## P

where P, M are second-order axial force and bending moment, Py is the squash load, and Mp is the plastic moment capacity.

### 5.2.1.3 CRC Tangent Modulus

The CRC tangent modulus concept is employed to account for the gradual yielding effect due to residual stresses along the length of members under axial loads between two plastic hinges. In this concept, the elastic modulus E, instead of moment of inertia I, is reduced to account for the reduction of the elastic portion of the cross-section since the reduction of elastic modulus is easier to implement than that of moment of inertia for different sections. The reduction rate in stiffness between weak and strong axes is different, but this is not considered here because rapid degradation in stiffness in the weak-axis strength is compensated well by the stronger weak-axis plastic strength. As a result, this simplicity will make the present methods practical. From Chen and Lui [18], the CRC Et is written as (Figure 5.4):

FIGURE 5.3 Strength interaction curves for wide-flange sections.

FIGURE 5.3 Strength interaction curves for wide-flange sections.

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