Report - PEER - University of California, Berkeley

Report - PEER - University of California, Berkeley Report - PEER - University of California, Berkeley

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Yield Strength Coefficient, Cy2.01.81.61.41.21.00.80.60.40.20.00.0 0.2 0.4 0.6 0.8 1.0 1.2Yield Displacement, mElastic Design SpectrumDuctility = 2Ductility = 4Ductility = 8Figure 4. Yield Point Spectra: determined using Nassar and Krawinkler R-µ-Trelationship (thick lines) and using ATC-55 R-C 1 -T relationship (thin lines).The author has a clear preference for the YPS format, primarily because thisformat uncouples the degree of inelastic response (associated with the hazard) fromthe properties of the structure (represented by the yield point). This uncoupling makesit easy to identify the domain of yield points that satisfies a given performanceobjective (termed an Admissible Design Region) and to identify the yield strengthrequired to satisfy multiple performance objectives (Aschheim and Black, 2000).Other advantages to the YPS format are: (1) YPS can be developed for arbitraryhysteretic relationships. (2) For design or evaluation, one can focus on the yield point,without concern for how the post-yield stiffness affects the intersection with theADRS curves. (3) Peak response can be estimated by interpolating between theconstant ductility curves, without iteration. (4) The YPS plots for individual groundmotions are clear and easily read, but the same data plotted as a function of peakdisplacement can be difficult to make sense of. (5 ) P-Delta effects can be representedin YPS format (Aschheim and Hernández-Montes, 2003).Figure 4 presents YPS determined by applying R-µ-T and R-C 1 -T relationships tothe NEHRP design spectra (2/3 of the MCE) for Site Class C conditions in Berkeley,California. The constant ductility curves were determined for ductilities of 2, 4, and 8.Shown by thick lines are results obtained using the R-µ-T relationship determined byNassar and Krawinkler (1991) for bilinear oscillators having a post-yield stiffnessequal to 2% of the initial stiffness. Shown by thin lines are results obtained using anR-C 1 -T relationship that was developed for the ATC-55 project using the constant Rfactor approach, given byR −1C1= 1+(1)290T485

where R = the ratio of the strength required for elastic response and the yield strengthof an oscillator having an identical initial period, T. In general, the two relationshipsare seen to produce consistent results. However, the R-C 1 -T approach results insomewhat higher design strengths for intermediate period oscillators having relativelylarge ductility demands.2.4 Performance LimitsDiscrete performance objectives, consisting of the pairing of performance limits andhazard levels, may be considered. The performance limits are interpreted in termsrelevant to the response of the ESDOF system:• The peak displacement limit of the ESDOF system is equal to ∆ u /Γ 1 , where ∆ u =the roof drift limit.• The displacement ductility limit of the ESDOF system is equal to the systemductility limit.Note that peak dynamic interstory drifts can be related to peak roof drifts (e.g.,Ghoborah, 2004), and that simple analytical expressions can relate code limits onstory drift to roof drift limits.2.5 Required Strength DeterminationIf an estimate of the roof displacement at yield is available, then the required strengthcan be determined using the procedure described in this section. For other cases,Admissible Design Regions may be used to identify a continuum of yield points thatsatisfy one or more performance objectives (Aschheim and Black, 2000).Using standard approaches such as those described in ATC-40 (1996), the yielddisplacement of the ESDOF oscillator, ∆ y * , is given by∆*= ∆ / Γ(2)y y 1YPS are used to determine the required yield strength coefficient of the ESDOFoscillator, C y * , from which the base shear coefficient (at yield) of the MDOF system isdetermined asC = (3)*yC yα1The base shear strength (at yield) of the MDOF system is given by V y = C y W. Thefundamental period of the MDOF system matches that of the ESDOF system, givenby*∆yT = 2π , (4)*C gy486

Yield Strength Coefficient, Cy2.01.81.61.41.21.00.80.60.40.20.00.0 0.2 0.4 0.6 0.8 1.0 1.2Yield Displacement, mElastic Design SpectrumDuctility = 2Ductility = 4Ductility = 8Figure 4. Yield Point Spectra: determined using Nassar and Krawinkler R-µ-Trelationship (thick lines) and using ATC-55 R-C 1 -T relationship (thin lines).The author has a clear preference for the YPS format, primarily because thisformat uncouples the degree <strong>of</strong> inelastic response (associated with the hazard) fromthe properties <strong>of</strong> the structure (represented by the yield point). This uncoupling makesit easy to identify the domain <strong>of</strong> yield points that satisfies a given performanceobjective (termed an Admissible Design Region) and to identify the yield strengthrequired to satisfy multiple performance objectives (Aschheim and Black, 2000).Other advantages to the YPS format are: (1) YPS can be developed for arbitraryhysteretic relationships. (2) For design or evaluation, one can focus on the yield point,without concern for how the post-yield stiffness affects the intersection with theADRS curves. (3) Peak response can be estimated by interpolating between theconstant ductility curves, without iteration. (4) The YPS plots for individual groundmotions are clear and easily read, but the same data plotted as a function <strong>of</strong> peakdisplacement can be difficult to make sense <strong>of</strong>. (5 ) P-Delta effects can be representedin YPS format (Aschheim and Hernández-Montes, 2003).Figure 4 presents YPS determined by applying R-µ-T and R-C 1 -T relationships tothe NEHRP design spectra (2/3 <strong>of</strong> the MCE) for Site Class C conditions in <strong>Berkeley</strong>,<strong>California</strong>. The constant ductility curves were determined for ductilities <strong>of</strong> 2, 4, and 8.Shown by thick lines are results obtained using the R-µ-T relationship determined byNassar and Krawinkler (1991) for bilinear oscillators having a post-yield stiffnessequal to 2% <strong>of</strong> the initial stiffness. Shown by thin lines are results obtained using anR-C 1 -T relationship that was developed for the ATC-55 project using the constant Rfactor approach, given byR −1C1= 1+(1)290T485

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