The British Code of Practice for the design of water-retaining structures, BS 8007, includes recommendations for the calculation of design crack widths for sections under flexure and for sections under direct tension. However, it does not provide recommendations for sections under combined flexure and direct tension

In an earlier article, Erhard Kruger has shown the separate equations for flexure and direct tension to be based on similar premises. A method was proposed to proportion the tensile stiffening force to the two layers of reinforcement by considering horizontal and moment equilibrium of the stiffening forces. This method results in the “neutral axis” of the stiffening strain diagram not coinciding with the neutral axis of section under the applied forces. Some literature suggests, however, that the stiffening strains should emanate from the neutral axis position.

In this article Erhard Kruger and Robin Atkinson proposes an improved method to proportion the stiffening strain to the reinforcement layers in the two faces of the section under combined flexure and direct tension.

It is shown how the design crack widths for these cases can be determined. Equations are developed with variables allowing for different reinforcement ratios and concrete cover at each face.

Erhard Kruger of HGK Consulting CC and Robin Atkinson of Howes Atkinson Crowder LLP discusses this in an article which appeared in The Structural Engineer of 17 May 2005.