The “Spectrum Dipâ€‌: Dynamic Interaction of System Components

Abstract

In the early 1960s, many full sized surface combatants, submarines and structural models were tested with underwater explosions in order to evaluate the shock load to the ship and internal equipment structures. Initially, shock spectra were calculated from base motion measurements of internal equipment and components. Attempts were made to envelop these spectra to develop shock design spectra inputs. At that time, earthquake engineers were using this enveloping method to develop design procedures from ground motion measurements to protect structures from earthquakes. However, for the measurements on ships, this procedure resulted in calculated loads that would have caused catastrophic failure of the equipment; yet the equipment had not failed on the ship tests. As a result, the data were reanalyzed over a period of over a year. It was concluded that the dynamic interaction of each component or structure reduced the measured spectral motion at the fixedbase frequencies of the structure by about an order of magnitude. In many cases, there was a dip in the shock spectra at the fixedbase frequencies: the “spectrum dipâ€‌ phenomenon. This reanalysis led to shock spectra design curves for navy ships. This paper presents a review of an experimental study and analytical demonstration to explain the effect of dynamic interaction on the shock or response spectrum. In addition, a practical example of interaction of four single mass dynamic systems mounted on a realistic deck and subjected to a high impact shock input was studied by the authors and some of the results of that study are included.