This paper investigates both experimentally and theoretically the free rocking of a prismatic block supported by a stationary , horizontal foundation: the block is tilted, almost to the point of overturning, and released from this position. It is shown that the standard mathematical model for this problem is often inaccurate. A critical review of the implicit assumptions behind the standard model reveals that the free-rocking response of short blocks depends crucially on bouncing after each impact; out-of-plane effects are significant in very short blocks. The response of slender blocks is found to be easier to predict. Rocking has been observed during earthquakes in structures that consist of fairly rigid, unbonded elements, e.g. stacks of graphite blocks in nuclear reactors. and ancient Greek columns, and also in slender structures with foundations unable to resist uplift.

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