Applicability of simple expressions for bridge system reliability assessment

The performance of a structural system can often be represented by a single limit state function based on probabilistic finite element analysis; in this case, the associated system reliability index can be computed by using first-order second-moment method (FOSM). This method is exact if both the load effects and the system resistance follow normal or lognormal distributions. However, the amount of error introduced can be significant when the random variables follow distributions other than normal or lognormal. In fact, it is reasonable to represent the maximum intensity of the live loads on bridge structures using extreme value distribution especially when supported by truck load survey data. In this paper, the amount of error associated with the simple expressions based on FOSM to compute system reliability index is investigated. The system reliability index, for a selected bridge superstructure under extreme value type I largest random live loads and lognormal system resistance, is computed for varying coefficients of variation and mean values of the load effect and the system resistance using the first order reliability method (FORM) and the simple expressions based on FOSM. The amount of error introduced by using these expressions instead of FORM is presented.