A509.4.4 Period Determination

The effective fundamental period, T_e , in the direction under consideration, shall be determined using the force-displacement relation of the nonlinear static pushover analysis. The nonlinear relation between the base shear and target displacement of the control node shall be replaced by a bilinear relation to estimate the effective lateral stiffness, K_e , and the yield strength, V_y , of the building. The effective lateral stiffness shall be taken as the secant stiffness calculated at a base shear force equal to 60 percent of the yield strength. The effective fundamental period, T_e , shall then be calculated as:


where:

Ti	=	Elastic fundamental period in the direction under consideration calculated by elastic dynamic analysis.
Ki	=	Elastic lateral stiffness of the building in the direction under consideration.
Ke	=	Effective lateral stiffness of the building in the direction under consideration.