Formulation of a Novel Opensees Element for FPS Bearings With Enhanced Friction Model

Abstract

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 modelled 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 velocity, 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-tocode 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.

Influence of directionality of spectral‐compatible Bi‐directional ground motions on critical seismic performance assessment of base‐isolation structures

“…Assuming the target isolator displacements are 201 mm for the DE level and 383 mm for the $$\mathrm{MC}{\mathrm{E}}_{R}$$ level, respectively, the radius of the concave sliding surface $${R}_{\mathrm{FPS}}$$ is selected to achieve the specified equivalent natural period under the displacement levels. The corresponding post yield period of FPS $${T}_{\textit{py}}$$ is 2.98 s. The velocity dependent friction model is used in representing the FPS, as Equation (), so that the dynamic friction coefficient at low velocity is $${\mu}_{\textit{LV}}=\ 0.03$$, that at high velocity is $${\mu}_{\textit{HV}}=\ 0.075$$, and the exponent to represent the velocity dependence is $$\lambda \ =\ 0.055$$ s/mm, as used in a previous study for FPS 45 $$\begin{equation} \mu \ ={\mu}_{\textit{HV}}\ -\left({\mu}_{\textit{HV}}-{\mu}_{\textit{LV}}\right)\exp\left(-\lambda \left|v\right|\right) \end{equation}$$…”

“…The corresponding post yield period of FPS 𝑇 py is 2.98 s. The velocity dependent friction model is used in representing the FPS, as Equation ( 12), so that the dynamic friction coefficient at low velocity is 𝜇 LV = 0.03, that at high velocity is 𝜇 HV = 0.075, and the exponent to represent the velocity dependence is 𝜆 = 0.055 s/mm, as used in a previous study for FPS. 45…”