Response Functions and System Reliability of Bridges

Abstract

Redundancy of a structural system is defined as the capability of the system to carry additional loads after the failure of one or more of its components. Research studies to include redundancy in the U.S. bridge design and evaluation codes are underway. These efforts are based on calibrating system factors that could be added to the checking equations. The purpose of these system factors is to “reward” redundant designs by allowing savings in required member capacities, and “penalize” nonredundant designs by requiring their members to be more conservative. The calibration process uses a target redundancy index as the safety criterion that bridges should satisfy. The redundancy index is defined as the difference between the reliability index of the bridge system and the reliability index of the members. This paper illustrates the method proposed to calculate the redundancy indices of typical bridges. The member safety index is calculated, as traditionally done, using the results of an elastic analysis of the bridge. The system reliability index accounts for the nonlinear behavior of the bridge system and the redistribution of the applied loads after the failure of a member. In this paper, the response function (response surface) method is used to calculate the system safety indices of typical multi-beam steel and prestressed concrete bridges. System factors are then calibrated to reflect the level of redundancy that different bridge configurations possess.