Nonlinear dynamic response analysis of vehicle-bridge interactive system under strong earthquakes

Seismic design codes for highway bridges and viaducts in most countries do not specify the simultaneous presence of live vehicle loads and earthquake loads. However, urban highway bridges and viaducts are loaded constantly by heavy traffic, therefore it is important to elucidate the effects of moving vehicles on the seismic response of bridges. This paper proposed a seismic response analysis framework incorporating vehicle-bridge interactions (VBI) with nonlinear dynamic analysis. The proposed method integrates a commercial FEA package, ABAQUS, into the authors' in-house VBI-solving programs developed using MATLAB. With the proposed method, the effects of vehicle dynamics on seismic responses of a highway bridge could be clarified and the seismic performance of the bridge could be checked. For the bridge and ground motions studied herein, it was observed that continuously moving vehicles might yield larger longitudinal displacement responses at pier tops and plastic deformations at pier bottoms than those of the bridge alone, implying that ignoring vehicles' additional mass effect and dynamic effects during earthquakes might be on the non-conservative side. Acceleration responses of pier tops and relative displacements of bearings were generally reduced by the moving vehicles. Besides the reason that the vehicles act like dampers on the bridge, it was most probably that the vehicle-bridge in-phase modes were excited by specific ground motions to a level lower than the original bridge-alone mode was, which could be indicated from their corresponding pseudo acceleration response spectra. Although the out-of-phase modes were excited to a similar level to the original bridge-alone mode was, these modes might dissipate some seismic energy that would have been dissipated by the bridge components and reduced displacement of the bearing.