Calibration of LRFD Bridge Code

This paper reviews the code development procedures used for the new load and resistance factor design (LRFD) bridge code. The new code is based on a probability-based approach. Structural performance is measured in terms of the reliability (or probability of failure). Load and resistance factors are derived so that the reliability of bridges designed using the proposed provisions will be at the predefined target level. The paper describes the calibration procedure (calculation of load and resistance factors). A new live load model is proposed, which provides a consistent safety margin for a wide spectrum of spans. The dynamic load model takes into account the effect of road roughness, bridge dynamics, and vehicle dynamics. Statistical models of resistance (load-carrying capacity) are summarized for noncomposite steel, composite steel, reinforced concrete, and prestressed concrete. The reliability indices for bridges designed using the proposed code are compared with the reliability indices corresponding to the current specification. The proposed code provisions allow for a consistent design with a uniform level of reliability.