Transverse earthquake response analysis of a seismically isolated regular bridge with partial restraint

Most of the seismically isolated bridges in Taiwan share a common feature in that both ends of the superstructures are restrained from transverse movement. They are defined as partially restrained seismically isolated (PRSI) bridges. From measured acceleration responses, effectiveness of the longitudinal seismic isolation of the PRSI bridges had been confirmed. Nevertheless, the seismic isolation behavior in the partially restrained direction was not observed. The effectiveness of the partially restrained seismic isolation is evaluated in this paper. Analytical expressions are derived to assess the effect of the partial restraint on the transverse dynamic characteristics of a seismically isolated regular bridge. A non-isolated, a seismically isolated and a PRSI bridge model are constructed for a highway bridge. Nonlinear time history analyses of the bridge models under earthquake excitations are carried out to investigate their transverse seismic responses. Analytical results indicate that the composite damping ratio of a seismically isolated regular bridge may be decreased due to the partial restraint. Comparisons of the transverse earthquake responses show that the partially restrained seismic isolation is effective in column shear reduction. However, increased seismic demands on the abutment shear and the bending moments in the superstructure of the PRSI bridge are observed. It may be necessary to inspect the transverse seismic capacities of the abutments and bridge girders for practical application of the partially restrained seismic isolation. In addition, an equivalent linear analysis procedure is modified to estimate the transverse earthquake response of the PRSI bridge. It is shown that the modified procedure may be applied to practical design and analysis of the PRSI bridges.