Vibration and impact due to multiple vehicles moving across rough bridge decks are studied in seven steel multigirder bridges with different span lengths. The bridges are modeled as grillage beam systems. The vehicle is simulated as a nonlinear vehicle model with 12 degrees of freedom according to the HS20-44 truck design loading specified by AASHTO. Four classes of road surface roughness generated from power spectral density function for the approach roadways and bridge decks are used in the analysis. The results indicate that the impact of exterior girders of shortspan bridges are highly sensitive to lateral loading position, vehicle weight, road roughness, and so forth. Maximum impact factors of girders were obtained for two trucks (side by side) through changing their transverse positions, with different speeds and road surface roughness. Results are useful for the bridge design and the further study of impact formula proposed by AASHTO.

The impact on highway bridges of vehicles passing across the spans is a significant problem of interest to bridge engineers. A considerable amount of literature exists on this subject. The literature most relevant to this study concerns code provisions, experimental impact values, and the models for vehicles and bridges used in analytical studies.· The 1989 AASHTO specifications (1) are the basis for the design of highway bridges in many countries. They specify