Crack width calculation to BS 8007 for combined flexure and direct tension

(The Structural Engineer, 17 Sep. 2002)

Synopsis

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. Furthermore, little guidance in this regard is given in the literature. Neither Anchor nor Batty provides a rational approach to allow for the effect of tension stiffening for the case of combined flexure and direct tension in their handbooks.

Since combined flexure and direct tension often occurs in structural elements of certain water-retaining structures,  for example in the horizontal direction of walls of rectangular or square tanks, a need for a rational approach to calculate crack widths for this case exists.

In this article Erhard Kruger examines the procedures given in the code, and shows the separate equations for flexure and tension in the code to be based on similar premises.  The situation where tension exists across the whole of the section under combined flexure and direct tension is then examined, and the limiting values of the depth of the neutral axis for this to occur, are calculated. Equations are developed for surface strains and the stiffening effect of the concrete.

Similar equations are developed for the second case where some compression is present on one face 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 discusses this in an article which appeared in The Structural Engineer of 17 September 2002.

(Please note: This article must be read in conjunction with the following article by Erhard Kruger and Robin Atkinson, Improved crack width calculation method to BS 8007 for combined flexure and direct tension (The Structural Engineer, 17 May 2005).)

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