Cs = The seismic response coefficient
SD1 = The design spectral response acceleration parameter at a period of 1.0 s
SDS = The design spectral response acceleration parameter in the short period range
R = The response modification factor in Table 12.2-1
Ie = The Importance Factor
Seismic analysis procedure is performed with reduced response spectra. At MCER level of ground motion, structural elements are prospective to yield, buckle or behave inelastically. In ASCE 7-16, the response modification coefficient, R is used to compute seismic design internal forces by dividing the forces. It is produced in a structure behaving elastically when subjected to the design earthquake ground motion.
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.
Due to the design rules and limits according to C184.108.40.206, 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.
Structures typically have a much higher lateral strength than that specified as the minimum by ASCE 7-16. The first yielding of structures may occur at lateral load levels that are 30% to 100% higher than the prescribed design seismic forces by the standard.