Your tasks for the analysis and design of the superstructure part of the building are over and you’re off for foundation design. You go through the geotechnical report and found out that the recommended foundation to use are either raft or pile foundation and notice that the soil bearing capacity is 50kN/m2. Since the geotechnical report recommends either of the two options in foundation design type, you’re confused, which is which? Suddenly, you remember that you are designing a high rise tower; there is no way a raft foundation may work here. This time you decided to go for a pile foundation.
Well, the above scenario in a structural design world is over exaggerated. In fact, choosing a foundation type on the building you are designing can be detected or should I say decided on the first stage of the structural design. It should be stated at first in the design criteria. Usually, the local official having jurisdiction can recommend what type of foundation is suitable depending on the soil type from where the project will rise.
In this article, we will be discussing piles and their consideration in foundation design. We will start with a little definition of piles. Piles are used to transmit loads of the building to the hardest part of the soil strata. This is usually done in the areas where there is a very low soil bearing capacity and it comes with its duo, the pile caps. A typical detail of the pile can be seen in the image on the right, kindly click for a clearer view.
Design Considerations using Piles
We all know that piles or a group of piles are used to support a pile cap. For this to work, we have to provide a “support” for specified locations. This support can be interpreted as a “pin support”, but doing so will give as a very conservative result. Perhaps the best way is to use a spring constant (ks) or the so-called pile vertical stiffness as support. But how can we determine the spring constant? The spring constant (ks) is usually mentioned and derived in the geotechnical report, but if you are wondering, the pile spring constant or the pile vertical stiffness can be determined according to the recommended procedure set forth in EM 1110-2-2906 “Design of Pile Foundation”, Depth of the Army, US Army Corps of Engineers which is shown on the following formula:
Axial pile stiffness b33 = c33.AE/L
- A=cross sectional area of the pile taken as πd^2/4
- L= Length of the pile
- E= Modulus Elasticity of the pile, usually taken as 4700
C33 is a constant that accounts for the interaction between the soil and the pile, evaluated as:
Δ/δ, by which Δ=PL/AE
- P=Axial pile working load
- δ=the axial movement of the pile head due to axial load
Using the equations stated above, we can safely determine the vertical stiffness or the spring constant of any desired pile that we are going to use in foundation design.
How to incorporate the Spring Constant(ks) in Pile Cap Design
Since the pile cap is like a slab (but on the ground) we can use the SAFE program by CSI to design a pile cap. We just need to model this as a slab with its properties and incorporating the spring constant (ks) and assign the value of whatever is the result corresponding to the pile that we are going to use. Assign the vertical stiffness in translation Z calculated considering the equation above or as per the geotechnical report. Horizontal stiffness can be assigned in translation X and Y, if not given in the geotechnical report, it can be assigned as 10% of the vertical stiffness. The image below shows how to incorporate this in SAFE.
In SAFE, model the locations of piles as a null point, select the point in consideration and go to Assign>Support Data>Point Springs and the following dialogue box will appear.
The spreadsheets for the Design of PILE CAPS for two, three, and four piles are also available. Kindly select the images below for download!
PILE CAP Design Spreadsheets
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