Numerical modelling of shear behaviour of reinforced concrete pile caps

The application of bending theory based methods and strut-and-tie models for the design of pile caps to resist shear is still a subject of debate, with the latest Eurocodes permitting both methods but not giving much guidance as to their use. The former UK design standards for concrete buildings and bridges, recently withdrawn, gave more guidance and it is likely that these methods will continue to be used by designers. However, there is considerable discrepancy between these standards, particularly with regards to the width of cap over which shear enhancement at short spans may be applied, and how much longitudinal reinforcement to take as a tie in the strut-and-tie method. Both standards are also seen as conservative. To gain a better understanding of the problem and assess the available design methods, nonlinear finite element analysis has been performed to investigate the shear behaviour of four-pile reinforced concrete pile caps, under full-width wall loading. The models were validated against an experimental programme that included an optical photogrammetric method for measuring full-field displacements. An extensive parametric study was carried out, varying shear span, cap width and reinforcement ratio over a practical range. The conservatism of the UK design standards, and the real shear capacity of the pile caps, were found to be a function of shear enhancement factor and the width of the cap over which shear enhancement is applied. Strut-and-tie behaviour was observed in the models, and a commonly used strut-and-tie method was found to give fairly good predictions. A modified strut-and-tie method is suggested for this particular configuration of a four-pile cap under full-width loading, which gives more accurate predictions. This is especially so for samples with large transverse pile spacing where a significant proportion of the longitudinal reinforcement over the width of the cap can be assumed to participate in the yielding ties