SEISMIC RETROFITTING OF DAMAGED TWO STOREY RC FRAMED SCHOOL BUILDINGS

Abstract

Many existing RC frame buildings in India have not been designed at all according to Indian standards IS 1893-2002. To achieve required performance level of building during earthquake, earthquake resistant design features need to be considered. These buildings lack in strength, stiffness and ductility. In past earthquakes many buildings have collapsed and became life threatening to occupants of buildings. It is not economically feasible to demolish all such buildings and construct earthquake resistant building in their place. Retrofitting is one of the techniques by which strength, ductility and stiffness of building can be enhanced. Retrofitted structures performed well during earthquakes. School Building have given special occupancy classification according to IS 1893:2002. These buildings are important from life safety point of children and due to post earthquake importance of these structures. In post earthquake phase, school building serve as relief centers or used as temporary shelter for disaster affected peoples. But many of these school buildings are not designed as per Indian standards to resist seismic forces. Retrofitting of these buildings is necessary to ensure safety of such buildings against future earthquake forces. As a case study of retrofitting of RC frame school building blocks Kendriya Vidyalaya, Gangtok, has been considered in this dissertation. These building blocks were constructed in 1985. The blocks are asymmetric in plan and constructed on the hill terrain. Slopes have retained by retaining walls. Extensive cracks have appeared in some blocks due to lateral movement of soil. Major cracks have appeared in some of the masonry infihl and columns of building blocks after 18th September 2011 earthquake. Since Gangtok has also experienced frequent earthquakes in recent past, building blocks safety has been evaluated against the Design Basis Earthquake (DBE) and Maximum Credible Earthquake (MCE). A three dimensional analytical model for the building blocks have been developed in SAP 2000, software for simulation of behaviour under gravity and earthquake loadings. To evaluate the effect of infill on building during earthquake infilled frame modelling and analysis is also carried out. Load and load combinations have been considered as per relevant IS codes. Building blocks have been analyzed for Design Basis Earthquake (DBE) and Maximum Considered Earthquake (MCE). Nonlinear Static Pushover Analysis has been done to find performance of the buildings. Building blocks have Immediate Occupancy (10) level at Performance Point under DBE. For MCE plastic hinges have been observed in the range of IO-LS while til masonry infill panels show cracks. Hinges have been found in columns of blocks which show deficiency of columns against lateral forces. Since building blocks are non-ductile reinforced concrete structures so columns are found to be deficient. After observing results of performance point for earthquakes, retrofitting techniques have been suggested. To enhance lateral load capacity of blocks global retrofitting techniques such as addition of shear wall can be done. Since most of the columns are damaged so member retrofitting can also achieve 10 level performance for MCE level seismic demand. Member level retrofitting is done in the form of Fibre Reinforced Polymer (FRP) jackets and thickness of jacket for each damaged column has been worked out. In this study, it has been observed that contribution of infill in lateral load resistance is substantial. Strength and stiffness of infill have effect on performance point and time period of buildings. Increased performance has been obtained due to inclusion of masonry. To improve in plane strength of building repair methods for infill are also suggested in the study.