AASHTO Specifications. For many years, the basic manual for design of highway bridges has been the Standard Specifications for Highway Bridges adopted by the American Association of State Highway and Transportation Officials (AASHTO). These specifications permit use of either allowable stress design or load factor design. In 1994, however, AASHTO published a completely new alternative volume, LRFD Bridge Design Specifications. It was subsequently updated with a second edition in 1998; third edition in 2004; fourth edition in 2007; and annual Interim Specifications thereafter. Based on the load and resistance factor design method, the LRFD Specifications represent a major step in improved bridge design and analysis methods. It is anticipated that usage of the new specifications will lead to bridges with improved serviceability, enhanced long-term maintainability, and more uniform levels of safety. The initial volume resulted from a 5-year research effort conducted under AASHTO’s National Cooperative Highway Research Program. Independent consultants, technical representatives from various industries, AASHTO members, and other engineers participated in the effort to develop a draft document. Then the provisions were tested in trial designs at 14 AASHTO member departments before final specifications were adopted. One of the most useful feature included is a detailed commentary that explains the specification provisions and gives references for further study.
The use of the new specifications has been increasing throughout the country. The Federal Highway Administration (FHWA) and AASHTO have established a goal that the LRFD Specifications be used for all new bridges designed after 2007 and for all culverts, retaining walls, and other standard structures after 2010. Most state DOTs have adopted LRFD specifications for the design of their bridges so as not to lose funding for federally funded bridge projects. States unable to meet these dates were required to provide justification and a schedule for completing the transition to LRFD. The Standard Specifications would be applicable only to structures designed prior to 2007, and could be used for the maintenance and rehabilitation of existing bridges. No technical revisions will likely be made to the Standard Specifications in the future as its usage is phased out.
AASHTO specifications are developed under the direction of the AASHTO Highway Subcommittee on Bridges and Structures. This subcommittee consists of all bridge engineers of states of the United States and of Canadian provinces and officials of selected turnpike and bridge authorities. The specification development process is a deliberate one. Nevertheless, changes are made on a regular basis (some would say too frequently for the average bridge designer to stay abreast of them). Between new editions, revisions are published under the title of Interim Specifications. When identifying the AASHTO specifications used for design on plans, some states refer to “AASHTO Standard Specifications for Highway Bridges, Current Edition.” A better practice is to refer to the specific edition, by number and year, along with any interims that were in effect at the time of design.
Unless there is a cogent reason for not meeting the minimum requirements of the AASHTO specifications, engineers designing bridges where they are in effect should apply and conform to them. Any exceptions should be noted on the plans. In case of litigation, one would have to explain why these recognized standards were not met.
Other AASHTO Publications. AASHTO offers numerous publications related to bridges and structures. A bridge designer should be aware of the availability of these publications and should use them where applicable.
In this chapter, references to the “AASHTO Specifications” or “AASHTO” will be to the AASHTO Standard Specifications for Highway Bridges unless otherwise noted.
Bridge Design Manuals. Many state departments of transportation publish bridge design manuals, which they develop for guidance of their own staff and consultants. States that do not have manuals often publish design memoranda. Before starting a bridge design project, a consultant should determine which of these aids are available, acquire and become familiar with them, and apply them in designing and preparing plans. Some state bridge design manuals are quite explicit, and are almost textbooks on bridge design.
