A novel OpenSees element with improved friction model for Friction Pendulum System isolators

The increasing use of sliding bearings with curved surfaces, like the Friction Pendulum System® (FPS), as seismic isolators benefits from the improvement of numerical models able to capture their experimental behavior and enhance the predictive capability of nonlinear response history analyses. Nevertheless, the effective implementation of the static coefficient of friction of FPS isolators in software programs for structural analysis has not yet been achieved, and the use of dynamic friction only is a common practice in design. In this study, a novel element has been formulated in the object-oriented finite element software OpenSees by modifying the standard “SingleFPSimple3d” element to represent the behavior of a FPS isolator comprising one concave sliding surface and a spherical articulation accounting for the static coefficient of friction at the breakaway; other features are the inclusion of the dependency of the dynamic coefficient of friction on the instantaneous values of axial load and slide velocity at the interface, and its degradation due to heating during cyclic motion. The new features of the friction model are verified in a code-to-code comparison. A case study dealing with to a reinforced concrete, moment resisting frame is eventually presented to demonstrate the improved prediction capability of the new FPS element over its standard counterpart