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FIGURE 5.22 Comparison of strength curves for a portal frame subject to strong-axis bending with Lc/rx = 60, Ga = 0 (further reduced tangent modulus method).

compressive residual stress is 0.3Fy when the ratio of depth to width (d/b) is greater than 1.2, and is 0.5Fy when the d/b ratio is less than 1.2 (Figure 5.25). The stress-strain relationship is elastic-plastic with strain hardening as shown in Figure 5.26. The geometric imperfections are Lc/450.

For comparison, the out-of-plumbness of Lc/450 is used in the explicit modeling method. The notional load factor of 1/450 and the reduced tangent modulus factor of 0.85 are used. The further reduced tangent modulus is equivalent to the geometric imperfection of Lc/500. Thus, the geometric imperfection of Lc/4500 is additionally modeled in the further reduced tangent modulus method, where Lc/4500 is the difference between the Vogel's geometric imperfection of Lc/450 and the proposed geometric imperfection of Lc/500.

The load-displacement curves in the proposed methods together with Vogel's plastic-zone analysis are compared in Figure 5.27. The errors in strength prediction by the proposed methods

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