The importance of accidental design eccentricity in seismic design of steel buildings with dual system under the effect of far- and near-fault ground motions

Seismic responses of buildings are amplified due to torsion. To account for the effects that cause torsion and are not considered in the design process of buildings, the seismic codes introduce “accidental design eccentricity (ADE)”. In this study, the adequacy of the Iranian Standard No. 2800 provisions about the design eccentricity is investigated. To this end, the 5-story torsionally-stiff and torsionally-flexible buildings with dual lateral load resisting system are studied. The mass eccentricity in plan-asymmetric buildings is assumed to be equal to 0.10b and 0.20b where b is the plan dimension. Nonlinear time history analyses are performed using far-field (FF), non-pulse (NP) and pulse-like (FD) near-field records for the models in two cases. In the first case, the effect of the ADE on the seismic demands of symmetric and asymmetric-plan building is investigated. Finally, to consider what happens when an actual accidental mass eccentricity (AME) is introduced in an already designed building, the mass centre of the buildings is shifted by ±0.05b simultaneously in both directions and the buildings are analyzed for the earthquake sets described above. The results indicate that the provision related to the accidental design eccentricity has little influence on the inelastic seismic responses for torsionally-stiff buildings and can be ignored. Also, the accidental mass eccentricity has more influence on inelastic seismic responses of torsionally-flexible buildings but the accidental design eccentricity has less influence on the reduction of seismic responses. Therefore, it seems that the accidental design eccentricity needs to be modified for torsionally-flexible buildings.