Investigating the Adequacy of Accidental Eccentricity Recommended by Seismic Design Codes to Consider the Rotational Components in Nonlinear Dynamic Analysis

Due to the lack of access to the data of three rotational components of earthquakes, seismic analysis of new buildings as well as assessment of the vulnerability of existing structures are usually carried out only by applying the translational components of earthquakes. Iranian Standard 2800 proposed an accidental eccentricity for considering the earthquake rotational component effect in the seismic analysis of building structures. The present investigation is focused on the effects of earthquake rotational excitation on the seismic response of buildings having various dynamic properties which situated on a rigid foundation. In addition, adequacy of the accidental eccentricity of 5% recommended by seismic design code for inclusion of the earthquake rotational component impact in the non-linear time history analysis of buildings is studied, as well. To achieve this, a large number of one-story torsionally stiff and flexible building models with a wide range of lateral vibration periods (T=0.05 to 2sec) and three different values of inherent eccentricity of 0, 15 and 25% were modeled. These models were once excited by the translational components of ground motions and once again by both translational and rotational components of ground motions. The building models were re-analyzed after applying the 5% accidental eccentricity based on the procedure presented by Standard 2800 (shifting the center of mass in the negative and positive directions by 0.05 of the plan dimension). For conducting the non-linear time history analyses, a number of earthquakes were selected and the rotational records for these events were generated by use of an indirect single station method based on the seismic wave propagation in an elastic and homogeneous medium. In total, over than 2600 nonlinear dynamic analyses have been conducted in this numerical research. In order to determine the role of earthquake rotational excitation in the seismic behavior of  buildings, the variations of displacement response for the left and right sides of diaphragm and the torsion of diaphragm about the mass center due to the effect of rotational component were evaluated. By comparing the results obtained in this study, it is found that the rotational component has a substantial influence on the structural responses, which this effect is a function of the fundamental dynamic characteristics of system such as uncoupled rotational to translational frequency ratio, lateral vibration period and irregularity. The displacement of diaphragm can be increased up to 50% when the rotational component of ground motion is included in the seismic load combinations. Decreasing the frequency ratio leads to increase of the rotational component effect for the stiff buildings with short periods, while in the other cases reduces the growth of displacement due to the rotational component. Furthermore, results indicate that the accidental eccentricity of 5% cannot increase the seismic responses as much as the earthquake rotational motion, and leads to unreal and underestimate results for the most of lateral vibration periods. Thus, the current Standard 2800 approach cannot be considered as an appropriate alternative for considering the accidental torsion induced by the rotational component of ground motion, and it seems that this approach needs to be re-evaluated.