Assessment of seismic response of vertically irregular RC moment resisting frame structures

Abstract

Now-a-days many structures are designed with vertical irregularity due to functional, aesthetic or economic reasons. However, damage or failure of a building may be triggered at the point of weakness induced by the vertical irregularity. In this thesis, a comprehensive numerical studies have been conducted to assess the seismic behavior of the structures with vertical irregularity. Stiffness, mass and geometric–three types of the vertical irregularities have been considered in this study. Stiffness irregularity cases have been considered due to sudden absence of masonry infill walls at open ground floor for parking facility. Bare, fully infilled and open ground floor (OGF) frames have been considered to assess the stiffness irregularity. Mass irregularity cases have been considered by applying a special superimposed dead load at mid storey level. Geometric irregularity cases have been considered due to presence of setback at top floor and mid floor level. 4, 6 and 10-storied reinforced concrete (RC) moment resisting frame buildings located at seismic zone 2 according to draft BNBC 2017 have been considered. All the frames were designed as an Intermediate Moment Resisting frame considering the response reduction factor (R) value of 5 as per draft BNBC 2017. Seismic analysis methods such as equivalent static, response spectrum, time history and nonlinear static or pushover analysis have been conducted. Open ground floor buildings were not always found with soft storey action as it depends on limits of storey stiffness ratio sets by the Code. Only 4 and 6-storied buildings with the open ground floor resulted extreme soft storey action. R values of 4, 6 and 10-storied bare frames and fully infilled frames were satisfied with the considered R = 5, whereas 4, 6 and 10-storied open ground floor (OGF) frames resulted R values of 3.01, 3.94 and 3.42, respectively were much lower than the considered R = 5. A decrease in R value in OGF frames was observed due to the stiffness irregularity caused by sudden absence of infill wall at the ground floor. Ratios of R values of 4, 6 and 10-storied bare frames to open ground floor (OGF) frames were found 1.71, 1.44 and 1.55, respectively. Therefore, special provision as per draft BNBC 2017 seems reasonably good agreement with the obtained results for the open ground floor buildings. Storey drifts of 4, 6 and 10-storied mass irregular frames were found 3.7%, 4.4% and 8.1%, respectively higher than those of the allowable drift limit. R value of 4, 6 and 10-storied mass irregular frames were found 2.31, 4.55 and 4.64, respectively lower than those of the regular one. R values of 4, 6 and 10-storied buildings with geometric irregular frames also provided lower values than those of the regular one. However, building with the setback at the top floor results higher reduction of R value than that of the mid floor level. A reduction of R value was obtained 39% for the 4-storied geometric irregular building with the setback at the top floor level.