Investigation of the breakaway friction influence on the seismic response of buildings isolated with Curved Surface Sliders

Under a seismic excitation the response of a building isolated with curved surface sliders (CSSs) is highly influenced by the breakaway friction coefficient, which is a parameter that governs the transition between the sticking and the sliding behaviour of the isolators. Its effect is detrimental especially in case of low-to-moderate earthquakes; in fact, the inertia forces are not able to overcome the breakaway frictional resistance of the CSS isolators and the structure behaves as a fixed-base building, thus experiencing higher acceleration, inter-storey drifts and internal forces than the isolated building. Usually this parameter is disregarded because the majority of structural analysis programs implement the dynamic friction coefficient only throughout the response history analysis; this leads to a possible overestimation of the displacement demand for the isolation system but a likely unsafe design for the superstructure. In this contribution, the frictional resistance to sliding before the breakaway is simulated through a bidirectional plasticity domain, coded in a finite element of the isolator formulated in OpenSees. Based on this formulation, an exhaustive parametric analysis comprising a range of friction coefficients and superstructure properties is implemented considering a number of natural spectrum-compatible bidirectional ground motions of different intensity levels and frequency contents. Through the review and discussion of the results, the study provides insight into the implications of including the breakaway coefficient in non-linear time history analyses, and recommendations useful to predict the trigger acceleration at which sliding motion starts.