Identifying damage mechanisms of gymnasium structure damaged by the 2011 Tohoku earthquake based on biaxial excitation

During the 2011 Tohoku Earthquake, gymnasium buildings sustained unexpected structural damage, preventing their use as evacuation shelters. This study examines one such severely damaged structure with fewer connections between the steel roof and reinforced concrete columns. Results of eigenvalue, pushover, and seismic response analyses elucidate the observed structural damage. The out-of-plane deformation is prominent because of the slight constraint at the column top, leading to severe damage at the column base. Moreover, a parametric study of the peak ground acceleration suggests that the biaxial excitation produces different failure modes of column top connections from those under uniaxial excitation. Moreover, variation of the failure mechanisms is evidenced by the variety of seismic motions with a broad range of frequency characteristics. Pushover analysis results demonstrated the predictability of initial connection failure. However, discrepancies between results of the pushover and seismic response analyses became greater with subsequent connection failures. The column base damage can be assessed accurately based on uniaxial pushover and seismic response analyses.