In an analysis by the matrix force method of a structure containing thin tubes of open cross-section, it is commonly assumed that the torsional stiffness of such members may be taken into account by considering the St. Venant shear stresses only. This effectively introduces the assumption that the cross-section of an open tube is able to warp freely, whereas in practice the warping may be constrained by the surrounding structure.

In this note a method is presented which allows such warping constraint effects to be taken into account in a finite element analysis using the Force Method. Because of the assumption of linear variation of direct stress along the element the full flexibility matrix does not have to be used, which may be an advantage in certain cases. The “exact”, but more complex, flexibility matrix has been derived and will be presented in a later note.