Abstract

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 Tand 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.

Abstract

Double concave surface slider (DCSS) is considered as an effective solution for baseisolation of existing structures located in a near-fault site, because of its capacity to notably increase horizontal displacements that can be accommodated in comparison to a single concave surface slider (SCSS) of identical in-plan dimensions. However, unexpected torsional pounding of in-plan irregular adjacent structures may be induced by variability of friction force and lateral stiffness of SCSS and DCSS, depending on the axial load and friction coefficient changes during an earthquake. Effectiveness of supplemental viscous damping at the base is studied in this work with the aim to analyse its effectiveness for limiting base displacement, so avoiding too large seismic gap requirement. Structural pounding between fixed-base and baseisolated L-shaped buildings, placed adjacent to form Tand C-shaped plans, is analysed. A simulated design of the original reinforced concrete (RC) fixed-base framed structure is preliminarily carried out in accordance to a former Italian code, for a medium-risk seismic zone. Then, seismic retrofitting with SCSSs is carried out, in order to attain performance levels imposed by the current Italian code in a high-risk seismic zone, while DCSSs have radius of curvature equal to half the SCSSs and the same friction coefficient. The insertion of additional fluid viscous dampers (FVD) at the base is examined, following damping distribution inversely proportional to the distance between the stiffness centre of the base-isolation system and the plane frame where each FVD is placed. Nonlinear modelling of SCSSs and DCSSs considers 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. Automated classification algorithms using wavelet analysis are adopted to compile three datasets of seismic input rotated in the range 0°-360°, with a constant step of 15°. Distinction is made between no-pulse and velocity-pulse, the latter further categorised into non-acceleration and acceleration-pulses.