The joists of a floor in a warehouse arc to be selected using square timber beams made of...

The joists of a floor in a warehouse arc to be selected using square timber beams made of oak. If each beam is to be designed to carry 90 lb/ft over a simply supported span of 25 ft, determine the dimension a of its square cross section to the nearest  in. The allowable bending stress is σallow = 4.5 ksi and the allowable shear stress is τallow = 125 psi.