Rocking isolation of bridge piers resting on spread foundations

Student: Panagiotis Mergos
Supervisor: Prof. K. Kawashima

Abstract

Spread foundations are widely used to support bridge piers where the soil condition is stable. It has been observed after some earthquakes that a number of structures resting on spread footings had responded to seismic excitation by rocking on their foundation and in some cases this enabled them to avoid failure. Through application to a specific bridge this report discusses the isolation effects of foundation rocking.

First, some modelling issues and difficulties are addressed. Then, the difference in response is examined when foundation uplift is taken into account. This report focuses on bidirectional excitation effects which seem to amplify foundation uplift.

It is found that foundation rocking has indeed an isolation effect but this can be jeopardized by the vertical accelerations caused by the impacts of the footing with the underlying soil.

The sensitivity of the results to the various data is emphasized and parametric analyses are carried out. More particularly, the influence of the following parameters is examined: (i) the moment capacity of the pier, (ii) the size of the footing, (iii) the stiffness of the underlying soil, (iv) the stiffness of the underlying soil in tension, (v) the yield strength of the underlying soil, (vi) the excitation pattern, (vii) the moment capacity of the pier in the perpendicular direction.

Finally, the high computational effort of performing such analyses is recognized and an alternative method is proposed. In particular, the use of a nonlinear spring is suggested that can fully replace, in two dimensions, Winkler foundation model on elastic soil.

You may download a digital version of this MSc dissertation here.