ABSTRACT
A pile cap, a thick reinforced concrete block whose function is to transfer loads from a column to a group of piles, is an expensive and important element in a pile foundation. However, the current pile cap design is rough and the researches on pile caps are rare. In particular, it is common engineering practice to apply the rigid cap assumption in pile cap design, the boundary between the rigid cap assumption and the flexible cap assumption is ambiguous and the depth design by conventional methods does not necessarily guarantee the rigid behaviours of a pile cap. This may lead to a mistaken and dangerous design of the pile cap and the piles if the rigid cap assumption is used to design a pile foundation with a pile cap that actually behaves as a flexible cap.4367

In this research, the differences of the structural behaviours between a pile group with piles linked by a rigid pile cap and those with piles linked by a flexible pile cap are studied, for the cases of a pile group subjected to a vertical load with or without bending moments. The advantages of using a rigid pile cap are shown outweighing the drawbacks, which explains the wide application of the rigid cap assumption in pile cap design. The need for additional torsional reinforcement for rigid pile caps, which is thought of as one of the major drawbacks of the rigid cap assumption, is proved a misconception by the use of finite element analysis of pile caps.

Finite element modelling techniques are discussed. The three-dimensional finite elements - the 3D solid element, the plate element and the shell element - are applied to analyse identical pile caps to select the one that outperforms the rest. The geometric configuration of the pile cap and the boundary conditions of the pile foundation models are also presented. Different loading cases and different shapes of the pile cap are compared to reveal the most critical combination.
By the finite element method, the internal stress distributions inside a pile cap are investigated, which provides a clear understanding of the force transfer mechanism and indicates the incorrect assumptions of the neutral axis always located at the mid-level of a pile cap and a linear strain distribution with depth at any section. The out-of-plane deflection curves at different levels of a pile cap are considered, demonstrating the uneven out-of-plane compressive strain within a pile cap.
     
A graph of the design curves for the depth of a pile cap required for the validity of the rigid cap assumption has been derived, according to the relationship between the depth of the cap and the cap curvature. In this study, the cap curvature is expressed as the out-of-plane deflection divided by the pile spacing, which is related to the extent of the rigid cap behaviours of a pile cap. The number of floors of the supported buildings and the centre-to-centre pile spacing are the only parameters used in the design curves, this is because of their great relationship with the required depth for a rigid cap as shown in the parametric studies. The design curves are compared with conventional design methods. It is concluded that for a tall building of a lower height, such as a 45-storey building, the depth designed by conventional methods are not sufficient for a pile cap to be regarded as rigid, and the rigid cap assumption governs the design of the depth of the pile cap in this case.
CHAPTER1   INTRODUCTION
1.1    BACKGROUND
Pile foundations are commonly adopted in tall buildings because in normal circumstances, the bearing capacity of the upper soil layers is insufficient for a shallow foundation to support such superstructures, but firmer strata are available at a greater depth. Pile foundations are also highly recommended for structures over shallow foundations when the upper soil layer is highly compressible, making it too weak to support the loads transmitted from the structures, or expansive and collapsible, which swells and shrinks as the moisture content changes. While in the foreseeable future the number of tall buildings to be constructed will increase and their heights might further rise, the demand for pile foundations will greatly increase. 高层建筑钢筋混凝土承台设计英文文献和翻译:http://www.youerw.com/fanyi/lunwen_1015.html