The outer dimensions of the hollow core beam cross section shown are b-24 in. and h-12 in. Two circular voids, each having a diameter of d = 8 in., have been formed inside the rectangular cross section. The circular openings are centered in the y direction, and symmetrically placed with respect to the y axis. (In other words, the shape is doubly symmetric.) If a bending moment of My - 129 kip-ft is applied about the z centroidal axis, what is the magnitude of the maximum bending stress in the beam cross section? 1/2 b