고체역학 (Beer) — Analysis and Design of Beams for Bending: Problem 5.92

5.92 A 54-kip load is to be supported at the center of the 16-ft span shown. Knowing that the allowable normal stress for the steel used is 24 ksi, determine (a) the smallest allowable length \( l \) of beam \( CD \) if the \( W12 \times 50 \) beam \( AB \) is not to be overstressed, (b) the most economical W shape that can be used for beam \( CD \). Neglect the weight of both beams.