As stated in the introduction to Section 6.5 the practically important case of an open section tube with no St. Venant torsional stiffness (see Fig. 38) will be treated as the limiting case of the open tube with J as J→0. It will be shown that the stress system g1h1over the open tube degenerates into a stress system gгhг which is statically equivalent to the torque TBS about the shear centre. This stress system is for a uniform cylindrical tube with no J identical to the Wagner-Kappus torsion-bending stress system in open tubes.