A rotating step shaft is loaded as shown, where the forces F_A and FB are constant at 600 lbf and 300 lbf, respectively, and the torque T alternates from 0 to 1800 lbf · in. The shaft is to be considered simply supported at points O and C, and is made of AISI 1045 CD steel with a fully corrected endurance limit of Se = 40 kpsi. Let K_f = 2.1 and K_fs = 1.7. For a design factor of 2.5 determine the minimal acceptable diameter of section BC using the 

a. DE-Gerber criterion.

b. DE-Goodman criterion.

6 in F = 600 lbf 6 in FB = 300 lbf 6 in kc T, n