Numerical Investigation of Frictional Heating in Curved Surface Sliders

The large diffusion worldwide of isolation systems with Curved Surface Sliders (also known as the Friction Pendulum System) requires detailed knowledge of their behavior and improved modeling capability under seismic conditions. An issue that has been addressed to in recent studies is the heat generation occurring at the sliding surface under large friction forces and high velocities, and the effect of the temperature rise on the friction material. In this paper the problem is approached by studying the heat state of a seismic isolation unit in numerical analyses. A three dimensional model of a curved surface slider is developed in a finite element code and subjected to the combined application of vertical load and horizontal movement according to arbitrary time histories. Heat generation is simulated through a heat source located on the pad surface, with heat flux depending on the coefficient of friction, the contact pressure, and the velocity. At the same time of the mechanical analysis, the thermal state of the isolator is calculated by solving the Fourier law, and the coefficient of friction is hence adjusted on the current temperature at the sliding surface. The numerical formulation is validated by comparing the calculated force and temperature histories to the relevant histories obtained in laboratory tests onreal scale isolators. The effect of the displacement path on the frictional heat flux is then investigated in analyses carried out under either unidirectional or arbitrary bidirectional displacement histories. The numerical approach may represent a useful tool for the selection of frictional materials accounting for their temperature-dependent characteristics. From the comparison of unidirectional respective to bidirectional displacement history analyses the importance of the loading protocol for a correct assessment of the behavior of the isolators under real earthquake attacks may be argued.