The design of a single pile cap is discussed in the article. When it is adequate to have a single pile, we can do the construction with a single pile cap. The single pile cap design procedure is not the same as the other designs.
Generally, the truss analogy is used to design. However, when we have a single pile the design concept is not the same.
Let’s see the basic arrangement of the single pile connecting to the column.
During the construction of the piles, we may not be able to construct the pile at the exact locations. In pile specifications, a 75mm allowance has been made generally as the construction tolerances.
Pile cap design needs to be done for this allowance as it is made from the specification, and also, we cannot expect the pile to be constructed at the exact point. After the construction, pile centers will be measured, and check whether the tolerances are less than 75mm. If they are within the limit and we have designed the pile caps for 75mm eccentricity, the pile cap is acceptable.
In addition to the design of the pile caps, we need to design the ground beams for this eccentricity (75mm). In the ground beam, the location of the pile (support for the ground beam; generally considered as pin support) can be shifted by 75mm on either side. We may create a different arrangement to get the critical bending moment.
When the deviation is greater than 75mm special attention shall be made and the design shall be revisited based on the site condition.
Pile Cap Desing Worked Example
In this pile cap design example, we will look into the connection of single pile and column by a pile cap.
Column Load = 900 kN
Pile eccentricity = 75mm
Bending Moment = Ne = 900 x 75 / 1000 = 67.5 kNm
Since we know the pile cap dimension, single pile cap reinforcement can be calculated based on the relevant standard you follow.
Generally, the pile cap size can be calculated as = pile diameter + 150 x 2
Here 150mm is the increase in the size of the pile cap from the pile. This value could be adjusted as you wish. However, generally, we maintained it around 150mm.
In addition, the design of a single pile cap the reinforcement for the ultimate limit state, and the pile could be designed for crack control based on the design crack specified in the design.