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Organization Title

OSMS 86-02

"Recorded Motion of a Base-Isolated Building During the 1985 Redlands Earthquake"

by M.J. Huang, A.F. Shakal and J.T. Ragsdale

Huang, M.J., A.F. Shakal and J.T. Ragsdale (1986). Recorded Motion of a Base-Isolated Building During the 1985 Redlands Earthquake, from ATC seminar on Base Isolation and Passive Engery - Dissipation Devices, San Francisco, March 12-13, 1986.

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Introduction

Low-level acceleration records were obtained in a recently-instrumented base-isolated building. The San Bernardino County Law and Justice Center in Rancho Cucamonga is a 4-story braced steel-frame structure isolated on elastomeric bearings located between the basement and foundation levels. The structure is approximately 414 by 110 feet in plan and 74 feet from the basement to the roof. The building is instrumented with a total of 16 accelerometers which are located a t the foundation, basement, 2nd floor and roof levels. An additional triaxial set of accelerometers is deployed in the free-field approximately 330 feet from the building. An earthquake of magnitude 4.8 (ML)occurred on October 2, 1985 near Redlands, at a distance of about 30 kilometers from the building. The records obtained at the building during that earthquake have been digitized and processed. The purpose of this paper is to highlight aspects of these data and present some preliminary interpretation of the structural response indicated by these low level records.

The peak horizontal acceleration was 0.036 g at the foundation level (below the isolators), 0.016 g at the basement level (above the isolators), and 0.029 g at the roof level. The peak vertical acceleration was approximately 0.02 g below and above the isolators. The computed peak horizontal dynamic displacement is 0.07 cm below the isolators, 0.08 cm above the isolators, and 0.19 cm at the roof.

Preliminary analysis of the data indicates that for this low-level motion the structure exhibited a fundamental made period of about 0.6 second in both the transverse and longitudinal directions and that the modal damping is about 4 percent of critical. The building underwent both torsional motion, roof relative to the basement, as well as rigid body rotation of the entire structure above the isolators. In terms of translational motion, lateral structural deformation dominated over rigid body translation and rocking. The structure above the isolators behaved as a 0.6 sec period conventional structure. The data indicate that the period of the entire structural system did not shift to longer period for this low-level input motion.