Preface
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 f