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Preface

The objective of the third edition is the same as the first two editions, that is, to provide the student or practitioner a meaningful introduction to the design of medium-and short-span girder bridges. However, the manner in which the material is presented has changed. Instead of the eight chapters of the second edition, the content has been spread out over twenty shorter chapters. This organization should lead to easier reading and simpler organization of classroom assignments.

To help understand how these changes have come about, it is informative to see how the process all started. It was in August 1990 that the two authors were at an International Conference on Short and Medium Span Bridges in Toronto, Canada, where both were presenting papers. They had often met at these bridge conferences and were familiar with each other's work—Puckett's on analysis and software development and Barker's fundamental application of LRFD to geotechnical materials. Both were classroom teachers in structural engineering.

At the time, a number of major changes were taking place in the design of highway bridges. Philosophically the most dramatic was the change from a deterministic (allowable stress) design approach to a probabilistic (limit state) design concepts. The other big change was a government edict that highway bridges that were built with federal dollars had to be constructed and designed in the metric system starting in 1997.

The timing was right for a comprehensive textbook on the design of highway bridges. The American Association of State Highway and Transportation Officials (AASHTO) were in the midst of a complete rewriting of their Bridge Design Specifications in a LRFD format. Finite-element analysis tools had matured, truck loads were better understood through weigh-in-motion studies, material behavior was being unified for prestressed and non-prestressed concrete by the American Concrete Institute (ACI), post-buckling strength of plate girder webs and fatigue strength of weld details were better understood.

The two professors decided that someone needed to write a textbook to present these changes to students and practicing civil engineers. So over dinner and a major league baseball game, they realized they could be the ones to do the writing. Puckett took his sabbatical with Barker at Virginia Tech in 1993, they wrote trial chapters, prepared a proposal that was accepted by John Wiley & Sons, and the first edition with ten chapters was published in 1997.

It was not long before the metric system requirement was dropped and the highway bridge designers needed a textbook written in U.S. Customary Units. Therefore, it became necessary to make revisions and to prepare a second edition of the book. Besides the units change, the LRFD specifications were in their third edition and the textbook needed to be updated. As new material was added, the number of pages was deemed too large and two chapters were dropped—Wood Bridges and Substructure Design. These two topics are found only in the metric system units of the first edition.

The remaining eight chapters of the second edition have been divided into four parts: General Aspects of Bridge Design (Chapters 1–7), Loads and Analysis (Chapters 8–12), Concrete Bridges (Chapters 13–16), and Steel Bridges (Chapters 17–20). Another change in the layout of the third edition is the addition of an insert of mainly color bridge photos. These photos have been selected to illustrate bridges of historical significance; the ones most aesthetically pleasing that are most beautiful in their surroundings, and noteworthy as the longest, tallest, or highest bridges of their type.

We suggest that a first course in bridges be based on Chapters 1–7 with Chapters 5, 6, and 7 compulsory reading. Loads and analysis should follow with required reading in Chapter 8 and selected portions of Chapter 9 and 10 depending upon the students' background and instructor's interest.

Design can be addressed with either the chapters on concrete (Chapters 13–16) or those on steel (Chapters 17–20). Instructor guidance is required to lead the student through these chapters and to address the topics of most interest. For example, concrete bridges could be addressed with nonprestressed bridges which would simplify the topic. However, teaching prestressed concrete within a bridge context could be an excellent way for students to gain broad-based knowledge in this area for both bridges and buildings. Similarly, teaching design using the steel chapter leads to a general knowledge of composite cross sections, staged construction, and plate girders. As the associated principles are common with buildings and bridges, again the bridge course can be used within a broader context.

How much of the material to present to a particular class is at the discretion of the professor, who is the best person to judge the background and maturity of the students. There is enough material in the book for more than one course in highway bridge design.

Practitioners who are entry level engineers will find the background material in Chapters 1–12 helpful in their new assignments and can use Chapters 13–16 and 17–20 for specific guidance on design of a particular bridge type. The same can be said for seasoned professionals, even though they would be familiar with the material in the loads chapter, they should find the other chapters of interest in providing background and design examples based on the AASHTO LRFD specifications.

Finally, those practitioners who just appreciate bridge history and aesthetics might find those chapters of interest from a personal enjoyment perspective. Bridges are art and so many are simply beautiful.

Acknowledgments

We would like to recognize those who have made the production of the third edition possible. The first person to be acknowledged is the editorial assistant at John Wiley & Sons who prepared a twenty chapter manuscript from the contents of the eight chapters of the second edition. This reorganized manuscript became the working document that the authors could edit and assign correct numbers to equations, figures, and tables.

To accompany the description of the I-35W Bridge collapse, the new figures drafted by Philip Jennings, a structural engineering graduate student at Virginia Tech, are gratefully acknowledged. Thanks also to the following state departments of transportation who supplied photographs of their bridges: Arizona, Colorado, Washington State, and West Virginia. The authors appreciate the computer modeling and project photos provided by Julie Smith of the FIGG Engineering Group.

The patience, understanding, and support shown us by Jim Harper, Bob Argentieri, Dan Magers, and Bob Hilbert at John Wiley & Sons, especially during the time of the senior author's health issues, are greatly appreciated.

Finally, we wish to thank Marilyn Barker and Kathy Puckett for their continued patience and strong support during our time of writing.

The authors would appreciate it that if the reader should have questions or if errors are found you would contact us at puckett@uwyo.edu.

Personal Acknowledgment to Richard Barker

I wish to recognize and thank Rich for his career of achievement in teaching, learning, research, and practice in bridge engineering, and most of all sharing it with me. Rich has made a tremendous difference to the professional lives of so many students and colleagues. I will be forever grateful for his friendship, guidance, selfless and thoughtful approach from which I have benefitted and learned so very much.

Rich was a professional in every sense of the term.

Happy trails, Rich.

