Fatigue reliability and redundancy in two-girder steel highway bridges.

摘要

The purpose of this dissertation is to present a comprehensive study of fatigue reliability and redundancy in two-girder steel highway bridges. To accomplish this study, a system reliability model is developed. The model, based on the Monte Carlo simulation method, is used to predict the time-dependent probabilities of girder failure, collapse, and damage detection for two-girder steel bridges.;Input to the system reliability model is developed in a series of preliminary analyses. These include investigations of truck headway, the dynamic response of a damaged bridge, truck load effects, and overload-induced collapse of a span with one failed girder. In these investigations, the finite element method is used to predict structural response.;An extensive series of system reliability analyses are performed. Two representative two-girder bridge spans are analyzed. Selection of these spans is based on a survey of welded two-girder highway bridges in New York State. The results of these analyses indicate that the representative bridges, in their as-built configurations, are structurally nonredundant. Their structural redundancy can be improved substantially by the addition of redundant bracing systems, designed in accordance with the guidelines provided in National Cooperative Highway Research Program Report 319. These guidelines are found to be entirely adequate, with respect to the strength of the bracing members.;The results also indicate that the system reliability of a nonredundant bridge is significantly affected by the number of fatigue-sensitive details present in the structure. It is noted that both structural and internal redundancy can be described as conditional reliabilities. The time-dependence associated with these reliabilities demonstrates that damage detection is a critical aspect of redundancy.;In addition, a Bayesian decision model is developed to provide a rational means of selecting the most cost-effective remedial action for a given bridge. This model uses expected cost as the basis for selection of a "best" remedial action. An example decision analysis is performed to demonstrate the application of the model. Selection of a best remedial action for a particular two-girder bridge is found to depend on the time period considered.