Many structures exhibit significant rate of loading effects that suggest that they need to be tested at or near real time. In the case of friction-isolated structures, this rate dependency poses a major problem for tests involving quasi-static hybrid simulations where tests are executed slower than real-time. In an effort to examine the behavior of seismically isolated structures with the isolation plane located in the upper stories of the structure rather than at the base, hybrid shake table tests were performed where the substructure was modeled numerically, and the isolation system and superstructure were tested experimentally. An isolation plane that is located in the upper stories provides a means to reduce higher mode effects on the response of the structure. Furthermore, the isolated portion of the structure provides supplemental inertial based energy dissipation on the overall seismic response of the tall building.
For this series of experimental tests, a seismically isolated two-story steel moment resisting frame, resembling the superstructure of the tall building, was placed on a specially designed and constructed unidirectional hybrid shake table. The moment frame test specimen was supported on six triple friction pendulum isolators, designed to minimize accelerations in the upper portions of the building and to increase energy dissipation at the isolation level. The structure was then subjected to a number of excitations where the height and period of the substructure below the isolation plane is varied to examine the effectiveness of the isolation concept for different building configurations, and the ability of the hybrid simulation method to accurately test midlevel seismic isolation concepts is also evaluated.