What is a Pile Cap?

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What is a Pile Cap?

Basically, a pile cap is a thick concrete mat.  This mat can be found on concrete or timber piles and those piles are driven into the unstable ground, or soft ground, to create a stable foundation.  The concrete pile cap will then distribute the entire load of the building.

The dimensions of the pile caps are determined by the loads and the arrangement of the connections of the structure and the foundation.  The shape of the pile cap is determined by the number of piles within the group and the spacing between each one of the piles.

The depth of the pile cap has a number of determining factors as well.  Those factors include the punching shear, the pile anchorage, the shrinkage and swelling of the soil, the amount of frost, and the groundwater table.

Pile caps also use secondary reinforcements, so the piles do not move outward from each one of the caps.  One of the secondary reinforcements is at the bottom of the pile cap and runs around the longitudinal reinforcement.  The other is changing the direction at the head of the piles.

One of the best ways to design pile caps is in SAFE, which is an excellent design tool.  There are six steps to designing pile caps in SAFE, which is not too many considering the importance of the caps.

The six steps of designing pile caps in SAFE:

  1. Determine the column and wall load assignment

In order to create pile caps, you must know what is needed for the foundation.  That means that you must know where the columns and walls will be located and where you need the load for each of them to be.  While you can do this all manually, it is always best to use a 3D analysis model.  This information can then be imported into SAFE, so you can see all the different vertical and lateral forces.  This will ensure that you know where all the dead loads and live loads are located and where pile caps will be needed.

  1. Determine which pile cap to use

It will be necessary for you to follow all the local codes and standards when you are determining which pile cap to use.  There is a basic formula to use when calculating the number of piles.  That formula is the number of piles is equal to the working load divided by the pile capacity.  You can then mark the type of pile cap on a layout so you can see where the pile caps should be.

  1. Model the pile caps

It is easy to model the pile caps in SAFE, especially if you utilize the dxf importing feature.  If you choose to do this manually in SAFE, it will take much longer.

  1. Run analysis and design

As soon as you have completed the pile cap modeling, you will need to mark the x’s and y’s when it comes to directions.  After that, you can finally run an analysis and design for review.

  1. Reviewing the analysis

When you are reviewing your analysis, you will want to keep an eye out for any anomalies from what was created with your soil report.

  1. Interpret all your results

The reinforcement results will show you where the pile caps need to be, as well as where the strip forces will be needed.  The slab strip design will also show where reinforcement is needed near the rebar.

If you have never done pile caps before, you must know the three most common shapes.  The first is triangular, which is for three piles.  Six or seven piles will need a hexagonal pile cap, while a rectangular pile cap is used for all other numbers of piles.

When it comes to pile diameters and the depth of the pile cap, please check out the numbers below:

Pile Diameters in mm                                          Depth of Pile Caps in mm

300                                                                                 700

350                                                                                 800

400                                                                                 900

450                                                                                 1,000

500                                                                                 1,100

550                                                                                 1,200

600                                                                                 1,400

750                                                                                 1,800

These pile cap depths are the most effective for the diameters they are associated with.  Therefore, you should use these as recommendations when you are creating pile caps in the future.

Pile caps can be very helpful in keeping buildings more stable than they would be without them.  So, when you are constructing newer buildings, or renovating older buildings, you are going to want to utilize pile caps to ensure that the building is as stable as it can possibly be.