Study on the Effect of Uplift-Restraint on the Seismic Response of Base-Isolated Structures

Under certain conditions isolation bearings can sustain undesirable tensile forces or uplift, the occurrence of which potentially entail detrimental effects on the isolation-system and the superstructure. This study reports on the effect of uplift-restraint, provided by the $XY$ -friction-pendulum (FP) sliding isolation-system, on the seismic response of base-isolated structures. Belonging to the new generation of isolation hardware, the uplift-restraining FP isolator $\left(XY\text{-FP}\right)$ is capable of providing uplift-restraint to the superstructure by virtue of its inherent property of developing tension. To better understand the phenomena of uplift or tension of isolators and their impact on the performance of structural and isolation-systems, nonlinear response-history analysis of a seismically-isolated real building subjected to bidirectional horizontal seismic excitation is conducted. The analysis was performed with an enhanced version of program 3D-BASIS-ME, which was modified to include an element representative of the mechanical behavior of the tension-capable $XY$ -FP isolator. The results demonstrate that tension in individual $XY$ -FP isolators, and by extension the increase of isolation-system friction force, did not have any appreciable effect on either the total isolation-system response or the superstructure response.