Rankine's formula

One of the most widely known formulae for the design and investigation of columns employed in engineering practice:

p=\frac{s}{1+a(\frac{l}{r})^2}

where

p = allowable unit stress for the column

s = allowable unit stress for short columns

a = a constant

l = length

r = radius of gyration in reference to an axis normal to a plane in which flexure takes place