An Approach for Modelling Accidental Eccentricity Effects in Symmetrical Frame Buildings Exhibiting Semi-Rigid Diaphragm Behavior

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The accidental eccentricity effect is specified, in earthquake codes, to account for the possible uncertainties in the mass and stiffness distribution of the structural system and the effect of the torsional component of the earthquake ground motion on the building. Türkiye Building Earthquake Code (TBEC-2018) considers the additional eccentricity effect only for the cases where rigid diaphragm behavior is provided in the slabs. However, in buildings with A2 and A3 irregularities or flexible diaphragms including insufficient strength and stiffness, the in-plane deformations and stresses on diaphragms may change the behavior of buildings under earthquake loads. In this study, a practical approach to be used in the equivalent earthquake load method was proposed to apply the accidental eccentricity effect on symmetrical frame buildings with semi-rigid diaphragms. The approach is based on distributing the total accidental torsion to the nodes as fictitious forces. Two numerical examples were presented. The first is a single-story precast industrial-type RC building, where the calculation steps of the procedure explained in detail. The building was modeled with both semi-rigid and rigid diaphragm assumptions, and a comparison of two modeling assumptions under accidental torsion was presented. Torsional irregularity factors obtained from building modeled by semi-rigid diaphragm assumption provided greater values with respect to those modeled by rigid diaphragm. This shows the significance of considering accidental eccentricity for semi-rigid diaphragms. The second numerical example, which is a RC concrete building, was used to validate the proposed methodology via finite element model (FEM) built-in algorithm. The obtained displacement demands by using the proposed methodology were very close to FEM results as a reference for the real solution. It is concluded from this study that the proposed methodology is reliable and can be used for modeling accidental eccentricity effects on symmetrical-plan buildings with semi-rigid diaphragms.