ESTIMATING SEISMIC DRIFTS OF MULTISTORY REINFORCED-CONCRETE FRAMES

In order to estimate maximum roof and story drifts occurring in major earthquakes, seismic design provisions usually specify a deflection am plification factor (DAF) to amplify elastic design drifts. A compariso n of several seismic design provisions indicated that the deflection a mplification factor in both the UBC and NEHRP Recommended Provisions, being equal to 3R(W)/8 and C-d, respectively, is very low and could le ad to unconservative drift estimations. Since a reliable estimate of a ctual seismic drifts is essential for developing a performance-based s eismic design provision to control both structural and non-structural damage, an analytical study of the seismic response of three instrumen ted reinforced concrete building frames located in California was firs t conducted. In addition to analytical studies, reported test results from shaking table testing of reduced-scale multistory reinforced conc rete frames were used to evaluate the deflection amplification factor. It is confirmed from both the analytical and experimental evidence th at drifts developed in major earthquakes are much higher than those pr edicted by the UBC or NEHRP approach. It is recommended that the defle ction amplification factor be increased to at least the seismic force reduction factor (R(W) in UBC and R in NEHRP) for estimating maximum d rifts.