Jay Puckett

Laramie, Wyoming

Preface to the Second Edition

This book has the same intent as the first edition and is written for senior-level undergraduate or first-year graduate students in civil engineering. It is also written for practicing civil engineers who have an interest in the design of highway bridges. The objective is to provide the reader a meaningful introduction to the design of medium- and short-span girder bridges. This objective is achieved by providing fundamental theory and behavior, background on the development of the specifications, procedures for design, and design examples.

This book is based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, Third Edition, and Customary U.S. units are used throughout. The general approach is to present theory and behavior upon which a provision of the specifications is based, followed by appropriate procedures, either presented explicitly or in examples. The examples focus on the procedures involved for a particular structural material and give reference to the appropriate article in the specifications. It is, therefore, suggested that the reader have available a copy of the most recent edition of the AASHTO LRFD Bridge Design Specifications.

The scope is limited to a thorough treatment of medium- and short-span girder bridges with a maximum span length of about 250 ft. These bridge structures comprise approximately 80% of the U.S. bridge inventory and are the most common bridges designed by practitioners. Their design illustrates the basic principles used for the design of longer spans. Structure types included in this book are built of concrete and steel. Concrete cast-in-place slab, T-beam, and box-girder bridges and precast–prestressed systems are considered. Rolled steel beam and plate girder systems that are composite and noncomposite are included.

Civil engineers are identified as primary users of this book because their formal education includes topics important to a highway bridge designer. These topics include studies in transportation systems, hydrodynamics of streams and channels, geotechnical engineering, construction management, environmental engineering, structural analysis and design, life-cycle costing, material testing, quality control, professional and legal problems, and the people issues associated with public construction projects. This reference to civil engineers is not meant to exclude others from utilizing this book. However, the reader is expected to have one undergraduate course in structural design for each structural material considered. For example, if only the design of steel bridges is of interest, then the reader should have at least one course in structural analysis and one course in structural steel design.

Chapter 1 introduces the topic of bridge engineering with a brief history of bridge building and the development of bridge specifications in the United States. Added to the second edition is an expanded treatment of bridge failure case histories that brought about changes in the bridge design specifications. Chapter 2 emphasizes the need to consider aesthetics from the beginning of the design process and gives examples of successful bridge projects. Added to the second edition are a discussion of integral abutment bridges and a section on the use of computer modeling in planning and design. Chapter 3 presents the basics on load and resistance factor design (LRFD) and indicates how these factors are chosen to obtain a desirable margin of safety. Included at the end of all the chapters in the second edition are problems that can be used as student exercises or homework assignments.

Chapter 4 describes the nature, magnitude, and placement of the various loads that act on a bridge structure. Chapter 5 presents influence function techniques for determining maximum and minimum force effects due to moving vehicle loads. Chapter 6 considers the entire bridge structure as a system and how it should be analyzed to obtain a realistic distribution of forces.

Chapters 7 and 8 are the design chapters for concrete and steel bridges. Both chapters have been significantly revised to accommodate the trend toward U.S. customary units within the United States and away from SI. New to the second edition of the concrete bridge design chapter are discussions of high-performance concrete and control of flexural cracking, changes to the calculation of creep and shrinkage and its influence on prestress losses, and prediction of stress in unbonded tendons at ultimate.

Chapter 8 includes a major reorganization and rewrite of content based upon the new specifications whereby Articles 6.10 and 6.11 were completely rewritten by AASHTO. This specification rewrite is a significant simplification in the specifications from the previous editions/interims; however, the use of these articles is not simple, and hopefully Chapter 8 provides helpful guidance.

The organization of the design chapters is similar. A description of material properties is given first, followed by general design considerations. Then a discussion is given of the behavior and theory behind the member resistance expressions for the various limit states. Detailed design examples that illustrate the LRFD specification provisions conclude each chapter.

We suggest that a first course in bridges be based on Chapters 1–6, either Sections 7.1–7.6, 7.10.1, and 7.10.3 of Chapter 7 or Sections 8.1–8.37, 8.45–8.10, and 8.11.2. It is assumed that some of this material will have been addressed in prerequisite courses and can be referred to only as a reading assignment. How much of the material to present to a particular class is at the discretion of the professor, who is probably the best person to judge the background and maturity of the students. There is enough material in the book for more than one course in highway bridge design.

Practitioners who are entry-level engineers will find the background material in Chapters 1–6 helpful in their new assignments and can use Chapters 7 and 8 for specific guidance on design of a particular bridge type. The same can be said for seasoned professionals, even though they would be familiar with the material in the loads chapter, they should find the other chapters of interest in providing background and design examples based on the AASHTO LRFD specifications.

Acknowledgments

In addition to the acknowledgements of those who contributed to the writing of the first edition, we would like to recognize those who have helped make this second edition possible. Since the publication of the first edition in 1997, we have received numerous emails and personal communications from students and practitioners asking questions, pointing out mistakes, making suggestions, and encouraging us to revise the book. We thank this group for their feedback and for making it clear that a revision of the book in Customary U.S. units was necessary.

We wish to acknowledge those who have contributed directly to the production of the book. The most important person in this regard was Kerri Puckett, civil engineering student at the University of Wyoming, who changed the units on all figures to Customary U.S., drafted new figures, catalogued the figures and photos, performed clerical duties, and generally kept the authors on track. Also assisting in the conversion of units was H. R. (Trey) Hamilton from the University of Florida who reworked design examples from the first edition in Customary U.S. units.

We also appreciate the contributions of friends in the bridge engineering community. Colleagues at Virginia Tech providing background material were Carin Roberts-Wollmann on unbonded tendons and Tommy Cousins on prestress losses. Thanks to John Kulicki of Modjeski & Masters for his continuing leadership in the development of the LRFD Specifications and Dennis Mertz of the University of Delaware for responding to questions on the rationale of the specifications. The authors appreciate the computer modeling and project photos provided by Linda Figg, Cheryl Maze, and Amy Kohls Buehler of Figg Engineers.

