Formulation of a novel OpenSees element for FPS bearings with enhanced friction model

The new “CSSBearing_BVNC” element has been coded in the object-oriented finite element software program OpenSees to represent the behavior of the Friction Pendulum System® (FPS) comprising one concave sliding surface and a spherical articulation, accounting for an enhanced formulation of the friction behavior. In the novel element, the hysteretic force – displacement relationship of the FPS bearing in the horizontal direction is mathematically mod-elled using the theory of plasticity, and two yield conditions are used to account for either the static or the dynamic friction coefficient. Other features of the friction model are the change of the dynamic coefficient of friction with the instantaneous values of axial load and slide ve-locity, and its degradation due to heat generated during cyclic motion. The primary assumptions in the development of the friction model are verified in a code-to-code comparison. A case study relevant to a base-isolated concrete, moment resisting frame is presented to demonstrate the improved prediction capability of the new bearing element over its standard counterpart, such as estimating a +40% increase in superstructure drift and column shear force and a +58% increase in displacement during high intensity earthquakes, and up to a +130% increase in internal forces and deformations of the structure under weak or medium intensity earthquakes due to the possible non-activation of the sliding isolators.