BRIDGE DESIGN SPECIFICATIONS FOR THE FUTURE

The American Association of State Highway and Transportation Officials (AASHTO)-sponsored National Coopperative Highway Research Program study to assess current U.S. bridge design specifications, foreign codes, and alternative specifications is discussed. The study findings led to the initiation of the NCHRP Project 12-33 to develop new bridge design specifications. The history of AASHTO Standard Specifications before and after the 1970s is reviewed including its modification to include the load factor design (LFD). The evolution of LRFD (load resistance factor design) specifications is described. LRFD-based specifications employ statistical methods and probability theory to define variations both in loading and material properties, and the likelihood that various combinations of loads will occur at the same time. It is anticipated that the final draft of the specifications will be considered for adoption by AASHTO in 1993. These specifications will result in construction of bridges that exhibit a more uniform level of safety and that provide better long-term serviceability and maintainability.