The Earthquake Engineering Online Archive

Estimating the seismic response of base-isolated buildings including torsion, rocking and axial-load effects

Ryan, Keri L.; Chopra, Anil K.

UCB/EERC-2005/01, Earthquake Engineering Research Center, University of California, Berkeley, 2005-06, 231 pages, pdf (400/E177/2005-01)

A rational procedure to estimate the peak response of a lead-rubber bearing isolation system, based on rigorous nonlinear analysis, is developed. The procedure offers an alternative to the iterative equivalent-linear methods used by current U.S. building codes. The governing equation is reduced to a form such that the median normalized deformation of the system due to an ensemble of ground motions with given corner period T subscript d is found to depend on only two parameters: the natural isolation period, defined from the post-yield stiffness, and the normalized strength, or strength normalized by peak ground velocity. The dispersion of normalized deformation for an ensemble of ground motions is shown to be small, implying that the median normalized deformation is a meaningful estimate of response. The simple trends shown by the median normalized deformation led to the development of suitable design equations for isolator deformation. The procedure is extended to more complex models of the isolated building, asymmetric-plan systems and buildings isolated with the friction pendulum system. The aspects of response neglected in existing models for isolation bearings are discussed, and improved bearing models are presented. The isolated structure model is modified to include rocking about one axis and to incorporate the improved bearing models. All previous modeling concepts are integrated in a final three-dimensional (3D) analysis of the response of the isolated block.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-2005-01.pdf (11 MB)