Report - PEER - University of California, Berkeley

outline and evaluate an improved procedure that retains the conceptual simplicity andcomputational attractiveness of current methods.2. NONLINEAR STATIC PROCEDURE: CURRENT PRACTICEThe nonlinear static procedure (NSP) in FEMA-356 requires development of apushover curve, a plot of base shear versus roof displacement, by nonlinear staticanalysis of the structure subjected first to gravity loads, followed by monotonicallyincreasing lateral forces with a specified, invariant height-wise distribution. Thelimitations of force distributions in the FEMA-356 nonlinear static procedure aredemonstrated in Figs. 1 and 2 where the resulting estimates of the median story driftand plastic hinge rotation demands imposed on the SAC buildings by the ensemble of20 SAC ground motions are compared with the “exact” median value determined bynonlinear RHA of the buildings. The first-mode force distribution grosslyunderestimates the story drifts, especially in the upper stories, showing that highermodecontributions are especially significant in the seismic demands for upper stories.Although the ELF and SRSS force distributions are intended to account for highermode responses, they do not provide satisfactory estimates of seismic demands. The“uniform” force distribution seems unnecessary because it grossly underestimatesdrifts in upper stories and grossly overestimates them in lower stories of fourbuildings. The FEMA-356 lateral force distributions either fail to identify, orsignificantly underestimate, plastic hinge rotations in beams at uppers floors.9BostonSeattleLos AngelesFloorNonlinear RHA6FEMA1st ModeELF3SRSSUniform9−StoryG0 0.5 1 1.5 220160 1 2 3 4 50 2 4 6 8Floor128420−StoryG0 0.5 1 1.5 2 0 1 2 3 4 5 0 1 2 3 4 5 6Story Drift, ∆ , or ∆ (%)NL−RHA FEMAFigure 1. Median story drifts determined by nonlinear RHA and four FEMA-356 force distributions: 1 st Mode, ELF, SRSS, and “Uniform.”334