SEISMIC EVALUATION AND RETROFITTING OF MULTISTOREYED RC FRAMED BUILDING

Abstract

The present study deals with the "Seismic Evaluation and Retrofitting of Multistoreyed RC Framed Building". A ground + eight storey (G+8), stilt RC framed building located at Nanamava near Rajkot in Gujarat (seismic zone-Ill) has been selected for the evaluation and retrofitting. The seismic evaluation has been carried out by calculating Demand Capacity Ratios (DCRs) for each RC member. The seismic design forces are calculated using response spectrum method for dynamic analysis as per IS:1893-2001 (draft). STAAD.Pro 2001 software of Research Engineer, USA has been used to calculate the seismic demands on the building components. Strength, stiffness and ductility criteria according to IS:456-2000, IS:1893- 2001 (draft) & IS:13920-1993 respectively has been used for the evaluation. The member capacities in shear & flexure have been calculated using programs written in 'C' language as per IS:456-2000 & IS:13920-1993. Demand Capacity Ratios (DCRs) of RC members at critical locations have been found out. Results obtained from the evaluation show that all frame members satisfy the strength (moment & shear) requirements according to lS:456-2000. Except the spacing of hoops/stirrups in beams & columns, all other ductility requirements have been satisfied according to IS:13920-1993. Inter-storey drifts in X (longitudinal) & Z (transverse) directions has been calculated and compared with permissible limit as per IS:1893-2001 (draft). All inter-storey drifts in both X & Z directions were found within the permissible limit. Although evaluation carried out show that the building is properly designed, however it can not be considered safe from seismic point of view due to the existence of soft ground storey. Therefore, retrofitting is needed to improve the seismic resistance by increasing the stiffness and strength of the soft ground storey. In order to apply retrofitting to soft storeys, two alternate methods proposed by clause 7.10 of IS:1893-2001 (draft) have been studied. The study presents a quantitative approach for seismic evaluation with recommendations to adopt any one proposed method for retrofitting of present building. The design of RC shear walls and the new member sizes for soft ground storey with additional 'reinforcement for the respective alternative has been carried out. The work also presents reinforcement detailing for retrofitting, with emphasis on connection of new reinforcement with existing reinforcement.