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The purpose of R is to reduce the demand determined, assuming that the structure remains elastic at the design earthquake level, to target the first significant yield. This reduction computes for ductility demand of the displacement, required by structural system and the inherent overstrength, Ω, of the seismic force-resisting system(SFRS) (Fig. C12.1-1). Significant yield point is where critical region is completely plastic (e.g., formation of the first plastic hinge in a moment frame), and the stiffness of the SFRS to further increases in lateral forces decreases as continued inelastic behavior spreads within the SFRS. This approach is suitable with member-level ultimate strength design practices.
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Due to the design rules and limits accroding to C12.9.1.3, including material strengths, structural elements are stronger than the required strength by analysis. The maximum strength developed along the curve is higher than that at first significant yield, and this point is referred to as the system overstrength capacity, Ω. The system overstrength described above is the direct result of overstrength of the elements that form the SFRS and the lateral force distribution used to evaluate the inelastic force–deformation curve.
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