D1.L.1.1.3 Compression
The allowable compressive stress for columns shall be as required by clause XVII2213 of NF3000 1974.

Gross Sections of Columns:

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


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 clause XVII2224 of NF3000 1974.
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 the clause XVII2225.2 of NF3000 1974.

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}, equal to the effective area divided by the actual area is introduced.
Combining both these factors, allowable stress for axially loaded compression members containing stiffened or unstiffened elements shall not exceed
${F}_{a}={F}_{y}\frac{{Q}_{s}{Q}_{a}[1\frac{{(KL/r)}^{2}}{2{C\text{'}}_{c}^{2}}]}{5/3+\frac{3(KL/r)}{8{C\text{'}}_{c}}\frac{{(KL/r)}^{3}}{8{C\text{'}}_{c}^{3}}}$Where:
${C\text{'}}_{c}=\sqrt{\frac{2{\pi}^{2}E}{{{Q}_{s}{Q}_{a}F}_{y}}}$ 
= 