Before and after retrofit - response of a building during ambient and strong motions

This paper presents results obtained from ambient vibration and strong-moti on responses of a thirteen-story, moment-resisting steel framed Santa Clara County Office Building (SCCOB) before being retrofitted by visco-elastic d ampers and from ambient vibration response following the retrofit. Understa nding the cumulative structural and site characteristics that affect the re sponse of SCCoB before and after the retrofit is important in assessing ear thquake hazards to other similar buildings and decision making in retrofitt ing them. The results emphasize the need to better evaluate structural and site characteristics in developing earthquake resisting designs that avoid resonating effects. Various studies of the strong-motion response records f rom the SCCOB during the 24 April 1984 (MHE) Morgan Hill (M-s = 6.1), the 3 1 March 1986 (MLE) Mt. Lewis (M-s = 6.1) and the 17 October 1989 (LPE) Loma Prieta (M-s = 7.1) earthquakes show that the dynamic characteristics of th e building are such that it (a) resonated (b) responded with a beating effe ct due to close-coupling of its translational and torsional frequencies, an d (c) had a long-duration response due to low-damping. During each of these earthquakes, there was considerable contents damage and the occupants felt the rigorous vibration of the building. Ambient tests of SCCOB performed f ollowing LPE showed that both translational and torsional periods of the bu ilding are smaller than those derived from strong motions. Ambient tests pe rformed following the retrofit of the building with visco-elastic dampers s how that the structural fundamental mode frequency of the building has incr eased. The increased frequency implies a stiffer structure. Strong-motion r esponse of the building during future earthquakes will ultimately validate the effectiveness of the retrofit method. (C) 1998 Published by Elsevier Sc ience Ltd. All rights reserved.