A simplified method to estimate the fundamental frequency of simple span bridges supported on lead rubber bearing

The impulsive load method applied on the mid-span of a girder is often used to measure the fundamental frequency of the bridge. This method commonly involves a truck that is dropped from a certain height to generate an impulsive load. In high seismicity zones, bridge structures often involve base isolators such as lead rubber bearing (LRB) to reduce the seismic force demand. However, the application of LRBs on the bridge girders may reduce its vertical fundamental frequency. This fundamental frequency may decrease further if the girder is supported on the frames or cantilever beams on high columns. In this study, the fundamental frequency of several bridge spans is computed through a simplified analytical study based on a single degree of freedom assumption. The results show that the application of LRBs does not significantly affect the fundamental frequency of the girder when compared to the identical girder supported on hinge-roller supports, due to the high vertical stiffness of the LRBs. In average, the values of fundamental frequency obtained through the simplified method and the FE modeling are lower than which obtained from the dynamic loading test. It indicates that the stiffness of the existing bridge is higher than the analyses.