The patience and understanding shown us by Jim Harper and Bob Hilbert at John Wiley & Sons is gratefully acknowledged.

Finally we wish to thank Marilyn Barker and Kathy Puckett for their patience and strong support during our time writing.

The authors would appreciate it if the reader should have questions or if errors are found that they be contacted at marichba@aol.com or puckett@uwyo.edu.

Preface to the First Edition

This book is written for senior level undergraduate or first year graduate students in civil engineering and for practicing civil engineers who have an interest in the design of highway bridges. The object of this book is to provide the student or practitioner a meaningful introduction to the design of medium- and short-span girder bridges. This objective is achieved by providing fundamental theory and behavior, background on the development of the specifications, procedures for design, and design examples.

This book is based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications and System International (SI) units are used throughout. The general approach is to present theory and behavior upon which a provision of the specifications is based, followed by appropriate procedures, either presented explicitly or in examples. The examples focus on the procedures involved for a particular structural material and give reference to the appropriate article in the specifications. It is, therefore, essential that the reader have available a copy of the most recent edition of the AASHTO LRFD Bridge Design Specifications in SI units. (For those who have access to the World Wide Web, addendums to the specifications can be found at http://www2.epix.net/~modjeski.)

The scope of this book is limited to a thorough treatment of medium- and short-span girder bridges with a maximum span length of about 60 m. These bridge structures comprise approximately 80% of the U.S. bridge inventory and are the most common bridges designed by practitioners, illustrating the basic principles found in bridges of longer spans. Structure types included in this book are built of concrete, steel, and wood. Concrete cast-in-place slab, T-beam, and box-girder bridges and precast–prestressed systems are considered. Rolled steel beam and plate girder systems that are composite and non-composite are included, as well as wood systems. This book concludes with a chapter on substructure design, which is a common component for all the bridge types.

Civil engineers are identified as primary users of this book because their formal education includes topics important to a highway bridge designer. These topics include studies in transportation systems, hydrodynamics of streams and channels, geotechnical engineering, construction management, environmental engineering, structural analysis and design, life-cycle costing, material testing, quality control, professional and legal problems, and the people issues associated with public construction projects. This reference to civil engineers is not meant to exclude others from utilizing this book. However, the reader is expected to have one undergraduate course in structural design for each structural material considered. For example, if only the design of steel bridges is of interest, then the reader should have at least one course in structural analysis and one course in structural steel design.

Chapter 1 introduces the topic of bridge engineering with a brief history of bridge building and the development of bridge specifications in the United States. Chapter 2 emphasizes the need to consider aesthetics from the beginning of the design process and gives examples of successful bridge projects. Chapter 3 presents the basics on load and resistance factor design (LRFD) and indicates how these factors are chosen to obtain a desirable margin of safety.

Chapter 4 describes the nature, magnitude, and placement of the various loads that act on a bridge structure. Chapter 5 presents influence function techniques for determining maximum and minimum force effects due to moving vehicle loads. Chapter 6 considers the entire bridge structure as a system and how it should be analyzed to obtain a realistic distribution of forces.

Chapters 7–9 are the design chapters for concrete, steel, and wood bridges. The organization of these three chapters is similar. A description of material properties is given first, followed by general design considerations. Then a discussion of the behavior and theory behind the member resistance expressions for the various limit states, and concluding with detailed design examples that illustrate the LRFD specification provisions.

Chapter 10 on substructure design completes the book. It includes general design considerations, an elastomeric bearing design example, and a stability analysis to check the geotechnical limit states for a typical abutment.

We suggest that a first course in bridges be based on Chapters 1–6, either Articles 7.1–7.6, 7.10.1, and 7.10.3 of Chapter 7 or Articles 8.1–8.4, 8.6–8.10, and 8.11.2, and conclude with Articles 10.1–10.3 of Chapter 10. It is assumed that some of this material will have been covered in prerequisite courses and can be referred to only as a reading assignment. How much of the material to present to a particular class is at the discretion of the professor, who is probably the best person to judge the background and maturity of the students. There is enough material in the book for more than one course in highway bridge design.

Practitioners who are entry level engineers will find the background material in Chapters 1–6 helpful in their new assignments and can use Chapters 7–10 for specific guidance on design of a particular bridge type. The same can be said for seasoned professionals, even though they would be familiar with the material in the loads chapter, they should find the other chapters of interest in providing background and design examples based on the AASHTO LRFD specifications.

Acknowledgments

Acknowledgments to others who have contributed to the writing of this book is not an easy task because so many people have participated in the development of our engineering careers. To list them all is not possible, but we do recognize the contribution of our university professors at the University of Minnesota and Colorado State University; our engineering colleagues at Toltz, King, Duvall, Anderson & Associates, Moffatt & Nichol Engineers, and BridgeTech, Inc.; our faculty colleagues at Virginia Tech and the University of Wyoming; the government and industry sponsors of our research work; and the countless number of students who keep asking those interesting questions.

The contribution of John S. Kim, author of Chapter 10 on Substructure Design, is especially appreciated. We realize that many of the ideas and concepts presented in the book have come from reading the work of others. In each of the major design chapters, the influence of the following people is acknowledged: Concrete Bridges, Michael Collins, University of Toronto, Thomas T.C. Hsu, University of Houston, and Antoine Naaman, University of Michigan; Steel Bridges, Sam Easterling and Tom Murray, Virginia Tech, and Konrad Basler, Zurich, Switzerland; and Wood Bridges, Michael Ritter, USDA Forest Service.

We also wish to acknowledge those who have contributed directly to the production of the book. These include Elizabeth Barker who typed a majority of the manuscript, Jude Kostage who drafted most of the figures, and Brian Goodrich who made significant modifications for the conversion of many figures to SI units. Others who prepared figures, worked on example problems, handled correspondence, and checked page proofs were: Barbara Barker, Catherine Barker, Benita Calloway, Ann Crate, Scott Easter, Martin Kigudde, Amy Kohls, Kathryn Kontrim, Michelle Rambo-Roddenberry, and Cheryl Rottmann. Thanks also to the following state departments of transportation who supplied photographs of their bridges and offered encouragement: California, Minnesota, Pennsylvania, Tennessee, Washington, and West Virginia.

