Numerical Analysis of the Ratio of Dynamic Torsion to Static Torsion in Isolated Irregular Structures under the Effect of Earthquake Records

Due to the population growth in recent years, we are witnessing an increase in construction of structures with different number of floors in different urban areas. The constraints of the existing lands as well as the significant population density, especially in urban areas, have led to an increase in the construction of buildings with irregularity in plan. While asymmetry and irregularity in different types of structures cause special phenomena such as torsion, the use of seismic isolators as one of the effective ways to deal with seismic force has been considered by many engineers. Therefore, in this study, irregularly isolated structures in plan and height under the effect of earthquake records have been non-linearly analyzed. Four-, eight- and twelve-floor structures with asymmetry in plan and ۲۵% and ۵۰% eccentricity in one and two directions as well as asymmetry in height with ۱۰% and ۳۵% reduction in stiffness, under the effect of seven records, were studied and evaluated. In this study, the values of .۸, .۱ and ۱.۷ have been chosen for the ratios of torsional frequency to lateral frequency of superstructure and isolation system. The analysis method used was nonlinear dynamic analysis through SAP۲۰۰۰. The main purpose was the numerical examination of the ratio of dynamic torsion to static torsion in three four-, eight- and twelve-floor structures with irregularly in plan and height in isolated mode. The results indicated the significant effect of the ratio of torsional frequency to lateral frequency of isolation system and superstructure on the ratio of dynamic torsion to static torsion of structures. The results of this study showed that the effect of eccentricity on the ratio of dynamic torsion to static torsion (TD/TS) is very significant, and that the dynamic torsion should not be neglected in the design process when eccentricity is low. Also, the TD/TS ratio of bidirectional eccentricity is almost the same in unidirectional eccentricity, while an increase is sometimes observed in dynamic torsion in some tall structures due to bidirectional eccentricity, which increases TD/TS ratio.

Keywords:

dynamic torsion , static torsion , irregular structures , seismic separator , nonlinear analysis , unidirectional and bidirectional eccentricity