Case Study of Dynamic Soil-Structure Interaction

Small but measurable foundation motions were recorded during dynamic tests (reported previously by Nielsen and Wiss) upon the Chimes Tower at the New York World's Fair. Using the measured motions of the entire tower when excited at the fundamental frequency, it is possible to compute the overturning moment and shear force at the foundation. Combining the moment and force with the measured motions of the foundation at this same frequency, the rocking and swaying spring constants of the foundation can be computed. When these foundation spring constants are added to the Nielsen-Wiss model for the structure itself, the agreement between the observed and computed fundamental frequency is improved. The model including foundation springs predicts a new mode (predominantly swaying of the foundation) that was not observed in the field tests. It is postulated that this mode was suppressed by the large radiation damping that theory predicts to be associated with swaying.