This research performs a finite element analysis of distortional behaviors of flyover steel bridges in Bangkok. Two types of superstructures are chosen. The first type is the composite steel I-girder superstructure of Prachanukul bridge having a span length of 25 m and two lanes. The second type is the orthotropic steel plate –girder superstructure of Wongsawang bridge having a span length of 35 m and two lanes. The effect of ten Thai truck types and AASHTO truck on the distortion behaviors including the lateral load distribution, relative deflection between adjacent girders and out-of-plane distortion-induced stress are studied. The results show that 1. the lateral load distribution factor is not dependent on the truck type. 2. the lateral load distribution factor by AASHTO LRFD 2007 equation seems to be appropriate for the design. 3. The relative deflection between adjacent girders is larger in case of one truck loading than two truck loading and 4. the out-of-plane distortion - induced tensile stress in the web gap due to all truck types is lower than fatigue threshold of category C for both superstructure types. For Prachanukul bridge, the critical girder is the exterior girder and the critical truck is truck type no. 10 (22 - wheel trailer), respectively. For Wongsawang bridge, the critical girder is the interior girder and the critical truck is truck type no. 8 (10 – wheel semi-trailer).
