Pile Cap Design For Helical Piles 1

[Eng_Struct]

Hi Group,

I am seeking some guidance on designing a concrete pile cap for a foundation supported by helical piles. I am adding a new column inside an existing building to allow roof framing modification for new roof openings. The column is interior and will not be resisting any lateral load.

For the factored column load of 200 kN, I only need 2 - 38mm diameter piles. I would like to know the following for pile cap design:

1. What is the typical embedment of the pile into the pile cap?
2. How to size the pile cap? Are there any rule of thumbs or good practice in terms of minimum dimensions for the pile?
3. With two piles, I can consider a pile cap with a rectangular configuration however my gut tells me that I should go for at least 3 piles for more stability. Any thoughts on this?
4. I suspect the pile will need to be designed as a beam in case of two pile configuration with point load from the column base plate being in the middle and two pile support on the side. In this situation, do I provide the tension reinforcement above the pile cap plate or can I keep them below the pile cap plate?

Thanks!

 

[cec17]

For pile caps in new construction I have seen them topped with a cap plate, like a micropile. See link below. Usually your pile cap is sized for shear (punching or one-way), and usually you will have ~ 6" embedment into the pile cap for a pin support. You can have tension steel that is below the cap plate, and moved around to "miss" the pile. I have also seen instances where steel is located above the cap plate so it's a mat footing supported by piles. Timothy Mays - Design Guide for Pile Caps is a good reference.

https://www.hubbell.com/hubbellpowe...rs3500-10x10in-3-hole-not-galvanized/p/143040

 

[Eng_Struct]

Thank you!

This is good information.

I did find the design guide you mentioned. It is useful and also comes with an Excel file for quick calculation of the pile cap.

For a 200 kN load, I am getting a pile depth of 762mm. It seems quite deep for a cap but I guess it is what it is.

 

[JAE]

Agree with cec17 on all points.

In addition - I'd suggest using 3 piling. Helical piles sometimes have some flexural strength/stiffness but I just feel better with a 3-pile system.
If the column gets built offset for some reason the piling would be under permanent bending with only a small amount of soil resisting the moment-induced movements in the piling.

The case where some bottom reinforcement is placed over the pile cap plate is typically, I think, to extend rebar across the shear punching failure planes and increase punching shear strength.

 

[EZBuilding]

Agree with JAE on the 3 piles, or at the very least needing some consideration for stability. In addition to the IBC requirements, I typically design pile caps to account for a 3 inch placement tolerance for the location of the piles. In a 1 or 2 pile set-up, the offset can only be resisted by bending in the pile...

Excerpt from FBC Chapter 18:

"1810.2.2 Stability.
Deep foundation elements shall be braced to provide lateral stability in all directions. Three or more elements connected by a rigid cap shall be considered braced, provided that the elements are located in radial directions from the centroid of the group not less than 60 degrees (1 rad) apart. A two-element group in a rigid cap shall be considered to be braced along the axis connecting the two elements. Methods used to brace deep foundation elements shall be subject to the approval of the building official.

Deep foundation elements supporting walls shall be placed alternately in lines spaced at least 1 foot (305 mm) apart and located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the foundation elements are adequately braced to provide for lateral stability.

Exceptions:

1.Isolated cast-in-place deep foundation elements without lateral bracing shall be permitted where the least horizontal dimension is no less than 2 feet (610 mm), adequate lateral support in accordance with Section 1810.2.1 is provided for the entire height and the height does not exceed 12 times the least horizontal dimension.

2.A single row of deep foundation elements without lateral bracing is permitted for one- and two-family dwellings and lightweight construction not exceeding two stories above grade plane or 35 feet (10 668 mm) in building height, provided the centers of the elements are located within the width of the supported wall."

 

[Eng_Struct]

Thank you for the feedback.

In terms of the pile cap design, Timothy Mays - Design Guide for Pile Caps analyzes the pile based on the sectional method instead of strut & tie. In the Canadian code, the concrete design handbook indicates a sectional method resulting in tension reinforcement that is 25% underdesigned as compared to the Strut & Tie Method.

The Canadian concrete design handbook allows us to use the sectional design method for a "shallow pile cap" but the definition of the shallow pile cap is not clear to me.

 

[bones206]

I’d probably assume span/depth ratio of 4 or more for “shallow” pile caps. Consistent with the definition of a deep beam.