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

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Section A-A