Effect of tuned mass damper on displacement demand of base-isolated structures

The third author of this paper has previously proposed the installation of a tuned-mass damper (TMD) to reduce the displacement demand on a base isolated structure. The TMD consists of a mass-dashpot-spring subsystem that is attached to the isolated superstructure, analogous to a pendulum. The present paper examines the effectiveness of this scheme and determines optimal parameters for the design of the TMD. Both the base-isolated structure and the TMD are modeled as single-degree-of-freedom, linear oscillators. The optimal TMD parameters are determined by considering the response of the base-isolated structure, with and without the TMD, to a white-noise base acceleration. Such an excitation is representative of broadband ground motions having a nearly constant intensity over a duration several times longer than the period of the base-isolated structure. It is found that, under such an excitation, a reduction of the order of 15%–25% in the displacement demand of the base-isolated structure can be achieved by adding the TMD. Next, the responses of an example base-isolated structure with and without an optimally designed TMD to selected suites of far- and near-field recorded accelerograms are determined. The study shows that for far-field ground motions the effectiveness of the TMD is more or less similar to that predicted by the white noise model, whereas for near-field ground motions the effectiveness of the TMD is less, i.e. of the order of 10% or less. Reasons for this result are described.