TORSIONAL EFFECT ON SOFT STORY FOR LIGHTLY REINFORCED CONCRETE STRUCTURES IN LOW SEISMIC ZONES

Abstract

This study deals with the performance of lightly reinforced concrete moment frames in low seismic zones. The frames under evaluation comprise vertical and/or plan irregularities and were designed for gravity loads only. Nonlinear time history analysis using scaled ground motions and pushover procedure as a supplement method are performed in this study. With the adoption of plastic hinge method, damage levels are addressed according to FEMA 356 definitions. Pivot model is considered for hysteresis behavior. The damage stage and number of formed hinges are classified for the beams and columns. In case of observed plastic hinge with collapse damage level stage, Fast Fourier Transform (FFT) method is applied to investigate the possible reason. Story drift is obtained based on inelastic behavior throughout of all story levels of archetype model inventories. Comparison between models demonstrates that the first story of symmetric plan models may suffer very minor up to moderate damage levels under low seismic intensity. However, the severity of damages to the asymmetric plan models can be noticeable, specifically for the lower structural models. The result of pushover method shows close to the results of time history analysis only for the vertical irregular frames without plan irregularity. Story drift illustrates that the lower structures suffer some degree of damage levels, especially for unsymmetrical plan models, while the taller models undergo lower drifts. As far as this study alone concerns, lightly reinforced concrete frame buildings may resist seismic events for the taller structures properly, whereas the lower structural models may suffer higher level of damages. The inherent frequency content of applied records affects the models' response more than their vertical and /or plan irregularity formations. Current US seismic design criteria in standard codes may need to be revised for low to moderate seismic zones in terms of vertical irregularity definitions and design criteria.