Gordon's formula

A column formula. Also called Rankine's formula:

$P={\frac {s}{1+a{\frac {l^{2}}{r^{2}}}}}$

where:

$P$ = the allowable unit stress for the column

$s$ = the allowable unit stress for blocks

$a$ = a constant depending on end conditions, etc.

$l$ = the unsupported length of the column

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