The Effects of Multiple Strong Ground Motions on the Steel Modular Bundled Tube Resistant Structures in Near-Field Areas

Study of the seismological aspects of major earthquakes occurred in California, Japan and New Zealand indicates that the structures located in regions with high level of seismicity, experience aftershocks with different intensities in addition to the mainshock. Multiple earthquakes create inelastic response in structures and lead to the accumulation of considerable damage in the structural and non-structural elements. The aim of this research is to determine the effect of aftershocks on the response parameters of a 10-story steel bundled tube frame structure. According to the analytical results of this study, the occurrence of severe aftershocks following the near-field earthquakes does not have a significant contribution to the changing maximum inter-story drift parameter. Additionally, by increasing the intensity of the aftershocks, the residual inter-story drift does not indicate a clear trend height-wise, obviously. Moreover, when the dominant period of the mainshock is close to the fundamental period of the structure, and the dominant period of the aftershock is close to the fundamental period of the damaged structure, then the occurrence of the aftershock increases the amplitude of the nonlinear response of structural elements. The response parameters studied in the current paper include maximum interstory drift, residual inter-story drift, plastic hinge mechanism and induced forces due to shear lag effect . It should be noted that the maximum inter-story drift in all stories of the studied structure subjected to the fault normal component of the Bam 2003 mainshock record has exceeded the allowable value prescribed by the Iranian seismic code 2800. This is due to the dominant period of the Bam record that is very close to the fundamental period of the studied structure. The findings of this study display that the occurrence of the aftershocks following the mainshock does not change the maximum inter-story drift considerably. Moreover, the strong aftershock (PGAas/PGAms=1.0) occurring after the Cape Mendocino 1992 mainshock i.e. PET record, increased the maximum inter-story drift at the middle and upper stories. Results apply that by changing the aftershock intensities, no clear trend in residual drift values is emerged. The reason could be attributed to the fact that the damaged structure may not have the more maximum displacement when it stops oscillating. However, the Bam mainshock record caused maximum residual drift equal to 0.024, which according to FEMA356 is beyond the Life Safety (LS) performance level.