Rankine Gordon formula for permissible stress in axial compression is (σc = crushing stress at failure, a = constant)
1
\( {{σ_c}\over {1+\alpha{k\over l_e}}}\)
2
\( {{σ_c}\over {1+\alpha{k\over l_e}^2}}\)
3
\( {{σ_c}\over {1+\alpha(L_e/k)^2}}\)
4
\( {{σ_c}\over {1+\alpha(L_e/k)}}\)