A formula expressing the resistance of long columns to buckling:
P = a π 2 E I l 2 {\displaystyle P={\frac {a\pi ^{2}EI}{l^{2}}}}
where
P = the external load or pressure
E = the modulus of elasticity
I {\displaystyle I} = the least moment of inertia
l {\displaystyle l} = length
a = constant depending on end conditions
π {\displaystyle \pi } = 3.14159
= the least moment of inertia
= length
= 3.14159