STRENGTHENING OF REINFORCED CONCRETE BUILDINGS FOR EARTHQUAKE FORCES

Abstract

Safety of lives in the event of an earthquake is the most important consideration in the design of buildings. There may be some buildings designed and constructed considering gravity load i.e. dead load and live load only. Again, some buildings, though they may be designed and constructed as earthquake resistant, may be situated in such a zone, where seismic zoning upgradation has taken place. In such buildings the seismic force resistance level of the primary structural system is bound to be low in some of the members. Therefore, such buildings might suffer extensive damage or even collapse if shaken by a severe ground motion. This calls for strengthening of such buildings to mitigate unacceptable hazrds which they pose in future earthquakes. The basic purpose of the present study is to make an endeavour on the field of strengthening of existing R.C. buildings for earthquake forces with the help of some examples. The example problems have been taken as three-storied and five-storied single bay buildings each designed considering gravity loads only. These buildings have been examined for various seismic force levels corresponding to zones III,IV & V computed by pseudo-static method and behaviour of uniform and tapered buildings was observed and the structural members of these buildings where distress is expected were identified. As 1st stage of strengthening arbitrary increase in concrete section was done in the columns and beams, where failure is indicated. Columns were strengthened by casing method and beams were strengthened by build-up-at-top method, observing their behaviour at the time of earthquake. Then steel reinforcement in the increased concrete sections of beams and columns were found out by modal analysis considering beam flexibility as final stage of strengthening for different dampings. Comparison of cost of strengthening of beams and columns of uniform and tapered buildings has also been carried out.