Estimation of period of vibration for concrete moment-resisting frame buildings

Most seismic design codes generally provide formulas to be used for the estimation of the base shear and lateral loads. For the determination of the lateral loads, it is required to estimate first the fundamental vibration period of the building theoretically or experimentally [1–4].In various current codes such as the current United States (US) and Egyptian building codes and also in the recommendations of many researches, empirical formulas relate the building fundamental period of vibration (T) to the building overall height (H) or the number of stories (N) without consideration of the two factors together. In this paper, improved formulas for estimating the fundamental period of vibration (T) of reinforced concrete moment-resisting frame buildings are developed by taking the effect of both building height (H) and number of stories together (N) (i.e. take the floor height (h) into consideration). The improved formula is based on regression analysis of the available data for the fundamental vibration period of reinforced concrete moment-resisting frame buildings measured from their motions recorded during eight California earthquakes. The results indicated that the value of coefficient Ct in the current US and Egyptian building codes’ formulas should be modified to be a function of the number of stories (N). Comparisons between the periods determined using the proposed formula and the measured values show good agreement.