"SEISMIC RETROFITTING OF STILT R.C. BUILDINGS",

Abstract

The present study deals with the seismic retrofitting of stilt reinforced concrete buildings. In order to study the stiffness change at first floor level in stilt type buildings and occurrence of soft storey phenomenon a parametric study has been performed. In the parametric study four types of frames: first a bare frame, second a frame with infill at all the storeys, third a frame with no infill at first storey and fourth a frame with stiffened first storey have been analyzed using STAAD III software. It has been seen from the parametric study that the stiffening of the ground storey by increasing the stiffness and strength of the members to 2.5 times the stiffness and strength calculated for bare frame, makes it safe against soft storey effects. A four storey stilt reinforced concrete building located in Ghaziabad (zone IV) has been selected for the evaluation and retrofitting. The building has been evaluated for the design level seismic force of the IS: 1893, draft code and the ductility requirements of IS: 13920-1993. Modal analysis based on response spectrum has been adopted for the dynamic analysis of the structure. Seismic demand on the building components has been obtained from the analysis. In order to determine the shear and flexure capacity of beams and columns, a software has been writtep in C++ language. The results obtained from the evaluation show that the building satisfies the strength requirements, however fails to meet the stiffness regularity criteria in vertical direction and results in the soft storey phenomenon. Therefore, retrofitting is needed to increase the stiffness and strength of ground storey of the building. For the retrofitting two methods as per IS: 1893, draft code, have been proposed. In the first method the ground storey beams and columns have been strengthened for 2.5 times the storey shears and moments obtained from the earthquake analysis of the iii building, neglecting the effect of infill walls in other storeys. In the second method, ground storey is strengthened by providing shear walls designed for 1.5 times the storey shear obtained from the earthquake analysis of the building, neglecting the effect of infill walls in other storeys. The reinforcement detailing for retrofitting with particular emphasis on connection of new reinforcement with existing reinforcement has been discussed in details. Different alternatives for anchoring the new reinforcement have been discussed. The details of construction and precautions against corrosion have also been discussed. iv