Effects of near‐fault acceleration and non‐acceleration pulses on pounding between in‐plan irregular fixed‐base and base‐isolated buildings

Abstract

Concave surface sliders (CSSs) are considered as an effective solution for retrofitting adjacent buildings irregular in plan that could undergo significant seismic pounding due to torsional displacements. This is because during the sliding phase there is coincidence between the projection of the gravity mass centre of the superstructure and the stiffness centre of the CSSs. However, unexpected torsional pounding, with amplification in a near-fault site, may be induced by variability of friction force and lateral stiffness of the CSSs, depending on the axial load and friction coefficient changes during an earthquake. In this work, structural pounding between fixed-base and base-isolated reinforced concrete (RC) L-shaped buildings, placed adjacent to form T- and C-shaped plans, is investigated. A simulated design of the original fixed-base framed structures is preliminarily carried out in accordance to a former Italian code, for a medium-risk seismic zone and a typical subsoil class. Then, seismic retrofitting with CSSs is carried out, to attain performance levels imposed by the current Italian code in a high-risk seismic zone and for moderately-soft subsoil. A computer code for the pounding analysis between fixed-base and base-isolated test structures is developed, in order to compare effects of nonlinear models of the CSSs that consider constant and variable axial load combined with friction coefficient at breakaway and stick–slip and as function of the sliding velocity, axial pressure and rising temperature at the sliding interface. Attention is focused on the pulse-type nature of near-fault earthquakes generally observed in the velocity time histories but largely overlooked in the acceleration ones. To this end, automated classification algorithms using wavelet analysis are adopted to compile three datasets of seismic input distinguishing between no-pulse and velocity-pulse, the latter further categorised into non-acceleration and acceleration pulses and rotated along the direction most likely to contain strong pulses.