12.07.2015 Views

Report - PEER - University of California, Berkeley

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Floor963SeattleNonlinear RHAFEMA1st ModeELFSRSSUniformLos Angeles9−StoryG0 0.02 0.04 0.06 0 0.02 0.04 0.062016Floor1284G20−Story0 0.02 0.04 0.06Beam Plastic Rotation (rad)0 0.02 0.04 0.06Beam Plastic Rotation (rad)Figure 2. Median plastic rotations determined by nonlinear RHA and fourFEMA-356 force distributions: 1 st Mode, ELF, SRSS, and “Uniform.”3. IMPROVED NSP: MODAL PUSHOVER ANALYSISIt is clear from the preceding discussion that the seismic demand estimated by NSPusing the first-mode force distribution (or others in FEMA-356) should be improved.One approach is to include the contributions <strong>of</strong> higher modes <strong>of</strong> vibration to seismicdemands.3.1 Basic ConceptThe equations <strong>of</strong> motion for a multistory building subjected to horizontal groundaccelerations u&& g () t or u&& gy () t along the x and y axes, respectively, are the same as ifthe ground were stationary and the excitation was replaced by external forces, knownas the effective earthquake forces:() t =− ι u&&() t or − ι u&&() tpeff M x gx M y gy(1)where the mass matrix M consists <strong>of</strong> three diagonal submatrices m, m, I o ; m isassociated with lateral degrees <strong>of</strong> freedom (DOF) and I o with torsional DOF. Theinfluence vectors ι x and ι y associated with x and y ground motions are as follows:335

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