Rankine's formula

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

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

where

<tex>p</tex> = allowable unit stress for the column

<tex>s</tex> = allowable unit stress for short columns

<tex>a</tex> = a constant

<tex>l</tex> = length

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