This article presents an overview on the design of pile caps to EN 1992-1-1-2004 (Eurocode 2). Where there are not much details given in Eurocode 2, the superseded British Standard, BS 8110-1-1997 is consulted.
Pile caps are transfer structures that transmit loads from superstructure through a wall or a column to one or more supporting piles. They serve as intermediary members between two compression/tension members.
Importance of Pile caps
- Pile caps are necessary as they correct any misalignment of piles and accommodate their deviations.
- They reduce the effective length of piles
- They reduce the adverse effect of a defective pile among pile group
Structural Integrity of Pile caps
For structural integrity of pile foundations, when pile caps sit on single or double piles, they should be interconnected by ground beams in both orthogonal directions. A pile cap on small-diameter piles can only be isolated without connecting beams as ties only when it is sitting on a minimum of three piles.
The head of piles projecting into pile caps should be minimum of 75mm. If the piles are to resist moment, the head of a concrete piles shall be exposed so that reinforcement bars are projecting into the pile cap for proper bonding. Dowel/starter bars should also be inserted into pile caps from the top to provide continuity for column reinforcement.
Pile cap shape and dimension on Plan
The size and shape of pile caps on plan is determined by the number and arrangement of piles beneath it. As a principle, piles should be arranged such that the center of gravity of the piles coincides with that of the column to be supported. Consequently, piles are distributed under pile caps mainly to fulfill this requirement. The pile cap in turn takes the shape formed by the distribution of piles underneath it.
As shown in the figure below, two and six number of piles would necessitate a rectangular pile cap. Three number of piles would require a triangular pile cap. Four and nine number of piles would call for a square shape pile cap, etc.
The centre-to centre distance between two piles in a row should be taken k x pile diameter (k∅), where k is 3. While the edge distance, which is the distance from the outer edge of a pile to the edge of the pile cap, should be at least 150mm.
Pile Cap depth
The depth of a pile cap is often governed by the magnitude of column load it is to transmit to supporting piles. The depth has to be deep enough to ensure rigidity so that the pile cap is capable of transmitting loads from the column shaft to the piles. The depth of the pile cap also has to allow for proper anchorage of column dowel bars as well as pile heads in the pile cap.
Tomilson et al recommend that for the depth of a cap with up to six piles then 2.2–2.4 times the pile diameter should be considered. When this ratio of this calculated depth to the length of the pile cap is greater than 2 then the pile cap should be considered as a deep beam but should be taken as a flexural beam if otherwise.
Design of Pile Cap Main Reinforcement
Pile caps can be designed based on their behavior (whether they obey bending theory or not). According to Eurocode 2, pile cap can be designed as deep beams, as solid slabs, or as beams. When they are considered as deep beams then their main reinforcement should be provided using struct-and-tie method (truss analogy), when they are considered as beams or slabs then they should be designed as flexural members that obey bending theory. Regardless of method for design of the pile cap main reinforcement, the reinforcement should be properly anchored beyond the head of the piles even if it means large-radius bends may have to be provided in the reinforcement to achieve the required anchorage length.
Pile Caps as deep beams
When pile caps have the ratio of their length to depth to be greater than 2, they are designed as deep beams using truss analogy (strut-and-tie method). The truss analogy assumes a concrete struct transfer the concentrated load from the column to the piles’ head, while the centers of the piles are connected by steel ties (steel reinforcement). The area of reinforcement required to form steel ties can be calculated from the tensile force in the pile cap. This is demonstrated below for a two-pile pile cap.
The tension to be resisted by the ties can be calculated thus:
N/2T = d/l
T = Nl/2d
Then the area of reinforcement can be calculated:
As = T/ fyd
fyd = fyk/1.15
As = Nl/2d
The tensile force in a pile cap for other arrangement of piles is shown in the figure below as extracted from Mosley et al:
Click here to study a Worked Example on the design of Pile Cap using Struct-and-Tie Method (Truss analogy)
Pile cap as beams
When the depth of a pile cap is considerably small so that its ratio to the pile cap’s length is less than 2, the pile cap behaves as a flexural member and is susceptible to substantial bending and shear. The bending is taken into account from the center of a pile to the face of nearest column. Hence a pile farthest from the nearest column generates the critical bending moment. Allowance should be made for 75mm deviation of pile heads when computing bending moment. Pile caps constructed over large groups of piles are designed as solid slab with differential settlement considered.
Click here to study a Worked example on the design of pile cap as beams using bending theory
Design for Shear in Pile Caps
Pile caps are to be designed for shear like other members subjected to point load. They are design for vertical shear, punching shear at column perimeter, and at basic control perimeter.
Vertical Shear (one-way shear) in Pile caps
One-way shear is critical in pile caps at a distance of Ф/5 from the surface of the piles within the piles. The design shear force (VEd) at this section depends on the reaction(s) of the piles within the critical section. According to clause 6.2.2 (6) of Eurocode 2, when loads are applied to the upper part of a member (pile cap) at a close distance to a support such that 0.5d ≤ av ≤ 2d (see fig (A) for the definition of av), then the shear force (VEd) may be reduced by av /2d. If, however av is less than or equal to 0.5d then av can be taken as 0.5d.
If the design shear force in the critical section is less than the shear resistance of the pile cap without shear reinforcement, minimum shear reinforcement is not required as declared by clause 3.11.4.4 (b) of BS 8110-1-1997.
Shear enhancement = av /2d
Where;
av is the distance from the column face to the critical section for shear as shown below
d is the effective depth of the pile cap
Punching Shear in Pile caps.
Punching shear is also critical as pile caps are essentially slabs which support concentrated loads. The punching shear is checked at basic control perimeter which is located at a distance of 2d away from the column surface, and also at the column face just like for pad foundations. However, if the distance between piles is less than or equals to 3d then the punching shear at 2d from column surface can be ignored according to clause 3.11.4.5 of BS 8110-1-1997, so that only punching shear at column face is verified.
Circumferential Reinforcement, and Top face Reinforcement.
Although Eurocode 2 recommends that the sides and the top surface of Pile caps may be unreinforced if there is no risk of tension developing in these parts of the member. Since this cannot be fully guaranteed, circumferential reinforcement should be provided around the pile cap following the guidance of surface reinforcement for deep beams in clause 9.7 of Eurocode 2. The circumferential reinforcement should be fully lapped, and the distance between two adjacent bars of the mesh should not exceed the lesser of twice the deep beam thickness or 300 mm. The area of the reinforcement should be 0.1% (the UK National Annex recommends 0.2%), but not less than 150 mm²/m in each face of the pile cap.
Minimum reinforcement should also be provided in both orthogonal direction at the top of the pile cap.
References:
EN 1992-1-1-2004: Design of concrete structures – Part 1-1: General rules and rules for buildings
BS 8110-1-1997: Part 1: Code of practice for design and construction
Pile design and Construction Practices – Micheal Tomilson and John Woodward
Reinforced Concrete Design to Eurocode 2 – Bill Mosley, John Bungey and Ray Hulse.
