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PROCEEDINGS OF THE ELEVENTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
An Insight on the Influence of Damping in Seismic Isolation
E. Figueiredo and R.C. Barros
Structural and Earthquake Engineering Division, Department of Civil Engineering, University of Porto, Portugal
E. Figueiredo, R.C. Barros, "An Insight on the Influence of Damping in Seismic Isolation", in B.H.V. Topping, (Editor), "Proceedings of the Eleventh International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 201, 2007. doi:10.4203/ccp.86.201
Keywords: seismic isolation, damping, coupled vs. uncoupled equations.
The purpose of this paper is to clarify the influence of damping in seismic isolation, limiting some current simplifications to the use of classical modal analysis with uncoupled equations and also limiting peak responses of isolated structures located at near-fault sites. The well known two-DOF system was used to carry out the seismic analysis for several real earthquakes, for which some conclusions are proposed. Firstly, the modal analysis with classical damping (uncoupled equations) is of good accuracy with respect to the exact solution (coupled equations) when the isolator damping is below 10% of the critical damping. On the other hand, the same results showed clearly that for isolator damping above 10% the responses start to diverge and consequently the classical damping is not valid. Thus, the equations cannot remain uncoupled and complex analysis should be done in order to include the off-diagonal terms of the damping matrix. Secondly, the addition of damping in the isolator system, while controlling the isolator displacements, has the counter effect of increasing the inter storey drift and total floor accelerations. It was pointed out that the increase of isolator damping to 50% of the critical damping can reduce the isolator displacements to about 50%, with just a slight increase of inter-storey drift and total acceleration of the floor. However this increase of total acceleration needs to be interpreted carefully because, although not being an issue of life safety, it is possible that large enough floor accelerations can be generated to damage non-structural elements and equipment, in addition to disturbing occupants .
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