Gravity well-inspired double friction pendulum system for bridges under pulse-like near-fault earthquakes

When a coin is tossed to a gravity well, it will spiral instead of falling directly to the center. Inspired by this phenomenon, a gravity well-inspired double friction pendulum system (GW-DFPS) is developed to extend the length of sliding trajectories of bridge superstructures during pulse-like near-fault earthquakes. As a result, a greater amount of energy will be dissipated due to the frictional sliding of the isolators. The GW-DFPS consists of a spherical surface and an outer surface described by a 1/x or logarithmic function to build gravity well. Full-scale isolators were fabricated and their response was characterized considering various parameters such as the friction material of slider, surface roughness of sliding surfaces, and applied vertical loads. Additionally, a finite element model of the isolator was created using the experimental test data. Numerical simulations were performed on a case-study bridge structure isolated using both a conventional DFPS system and the proposed GW-DFPS systems. The experimental results reveal that the proposed isolators exhibit stable response under vertical loads varying from 200 kN to 1000 kN with a negative stiffness response when the isolator slides at the outer sliding surface. The numerical simulations of the selected bridge structure demonstrate that the GW-DFPS significantly extends the sliding trajectory lengths of the superstructure during half of the earthquake pulses, resulting in increased energy dissipation during this interval. The kinetic energies of the bridge isolated by GW-DFPS are consistently lower than those of the bridge isolated by the other two kinds of isolators, resulting lower shear forces on the bridge.