C-F5F5 Doubly symmetric and singly symmetric I-shaped members with slender webs bent about their major axis

This section applies to doubly and singly symmetric I-shaped members with a slen- der web, that is, h c t w > λ r = 5.70 E F y . As is the case in Section F4, four limit states are considered: (a) compression flange yielding; (b) lateral-torsional buckling; (c) compression flange local buckling; and (d) tension flange yielding. The provi- sions in this section have changed little since 1963. The provisions are based on research reported in Basler and Thürlimann (1963).

There is no seamless transition between the equations in Section F4 and Section F5. The bending strength of a beam with $F_{y}=50 \mathrm{ksi}(345 \mathrm{MPa})$ and a web slenderness, $h / t_{w}=137$, is not close to that of a beam with $h / t_{w}=138$. These two slenderness ratios are on either side of the limiting ratio. This gap is caused by the discontinuity between the lateral-torsional buckling resistances predicted by Section F4 and those predicted by Section F5 due to the implicit use of $J=0$ in Section F5. However, for typical I-shaped members with webs close to the noncompact web limit, the influence of $J$ on the lateral-torsional buckling resistance is relatively small (for example, the calculated $L_{r}$ values including $J$ versus using $J=0$ typically differ by less than $10 \%$ ). The implicit use of $J=0$ in Section F5 is intended to account for the influence of web distortional flexibility on the lateral-torsional buckling resistance for slenderweb I-section members.