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Report - PEER - University of California, Berkeley

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W14x68W14x68W14x99W14x99W14x120W14x120W14x145W14x145W14x159W14x159W14x193W14x193(a)W18x46W18x46W21x57W21x57W21x68W21x68W24x68W24x68W24x76W24x76W24x76W24x763 @ 8.00 m11 @ 4.00 m5.00 m00000Figure 7. PCA and elastic mode shapes computed for a 12-story steel momentframe (a) under a beam-hinging mechanism (b) and a weak-story mechanism (c).(b)ElasticPCA00000(c)ElasticPCAsustained (in force-controlled components) to enable the development <strong>of</strong> the intendedmechanism.Many approximate techniques for accounting for MDOF effects in inelasticsystems have been suggested, but experience within the ATC-55 project suggeststhese techniques are not universally applicable. One promising technique identifiedwithin the ATC-55 project is the Scaled Nonlinear Dynamic Procedure, so namedbecause ground motion records are scaled so that the peak ro<strong>of</strong> displacement matchesthe estimate made on the basis <strong>of</strong> the ESDOF system. It is suggested that thistechnique can be used to determine statistical distributions <strong>of</strong> performance parametersand to determine the strengths that must be provided to force-controlled components.Further information on this procedure can be found in Aschheim et. al (2004).Finally, a characterization <strong>of</strong> the predominant mode and an assessment <strong>of</strong> thepredominant mechanism can be made using a technique <strong>of</strong> multivariate statisticsknown as Principal Components Analysis (PCA). When applied to the displacementresponse data <strong>of</strong> an elastic dynamic system, the principal components coincide withor are related to the elastic mode shapes, depending on the mass distribution. PCAmay be applied to data obtained from systems responding inelastically, to identify thepredominant “modes” <strong>of</strong> such systems. Figure 6 illustrates the first PCA mode shapesand elastic mode shapes for a 12-story steel moment-resistant frame having a uniformdistribution <strong>of</strong> mass. The first PCA mode shape nearly coincides with the elasticmode shape when a beam-hinging mechanism develops (Figure 7b). The development<strong>of</strong> a weak-story mechanism after the lowest-story columns were numericallyweakened is easily identified in the PCA mode shape <strong>of</strong> Figure 7c. Furtherinformation can be found in (Aschheim, Black, and Cuesta, 2002).490

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