Ry = Ratio of the expected yield stress to the specified minimum yield stress, Fy
Rt = Ratio of the expected tensile strength to the specified minimum tensile strength, Fu
AISC 341-16 Provision uses a methodology for special moment frames, special concentrically braced frames, and eccentrically braced frames seismic systems that can be defined as “capacity design.” The capacity design philosophy is aimed at certain members intended to undergo inelastic deformations to be able to absorb energy efficiently. So, the required strength of members that are referred to behave essentially elastically is defined by forces corresponding to the capacity (expected strength) of certain members intended to undergo inelastic deformations.
Capacity-design methodology requires a realistic estimate of the expected strength of the members intended to undergo inelastic deformations. According to A3.2, the ratio of expected to nominal yield stress has been included in the Provisions as Ry to reach this aim. The expected capacity of the designated yielding member is defined as Ry times the nominal strength of the member based on the desired yield mode. This expected strength is amplified to account for strain-hardening in some cases.
Similarly, rupture limit states within the designated yielding element are affected by increased material strength.