A model order reduction via Krylov subspace has been utilized as a booster of the finite element analysis in this paper. In the seismic design of structures, low frequency responses up to about 20Hz should be accurately predicted. A stepwise iterative frequency response analysis is utilized to evaluate an effective reduced order to maintain the accuracy at the target frequency range. In the frequency analysis, simple error estimation is performed to check the comparison is supported by the Moment Matching (MM), which is one of the important approximation properties of the KS-MOR. In the numerical example, a 3D bridge model with different bearing system was solved to show performance of KS-MOR. One of the bearing system is classical steel hinge, the other is steel laminated rubber bearing. The KS-MOR can apply to the general dynamic structure analysis including the damping matri. The efficiency in the CPU time is about 20-30 times faster than the conventional FEM calculation.