Beyond conventional capacity design: towards a new design philosophy

The paper illuminates a newseismic design philosophy,which takes advantage of soil "failure" to protect the superstructure.A reversal of conventional "capacity design" is introduced, through intentional underdesigning of the foundation.A simple but realistic bridge is used as an illustrative example of the effectiveness of the new philosophy. Two alternatives are compared: one in compliance with conventional capacity design, with over-designed foundation so that the plastic "hinge" develops in the superstructure ; and one with under-designed foundation, so that the plastic "hinge" may occur in the soil. The seismic performance of the two alternatives is investigated through numerical (finite element) and experimental (shaking table) simulation. It is shown that the performance of both alternatives is totally acceptable for moderate seismic motions. For large intensity seismic motions, the performance of the new scheme is shown to be advantageous, not only avoiding collapse but hardly suffering any inelastic structural deformation. Naturally, there is always a price to pay, which is none other than increased foundation settlement and residual rotation.