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Pile foundations can be comprised of a single pile or a pile group constructed with or without a pile cap, a thick reinforced concrete block whose function is to transfer loads from columns to a group of piles. It has been shown that the existence of a pile cap can increase the load efficiency in terms of the skin friction of the pile group (Lee and Chung, 2000). Therefore, pile cap design is usually associated with the pile foundation design, and a pile cap is an important structural member in pile foundation, in contact with the soil as in most cases, or well above the ground as in the case of offshore platforms.
The pile cap is an expensive item in construction and appreciable savings in the cost can be achieved by performing a more realistic modelling of the problem (Cheng, 2003). In addition, pile caps are basic to the safety of structures, and a sound knowledge of their exact behaviour is essential (Sam and Iyer, 1995). Furthermore, design rules developed for pile caps may have applicability to other similar structural configurations such as reinforced concrete corbels and transfer plates. However, the current design procedures for pile caps do not provide engineers with a clear understanding of the physical behaviours of these elements, and the generally used rigid cap assumption in the pile cap design is not stated in any of the design codes.A detailed analysis of the pile caps is thus required.
BS 8110 (1997) and BS 5400:Part 4 (1990) are respectively the design codes for reinforced concrete buildings and bridges in the UK. These two codes are broadly consistent, as they are both based on CP 110 (1972). Nevertheless, for shear in large pile caps, BS 8110 (1997) could give strengths as much as three times those given in BS 5400 (1990), implying that either pile caps in bridges are over-designed and uneconomic, or pile caps in buildings are unsafe. Besides the disparity, both codes will eventually be replaced by a European Document, EuroCodes 2 (2002), which does not have specific rules for pile caps at present. This is an example of inconsistency of different design codes in pile cap design and shows that there is a need for a united pile cap design practice.
Two basic methods of analysing pile caps are in common use - the beam theory and the truss analogy. The beam theory states that a pile cap can be considered to behave as a short deep beam, transferring the load from the column to the piles by bending actions. This method seems the most appropriate for a two-pile cap. Alternatively, the truss analogy states that a pile cap may be imagined to act as a space frame, the inclined lines of force linking the underside of the column to the top of the piles being assumed as compression members, and the reinforcement linking the pile heads being imagined as horizontal tension members. This assumption appears particularly appropriate for analysing the more ‘three-dimensional’ pile caps, such as three-pile caps and four-pile caps. Both BS 8110 (1997) and BS 5400 (1990) permit the design of a pile cap either as a simple beam or as a truss. The codes, however, give no guidance as to which of these methods is the appropriate or economic assumption.
As early as in the 1950s, it is evident that the ordinary laws of flexure do not apply to the pile cap, but in the absence of a more rational design procedure, pile caps are still designed as simple beams and the latticed frame analogy has remained a concept only, without attempt to apply in the design and extension to more complex cases involving three-dimensional framework (Yan, 1954). As the strut-and-tie analogy developed, Adebar et al. (1990) concluded that the strut and tie model can accurately represent the behaviour and failure loads of pile caps. Using eight-node isoparametric elements to conduct a nonlinear finite element analysis of a reinforced concrete four-pile cap with square column and piles, Sam and Iyer (1995) showed that at the initial stages of loading the beam action is predominant in the pile cap whereas at the final stages of loading the pile caps resists forces by strut action. The truss analogy is now commonly used in pile cap design.