The effect of modifying the fixity conditions of the bearings on the seismic response and vulnerability of existing bridges with heavy substructures is studied. For this purpose, nonlinear seismic analysis of a typical bridge with heavy substructures is conducted to assess its seismic response and vulnerabilities. Next, the bearings are modified to obtain four different configurations of bearing fixities over the substructures. Nonlinear seismic analyses of the bridge are conducted to assess its seismic response and vulnerability for each bearing fixity configuration. It is found that changing the fixities of the bearings may be an effective response modification technique to mitigate the effect of seismic forces on vulnerable substructures of the bridge under consideration. The thermal load effects on the bridge with fixed bearings are found to be generally negligible compared to seismic load effects. Thus such a response modification technique may be used for the economical design of new or seismic retrofitting of existing short- to medium-length bridges similar to that considered in this study and subjected to low- to moderate-intensity ground motions.