D1.L.4.1.3 Compression
The allowable compressive stress for columns, except those fabricated from austenitic stainless steel shall be as required by NF3322.1(c)(1). The allowable compressive stress for columns fabricated from austenitic stainless steel shall be as required by NF3322.1(c)(2). The allowable compressive stress for member elements other than columns constructed by any material, including austenitic stainless steel, shall be as required by NF3322.1(c)(3).

Gross Sections of Columns, except those fabricated of austenitic stainless steel:

On gross section of axially loaded compression members, when (Kl/r) < C_{c} ,
${F}_{a}={F}_{y}\frac{[1\frac{{(KL/r)}^{2}}{2{C}_{c}^{2}}]}{5/3+\frac{3(KL/r)}{8{C}_{c}}\frac{{(KL/r)}^{3}}{8{C}_{c}^{3}}}$ (Eq. A1) Where:
${C}_{c}=\sqrt{\frac{2{\pi}^{2}E}{{F}_{y}}}$ 
When (Kl/r) > C_{c} ,
${F}_{a}=\frac{12{\pi}^{2}E}{23{(KL/r)}^{2}}$ (Eq. A2) 
When (Kl/r) > 120,
${F}_{as}=\frac{{F}_{a}\left[Eq\mathrm{.}\right(a1\left)\text{or}Eq\mathrm{.}\right(a2\left)\right]}{1.6\left(\frac{l}{200r}\right)}$


Gross sections of columns fabricated from Austenitic Stainless steel:

When (Kl/r) ≤ 120,
F_{a} = F_{y}[0.47  (Kl/r)/444]

When (Kl/r) > 120,
F_{a} = F_{y}[0.40  (Kl/r)/600]


Member elements other than columns:

For Plate Girder Stiffeners,
F_{a} = 0.60·F_{y}

For webs of rolled shapes,
F_{a} = 0.75·F_{y}
The above clauses are applicable only when the widththickness ratio of the element satisfies all the subsections of NF3322.2(d)..

If the abovementioned clauses are not satisfied,

For unstiffened compression element,
A reduction factor Q_{s} is introduced. Detailed values of Q_{s} for different shapes are given in NF3322.2(e)(2)(a) to NF3322.2(e)(2)(d).
In the case for angles or plates projecting from compression members and for projecting elements of compression flanges of girder,
When 95/(F_{y}/kc)^{1/2} < b/t < 195/(F_{y}/kc)^{1/2} , Q_{s} = 1.293  0.00309·(b/t)·(F_{y}/kc)^{1/2}
When b/t > 195/(F_{y}/kc)^{1/2} , Q_{s} = 26,200·kc/[F_{y}(b/t)^{2})]
Where:
 kc = 4.05/[(h/t)^{0.46}] if h/t > 70, otherwise kc = 1.0.

For stiffened compression element,
A reduced effective width b_{e} is introduced.

For the flanges of square and rectangular sections of uniform thickness:
${b}_{e}=\frac{253t}{\sqrt{f}}[1(\frac{50.3}{(b/t)\sqrt{f}}\left)\right]\le b$ 
For other uniformly compressed elements:
${b}_{e}=\frac{253t}{\sqrt{f}}[1(\frac{44.3}{(b/t)\sqrt{f}}\left)\right]\le b$
Consequently, a reduction factor Q_{a} is introduced and is equal to the effective area divided by the actual area. Combining both these factors, allowable stress for axially loaded compression members containing stiffened or unstiffened elements shall not exceed
${F}_{a}={F}_{y}\frac{{Q}_{s}{Q}_{a}[1\frac{{(KL/r)}^{2}}{2{C\prime}_{c}^{2}}]}{5/3+\frac{3(KL/r)}{8{C\prime}_{c}}\frac{{(KL/r)}^{3}}{8{C\prime}_{c}^{3}}}$Where:
${C\prime}_{c}=\sqrt{\frac{2{\pi}^{2}E}{{{Q}_{s}{Q}_{a}F}_{y}}}$ 
= 