Concave surface base-isolation system against seismic pounding of irregular adjacent buildings

The application of the concave surface sliders (CSSs) as seismic-isolation system of buildings is growing due to the automatic coincidence between the projection of the gravity mass centre of the superstructure and the stiffness centre of the CSSs, during the sliding phase, and self-re-centring properties, after an earthquake. These advantages make them attractive for the retrofitting of adjacent fixed-base framed buildings with irregular plan that may experience significant seismic pounding induced by torsional displacements. However, friction force and lateral stiffness of the CSSs present continuous variation during an earthquake because they are proportional to the axial load. Moreover, further changes of the friction force result from variation of the friction coefficient depending on the sliding velocity, with reduction at the onset of motion of the CSS and motion reversals, axial pressure and temperature at the sliding surface. In this work, structural pounding incidences are investigated with reference to five-storey reinforced concrete (r.c.) framed structures with an L-shaped plan placed adjacent to form T- and C-shaped plans. A simulated design of the original fixed-base buildings is preliminarily carried out in accordance to an old Italian code, for a medium-risk seismic zone and a typical subsoil class. Then, the seismic retrofitting of the residential buildings is carried out with the CSS bearings, for attaining performance levels imposed by current Italian code in a high-risk seismic zone and for moderately-soft subsoil. The design of the base-isolation systems is carried out on the assumption that the same radius of curvature is considered for all the isolators, with constant or variable dynamic-fast friction coefficients. A computer code for the nonlinear dynamic analysis of the fixed-base and base-isolated test structures is developed, in order to compare different models of the CSS bearings 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 temperature. The inelastic response of the superstructure is also taken into account by a lumped plasticity model at the end sections of r.c. frame members, where flat surface modelling of the axial load-biaxial bending moment elastic domain is adopted. Attention is focused on the pulse-type and non-pulse-type nature of near-fault earthquakes.