Torsion of a Constrained Rectangular Box Section

The paper describes an investigation of the stresses due to torsion of a doubly-symmetrical thin-walled rectangular box section with four corner spar flanges. The box section was subjected to a pure torque at the free end. The other end of the box section was constrained to remain plane. The shear stresses in the top and bottom skins and in the vertical webs and the direct stresses in the corner spar flanges were measured. The measured values were compared with theoretical predictions using the 'linear' and 'closely spaced rigid diaphragm' assumptions. The torsional stiffness was also measured and compared with theory. The experimental results show the marked effect of buckling of the skin covering on the stresses in the flanges. They also illustrate the importance of using the secant modulus in calculations.