The patience and understanding that Charles Schmieg, Associate Editor, Minna Panfili, editorial program assistant, and Millie Torres, Associate Managing Editor at John Wiley & Sons, have shown us during the preparation and production of the manuscript are gratefully acknowledged. We also recognize the assistance provided by editors Dan Sayre and Robert Argentieri of John Wiley & Sons during the formative and final stages of this book.

Finally, on behalf of the bridge engineering community the authors wish to recognize John Kulicki of Modjeski & Masters and Dennis Mertz of the University of Delaware for their untiring leadership in the development of the LRFD Specification. The authors wish to thank these professionals for providing support and encouragement for the book and responding to many questions about the rationale and background of the specification. Others who contributed to the development of the LRFD Specification as members of the Code Coordinating Committee or as a Chair of a Task Group have also influenced the writing of this book. These include: John Ahlskog, Ralph Bishop, Ian Buckle, Robert Cassano, Paul Csagoly, J. Michael Duncan, Theodore Galambos, Andrzej Nowak, Charles Purkiss, Frank Sears, and James Withiam. A complete listing of the members of the task groups and the NCHRP panel that directed the project is given in Appendix D.

As with any new book, in spite of numerous proofreadings, errors do creep in and the authors would appreciate it if the reader would call them to their attention. You may write to us directly or, if you prefer, use our e-mail address: barker@vt.edu or puckett@uwyo.edu.

Part I

General Aspects of Bridge Design

Chapter 1

Introduction to Bridge Engineering

Bridges are important to everyone. But they are not seen or understood in the same way, which is what makes their study so fascinating. A single bridge over a small river will be viewed differently because the eyes each one sees it with are unique to that individual. Someone traveling over the bridge everyday may only realize a bridge is there because the roadway now has a railing on either side. Others may remember a time before the bridge was built and how far they had to travel to visit friends or to get the children to school. Civic leaders see the bridge as a link between neighborhoods, a way to provide fire and police protection, and access to hospitals. In the business community, the bridge is seen as opening up new markets and expanding commerce. An artist may consider the bridge and its setting as a possible subject for a future painting. A theologian may see the bridge as symbolic of making a connection with God. While a boater on the river, looking up when passing underneath the bridge, will have a completely different perspective. Everyone is looking at the same bridge, but it produces different emotions and visual images in each.

Bridges affect people. People use them, and engineers design them and later build and maintain them. Bridges do not just happen. They must be planned and engineered before they can be constructed. In this book, the emphasis is on the engineering aspects of this process: selection of bridge type, analysis of load effects, resistance of cross sections, and conformance with bridge specifications. Although very important, factors of technical significance should not overshadow the people factor.

1.1 A Bridge Is the Key Element in a Transportation System

A bridge is a key element in a transportation system for three reasons:

check box It likely controls the capacity.

check box It is the highest cost per mile.

check box If the bridge fails, the system fails.

If the width of a bridge is insufficient to carry the number of lanes required to handle the traffic volume, the bridge will be a constriction to the traffic flow. If the strength of a bridge is deficient and unable to carry heavy trucks, load limits will be posted and truck traffic will be rerouted. The bridge controls both the volume and weight of the traffic carried.

Bridges are expensive. The typical cost per mile of a bridge is many times that of the approach roadways. This is a major investment and must be carefully planned for best use of the limited funds available for a transportation system.

When a bridge is removed from service and not replaced, the transportation system may be restricted in its function. Traffic may be detoured over routes not designed to handle the increase in volume. Users of the system experience increased travel times and fuel expenses. Normalcy does not return until the bridge is repaired or replaced.

Because a bridge is a key element in a transportation system, balance must be achieved between handling future traffic volume and loads and the cost of a heavier and wider bridge structure. Strength is always a foremost consideration but so should measures to prevent deterioration. The designer of new bridges has control over these parameters and must make wise decisions so that capacity and cost are in balance, and safety is not compromised.

1.2 Bridge Engineering in the United States

Usually a discourse on the history of bridges begins with a log across a small stream or vines suspended above a deep chasm. This preamble is followed by the development of the stone arch by the Roman engineers of the second and first centuries bc and the building of beautiful bridges across Europe during the Renaissance period of the fourteenth through seventeenth centuries. Next is the Industrial Revolution, which began in the last half of the eighteenth century and saw the emergence of cast iron, wrought iron, and finally steel for bridges. Such discourses are found in the books by Brown (1993), Gies (1963), and Kirby et al. (1956) and are not repeated here. An online search for bridge engineering history leads to a host of other references on this topic. Instead a few of the bridges that are typical of those found in the United States are highlighted.

1.2.1 Stone Arch Bridges

The Roman bridge builders first come to mind when discussing stone arch bridges. They utilized the semicircular arch and built elegant and handsome aqueducts and bridges, many of which are still standing today. The oldest remaining Roman stone arch structure is from the seventh century bc and is a vaulted tunnel near the Tiber River. However, the oldest surviving stone arch bridge dates from the ninth century bc and is in Smyrna, Turkey, over the Meles River. In excavations of tombs and underground temples, archaeologists found arched vaults dating to the fourth millennium bc at Ur in one of the earliest Tigris–Euphrates civilizations (Gies, 1963). The stone arch has been around a long time and how its form was first discovered is unknown. But credit is due to the Roman engineers because they are the ones who saw the potential in the stone arch, developed construction techniques, built foundations in moving rivers, and left us a heritage of engineering works that we marvel at today such as Pont du Gard (Exhibit 1 in the color insert).

