SEISMIC EVALUATION OF EXISTING R.C. STRUCTURES : A CASE STUDY

Abstract

Seismically deficient buildings are susceptible to damages due to the effect of earthquake ground motion. Intensity of damage depends on the severity of the earthquake shaking and the level of seismic deficiency the building possesses. These deficient buildings pose serious seismic hazards to their occupants. The damaging effects from the building like partial or total collapse would induce injury to the people living in the vicinity and harm their properties. Seismic evaluation is necessary to identify the weak buildings either to strengthen them for enhanced seismic performance or to have a proper planning and strategy for earthquake disaster preparedness. This dissertation focuses on the seismic performance of two Reinforced Concrete (RC) buildings. These buildings are located in Bombay and Jabalpur which falls in the seismic zone II of the Draft Code IS: 1893 [BIS 1996]. The building at Jabalpur was damaged in the May 22, 1997 Jabalpur earthquake. The buildings were evaluated for the design level seismic force of the Draft Code IS: 1893 and ductility requirements of IS:13920-1993 [BLS 1993]. The methodology followed in this study involved two stages. The first stage involved the study of the building configuration to check for the presence of irregularities as defined in the Draft Code. Redundancy and proper load path characteristics of the building were also included. Geotechnical aspects were also discussed with whatever little information was available for these structures. The second stage involved the dynamic analysis to determine the structural response to the design level earthquake forces. Seismic demands on the building components were obtained from the analysis. Capacity of the sections were determined for moment resistance and shear strength as per IS:456-1978 [BIS 1981] and IS:13920-1993. Demand Capacity Ratio (DCR) less than unity identified the weak iii components. The application of weak beam-strong column concept was also investigated and the demand on the column members to enforce this criterion was also studied. The results obtained showed the building elements suffer from brittle shear failure. This unsatisfactory behaviour results from the non-compliance of the ductility requirements. Damages to the building stresses the need for proper seismic planning and design of building even in the regions of low and moderate seismicity. The study concludes with a strong emphasis on the need for a comprehensive methodology for seismic evaluation of buildings in India, strict compliance of ductility requirements, adherence to good seismic planning and identification of minimum number of seismically active primary components in a building.