Post-Elastic Dynamic Response of Mild Steel Structures

Citation

Hanson, Robert Duane (1965) Post-Elastic Dynamic Response of Mild Steel Structures. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/76XX-4687. https://resolver.caltech.edu/CaltechETD:etd-04152003-093000

Abstract

The current philosophy of earthquake resistant design is that a structure should be able to withstand an extremely strong motion earthquake without collapsing, even though a certain amount of damage is incurred. To make such a design requires a knowledge of the dynamic behavior of structures under large amplitude vibrations. The objective of the work reported here was to investigate the dynamic frequency response characteristics of a mild steel structure vibrating in the plastic range. Comparisons were made of the static, the dynamic, and the theoretical responses of the yielding structure. The behavior of single-story structures having structural steel columns was investigated experimentally by means of horizontally applied forces generated by a shaking machine. The experimentally determined dynamic response showed the decrease in resonant frequency for increases in deflection amplitude which is characteristic of a "softening spring" type of nonlinearity. Ultraharmonic response was also observed. Under steady-state oscillations the fraction of equivalent viscous damping varied from 0.0016 at small, elastic deflections to 0.089 at large, plastic deflections. Structural deterioration was observed in both the static and dynamic experiments with a recovery of strength occurring between tests. A completely stable hysteretic loop was not attained at large deflection amplitudes, and the hysteresis loop did not become completely stable until the deflection amplitude was reduced to almost the initial yield value. However, the change in the hysteretic loop per cycle for the large deflections was small enough to permit assuming that a steady-state dynamic condition existed over a limited number of cycles. Discrepancies between the theoretical and the experimental virgin force-deflection curves were found and these are shown to be the result of simplifying approximations introduced in the structural analysis. It was found that the resonant vibrational amplitude of the structure can be predicted within 20%, and the resonant natural frequency within 2-1/2%, on the basis of the static virgin force-deflection curve. For large, plastic deflections at an excitation frequency of 3 cps, it was found that the differences between the dynamic and the static hysteresis loops were less than the changes in the static loops resulting from the deterioration caused by repeated cycles of loading.

Thesis Files