Yield Point Spectra (YPS) are used to indicate the influence of P-∆ effects on the lateral strengths of structures with constant ductility demands. The intensity of P-∆ effects is represented by an index, called the effective height, heff . Both the yield displacement and the effective height are stable parameters, even when the structure shows nonlinear behavior. The strength ratio, n, is defined as the ratio of lateral strength associated with P-∆ effect at the peak displacement and the total strength developed at the peak displacement response. In this paper by Ploting strength ratio(n) vs. yield displacement , the influence of P-∆ effects has been verified.