Seismic Fragility Assessment of Asymmetric Structures Supported on TCFP Bearings Subjected to Near-field Earthquakes

Seismic Fragility curves of asymmetric steel structures isolated with Triple Concave Friction Pendulum (TCFP) bearings are studied in this paper. To consider the record-to-record uncertainty an ensemble of 45 sets of nearfault ground motion is selected. The main sources of eccentricity in torsionally coupled base-isolated structures, namely mass eccentricity in the superstructure and stiffness eccentricity in isolators are investigated elaborately. The seismic performance of structures is studied through the damage states (DS) framework introduced by HAZUS-2003 including slight, moderate, extensive, and complete structural damage states. In order to quantify the damage states, two engineering demand parameters i.e. interstory drift and acceleration are implemented. Three-dimensional nonlinear analysis of steel moment-frame superstructures mounted on a reasonable variety of TCFP bearings shows that among different types of eccentricity, mass eccentricity of superstructure affects the fragility characteristics more than the other. Generally, torsion due to mass eccentricity decreases the seismic intensity of different damage states; however, this reduction is more noticeable in slight and moderate ones. The 5% damped spectral acceleration at the isolation system effective period, Sa(Teff) reduces up to 20% in slight DS because 30% mass eccentricity, while this reduction is nearly 2% in the collapse DS. It affirms that TCFP bearing is capable of controlling great responses through strong ground motions. Furthermore, it is demonstrated that mass eccentricity will increase the risk of collapses calculated through the FEMA P-695 method which considers earthquake, modeling, design and test data uncertainties.