Differential Settlement Effect On Reliability Of Bridge Superstructure Based On LRFD Design Theory

Maximum deterministic differential settlement is used in current LRFD design specification. However, the deterministic differential foundation settlement is quite different from the actual settlement due to the soil’s large variability. This thesis investigates the effects of the random differential foundation settlement on the reliability of bridge superstructure. A lognormal distribution with a coefficient of variation of 0.25 of random settlement is used in reliability analysis. Dead and live loads are modeled as random variables with normal and Extreme Value I Distribution, respectively. Then, the reliability of bridge superstructure considering random differential settlement effect is calculated by using the First Order Reliability Method (FORM). The effect of a live load reduction factor on bridge reliability is investigated. The reliability indices of 2 existed bridges and 31 new designed multi-span girder bridges are calculated.It is demonstrated that small change in differential settlement can significantly reduce the reliability of the bridge superstructure, depending upon the span length and rigidity of the girder. The second moments and shears of the bridges A6569 and A3973 due to settlement effect are analyze. The results show that the bridge reliability index is significantly reduced when the settlement is increased.For the 31 new designed bridges, the cross section is based on the minimum requirement of the worst load combination, therefore, the reliability indices of the new designed bridges are smaller than the values for real bridges. The bridge can can tolerate an extreme settlement of L/500 without reduced live loads and L/2500 with reduced live loads in the case of a bridge reliability index of 3.5. The reliability analysis results also indicate that without reduced live loads, current AASHTO specification with a load factor of 1.0 for the differential settlement has enough safe margins within an allowable settlement of L/250. However, when live loads are reduced, the load factor for settlement has to be amplified.