Semi-circular yieldable steel arches with a nominal radius of 6.75 ft are used in main entries to a coal mine developed from outcrop under plateau overburden. Seam depth at a point of interest is 2,350 ft where ground pressure on the supports is estimated to be 20 psi. Determine cross-sectional area (square inches) of A36 steel needed and a reasonable set spacing for an appropriate steel safety factor.