An aluminum box column (E=73 GPa) of square cross-section is fixed at the base and free at the top. The outside dimension b of each side is 100mm and the thickness of the wall is 8mm. The resultant of the compressive loads on the top of the column is a force P=50KN acting on the outer edge of the column at the midpoint one side. What is the longest permissible length Lmax of the column if the deflection at the top is not to exceed 30mm?