F3Doubly symmetric I-shaped members with compact webs and noncompact or slender flanges bent about their major axis

This section applies to doubly symmetric I-shaped members bent about their major axis having compact webs and noncompact or slender flanges as defined in Section B4.1 for flexure.

User Note: The following shapes have noncompact flanges for $F_{y}=50 \mathrm{ksi}(345$ MPa): W21x48, W14x99, W14x90, W12x65, W10x12, W8x31, W8x10, W6x15, W6x9, W6x8.5, and M4x6. All other ASTM A6/A6M W, S, and M shapes have compact flanges for $F_{y} \leq 50 \mathrm{ksi}(345 \mathrm{MPa})$.

The nominal flexural strength, $M_{n}$, shall be the lower value obtained according to the limit states of lateral-torsional buckling and compression flange local buckling.

F3.1 Lateral-Torsional Buckling

For lateral-torsional buckling, the provisions of Section F2.2 shall apply.

F3.2 Compression Flange Local Buckling

(a) For sections with noncompact flanges

(F3-1)

(b) For sections with slender flanges

(F3-2)

where

$k_{c}=\frac{4}{\sqrt{h / t_{w}}}$ and shall not be taken as less than 0.35 nor greater than 0.76 for calculation purposes

$h=$ distance as defined in Section B4.1b, in. (mm)

$t_{w}=$ thickness of the web, in. (mm)

$\lambda=\frac{b_{f}}{2 t_{f}}$

$b_{f}=$ width of the flange, in. (mm)

$t_{f} \quad$ = thickness of the flange, in. (mm)

λpf = λp, the limiting width-to-thickness ratio for a compact flange as defined in Table B4.1b

$\lambda_{r f}=\lambda_{r}$, the limiting width-to-thickness ratio for a noncompact flange as defined in Table B4.1b