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Bridge support boundaries usually change in an operating environment, such as the deterioration of elastic constraints caused by bearing damage and pier scouring. The dynamic response of a bridge under a moving vehicle load may be amplified due to changes in the support boundaries. In this paper, the vibration characteristics of elastically supported bridges under moving vehicle loads are investigated through a numerical model accompanied by a dynamic impact factor (IM) to estimate the amplification effect. A beam elastically supported at both ends and a simplified two-degree-of-freedom moving spring mass system are used to construct a vehicle–bridge interaction model, followed using the road surface condition (RSC) as an input and the Newmark-beta method to solve the interaction equation to obtain the dynamic response of the bridge. Finally, the impact factor is calculated, and a case study including bridge and vehicle parameters is carried out to evaluate the impact of the boundary deterioration on the dynamic response of the bridge. The results show that the dynamic response of the elastically supported bridge increases with decreasing bridge span and vehicle weight, as well as the deterioration of the RSC and support boundary, when the degradation of the support boundary is serious, the IM may exceed 1.0 even if the RSC is in good condition, which suggests that maintaining the flatness of the bridge deck and the integrity of the support boundaries can reduce the impact of the bridge dynamic effect.

Many of the existing multi-span simply supported bridges in New York State, U.S., are susceptible to earthquake damage and need to be retrofitted to reduce their seismic risk. In this study, seismic retrofit of a five-span simply supported bridge with typical high-type fixed and expansion steel rocker bearings is conducted. A refined three-dimensional (3D) finite element model of the bridge is developed in ANSYS by considering foundation impedances. Multi-support time history analyses have been implemented in the seismic retrofit design for two levels of ground motions: 1,000- and 2,500-year return period earthquakes. The site-specific ground motions with consideration of the spatial variation are generated based on the geotechnical information. Seismic retrofit by replacing existing steel bearings with lead-rubber bearing (LRB) isolators has been adopted. The parameters of the isolators are determined by considering factors such as the seismic performance and translational resistance during normal service. The vulnerability of structural members and seismic retrofit effectiveness are quantified by the demand-to-capacity (D/C) ratio for the combined demands at the extreme limit state. The analyses show that after seismic isolation retrofit the pervasive vulnerabilities in pier columns and cap beams are eliminated. Comparing with strengthening the vunerable structural members, seismic isolation is proved a cost-effective retrofit solution. The overall seismic isolation design and analysis procedures presented in this study can help guide future seismic retrofit of similar types of bridges.