PLANAR AND SPATIAL INVESTIGATION OF EARTHQUAKE INDUCED POUNDING OF BASE ISOLATED BUILDINGS

Seismic isolation is utilized in relatively stiff buildings to avoid resonance with the predominant frequencies of typical seismic excitations and reduce the induced seismic loads, floor accelerations and interstory drifts, while large strains are confined at the isolation level. Although a wide seismic gap is provided around a base isolated building (BIB) to accommodate the expected large relative displacements at the isolation level, since its width is often finite, there is a possibility of pounding with the surrounding moat wall or adjacent structures during a stronger than expected earthquake excitation. This research work investigates, through both planar and spatial simulations, that possibility and assesses how potential structural pounding may affect the effectiveness of seismic isolation. Conducted planar analyses reveal that the peak interstory drifts ratio of a BIB increases significantly due to structural impact with the adjacent moat wall, reaching values that are several times the corresponding peak response values without pounding, and that such an increase is, in general, amplified as the width of the seismic gap between the BIB and the adjoining moat wall reduces. Subsequently, three-dimensional (3D) analyses are used to take into account other spatial factors, such as the incidence angle of the imposed seismic excitations, potential mass eccentricities and torsional effects, which cannot be considered through planar simulations. Multi-degree-of-freedom (DOF) systems of 3 dynamic DOF at each floor level, representing base isolated buildings, with impact capabilities, subjected to two orthogonal seismic components, of which the incidence angle may vary are simulated using a specially designed 3D software. The developed software enables the spatial simulation of buildings modeled as 3D MDOF systems with shear-type behavior, while the nonlinear inelastic bidirectional coupled Bouc–Wen model is employed for simulating the isolation system, which is assumed to consist of lead rubber bearings (LRBs). The software enables the consideration of pounding with both the surrounding moat wall and adjacent conventionally fixed-supported buildings. The conducted spatial simulations show that the critical angle of incidence differs, depending on the characteristics of the imposed seismic excitations. Thus, the customary practice of imposing the earthquake excitations along the major construction axes of the BIB may lead to significant underestimations of the peak structural response in case of pounding during strong earthquake excitations. Moreover, the seismic incidence angle influences significantly the width of the required seismic gap that should be provided as clearance to avoid structural impact. In addition, the effect of the directionality of the imposed seismic ground motions, as well as the number of floors and the fundamental eigenperiod of the adjacent structures, influence the possibility of structural pounding and the severity of the peak structural response in case of impact during severe seismic excitations.