Coupled Vertical-Rocking Response of Base-Isolated Structuresby T.-C. Pan, Bechtel Natl Inc, San Francisco, CA, USA,
J. M. Kelly, Bechtel Natl Inc, San Francisco, CA, USA,
Document Type: Proceeding Paper
Part of: Structural Engineering in Nuclear Facilities
A base-isolated building can have a small horizontal eccentricity between the center of mass of the superstructure and the center of rigidity of the supporting bearings. The structure can be modeled as a rigid block with tributary masses supported on massless rubber bearings placed at a constant elevation below the center of mass. Perturbation methods are implemented to find the dynamic characteristics for both the detuned and the perfectly tuned cases. The Green's functions for the displacement response of the system are derived for the undamped and the damped conditions. The response spectrum modal superposition method is used in estimating the maximum acceleration. A simple method, accounting for the effect of closely spaced modes, is proposed for combining modal maxima and results in an approximate single-degree-of-freedom solution. The approximate solution may be used for the preliminary design of a base-isolated structure. Numerical results for a base-isolated building subjected to the vertical component of the El Centro earthquake of 1940 were carried out for comparison with analytical results. It is shown that the effect of rocking coupling on the vertical seismic response of base-isolated structures can generally be neglected because of the combined effects of the time lag between the maximum translational and rotational responses and the influence of damping in the isolation system.
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