Effects of soil–structure interaction on inelastic response of torsionally-coupled structures

In this paper, effect of torsion on the inelastic responses of structures resting on a nonlinear flexible medium is studied. Similar studies have shown that soil–structure interaction can result in augmentation of nonlinear responses of lower stories. The focus here is on the plan-wise distribution of inelastic responses of a torsional structure considering soil–structure interaction (SSI). For this purpose, 4, 8, and 12-story steel structures consisting of special moment frames are considered on a relatively soft soil. A nonlinear set of non-uniform springs is used for modeling of SSI. For each building, different structural responses are calculated under 11 suitably scaled earthquake ground motions at the design basis and maximum considered earthquake hazard levels. The mass eccentricity ratio is varied from zero to 30%. The inelastic responses include the story drift ratios and distribution of plastic hinge rotations and performance levels of each story and frame. The results clearly show that SSI increases the drift ratio of the first story up to 30% regardless of the eccentricity value. On the other hand, SSI amplified the cumulative plastic hinge rotation of the upper stories, especially in the taller buildings. The maximum value of amplification exceeded 3 and it was very sensitive to the extent of eccentricity. The local amplification effect of combination of torsion and SSI was much more severe where it reached values over 8 in the outer frames of the buildings, with SSI having the larger share in amplification.