Proposed load combination factors for bridges with high dead-to-live load ratios

Abstract

The combination of dead load and live load is very important in design of bridge superstructure as, in practice, it controls the strength limit states. The basic set of load factors for the Strength I limit state is 1.25 for dead load and 1.75 for live load and dynamic load. For design cases when the dead load dominates, i.e. for Strength IV limit state, the dead load factor is 1.5. The acceptability criterion for load and resistance factors in the AASHTO LRFD Code is closeness to the target reliability index, which is assumed to be 3.5 for steel and concrete girder bridges. However, the reliability analysis performed for a full range of dead load to live load ratios indicates that when live load is about 10-20% of the total load, the reliability indices are about 3.0 which is lower than the target value of 3.5. This is an indication that the reliability level is insufficient and there is a need for increasing load factors. On the other hand, for dead load constituting about 100% of the total load (i.e. no live load), the reliability index is much higher than 3.5, which means that the load factor 1.5 can be reduced. Therefore, it is proposed to change the current Strength IV load factors to dead load of 1.4 and live load factor 1.4. The result is a more uniform reliability level for all combinations of dead load and live load. The results of reliability analysis are presented in graphs.