LOCAL-MODE RESONANCE AND ITS STRUCTURAL EFFECTS UNDER HORIZONTAL GROUND SHOCK EXCITATIONS

The dynamic response of building structures has been studied extensively for relatively low-frequency seismic actions, and it is established that the seismic response generally is governed by the global-mode vibration, i.e., the vibration in terms of the floor movement. Much less fundamental study has been done regarding the structural response to ground shock excitations with principal frequencies many times of the fundamental frequency of the structural system. Most of the existing code provisions on ground shock control have U001remained empirical. In this paper, it is demonstrated through numerical study and laboratory model testing that the structural response to high-frequency ground shocks have distinctive characteristics as compared to the seismic response, and most significant is the participation of the vibration at the local elemental level. Local-mode resonance could occur when the shock frequency is sufficiently high, and to a large extent it can be uncoupled from the global floor vibration. As a result, large force effects can develop at relatively small floor displacement, rendering the conventional displacement-based criteria inapplicable, while more focus on the stress-strain response is deemed necessary. The results pave a way for further development of more rational criteria for this category of the structural vibration problems.