Since initial adoption more than 50 years ago, the AASHTO Standard Specifications for Highway Bridges have been modified annually by the AASHTO Subcommittee on Bridges and Structures. These specifications are relied on by engineers in state highway agencies, consulting firms, and other organizations responsible for design, construction, and maintenance of bridges. Because of the piecemeal development of the current specifications, extra care is required to avoid inconsistencies, fragmentation, and internal conflicts as individual sections of the specifications are revised each year. This problem is compounded by the fact that a comprehensive commentary is not available to clarify the intent and record the origin of key provision. Some of the specification's shortcomings were corrected by complete editorial revision of the specification in 1984.

The AASHTO Subcommittee on Bridges and Structures recognizes the need for clear, practical specifications based on the best current technology, and state bridge engineers and others devote a substantial amount of time and attention to this end. In recent years, some bridge engineers have called attention to the potential advantages of developing a completely new comprehensive specification and an accompanying commentary.

In response to a high level of interest among state bridge engineers, the AASHTO Subcommittee on Bridges and Structures requested the NCHRP to conduct a study to recommend an outline for an updated AASHTO bridge specification. The scope of the study required an identification of the gaps and inconsistencies in the present specifications and an assessment of the feasibility of basing the revised specifications on a probabilistic load and resistance factor design (LRFD) philosophy.

The study was completed in 1987. It identified many areas where current bridge design technology and design practice are not reflected in the existing AASHTO specifications. Additionally, it recommended that new specifications be developed based on LRFD concepts. The study also recommended that a comprehensive companion commentary be developed.

NCHRP Project 12-33 was initiated in mid-1988 with the objective of developing recommended LRFD-based bridge design specifications and commentary for consideration by the AASHTO Subcommittee on Bridges and Structures. The new specifications are expected to draw heavily from recent developments in bridge design practice throughout the world as well as from recently completed and current bridge research.

Thirteen task groups were responsible for developing the recommended specifications. The task groups are: general features; loads; analysis and evaluation; deck systems; concrete structures; metal structures; timber structures; joints, bearings, and accessories; foundations; soil-structure interaction systems; moveable bridges; bridge rail; and specification calibration.

Four contractors and 47 consultants have been employed for various tasks on the project. In addition, more than 20 state, federal, and industry engineers have worked on the project on a volunteer basis.

The project is complete. The LRFD specifications have been adopted by AASHTO and published as the AASHTO LRFD Bridge Design Specifications,First Edition, 1994. Both SI unit and customary U.S. unit versions of the specifications are available. A report describing the specification development effort has been published in NCHRP Research Results Digest Number 198.