A Method for Correlating Severity of Different Seismic Qualification Tests

A general methodology has been developed for correlating the severity of various seismic qualification motions that can have very different dynamic characteristics. The approach is based on a vibrational equivalence concept which allows a severity comparison between two different waveforms. The comparison is in terms of a fragility ratio, D_FR, which is a ratio of vibrational input level to that which the specific equipment is capable of sustaining at its fragility level. Measurement of the severity at both levels can be characterized in terms of response spectrum, power spectrum, or other parameters which may be used, or have been used, in typical equipment qualification procedures. Relationships among the various parameters are defined so that transformations from one to another are possible. The inherent use of the fragility function for the methodology causes some problem in that such data are not generally included in previous qualification information. This problem is overcome by defining a lower-bound, or acceptable approximate fragility function, which is based on the previous qualification levels. A method of measuring relative severity of two motions is also developed in terms of a relative severity ratio, D_SR. This ratio is proportional to several other relative severity factors that have been established by other researchers.