PERIOD FORMULAS FOR MOMENT-RESISTING FRAME BUILDINGS

Most seismic codes specify empirical formulas to estimate the fundamen tal vibration period of buildings. Evaluated first in this paper are t he formulas specified in present U.S. codes using the available data o n the fundamental period of buildings ''measured'' from their motions recorded during eight California earthquakes, starting with the 1971 S an Fernando earthquake and ending with the 1994 Northridge earthquake. It is shown that, although the code formulas provide periods that are generally shorter than measured periods, these formulas can be improv ed to provide better correlation with the measured data. Subsequently, improved formulas for estimating the fundamental periods of reinforce d concrete (RC) and steel moment-resisting frame buildings are develop ed by regression analysis of the measured period data. Also recommende d are factors to limit the period calculated by a rational analysis, s uch as Rayleigh's method.