An evaluation of P-Delta amplification factors in multistorey steel momentresisting frames

Three different amplification factors that have been proposed to account fo r P-Delta effects in the seismic design of multistorey building structures are described and compared. Nonlinear dynamic analyses of a typical 20-stor ey steel moment resisting frame are carried out under earthquake ground mot ions typical of eastern and western Canada to evaluate the gravity load eff ects and to assess the effectiveness of each type of amplification factor i n accounting for these effects. All three approaches maintain the ductility demand within the level computed without P-Delta effects, but lateral defo rmations are generally larger than those obtained neglecting the gravity lo ads. Nonlinear dynamic analyses are also performed on a shear-beam (stick) model of the same building to examine the possibility of using such simple models for studying the dynamic stability of buildings subjected to ground motions. The shear-beam model does not predict adequately the seismic behav iour of steel moment resisting frames for which P-Delta effects are signifi cant.