Global and local seismic drift estimates for RC frames

The relationship between reinforced concrete building displacement responses to seismic loading calculated using multi-degree-of-freedom (MDOF) nonlinear models and equivalent single-degree-of-freedom (SDOF) and MDOF linear models is examined. Displacement response is characterized by the peak roof drift (global deformation), maximum story distortion (local deformation) and the distortion location. A simple relationship between nonlinear peak roof drift, linear response spectrum, and effective building period is determined from parametric analysis of a set of frames subjected to a suite of ground motions. The relationship is expressed in terms of initial period using uncracked sections, a period factor, and a linear displacement response spectrum of particular equivalent damping. The best relationship was found to be in terms of the initial period factored by 2.0 and 2.3 for moderate and high seismicity ground motion, respectively, and using a 10%-damped linear response spectrum. The optimal period factor remained nearly constant for linear spectra with damping between 6% and 12%. A relationship between maximum story distortion for linear MDOF systems and nonlinear MDOF systems was sought in terms of maximum value and location. The ratios of maximum story distortions for nonlinear and linear analyses were found to vary significantly, as did the ratios between locations of these values. On average, the maximum story drift ratio from nonlinear analysis was 1.5 times the value calculated from linear analysis.