Equipment fragility due to shock response: Fid-Bau Portal

Equipment fragility due to shock response

Tuan, Christopher Y.

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. This study points out the shortcomings of the SRS approach to characterizing equipment shock fragility. (1) SRS-based fragility spectra are not unique. Even for equipment that can be modeled closely by a simple spring-mass oscillator, different base excitations generally produce different fragility spectra. (2) Assuming the total response of an equipment is predominantly in the first mode can lead to significant error. In order to rigorously quantify the errors due to ignoring responses from higher modes, the exact solution to the transverse vibration problem of a cantilever beam with a tip mass having rotary inertia was presented in the shock response analyses. In conclusion, this study has revealed the inadequacy of the SRS for characterizing the shock fragility of equipment. A more rigorous analytical approach for assessing equipment fragility is needed.