Compared to these early beginnings, the stone arch bridges in the United States are relative newcomers. One of the earliest stone arch bridges is the Frankford Avenue Bridge over Pennypack Creek built in 1697 on the road between Philadelphia and New York. It is a three-span bridge, 73 ft (23 m) long and is the oldest bridge in the United States that continues to serve as part of a highway system (Jackson, 1988).

Stone arch bridges were usually small scale and built by local masons. These bridges were never as popular in the United States as they were in Europe. Part of the reason for lack of popularity is that stone arch bridges are labor intensive and expensive to build. However, with the development of the railroads in the mid- to late-nineteenth century, the stone arch bridge provided the necessary strength and stiffness for carrying heavy loads, and a number of impressive spans were built. One was the Starrucca Viaduct, Lanesboro, Pennsylvania, which was completed in 1848, and another was the James J. Hill Stone Arch Bridge, Minneapolis, Minnesota, completed in 1883.

The Starrucca Viaduct (Exhibit 2 in the color insert) is 1040 ft (317 m) in overall length and is composed of 17 arches, each with a span of 50 ft (15 m). The viaduct is located on what was known as the New York and Erie Railroad over Starrucca Creek near its junction with the Susquehanna River. Except for the interior spandrel walls being of brick masonry, the structure was of stone masonry quarried locally. The maximum height of the roadbed above the creek is 112 ft (34 m) (Jackson, 1988) and it still carries heavy railroad traffic.

The James J. Hill Stone Arch Bridge (Fig. 1.1) is 2490 ft (760 m) long and incorporated 23 arches in its original design (later, 2 arches were replaced with steel trusses to provide navigational clearance). The structure carried Hill's Great Northern Railroad (now merged into the Burlington Northern Santa Fe Railway) across the Mississippi River just below St. Anthony Falls. It played a key role in the development of the Northwest. The bridge was retired in 1982, just short of its 100th birthday, but it still stands today as a reminder of an era gone by and bridges that were built to last (Jackson, 1988).

Fig 1.1 James J. Hill Stone Arch Bridge, Minneapolis, Minnesota.

(Hibbard Photo, Minnesota Historical Society, July 1905.)

1.2.2 Wooden Bridges

Early bridge builders in the United States (Timothy Palmer, Lewis Wernwag, Theodore Burr, and Ithiel Town) began their careers as millwrights or carpenter-mechanics. They had no clear conception of truss action, and their bridges were highly indeterminate combinations of arches and trusses (Kirby and Laurson, 1932). They learned from building large mills how to increase clear spans by using the king-post system or trussed beam. They also appreciated the arch form and its ability to carry loads in compression to the abutments. This compressive action was important because wood joints can transfer compression more efficiently than tension.

The long-span wooden bridges built in the late-eighteenth and early-nineteenth centuries incorporated both the truss and the arch. Palmer and Wernwag constructed trussed arch bridges in which arches were reinforced by trusses (Fig. 1.2). Palmer built a 244-ft (74-m) trussed arch bridge over the Piscataqua in New Hampshire in the 1790s. Wernwag built his Colossus in 1812 with a span of 340 ft (104 m) over the Schuylkill at Fairmount, Pennsylvania (Gies, 1963).

Fig 1.2 Trussed arch—designed by Lewis Wernwag, patented 1812.

In contrast to the trussed arch of Palmer and Wernwag, Burr utilized an arched truss in which a truss is reinforced by an arch (Fig. 1.3) and patented his design in 1817. An example of one that has survived until today is the Philippi Covered Bridge (Fig. 1.4) across the Tygant's Valley River, West Virginia. Lemuel Chenoweth completed it in 1852 as a two-span Burr arched truss with a total length of 577 ft (176 m) long. In later years, two reinforced concrete piers were added under each span to strengthen the bridge (Exhibit 3 in the color insert). As a result, it is able to carry traffic loads and is the nation's only covered bridge serving a federal highway.

Fig 1.3 Arched truss—designed by Theodore Burr, patented 1817.

(From Bridges and Men by Joseph Gies. Copyright © 1963 by Joseph Gies. Used by permission of Doubleday, a division of Bantam Doubleday Dell Publishing Group, Inc.)

Fig 1.4 Philippi covered bridge.

(Photo by Larry Belcher, courtesy of West Virginia Department of Transportation.)

One of the reasons many covered bridges have survived for well over 100 years is that the wooden arches and trusses have been protected from the weather. Palmer put a roof and siding on his permanent bridge (called permanent because it replaced a pontoon bridge) over the Schuylkill at Philadelphia in 1806, and the bridge lasted nearly 70 years before it was destroyed by fire in 1875.

Besides protecting the wood from alternating cycles of wet and dry that cause rot, other advantages of the covered bridge occurred. During winter blizzards, snow did not accumulate on the bridge. However, this presented another problem; bare wooden decks had to be paved with snow because everybody used sleighs. Another advantage was that horses were not frightened by the prospect of crossing a rapidly moving stream over an open bridge because the covered bridge had a comforting barnlike appearance (so says the oral tradition). American folklore also says the covered bridges became favorite parking spots for couples in their rigs, out of sight except for the eyes of curious children who had climbed up and hid in the rafters (Gies, 1963). However, the primary purpose of covering the bridge was to prevent deterioration of the wood structure.

Another successful wooden bridge form first built in 1813 was the lattice truss, which Ithiel Town patented in 1820 (Edwards, 1959). This bridge consisted of strong top and bottom chords, sturdy end posts, and a web of lattice work (Fig. 1.5). This truss type was popular with builders because all of the web members were of the same length and could be prefabricated and sent to the job site for assembly. Another advantage is that it had sufficient stiffness by itself and did not require an arch to reduce deflections. This inherent stiffness meant that horizontal thrusts did not have to be resisted by abutments, and a true truss, with only vertical reactions, had really arrived.

Fig 1.5 Lattice truss—designed by Ithiel Town, patented 1820.

(From Bridges and Men by Joseph Gies. Copyright © 1963 by Joseph Gies. Used by permission of Doubleday, a division of Bantam Doubleday Dell Publishing Group, Inc.)

