Since the first edition of the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications in 1994, U.S. highway bridge design has been moving towards load andn resistance factor design (LRFD). Prior to that, for more than a half of century, highway bridge design practice has been using the load factor design (LFD) and service load design (SLD), according to the AASHTO Standard Specification for Highway Bridges. Furthermore, in 2003, the first edition of AASHTO Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges was issued for bridge evaluation and load rating. In 2007, AASHTO mandated the LRFD specifications in the United States after more than a decade of preparation, including trial use and software development. With the Federal Highway Administration’s (FHWA’s) requirement and also for consistency, a number of states have started the practice of LRFR for bridges designed according to LRFD. This introductory book covers the LRFD and LRFR methods for highway bridges as a textbook for a first undergraduate and/or graduate course on highway bridge design and/or evaluation.
The sixth edition of the AASHTO LRFD Bridge Design Specifications issued in 2012 was the latest design code when this book was prepared. Therefore, in this book, this edition of specifications is referred to as the AASHTO specifications, AASHTO design specifications, or AASHTO LRFD specifications, depending on what is being emphasized in the context.
For bridge evaluation and load rating, the second edition of the AASHTO Manual for Bridge Evaluation dated 2011 was the latest when this book was prepared, which is referred to as the AASHTO manual or AASHTO evaluation specifications, depending on context. This set of specifications is also sometimes referred to as the AASHTO specifications collectively along with the LRFD specifications.
While some information provided here may be used for preliminary design of highway bridges, this book mainly covers the detailed design of short- to medium-span highway bridges according to the AASHTO specifications. In addition, structural design is the main focus in detailed design. In other words, detailed design covered in this book proceeds with given or already optimized structure type, span arrangement, span length, and so on. This situation is assumed so that the student will not have to be concerned with whether the being-designed bridge needs to be optimized, with respect to span type, length, width, geometric parameters, and so on, which would unlock everything and leave no direction for the student to follow. On the other hand, the student needs to understand that determination of these factors may require additional information and/or knowledge not completely covered in this book, such as cost effectiveness of alternative span lengths and/or arrangements associated with different materials and climate conditions, and so on.
Furthermore, this book has a focus on short- to medium-span highway bridges for detailed design. The short- and medium-span lengths here are defined as those in the range of 20 to about 200 ft, within the application range of the specifications. This book also covers those bridges’ evaluation (particularly load rating) according to the AASHTO manual. For the given span length range, wind is usually not the major governing load. However, longer highway bridge spans whose design often is controlled by wind load effect still need to meet the requirements of the AASHTO specifications presented here. Therefore, understanding the requirements covered in this book may be viewed as a prerequisite to studying longspan bridge design and evaluation.
This book has seven chapters. Chapter 1 is an introduction to bridge engineering, including design and evaluation. Chapter 2 covers both the general and specific requirements in the AASHTO specifications for designing highway bridges. The concepts of structural reliability are also presented, which were used for the calibration of both sets of AASHTO specifications focused on here. Chapter 3 presents further detailed specific requirements of the specifications for loads, load effects, and their combinations for highway bridge components. Chapter 4 of this book covers the superstructure part of bridge design, Chapter 5 the bearings, and Chapter 6 the substructure. Chapter 7 shifts focus from design to evaluation (load rating) of the same bridges covered in the book.
This book is designed as a textbook for first courses of undergraduate and/or graduate studies on highway bridge design and/or evaluation according to current AASHTO specifications. For an undergraduate course of three credit hours, Chapters 1, 3, and 4 (or along with 5) are recommended to be covered. If the undergraduate course is designed
for four credit hours, Chapter 5 (or 6) may be added. For a graduate course, Chapters 1 through 6 may be covered for three credit hours, and Chapter 7 can be added for four credit hours. Structural analysis will be a prerequisite for using this textbook, and steel and concrete designs are preferred to be prerequisites but may be allowed as corequisites. Successful completion of the course will enable the student to perform duties of an entry-level engineer in bridge design and evaluation, according to the current AASHTO specifications.
Another alternative way of using this book is to teach only its bridge analysis and the related examples or portions of the examples. It can be part of a structural analysis course or an independent course at either undergraduate or graduate level.
The examples included in this book can be used without referring to the text. Such use can be particularly convenient for review after a level of understanding of the relevant text material is established. Therefore, they may also serve as a helpful reference for junior engineers with limited familiarity with bridge design and/or evaluation and those who are preparing for a professional engineer license exam, particularly where bridge design is a required subject.
