# Correspondence on The Derivation of Torsional Stiffness Using a Modern Spreadsheet and an Improved Finite Difference Scheme by A.G. Smyrell

*Dr D. Johnson (M) (Nottingham Polytechnic)
The author has presented an interesting spreadsheet application in tackling the torsion problem by the stress function formulation, using the finite difference method. While agreeing that such an approach has the attraction of simplicity, it should perhaps be recognised that it also has some limitations. Box and other 'multiply-closed' sections, for instance, are cumbersome to analyse by a stress function formulation and are more conveniently analysed by the use of a warping function. It may also be argued that it is desirable to produce solutions by both warping and stress function based approaches so that upper and lower bounds on the torsional stiffness are obtained, so minimising the need for successively refined analyses. Unfortunately, the warping function approach is much less amenable to finite difference modelling, although either function may be readily used with the finite element method, which would appear to be more versatile in this respect. *