The next step toward simplicity in wooden bridge truss types in the United States is credited to an army engineer named Colonel Stephen H. Long who had been assigned by the War Department to the Baltimore and Ohio Railroad (Edwards, 1959). In 1829, Colonel Long built the first American highway–railroad grade separation project. The trusses in the superstructure had parallel chords that were subdivided into panels with counterbraced web members (Fig. 1.6). The counterbraces provided the necessary stiffness for the panels as the loading changed in the diagonal web members from tension to compression as the railroad cars moved across the bridge.

Fig 1.6 Multiple king-post truss—designed by Colonel Stephen H. Long in 1829.

(From Bridges and Men by Joseph Gies. Copyright © 1963 by Joseph Gies. Used by permission of Doubleday, a division of Bantam Doubleday Dell Publishing Group, Inc.)

The development of the paneled bridge truss in wooden bridges enabled long-span trusses to be built with other materials. In addition, the concept of web panels is important because it is the basis for determining the shear resistance of girder bridges. These concepts are called the modified compression field theory in Chapter 14 and tension field action in Chapter 19.

1.2.3 Metal Truss Bridges

Wooden bridges were serving the public well when the loads being carried were horse-drawn wagons and carriages. Then along came the railroads with their heavy loads, and the wooden bridges could not provide the necessary strength and stiffness for longer spans. As a result, wrought-iron rods replaced wooden tension members, and a hybrid truss composed of a combination of wood and metal members was developed. As bridge builders' understanding of which members were carrying tension and which were carrying compression increased, cast iron replaced wooden compression members, thus completing the transition to an all-metal truss form.

In 1841, William Howe, uncle of Elias Howe, the inventor of the sewing machine, received a patent on a truss arrangement in which he took Long's panel system and replaced the wooden vertical members with wrought-iron rods (Gies, 1963). The metal rods ran through the top and bottom chords and could be tightened by turnbuckles to hold the wooden diagonal web members in compression against cast-iron angle blocks (Fig. 1.7). Occasionally, Howe truss bridges were built entirely of metal, but in general they were composed of both wood and metal components. These bridges have the advantages of the panel system as well as those offered by counterbracing.

Fig 1.7 Howe truss—designed by William Howe, patented in 1841.

(From Bridges and Men by Joseph Gies. Copyright © 1963 by Joseph Gies. Used by permission of Doubleday, a division of Bantam Doubleday Dell Publishing Group, Inc.)

Thomas and Caleb Pratt (Caleb was the father of Thomas) patented a second variation on Long's panel system in 1844 with wooden vertical members to resist compression and metal diagonal members, which resist only tension (Jackson, 1988). Most of the Pratt trusses built in the United States were entirely of metal, and they became more commonly used than any other type. Simplicity, stiffness, constructability, and economy earned this recognition (Edwards, 1959). The distinctive feature of the Pratt truss (Fig. 1.8), and related designs, is that the main diagonal members are in tension.

Fig 1.8 Pratt truss—designed by Thomas and Caleb Pratt, patented in 1844.

(From Bridges and Men by Joseph Gies. Copyright © 1963 by Joseph Gies. Used by permission of Doubleday, a division of Bantam Doubleday Dell Publishing Group, Inc.)

In 1841, Squire Whipple patented a cast-iron arch truss bridge (Fig. 1.9), which he used to span the Erie Canal at Utica, New York (Note: Whipple was not a country gentleman, his first name just happened to be Squire.) Whipple utilized wrought iron for the tension members and cast iron for the compression members. This bridge form became known as a bowstring arch truss, although some engineers considered the design to be more a tied arch than a truss (Jackson, 1988). The double-intersection Pratt truss of Figure 1.10, in which the diagonal tension members extended over two panels, was also credited to Whipple because he was the first to use the design when he built railroad bridges near Troy, New York.

Fig 1.9 Bowstring arch—designed by Squire Whipple, patented in 1841.

Fig 1.10 Double-intersection Pratt—credited to Squire Whipple.

To implement his designs, it is implied that Squire Whipple could analyze his trusses and knew the magnitudes of the tensile and compressive forces in the various members. He was a graduate of Union College, class of 1830, and in 1847 he published the first American treatise on determining the stresses produced by bridge loads and proportioning bridge members. It was titled A Work on Bridge Building; consisting of two Essays, the one Elementary and General, the other giving Original Plans, and Practical Details for Iron and Wooden Bridges (Edwards, 1959). In it he showed how one could compute the tensile or compressive stress in each member of a truss that was to carry a specific load (Kirby et al., 1956).

In 1851, Herman Haupt, a graduate of the U.S. Military Academy at West Point, class of 1835, authored a book titled General Theory of Bridge Construction, which was published by D. Appleton and Company (Edwards, 1959). This book and the one by Squire Whipple were widely used by engineers and provided the theoretical basis for selecting cross sections to resist bridge dead loads and live loads.

One other development that was critical to the bridge design profession was the ability to verify the theoretical predictions with experimental testing. The tensile and compressive strengths of cast iron, wrought iron, and steel had to be determined and evaluated. Column load curves had to be developed by testing cross sections of various lengths. This experimental work requires large-capacity testing machines.

The first testing machine to be made in America was built in 1832 to test a wrought-iron plate for boilers by the Franklin Institute of Philadelphia (Edwards, 1959). Its capacity was about 10 tons (90 kN), not enough to test bridge components. About 1862, William Sallers and Company of Philadelphia built a testing machine that had a rated capacity of 500 tons (4500 kN) and was specially designed for the testing of full-size columns.

Two testing machines were built by the Keystone Bridge Works, Pittsburgh, Pennsylvania, in 1869–1870 for the St. Louis Bridge Company to evaluate materials for the Eads Bridge over the Mississippi River. One had a capacity of 100 tons (900 kN) while the other a capacity of 800 tons (7200 kN). At the time it was built, the capacity of the larger testing machine was greater than any other in existence (Edwards, 1959).

