(Torsional stiffness calculated by means of statically admissible stress fields)
The present report shortly describes the theory of calculation of torsion stiffness by means of the energy equation applied to statically admissible stress fields. Moreover it describes an easy way of determining these stress fields by means of Prandtl's stress function.The calculations are carried through for two solid cross-sections, a square and a rectangular one. Different solutions are suggested, and comparison with the exact solution is made. The comparison shows that even with very simple assumptions about the stress field, fairly accurate approximations to the torsion stiffness (lower bound values) can be achieved. The best solution for the rectangular cross-section has a maximum deviation from the exact solution of 11%. This solution also provides an acceptable approximation to the maximum shear-stress. The deviation from the exact maximum stress is 17%.