Reliability of Inelastic Load Capacity Rating Limits for Steel Bridges

To achieve consistent levels of safety over the bridge inventory, consideration should be given to the ultimate capacity of the system. Although multispan, longitudinally redundant bridges show significant reserve capacity beyond first-hinge limits, it is important to examine the level of safety related to this reserve capacity. If inelastic limits are to be used to evaluate existing bridges, the reliability at the inelastic limit must meet or exceed target levels associated with single-hinge limit state methods. This paper examines load capacity ratings and reliability indices of a simple-span and a three-span bridge for the first-hinge limit, the single-girder shakedown limit, and the system shakedown limit. First-order second-moment reliability models are presented that consider lognormally distributed variates and correlation between variables to determine the safety index β for the load capacity limit states. Results show that the three-span bridge possesses significant available reserve capacity beyond the first-hinge limit. Incorporating the reserve capacity in the load rating evaluation also results in levels of safety above target levels and those associated with the first-hinge limit.