12.5 Figure P12.5 shows a fully opened trap door covering a stairwell. The door weighs 60 lb, with center of gravity 2 ft from the hinge. A torsion bar spring, extending along the hinge axis, serves as a counterbalance. Determine the length and diameter of a solid steel torsion bar that would counterbalance 80% of the door weight when closed, and provide a 6-lb-ft torque holding the door against the stop shown. Use a maximum allowable torsional stress of 50 ksi. Make a graph showing gravity torque, spring torque, and net torque all plotted against the door-open angle. [Ans.: 115 in., 0.49 in.] Center of gravity of 60-lb door Torsion bar Door stop Fixed end 110 End attached to door FIGURE P12.5 24 in.