Shaking Table Testing of Rocking—Isolated Bridge Pier on Sand

Abstract

This article studies the seismic performance of a rocking-isolated bridge pier on surface foundation, resting on sand. A series of reduced-scale shaking table tests are conducted, comparing the performance of a rocking-isolated system to a pier founded on conventionally designed foundation. The two design alternatives are subjected to a variety of shaking events, comprising real records and artificial motions of varying intensity. In an effort to explore system performance in successive seismic events, three different shaking sequences are performed. Rocking isolation is proven quite effective in reducing the inertia forces transmitted onto the superstructure. The rocking-isolated pier is effectively protected, surviving all seismic excitations without structural damage, at the cost of increased foundation settlement. In contrast, a certain degree of structural damage would be unavoidable for the same system founded on a conventionally designed foundation. The rocking-isolated system is proven remarkably resistant to cumulative cyclic loading, exhibiting limited strength degradation even when subjected to cyclic drift ratio in excess of 5.5%. Due to soil densification, the rate of settlement accumulation is found to decrease with repeating seismic excitations. The rotational response is practically insensitive to the shaking history when the preceding seismic motions are symmetric (such as sinusoidal motions). In stark contrast, when the preceding seismic motions are non-symmetric (such as the directivity-affected records of this study), the system tends to accumulate rotation after each event, progressively worsening its performance. Nevertheless, the rocking-isolated system survives toppling collapse, even when subjected to a highly improbable, unrealistically harsh, sequence of seismic events.