During the last half of the nineteenth century, the capacity of the testing machines continued to increase until in 1904 the American Bridge Company built a machine having a tension capacity of 2000 tons (18,000 kN) (Edwards, 1959) at its Ambridge, Pennsylvania, plant. These testing machines were engineering works in themselves, but they were essential to verify the strength of the materials and the resistance of components in bridges of ever increasing proportions.

1.2.4 Suspension Bridges

Suspension bridges capture the imagination of people everywhere. With their tall towers, slender cables, and tremendous spans, they appear as ethereal giants stretching out to join together opposite shores. Sometimes they are short and stocky and seem to be guardians and protectors of their domain. Other times, they are so long and slender that they seem to be fragile and easily moved. Whatever their visual image, people react to them and remember how they felt when they first saw them.

Imagine the impression on a young child on a family outing in a state park and seeing for the first time the infamous swinging bridge across the raging torrent of a rock-strewn river (well, it seemed like a raging torrent). And then the child hears the jeers and challenge of the older children, daring him to cross the river as they moved side to side and purposely got the swinging bridge to swing. Well, it did not happen that first day, it felt more comfortable to stay with mother and the picnic lunch. But it did happen on the next visit, a year or two later. It was like a rite of passage. A child no longer, he was able to cross over the rock-strewn stream on the swinging bridge, not fighting it, but moving with it and feeling the exhilaration of being one with forces stronger than he was.

Suspension bridges also make strong impressions on adults and having an engineering education is not a prerequisite. People in the United States have enjoyed these structures on both coasts, where they cross bays and mouths of rivers. The most memorable are the Brooklyn Bridge (Exhibit 4 in the color insert) in the east and the Golden Gate Bridge (Exhibit 5 in the color insert) in the west. They are also in the interior of the country, where they cross the great rivers, gorges, and straits. Most people understand that the cables are the tendons from which the bridge deck is hung, but they marvel at their strength and the ingenuity it took to get them in place. When people see photographs of workers on the towers of suspension bridges, they catch their breath, and then wonder at how small the workers are compared to the towers they have built. Suspension bridges bring out the emotions: wonder, awe, fear, pleasure; but mostly they are enjoyed for their beauty and grandeur.

In 1801, James Finley erected a suspension bridge with wrought-iron chains of 70-ft (21-m) span over Jacob's Creek near Uniontown, Pennsylvania. He is credited as the inventor of the modern suspension bridge with its stiff level floors and secured a patent in 1808 (Kirby and Laurson, 1932). In previous suspension bridges, the roadway was flexible and followed the curve of the ropes or chains. By stiffening the roadway and making it level, Finley developed a suspension bridge that was suitable not only for footpaths and trails but for roads with carriages and heavy wagons.

Most engineers are familiar with the suspension bridges of John A. Roebling: the Niagara River Bridge, completed in 1855 with a clear span of 825 ft (250 m); the Cincinnati Suspension Bridge, completed in 1867 with a clear span of 1057 ft (322 m); and the Brooklyn Bridge, completed in 1883 with a clear span of 1595 ft (486 m). Of these three wire cable suspension bridges from the nineteenth century, the last two are still in service and are carrying highway traffic. However, there is one other long-span wire cable suspension bridge from this era that is noteworthy and still carrying traffic: the Wheeling Suspension Bridge completed in 1849 with a clear span of 1010 ft (308 m) (Fig. 1.11).

Fig 1.11 Wheeling Suspension Bridge.

(Photo by John Brunell, courtesy of West Virginia Department of Transportation.)

The Wheeling Suspension Bridge over the easterly channel of the Ohio River was designed and built by Charles Ellet who won a competition with John Roebling; that is, he was the low bidder. This result of a competition was also true of the Niagara River Bridge, except that Ellet walked away from it after the cables had been strung, saying that the $190,000 he bid was not enough to complete it. Roebling was then hired and he completed the project for about $400,000 (Gies, 1963).

The original Wheeling Suspension Bridge did not have the stiffening truss shown in Figure 1.11. This truss was added after a windstorm in 1854 caused the bridge to swing back and forth with increased momentum, the deck to twist and undulate in waves nearly as high as the towers, until it all came crashing down into the river (very similar to the Tacoma Narrows Bridge failure some 80 years later). A web search for Tacoma Narrows Movie will provide several opportunities to view movies that illustrate the failure.

The Wheeling Bridge had the strength to resist gravity loads, but it was aerodynamically unstable. Why this lesson was lost to the profession is unknown, but if it had received the attention it deserved, it would have saved a lot of trouble in the years ahead.

What happened to the Wheeling Suspension Bridge was not lost on John Roebling. He was in the midst of the Niagara River project when he heard of the failure and immediately ordered more cable to be used as stays for the double-decked bridge. An early painting of the Niagara River Bridge shows the stays running from the bottom of the deck to the shore to provide added stability.

In 1859 William McComas, a former associate of Charles Ellet, rebuilt the Wheeling Suspension Bridge. In 1872 Wilhelm Hildenbrand, an engineer with Roebling's company, modified the deck and added diagonal stay wires between the towers and the deck to increase the resistance to wind (Jackson, 1988) and to give the bridge the appearance it has today.

The completion of the Brooklyn Bridge in 1883 brought to maturity the building of suspension bridges and set the stage for the long-span suspension bridges of the twentieth century. Table 1.1 provides a summary of some of the notable long-span suspension bridges built in the United States and still standing.

