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