Abstract
Attempts to combine shock spectra, by envelope of fiducial limit types of analysis, to produce curves useful for design purposes have led to design shock spectra which are extremely overconservative. It was noticed that this technique assumes that the impedance of any structure at its foundation for its fixed base natural frequencies is very low, and therefore unable to affect the foundation motion. This was in contrast to test evidence that a dip would appear in the regions of these frequencies in the spectrum plot. This paper examines the theory of shock spectra, Fourier spectra, and mechanical impedance to determine qualitatively the effect upon shock spectra of the dynamic reaction of structures. The concept of shock spectrum dip is presented, explained, and experimental verification exhibited. Several theorems regarding shock and Fourier spectra are advanced, and a definition of shock spectrum proposed. The necessary distinction between shock spectrum values at fixed base natural frequencies of a structure and at natural frequencies of a structure-foundation combination is demonstrated. The potentially extreme overconservatism in design resulting from incorrect usage of shock spectra is pointed out.
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