Internal Pounding between Structural Parts of Seismically Isolated Buildings

Pounding among internal structural parts of seismically isolated structures may happen because of high horizontal deformability of the isolation system. To study this poorly investigated issue, a reinforced concrete (r.c.) framed structure built in Sicily (Italy) is considered, with a hybrid isolation system combining elastomeric and sliding bearings. The building consists of four-storey, basement included, with the steel-framed structure of the elevator crossing the isolation level. Two structural configurations are considered for the elevator shaft: base-isolated, with rigid link to the surrounding building; fixed-base, with a seismic gap to prevent internal pounding. Effectiveness of the Italian and European code provisions, based on setting up adequate separation distance, is assessed. Four modelling assumptions of the highly nonlinear behavior of elastomeric bearings are examined and a mathematical model is calibrated on experimental data. Nonlinear seismic analysis of the test structure is carried out considering eight cases, which are obtained combining four position of the elevator (i.e., at the basement and upper three levels) and two load conditions (i.e., elevator empty and with maximum load). Torsional effects induced by an asymmetric position of the elevator shaft are quantified by means of polar plots, representing the critical envelope for all excitation angles of bi-directional earthquakes.