The hollow box beam shown in the figure is subjected to a bending moment M of such magnitude that the flanges yield but the webs remain linearly elastic.
(a) Calculate the magnitude of the moment M if the dimensions of the cross section are h = 15 in., h1 = 12.75 in.,
(b) b = 9 in., and b1 = 7.5 in. Also, the yield stress is σY = 33 ksi.