Damage suffered by slab-on-girder steel bridges during low-to-moderate seismic excitations reveal the deficiency of standards and codes and indicate the necessity of obtaining insight to the lateral load path and the capacity of individual component and assembled systems. In this paper, to improve the seismic performance of this type of bridges, the lateral load path was initially studied. Subsequently, thin steel infill plates were placed on the seismic load path to dissipate the imposed energy. The results indicate that the use of stocky infill plates with common yielding stress as end diaphragms leads to elastic behaviour of the end diaphragm, at least up to displacement corresponded to the ultimate strength of the substructures, whereas thinner infill plates with low yield stress exhibit an appropriate energy dissipation capacity, and a significant amount of transmitted lateral load will be decreased through their plastic deformation.