 through 12. I also want to thank those who
reviewed the draft manuscripts for their valuable suggestions and those faculty members
who have chosen to class test the draft of this text prior to the actual publication of the
work.
REVIEWERS
Sonya Cooper, New Mexico State University
Jose Gomez, Virginia Transportation Research Council
Jeffery A. Laman, Penn State University
Dr. Craig C. Menzemer, The University of Akron
Levon Minnetyan, Clarkson University
Candace S. Sulzbach, Colorado School of Mines
Preface ix
CLASS TESTERS
Dr. Chris Tuan, University of Nebraska at Omaha; Catherine Frend, University of Min
nesota; David G Pollock, Washington State University; Kelly Salyards, Bucknell University;
P. K. Saha, Alabama A&M University; Marc Leviton, Louisiana State University; Chai H.
Yoo, Auburn University; Dr. Anil Patnaik, South Dakota School of Mines and Technology;
Bozidar Stjadinovic, University of California; Dimitris C. Rizos, University of South
Carolina; Chia-Ming Uang, University of California, San Diego.
Finally, I want to thank my wife, Judy, for her understanding and that not-so-subtle
nudge when it was really needed. Her continued support has permitted me to complete this
project.
Louis F. Geschwindner
State College, Pennsylvania