Report - PEER - University of California, Berkeley

Yield Strength Coefficient, Cy*1.61.41.21.00.8Elastic Design SpectrumDuctility = 2Ductility = 4Ductility = 80.60.40.2µ=2.30.0*∆ y = 0.104* ∆u0.0 0.1 0.2 0.3 0.4 0.5Yield Displacement, m(a)Yield Strength Coefficient, Cy*1.61.4Elastic Design Spectrum1.2Ductility = 21.0Ductility = 40.8T= 1.05 sec Ductility = 80.60.40.20.0*∆ y*∆ u0.0 0.1 0.2 0.3 0.4 0.5Yield Displacement, mFigure 6. The influence of strength on peak displacement response (indicated by an“x”) for (a) a given yield displacement, and (b) a given period.demands are represented using YPS and design is based on an estimate of the yielddisplacement. As indicated by Equation (4), the period is a consequence of thestrength required to satisfy the performance specification. Figure 2 illustrates that theyield displacement typically is independent of the strength provided. Therefore, had adifferent roof drift limit, a different hazard level, or a number of performanceobjectives been considered, the required base shear strength would have differed, butthe associated yield displacement would have remained constant or nearly so.More generally, the influence of lateral strength on peak displacement response isillustrated in Figure 6. Given a yield displacement estimate, one simply selects thestrength that results in acceptable displacement and ductility demands (Figure 6a). Incontrast, if the period is presumed to be known, the influence of strength ondisplacement is as illustrated in Figure 6b. In this case, the equal displacement ruleapplies, indicating that strength affects displacement ductility demands but not peakdisplacement. Consequently, proposals for design that use period as a fundamentaldesign parameter must assume the period is unknown and can be varied (inconjunction with strength) to achieve the desired performance. In reality, a continuumof yield points exist that will satisfy the performance objective. The engineer,however, generally does not have the latitude to provide the strength and stiffnessassociated with a particular yield point, selected arbitrarily from the continuum thatsatisfies the performance objective. Thus, proposals that use period as a fundamentaldesign parameter inevitably will be iterative, as infeasible yield points are tried anddiscarded in the search for an acceptable design. The stability of the yielddisplacement is exploited here to reduce or eliminate the need for iteration in design.The base shear strength required of alternative structural systems may beevaluated in this fashion to determine the system best-suited to any particularapplication. Once the best-suited system has been identified, member depths andspans can be adjusted (within architectural constraints) to modify the yielddisplacement in order to reduce the required base shear strength (and cost).(b)488