The Substitute Structure Method is based on the procedure presented in Priestley et al. [4.4] and is briefly summarized below.
- Idealize the pushover curve from nonlinear pushover analysis, as described in Section 3104F.184.108.40.206, and estimate the yield force, Fy, and yield displacement, Δy.
- Compute the effective elastic lateral stiffness, ke, as the yield force, Fy, divided by the yield displacement, Δy.
- Using the curve applicable to the effective structural damping, ξeff, find the effective period, Td (see Figure 31F-4-6).
- In order to convert from a design displacement response spectra to another spectra for a different damping level, the adjustment factors in Section 3103F.4.2.9 shall be used.
- Fu and Δd can be plotted on the force-displacement curve established by the pushover analysis. Since this is an iterative process, the intersection of Fu and Δd most likely will not fall on the force-displacement curve and a second iteration will be required. An adjusted value of Δd, taken as the intersection between the force-displacement curve and a line between the origin and Fu and Δd, can be used to find μΔ.
- Repeat the process until a satisfactory solution is obtained (see Figure 31F-4-7).
- From pushover data, calculate the displacement components of an element along the two axis of the system.