The Earthquake Engineering Online Archive

A practical soft story earthquake isolation system

Kelly, James M.; Eidinger, John M.; Derham, C. J.

UCB/EERC-77/27, Earthquake Engineering Research Center, University of California, Berkeley, 1977-11, 150 pages (530/K38/1977/(27))

This report describes the experimental and analytical results of a practical earthquake isolation system. The experimental work was carried out using a 20 ton three-story single-bay moment-resistant steel frame structure on the 20 by 20 ft shaking table at the Earthquake Engineering Research Center at the Univ. of California, Berkeley. The soft-story isolation system is composed of elastic natural rubber bearings and a highly nonlinear energy-absorbing device, all placed beneath the base floor of the model structure. The bearings allow for lateral movement of the base of the model and are designed so that no adverse column P-Delta effects can occur. The energy-absorbing devices act as highly efficient dampers and are based upon the two-way plastic torsion of steel bars. For smaller earthquakes, the structure behaves as it would with a rigid foundation. For large earthquakes, the structure's first mode period increases from 0.6 to 1.0 sec and equivalent first mode damping is between 30% and 35%. Thus, for destructive earthquakes, the use of the isolation system typically reduces the structure's response by over 50% of that of a conventional rigid foundation structure. An inelastic time-history analysis gives good correlation with experimental test data. A simple design procedure based upon elastic response spectra is suggested. A full-scale structure located in a seismic zone and built with such an isolation system achieves two major cost benefits over a conventional structure: (1) lower initial construction costs due to reduced lateral load requirements; (2) lower earthquake-caused repair costs due to decreased structural and nonstructural damage.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-77-27.pdf (73 MB)