LOAD MODEL FOR HIGHWAY BRIDGES

The paper deals with the development of load models for LRFD bridge design code. The considered load components include dead load, static live load and dynamic live load. The statistical data is based on the available truck surveys. Distribution functions are derived for moments and shears. The distribution functions are extrapolated to calculate maximum moments and shears for longer time periods, up to 75 year life time for a newly designed bridge. Live load model is derived for one lane and multi-lane bridges. It was observed that for the spans up to about 30-40m lane loading is governed by one truck. For longer spans, the largest effect is produced by two trucks following behind each other. The correlation between truck weights is considered. For two lane bridges, the maximum load effect is caused by two fully correlated trucks, side-by-side in adjacent lanes. The maximum 75 year moment in a grider is caused by two maximum 2 month trucks. Each such a truck is about 85% of the maximum 75 year truck. Dynamic portion of live load is modeled by simulations. Three parameters are considered: bridge dynamics, vehicle dynamics and road roughness. Extensive simulations indicate that dynamic load, as a fraction of live load, decreases with increasing truck weight. It was also found that dynamic load caused by two trucks side-by-side is lower (as a fraction of live load) than the effect of a single truck.