Alternative Load Paths in Steel through-Truss Bridges: Case Study

Abstract

Conventional design and evaluation procedures usually classify steel through-truss bridges as single-load-path structures; however, their historic performance has demonstrated considerable structural resiliency. This paper presents a study of the Grand River Bridge, a Pratt through-truss bridge in Cayuga, Ontario, Canada, that systematically investigates the load paths not conventionally assumed in design and evaluation. The bridge with individual truss members removed was analyzed using nonlinear finite-element analyses to investigate the alternative load paths and associated critical members and responses. A system reliability analysis was conducted to evaluate the failure probabilities of the bridge accounting for the member resistance uncertainties. The bridge was found to be sufficient to carry its nominal dead load, even for the worst case of removal of an end post. The end bottom chord and hanger vertical were found to be the critical tension chord and web member, respectively. If multiple alternative load paths exist, the variability of the system collapse load is much less than that of the critical member resistance.