SYMBOLS
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
The reduced response spectra from ground motions is used basic seismic analysis procedure. Structural elements are anticipated to yield, buckle or otherwise behave inelastically at the MCER level of ground motion. In the ASCE 7-16, seismic design internal forces are computed by dividing the forces the response modification coefficient, R, would be 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, 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 standart.
The energy dissipation resulting from hysteretic behavior can be measured as the area enclosed by the force–deformation curve of the structure as it experiences several cycles of excitation. Some structures have far more energy dissipation capacity than others. The extent of energy dissipation capacity available depends largely on the amount of stiffness and strength degradation the structure undergoes as it experiences repeated cycles of inelastic deformation. Below figure shows representative load deformation curves for two simple substructures, such as a beam–column assembly in a frame. Hysteretic curve (a) in the figure represents the behavior of substructures that have been detailed for ductile behavior. The substructure can maintain almost all of its strength and stiffness over several large cycles of inelastic deformation. Hysteretic curve (b) represents the behavior of a substructure that has much less energy dissipation than that for the substructure (a) but has a greater change in response period. The structural response is determined by a combination of energy dissipation and period modification.
