The main purpose of bridge design specifications is to ensure bridge safety so that the capacities of each component and the entire structural system exceed the associated load effects under different load combinations.As the understanding of structural behavior and loading phenomena increased,the bridge design philosophy has evolved from allowable stress design (ASD) to load factor design (LFD),and to the probability-based load and resistance factor design (LRFD).In the LRFD method the uncertainties associated with both loads and resistances are accounted for in a rational way.