The concept of pile foundation is incomplete without discussing pile caps because the loads from the superstructure of a structure cannot be transferred directly on piles as it is almost unlikely for the vertical element consisting the superstructure to require single piles to support the actions to which they are subjected to. It is common, however, to use a group and in order to distribute the load on the piles, a pile cap is required to transmit the loads from the superstructure into the piles.
This post concerns the design for pile caps for a small group of piles e.g. 2-4 piles purely axially loaded. It doesn’t consider large pile group and pile caps subjected to horizontal loads and/or moments due to the complexity in obtaining the distribution of loads on the pile groups and high influence of differential settlement further complicating their design. However, guidance can be obtained from the reference and further reading section of the post.
Pile Cap Design
The layout of piles is largely determined from the magnitude and location of actions they are to support from the superstructure.
They are grouped together based on the pile’s capacity to support axial forces that can be either tension or compression, depending on the direction of the axial forces induced into the pile-cap from actions generated by the superstructure.
The location of piles with reference to the point of axial force application should be symmetrical. The proximity of piles to one another is at least 3 × diameter of the pile. For pile-caps with 1 or 2 piles, some restraint needs to be provided orthogonally to the pile(s), which is usually achieved via ground beams. Figure 1 provides guidance on the location of piles in 2, 3 and 4 pile pile-caps where s is the spacing between piles.
Determining axial forces in piles within pile-caps
When a pile-cap is supporting an axial force N that is placed within the centroid of the pile group, the axial force in each pile is defined as:
Where: n is the number of piles.
This only applies to piles within
Design of Tension Reinforcement
Two methods of design are common: design using beam theory or design using a strut-tie approach. In the former case the pile cap is treated as an inverted beam and is designed for the usual conditions of bending and shear. The strut-tie assumes the reinforcement within the pile-cap is acting as if it were part of a truss, with the compression stresses being withstood by the concrete and the tension by the steel reinforcement. This method is applicable for pile-caps with less than 6 piles and is therefore the focus of this post.
Figure 2 shows the forces that pass through a pile-cap into the piles and how the strut and tie method is applied to it. It indicates how the depth of the pile-cap determines the magnitude of the forces within the concrete and the tension reinforcement. Typically the angle of the truss is set at 45º, which is then used to determine the depth of the pile-cap. Further iterations of this angle may be necessary as the size of the pile-cap is altered and/or the element it is supporting is modified to overcome geometry constraints and other extraneous design criteria. This can result in having a shallower angle that increases the tension in the reinforcement – as does the compression stress in the struts within the pile-cap.
When the location of the axial force is not eccentric, the applied tension force between each pile can be calculated using Table 1
The tension reinforcement As required in the pile-cap is then defined as
Where fyk is the tension strength of the reinforcement, which is typically 500 N/mm2.
Design of Shear Reinforcement
A critical shear plane adjacent to the vertical element that the pile-cap is supporting needs to be checked to determine whether or not it fails in shear. The plane’s location is based on the distance av, which is the dimension from the face of the vertical element the pile-cap is supporting and the face of a pile plus 0.2 × the pile diameter. This plane’s location is further explained in Figure 7. The shear verification is then carried out the same way we did for concrete beams.
An additional check with respect to shear is required at the face of the vertical element the pile-cap supports.
Ned is the design axial load on the pile cap while VRd,max is the maximum shear resistance defined as
- p is the perimeter length of the vertical element of the superstructure
- d is the depth of the pile-cap
Pile Cap Reinforcement Detailing
There are several unique detailing requirements that are specific to pile caps. The anchorage length of the tension reinforcement is dependent on the bond conditions between the concrete and the steel. In the case of pile-caps, a good condition bond requires the reinforcement to be located within the 250mm depth of the concrete pour. For anything placed outside of that zone, the bond is considered to be ‘poor’. For more information on this see Clause 8.4.2 in BS EN 1992-1-1. Table 2 provides anchorage lengths for reinforcement bars based on concrete strength and bond conditions. A more accurate tension reinforcement anchorage length can be calculated using the guidance provided in Clause 8.4.3 of EN 19921-1. The minimum diameter of reinforcement used in a pile-cap is 8mm
Also, the minimum area of steel as defined for slabs & beams should also be verified to ensure that it is not critical
A 500x 500 concrete column carries an axial design action of 4250 kN. Design a 4 pile pile-cap to support the column. The piles are 500mm in diameter cylindrical concrete. Design the pile cap completely using C30/37 concrete with 500mpa high tensile steel assuming the column to be placed in the centroid of the pile group.
Geometry of the pile-cap
Try an overall depth h=1000mm with an average effective depth of 900mm, the spacing between piles = 3×500=1500mm and assuming an overhang of 400mm both ways. width of pile cap = 400+1500+400 = 2300mm. Figure 4 shows the trial geometry of the pile cap.
Verify Minimum Area of Steel
Since pile spacing as been taking at exactly 3times the pile diameter, no punching verification is necessary, except at the column face.
Further Reading & References
- Mosley W. H., Hulse R. and Bungay J.H. (2012) Reinforced Concrete Design to Eurocode 2 (7th ed.) Basingstoke, UK: Palgrave MacMillan
- Tomlinson M. and Woodward J. (2007) Pile Design and Construction Practice (5th ed.) Boca Raton, Florida: CRC Press
- Webster R. and Brooker O. (2007) How to design concrete structures using Eurocode 2 – Part 6. Foundations [Online] Available at: www.concretecentre.com/pdf/ publicationlibrary/how2_foundations.pdf (Accessed: November 2013)
- The Institution of Structural Engineers (2013). Designing a Pile Cap. Technical guidance notes (level 2).