Equipment Fragility Due to Shock Response

Because of its simplicity, the shock response spectrum has become widely used as a means of describing the shock responses and fragilities of structures and equipment. This paper focuses on the drawbacks of using the shock response spectrum for defining equipment shock tolerance. A cantilever beam with a tip mass was used to model a hypothetical piece of equipment subjected to strong ground motion such as that caused by an explosion. The exact solution from a detailed modal analysis shows that multiple modes of response were excited. Contributions from higher modes can be more predominant than that from the fundamental mode. Assuming the total response of equipment is predominantly in the first mode can lead to significant error. Current shock spectrum procedures for equipment fragility assessment are inadequate, not only because of the physical limitations of shake table tests, but also because of the lack of a reliable analytical model.