Design of RHS for torsion using simplified thick wall theory.

The theory of torsion in Rectangular Hollow Sections (RHS) is reviewed and discussed with reference to design code treatments in the UK, Europe and the USA. The classical theories based on thin wall and thick wall assumptions are compared, and thick wall theory is shown to be a better representation of the torsional behaviour of RHS with reference to the results of finite element analysis. Simplified thick wall analysis is then extended to create expressions for the complete torque–rotation relationship, which are shown to produce predictions that are almost identical to those produced by finite element models. Since thin wall theory overestimates the torque required to cause permanent deformation, the use of thick wall theory in design is advocated. The effects of warping and shear buckling of the tube wall are also examined and are shown to be insignificant for most practical cases, although local instability can lead to a reduction in torsional capacity for extremely slender tube walls