This project helps engineers understand new state-of-the-art LRFD bridge design codes developed and adopted on a trial basis by AASHTO. The LRFD method applies statistically determined factors to bridge design parameters. It uses a series of load factors and resistance factors to account for variabilities in properties of loads and material resistances. The LRFD specifications employ statistical methods and probability theory to define the variations in loading and material properties, and the likelihood that various loads combinations will occur simultaneously.

Since 1931, the design and construction of highway bridges in the U.S. has been governed by the Standard Specifications for Highway Bridges published by AASHTO. Over time, there have been numerous revisions and updates and the specifications have begun to resemble a patchwork quilt. A number of major gaps and inconsistencies were compounded by a lack of commentary to explain the intent or background of any of the specification provisions. After reviewing several options, AASHTO decided to develop an entirely new bridge code that would incorporate state-of-the-art bridge engineering and would be based on the load and resistance factor design approach. Following review and consideration, the new specification is expected to be adopted by AASHTO for use by any agency that designs bridges in the U.S. This new specification is a radical departure from the traditional design philosophy of Working Stress Design (WSD), which requires that the total stress caused by all design loads be less than a specified percentage of the bridge material's stress limit. While this design concept has worked well for many years, engineers have known that neither loads nor material properties can be defined with accuracy and precision. The LRFD code, with its extensive commentary, is seen as state of the art and technically easy to maintain, as well as ultimately easy to understand and use by bridge designers.

A 5-day training course on LRFD design of bridge superstructures is available. A contractor began work in September 1995 on development of a companion course on LRFD design for bridge foundations. The course is expected to be available in mid-1998.

Not available.

---

HIGHWAYTECHNET   VIRTUAL EXPO   STRUCTURES INDEX

---

U.S. Department of Transportation
Federal Highway Administration