THE SHEAR BEHAVIOUR OF THE REINFORCED CONCRETE FOUR-PILE CAPS

Abstract

There has been a consistent discrepancy between UK design standards BS5400 and BS8110 in the prediction of the shear capacity of 2-way spanning reinforced concrete pile caps from bending theory-based empirical design formulae. This causes designers difficulty to predict an accurate shear capacity of the pile cap. The inherently empirical character of the formulae is due to the fact that the formulae have been extrapolated from semi-empirical shear formulae for simply supported deep 1-way spanning beam structures, and been further empirically developed for 2-way spanning caps. Thus the essential cause of the discrepancy is that the formulae lack both physical explanation in terms of the cap’s shear behaviour, and sufficient basis as empirical formulae due to the shortage of experimental data.
This research focuses on the revelation of the true shear capacity and failure mechanism of pile caps by consideration of a particular prototype form, namely a singly reinforced four-pile concrete cap under wall loading. It is aided by a series of laboratory experiments which are validated by an advanced non-linear numerical modelling for the reinforced concrete structure. The experience from the numerical modelling is taken further to carry out a parametric study expanding the sample size to a range covering more practical samples and covering different load patterns in order to enrich the limited data from the experiments.
The results give a verdict that both BS5400 and BS8110 are conservative with the former one most conservative. The level of conservatism of the standards, the actual shear capacity and failure mechanism of the cap vary with key pile cap dimensions such as longitudinal and transverse pile spacing, shear enhancement factor, and the width of the cap over which the shear enhancement factor is applied. The shear behaviour of pile caps is also influenced by the load patterns. In this research, the strut-and-tie method has been proved to be a more efficient and precise method than the empirical formulae because it presents a physical explanation of the shear mechanism. Suggestions to improve the design method are given.
A particular feature of this research is the application of a digital photogrammetry technique (PIV), normally applied in soil and fluid mechanics, to a solid mechanics situation. The tool has successfully detected the full-field displacement on the concrete surface and strains which are of high magnitude. The outputs have been compared with those from numerical modelling and they are in the same order of magnitude. The thesis describes the procedure of the application and an analysis of errors expected to occur in its application.

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