Pile caps are designed to hold the pile and superstructure together at or below ground level while transferring loads from the superstructure to the foundation.
Generally, pile heads are designed to connect one pile, two piles, three piles, four piles, or a group of lines. The dimensions of the tops of the piles are determined based on the loads and the arrangement of the superstructure and foundation pile connections.
There are basically two methods for building block boards.
- Using the truss analogy
- Use of bending theories
Most often, up to four piles are connected by a block plate to accommodate the concentrated load of the superstructure. Lattice theory is used to construct the block plates.
Equations derived from the truss model and stress range are available to calculate the area of reinforcement.
The following figure shows typical equations that can be used to calculate the area of tensile reinforcement. It was copied from the book “Reinforced concrete design to BS 8110”.
If there are more piles, the design can be created using finite element software, facilitating analysis.
Generally, the distance between piles is 2.5 times the diameter of the pile. This is done to avoid interaction of one stake with others.
In addition to the calculation of tensile reinforcement, various shear tests, such as punching and vertical line shear, must be carried out. If there are more than two piles, the selection of the critical shear range must be done very carefully. BS 5400 guidelines can be used to select the amount of shear.
Worked Example: Design of a Block Plate
Consider constructing a two-pile, single-column pile head support on the pile head plate.
Data
- Post diameter 600 mm
- Design load 3,000 kN
- Reinforcement cover 50mm
- Concrete grade 30
- The characteristic strength of steel is 500 N/mm 2
- Column size in pile head plate 500x500mm
Calculate the dimensions of the block plate
Allow a pile displacement of 150 mm and a distance between piles of 2.5 times the pile diameter.
Width = 500 + 150 + 150 = 800 mm
Length = 2.5 x 600 + 250 + 250 + 150 + 150 = 2150 mm
Consider a depth of 1000 mm and a main rod diameter of 20 mm.
Effective depth, d = 1000-50-20/2 = 940 mm > 750 mm; (2.5 × 600/2) OK.
Consider lattice theory
Traction force, T = Nl / 2d
T = 3,000 x 0.75 / (2 x 0.94) = 1,197 kN
A S =T/0.87f j
A S = 1197 x 1000 / (0.87 x 500) = 2752 mm 2
Make 7 T25 (A is given = 3430 mm 2 )
Check for perforation
V = 3000 x 10 3 / (4 x 500 x 940) = 1.596 N/mm 2
v all = 0.8 (fcu) 0.5 = 0.8 x (25) 0.5 = 4 N/mm 2 <5 N/mm 2
So drilling is ok
Check for shear in the vertical line
The critical section of the pile is considered to be 20% of the pile diameter within the pile area . The following illustrations taken from the code (BS 8110) provide a clear position to consider in shear design.
If this section is considered critical and the distance between the piles is less than or equal to three times the diameter of the pile, reinforcement will be required (2 days/year against ) for V C to be taken into account. Here you can compare the distance from the column surface to the critical section.
In this example, we only consider two stacks as shown in the following image.
v C and can be calculated from Table 3.8 of BS 8110 based on the value 100As/bd.
100As / bd = 100 x 3430 / (1000 x 940) = 0.365
v C = 0.446 N/mm 2
Better shear capacity; (2d/a against )V C
(2 days/year versus )V C = (2 x 940/320) x 0.446 = 2.62N/mm 2
Nominal shear stress = 1500 x 10 3 / (1000 x 940) = 1.596 N/mm 2
So, okay.
Calculate the steel distribution area
Pay attention to the minimum reinforcement requirement.
100As/Ac = 0.13
As = 0.13 x 1000 x 1000/100 = 1300 mm 2 /M
Providing T16@150mm c/c (as supplied = 1340mm 2
Horizontal truss area
Providing 25% of design reinforcements
As binder = 0.25 x 2752 = 688 mm 2
Specify T12@150 (As specified = 754 mm 2 )
Similarly, other types of pile caps can also be designed using the same process. However, as the number of piles in a block increases, manual calculation becomes more complicated. Computer packages can be used to simplify the design while verifying the design result with simple calculations.
The design of a single block is not the same. The column and pile are connected via a pile top plate. The article Design of a block plate Design of a single block Further information on the design of pile caps will be discussed.
