Evaluation of Seismic Performance of Tall Bundled Tube Structural System Under Multiple Earthquakes

Provisions and recommendations of seismic codes are generally based on the determination of design earthquake and the effects of aftershocks is neglected. Moreover, in regions with high seismicity risk, aftershocks with different magnitude and occurrence time would take place following the main-shock. The main purpose of this paper is to evaluate seismic response parameters of a twenty-story bundled tube frame subjected to three-components near-field ground motions considering the effects of aftershocks. Spectral and physical characteristics of aftershocks encompass a wide range of magnitudes. Hence, the ground shaking modeling procedure is conducted via different ratios of aftershock peak ground acceleration divided by that of the proposed main-shock. The main characteristic of strong ground motion records used in this research is the existence of consecutive long duration, high amplitude velocity pulses. Occurrence of successive strong aftershocks leads to accumulation of permanent displacement as well as residual drift in the structures. In order to generate aftershocks, the randomized approach was applied in this study. The response parameters studied in the current paper include maximum story drift envelope, story residual drift, plastic hinge mechanisms and induced axial forces due to shear lag effect. The results obtained from analyses reveal that the amount of story drift and its variations, are directly proportional to the presence of rupture directivity effects in earthquake record as well as the ratio of aftershock peak ground acceleration to that of the main-shock, i.e. PGAas/PGAms. Moreover, residual drift under earthquake tremors containing successive aftershocks, either free field records (related to the actual dynamic behavior viewpoint) or scaled ones (related to the performance based design viewpoint) represents considerable variations.