Fitting an ellipse into a triangle

In a concrete pile or circular column, only a component of the force in a circular or spiral shear link resists the applied shear. However, some approaches to the design of these links make no allowance for their curvature or the asymmetry of spirals. Although the author is unaware of failures of piles or columns designed using these methods, elements so designed are likely to have a reduced factor of safety. This paper recommends alternative expressions that consider the shape of the links more rigorously. It also recommends that the method for calculating shear stress in the Highways Agency’s design manual should be reviewed. Ian Feltham, MA, CEng, MIStructE, MICE Arup Research + Development, London