Shear capacity of reinforced concrete pile caps

Reinforced concrete pile caps may be considered to act either as deep beams, or analogous to a truss. When designed as a deep beam, there is currently a contradiction in the shear design between two UK structural engineering codes of practice – the design code BS 8110 for reinforced concrete buildings, and BS 5400: Part 4 for bridges. The majority of this difference in shear design is concerned with the width of the cap for which a shear enhancement factor for short shear spans may be applied. BS 8110 permits the factor to be applied across the full width of the cap if the pile spacing is no more than three pile diameters, whereas BS 5400 allows the factor to be applied only for the width of the pile diameters. Given that the shear enhancement factor is a very significant component of the design strength of the cap, this difference can give a factor of two or three between the shear strengths according to the two codes of practice.This paper describes research that has been carried out with the aim of resolving the issue of the width of shear enhancement. A series of sixteen four-pile caps of close to full size, with spans in the range 500mm – 1200mm and depths from 230mm – 400mm have been tested to failure in the laboratory. The test results have been used to verify three-dimensional nonlinear finite element analyses conducted using the commercial package DIANA. The results have shown good agreement of behaviour between the tests and the numerical analyses, and have also indicated that the BS 8110 shear enhancement approach is safe. It is anticipated that the results of this research will inform the next revision of the Structural Eurocode, EN 199