VERIFICATION OF CODE RECOMMENDED FUNDAMENTAL TIME PERIOD FOR LOW AND MEDIUM RISE R.C.C BUILDINGS

Abstract

Fundamental time period of any structure is one of the most important aspect as it determines the amount of base shear and all other design forces that are to be considered in the analysis and design of the structure. If a structure has a higher value of time period making it fairly flexible, it will attract lesser forces compared to its stiffer counterpart with smaller time period. The empirical formulae suggested by IS 1893 (2002) are based on observed natural period values on real buildings during the 1971 San Fernando earthquake in California which are very general in nature and does not incorporate the inherent variety of unsymmetry, irregularities existing in different buildings. The time period obtained using these formulae often gives large variations when compared with the fundamental mode time periods of dynamic analysis. As a result of this variation the base shear calculated using dynamic analysis is often lower than the static analysis. Due to this the code recommends to scale the dynamic analysis base shear, so that it matches with the static one. This approach however conservative may be, but is not accurate. In the present study we are trying to find a rational approach by studying different models and investigate the variation in time period and forces between dynamic analysis results and code recommended empirical formulae results. An effort has been made to incorporate different kind of buildings along with some unsymmetry and irregularities; and investigate their vibrational behaviour. Regression analysis has also been carried out to generate empirical expressions from the dynamic analysis results and their variation with the codal formulae have been investigated. After studying these variations it was realized that the basic issue with our code still remains in its empirical formula approach. However large be the sample size, there would always be buildings that are not part of that sample size. In fact, every other buildings may behave differently under dynamic loads. Thus a more rational approach would be to drop the empirical formula and analyse every building rigorously. A more rigorous dynamic analysis, pushover analysis or performance based analysis would be more suited for the purpose.