Determine the allowable axial load Pallow for a steel pipe column that is fixed at the base and free at the top (see figure) for each of the following lengths: L = 2.6 m, 2.8 m, 3.0 m, and 3.2 m. The column has outside diameter d = 140 mm and wall thickness t = 7 mm. (Assume E = 200 GPa and (Y = 250 MPa.)