Keywords: base isolation, seismic isolation, peak seismic response, near vs. far fault, incidence angle, torsional effects.

This research work investigates the seismic behavior of a typical 2-story steel base-isolated building (BIB) under different seismic ground motions, using the SAP2000 software, while the effect of near-fault ground motions is studied by imposing 4 pairs of near-fault (NF) and 4 pairs of far-fault (FF) seismic excitations to the building. Furthermore, the influence of the direction of the imposed earthquake excitations and potential accidental mass eccentricities are also studied by rotating the imposed pairs of seismic records from 0 to 360 degrees and by considering 5% and 10% accidental mass eccentricities, respectively, in the conducted parametric analyses. The results indicate that the NF seismic components are more likely to increase the peak seismic response of the building than the corresponding FF components. The computed response, while rotating the imposed pairs of seismic records from 0? to 360?, with respect to the major construction axes, shows that the peak seismic response, in general, occurs along incidence angles other than the horizontal construction axes of the building. Moreover, it is observed that, under each seismic event a different incidence angle is the critical one. The determination of the critical incidence angle is hence complicated, and different dynamic simulations should be performed for each building, especially if it is a high-importance building, in order to obtain a more reliable assessment of the peak seismic response under the worst-case scenario regarding the incidence angle. Furthermore, considering the influence of 5% and 10% accidental mass eccentricities indicates that when accidental mass eccentricities are taken into account the peak relative displacements of the base isolated structure are, in general, increased, which is very critical for the proper estimation of the required seismic gap that should be provided around a BIB to avoid potential structural pounding with adjacent structures or the perimetric moat wall. Nevertheless, the peak floor accelerations at the superstructure have minor differences between the symmetric building and the two buildings with accidental mass eccentricities, which leads to the conclusion that the superstructure of a base-isolated building may not be significantly affected by the accidental mass eccentricities.

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