A complete highway bridge will be visible to the student when several of these examples are integrated together, including its load rating of primary members. For instance, the following examples can make a typical highway commonly seen in one of today’s bridge design offices: the reinforced concrete deck designed in Examples 4.1, 4.3, and 4.4 plus the steel plate girders designed in Examples 4.9 to 4.11 plus the shear studs designed in Example 4.8 plus the abutment designed in Examples 6.1 to 6.4. Examples 7.1 and 7.2 then provide the bridge’s superstructure member load rating. Of course, the deck design in Example 4.1 may be replaced by another deck design in Example 4.2. Furthermore, the superstructure steel beams in Examples 4.9 to 4.11 may be replaced by the prestressed concrete beams in Examples 4.12 to 4.14 or another steel beam superstructure in Examples 4.5 to 4.7. To minimize inconvenient cross referencing between different examples, a few steps of calculation in these examples may have been repeated in other examples, so that such cross referencing will involve only nearby pages if at all.
The student is not required to have the AASHTO LRFD or LRFR specifications while taking a course using this book. However, reference to the AASHTO specifications provisions are provided in the text so that the instructor and/or student can easily find more information in the specifications if needed. It is also worth noting that AASHTO offers U.S. faculty three free AASHTO publications a year, while the faculty needs to pay for shipping and handling.
This book is based on the author’s experience of teaching bridge design and evaluation at Wayne State University and Illinois Institute of Technology for 16 years. Many students of the author have assisted in preparing the manuscript. He would like to express gratitude to them: Drs. Dinesh Devaraj and Pang-jo Chun and Messrs. Tapan Bhatt, Jason Dimaria, Alexander Lamb, and Justin Sikorski. Bridge design plans and/or calculations also have been received from state bridge owners of California, Georgia, Idaho, Illinois, Michigan, New York, and New Jersey. The author is very grateful to the state agencies for this set of information related to current practice of bridge design and evaluation. It certainly has influenced the design and preparation of this book.
The following unit abbreviations have been used:
in. = inch
ft = foot
k = 1000 pounds
kin = 1000 pound inch
kft = 1000 pound foot
k/ft = 1000 pounds per foot
k/in2 = 1000 pounds per square inch or ksi
Bridge design and evaluation is a widely spread subject, and many engineers in the world have spent their entire career on it. An introductory text book as this may easily miss some relevant and/or important information or contain errors on a topic. The author would be grateful if the reader could identify them to him. He can be reached at email@example.com.
- 1.1 Bridge Engineering and Highway Bridge Network
- 1.2 Types of Highway Bridges
- 1.3 Bridge Construction and Its Relation to Design
- 1.4 AASHTO Specifications and Design and Evaluation Methods
- 1.5 Goals for Bridge Design and Evaluation
- 1.6 Preliminary Design versus Detailed Design
- 1.7 Organization of This Book
2 Requirements for Bridge Design and Evaluation
- 2.1 General Requirements
- 2.2 Limit States
- 2.3 Constructability
- 2.4 Safety
- 2.5 Serviceability
- 2.6 Inspectability
- 2.7 Economy
- 2.8 Aesthetics
- 2.9 Summary
3 Loads, Load Effects, and Load Combinations
- 3.1 Introduction
- 3.2 Permanent Loads
- 3.3 Transient Loads
- 3.4 Load Combinations
4 Superstructure Design
- 4.1 Introduction
- 4.2 Highway Bridge Superstructure Systems
- 4.3 Primary Components of Highway Bridge Superstructure
- 4.4 Deck Systems
- 4.5 Deck-Supporting Systems
- 4.6 Design of Reinforced Concrete Deck Slabs
- 4.7 Design of Steel I Beams
- 4.8 Design of Prestressed Concrete I Beams
5 Bearing Design
- 5.1 Introduction
- 5.2 Types of Bridge Bearing
- 5.3 Appropriate Selection of Bearings
- 5.4 Design of Elastomeric Bearings
6 Substructure Design
- 6.1 Introduction
- 6.2 Piers
- 6.3 Abutments
- 6.4 Foundations
- 6.5 Design of Piers
- 6.6 Design of Abutments
7 Highway Bridge Evaluation
- 7.1 Introduction
- 7.2 Inspection and Condition Rating
- 7.3 Load Rating
- 7.4 Fatigue Evaluation for Steel Components