عنوان مقاله [English]
One of the most important and, at the same time, unpredictable type of lateral loads is the earthquake load. Extensive research on safe and economical seismic design of structures has been carried out in this field. The research works can be divided into two general categories: force-based design and performance-based design. Displacement-based design is among the performance-based methods.
In this paper it is purposed to investigate and a major drawback in this approach, namely lack of precision in estimation of torsional response of buildings. For this purpose, the model code of displacement-based design (DBD12) is adopted as the basis for studying and modifying methodology. In this code, there are equations that take into account involvement of torsional effects in the structural analysis. It is pointed out that the main weak point of the mentioned relationships is consideration of nonlinear behavior only in the earthquake direction while a linear behavior is considered in the perpendicular direction when calculating the torsional stiffness. Therefore, it is needed to evaluate and modify these relations.
For the purposes of this study, three buildings with 4, 7, and 10 floors are selected. Eccentricity in the buildings is assumed to be equal to 0, 5, 10, 15, 20 and 30% of the plan dimension in transverse direction. The structural system is selected to be steel moment resisting frames and the diaphragm of the floors is assumed to be rigid. Studying the accuracy of the design equations for torsion and implementation of the necessary corrections are done using the nonlinear time history analysis. In this analysis, utilizing the OpenSees software, the structures are subjected to 11 consistent records that are properly scaled.
After obtaining the results of exact nonlinear analysis and the displacement-based design method, the equation of the code for calculating the story twist angle is corrected against the average values determined by the nonlinear dynamic analysis. The accuracy of corrected relationships in comparison with the exact values is evaluated. It is shown that the corrected equations successfully predict the torsional response of the investigated buildings with good accuracy such that it reduces the estimation errors to less than 10%.