Ambient vibration studies of three steel-frame buildings strongly shaken by the 1994 Northridge earthquake

Abstract

Ambient vibration surveys (AVS) can be used efficiently, cheaply and unobtrusively to identify the small-amplitude periods and modeshapes of lower modes of vibration of structures. Under Task 3.2 of Phase I of the SAC Steel Building Program, AVS were performed on three steel-frame buildings which experienced strong shaking during the January 17, 1994 Northridge earthquake. The primary objective of this study was to identify the small-amplitude modal parameters of the building for assessment of the analytical models constructed by others under SAC Task 3.1. A preliminary comparison of the modal periods obtained from the AVS with those calculated form the analytical models is presented. Although the periods identified from the AVS were considerably shorter than the model periods, the ratios of the identified to model periods for each mode were quite similar. The differences in periods are thought to be primarily because the analytical models treat only the structural frame, ignoring the stiffness of nonstructural components such as architectural partitions and building cladding. This study also serves as a first step in the testing of proposed structural health monitoring methodologies. Since one of the tested steel-frame buildings was damaged and has not yet been repaired, its data represent an "after" damage state. The tests on this steel-frame building can be followed later by an AVS if the structure is repaired. These "before" and "after" AVS results would provide valuable data to test proposed global structural health monitoring methodologies whose goal is to detect, locate and assess damage by monitoring ambient vibrations. A successful structural health monitoring method would allow "hidden" damage to be detected almost immediately rather than weeks after an earthquake.