Table 1.1 Long-Span Suspension Bridges in the United States

Some comments are in order with regard to the suspension bridges in Table 1.1. The Williamsburg Bridge and the Brooklyn Bridge are of comparable span but with noticeable differences. The Williamsburg Bridge has steel rather than masonry towers. The deck truss is a 40-ft (12.5-m) deep lattice truss, compared to a 17-ft (5.2-m) deep stiffening truss of its predecessor. This truss gives the Williamsburg Bridge a bulky appearance, but it is very stable under traffic and wind loadings. Another big difference is that the wire in the steel cables of the Brooklyn Bridge was galvanized to protect it from corrosion in the briny atmosphere of the East River (Gies, 1963), while the wire in its successor was not. As a result, the cables of the Williamsburg Bridge have had to be rehabilitated with a new protective system that cost $73 million (Bruschi and Koglin, 1996). A web search for Williamsburg Bridge image, or other bridge names listed in Table 1.1, provides a wealth of information and illustration.

Another observation of Table 1.1 is the tremendous increase in clear span attained by the George Washington Bridge over the Hudson River in New York. It nearly doubled the clear span of the longest suspension bridge in existence at the time it was built, a truly remarkable accomplishment.

One designer, Leon Moisseiff, is associated with most of the suspension bridges in Table 1.1 that were built in the twentieth century. He was the design engineer of the Manhattan and Ben Franklin bridges, participated in the design of the George Washington Bridge, and was a consulting engineer on the Ambassador, Golden Gate, and Oakland–Bay bridges (Gies, 1963). All of these bridges were triumphs and successes. He was a well-respected engineer who had pioneered the use of deflection theory, instead of the erroneous elastic theory, in the design of the Manhattan Bridge and those that followed. But Moisseiff will also be remembered as the designer of the Tacoma Narrows Bridge that self-destructed during a windstorm in 1940, not unlike that experienced by the Wheeling Suspension Bridge in 1854.

The use of a plate girder to stiffen the deck undoubtedly contributed to providing a surface on which the wind could act, but the overall slenderness of the bridge gave it an undulating behavior under traffic even when the wind was not blowing. Comparing the ratio of depth of truss or girder to the span length for the Williamsburg, Golden Gate, and Tacoma Narrows bridges, we have 1 : 40, 1 : 164, and 1 : 350, respectively (Gies, 1963). The design had gone one step too far in making a lighter and more economical structure. The tragedy for bridge design professionals of the Tacoma Narrows failure was a tough lesson, but one that will not be forgotten.

1.2.5 Metal Arch Bridges

Arch bridges are aesthetically pleasing and can be economically competitive with other bridge types. Sometimes the arch can be above the deck, as in a tied-arch design, or as in the bowstring arch of Whipple (Fig. 1.9). Other times, when the foundation materials can resist the thrusts, the arch is below the deck. Restraint conditions at the supports of an arch can be fixed or hinged. And if a designer chooses, a third hinge can be placed at the crown to make the arch statically determinate or nonredundant.

The first iron arch bridge in the United States was built in 1839 across Dunlap's Creek at Brownsville in southwestern Pennsylvania on the National Road (Jackson, 1988). The arch consists of five tubular cast-iron ribs that span 80 ft (24 m) between fixed supports. It was designed by Captain Richard Delafield and built by the U.S. Army Corps of Engineers (Jackson, 1988). It is still in service today.

The second cast-iron arch bridge in this country was completed in 1860 across Rock Creek between Georgetown and Washington, DC. It was built by the Army Corps of Engineers under the direction of Captain Montgomery Meigs as part of an 18.6-mile (30-km) aqueduct, which brings water from above the Great Falls on the Potomac to Washington, DC. The two arch ribs of the bridge are 4-ft (1.2-m) diameter cast-iron pipes that span 200 ft (61 m) with a rise of 20 ft (6.1 m) and carry water within its 1.5-inch (38-mm) thick walls. The arch supports a level roadway on open-spandrel posts that carried Washington's first horse-drawn street railway line (Edwards, 1959). The superstructure was removed in 1916 and replaced by a concrete arch bridge. However, the pipe arches remain in place between the concrete arches and continue to carry water to the city today.

Two examples of steel deck arch bridges from the nineteenth century that still carry highway traffic are the Washington Bridge across the Harlem River in New York and the Panther Hollow Bridge in Schenely Park, Pittsburgh (Jackson, 1988). The two-hinged arches of the Washington Bridge, completed in 1889, are riveted plate girders with a main span of 508 ft (155 m). This bridge is the first American metal arch bridge in which the arch ribs are plate girders (Edwards, 1959). The three-hinged arch of the Panther Hollow Bridge, completed in 1896, has a span of 360 ft (110 m). Due to space limitations, not all bridges noted here can be illustrated in this book; however, web searches for the bridge name and location easily takes the reader to a host of images and other resources.

One of the most significant bridges built in the United States is the steel deck arch bridge designed by James B. Eads (Exhibit 6 in the color insert) across the Mississippi River at St. Louis. It took 7 years to construct and was completed in 1874. The three-arch superstructure consisted of two 502-ft (153-m) side arches and one 520-ft (159-m) center arch that carried two decks of railroad and highway traffic (Fig. 1.12). The Eads Bridge is significant because of the very deep pneumatic caissons for the foundations, the early use of steel in the design, and the graceful beauty of its huge arches as they span across the wide river (Jackson, 1988).

Fig 1.12 Eads Bridge, St. Louis, Missouri.

(Photo courtesy of Kathryn Kontrim, 1996.)

Because of his previous experience as a salvage diver, Eads realized that the foundations of his bridge could not be placed on the shifting sands of the riverbed but must be set on bedrock. The west abutment was built first with the aid of a cofferdam and founded on bedrock at a depth of 47 ft (14 m). Site data indicated that bedrock sloped downward from west to east, with an unknown depth of over 100 ft (30 m) at the east abutment, presenting a real problem for cofferdams. While recuperating from an illness in France, Eads learned that European engineers had used compressed air to keep water out of closed caissons (Gies, 1963). He adapted the technique of using caissons, or wooden boxes, added a few innovations of his own, such as a sand pump, and completed the west and east piers in the river. The west pier is at a depth of 86 ft (26 m) and the east pier at a depth of 94 ft (29 m).

However, the construction of these piers was not without cost. Twelve workmen died in the east pier and one in the west pier from caisson's disease, or the