Influence of Massive Substructures on Capacity Design Method

When adopting “capacity protected design method”, inertia force of piers is neglected and inertia force of pile cap is calculated by static method in the current code, which may be appropriate in terms of short and medium span bridges as the mass of pier and pile cap in those bridges is negligible and can be ignored. However, with respect to long-span continuous bridges, the bridge columnsand pile caps are large dimensional and massive,and definitely causes considerable seismic forces, which cannot simply be neglected. This paper mainly deals with the analysis of the seismic behavior of bridges characterized by massive piers and pile caps, focusing on the validity of current “capacity protected design method”. It has been shown that this design strategy may prove inadequate in long-span continuous girder beams whose seismic behavior is governed by pier or pile cap dynamics and, specifically, in case of massive piers and pile caps. Take a typical long-span continuous bridge(with massive piers and pile caps) as the project background, a series of nonlinear dynamic time-history analyses were carried out in order to assess the influence of the piers and pile caps. Compared with the results of nonlinear time-history method, the shear and moment values calculating from code-specified capacity protected design method underestimate the inertia force contribution of piers and pile caps. Therefore,as for ductile and seismic isolation design strategy, there is a need to adjust existing design guidelines to better capture the seismic response of long-span continuous bridges.