You work for a fabricator and are reviewing a set of structural drawings for bid. You notice that there are many HSS7.500x0.375 ASTM A500 Gr. C columns with effective length (Lc) equal to 15 ft called out on the drawings and recall that you are able procure ASTM A53 Gr. B pipe material at a much cheaper price (this is not generally true). What is the lightest pipe section that you can propose to the engineer of record that is a suitable alternative for the HSS (i.e., has the same or greater axial strength)?

You may use the tables in Part 4 of the AISC Steel Manual, but you must show full calculations for your final answer.