DEFLECTION AMPLIFICATION FACTOR FOR SEISMIC DESIGN PROVISIONS

Seismic design provisions estimate the maximum roof and story drifts o ccurring in major earthquakes by amplifying the drifts computed from e lastic analysis at the prescribed design seismic force level with a de flection amplification factor (DAF). A comparison of several seismic d esign provisions indicated that the deflection amplification factor in both the UBC and NEHRP Recommended Provisions, being equal to 3R(w)/8 and C(d), respectively, is very low and could lead to unconservative drift estimations. An analytical study of the seismic responses of fou r instrumented buildings confirmed that drifts developed in major eart hquakes are much higher than those predicted by the UBC or NEHRP appro ach. It is recommended that the deflection amplification factor be inc reased to at least the seismic force reduction factor (R(w) in UBC and R in NEHRP) for estimating maximum drifts. The effects of the ratio b etween building period and earthquake predominant period, types of yie ld mechanisms, and structural overstrength on the DAF are also present ed.