Use of single pile under a pile cap 9

[NewbieInSE]

Dear Engineers,

Is it prohibited in any code (ACI, ASCE, IBC, UBC) regarding the use of a single pile under a column if the support condition is considered a pin? Or is this matter discussed anywhere?

Note: Assume that, it is required to use pile foundation than shallow foundation due to some reasons.

 

[EZBuilding]

Review this section of the IBC:

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.

 

[NewbieInSE]

EZBuilding,
Thanks.

What if the pile dia is 610mm (Cast-in-Situ), and the pile length is 21.0 m.

As per the above-mentioned exception no. 01, this isolated pile is not allowed, right?

What if I don't transfer moment to the pile cap? The isolated pile is also not able to resist building transferred moment.

 

[NewbieInSE]

Why should piles be braced (by increasing their number, which will eventually resist building transferred moments (if considered fixed at base)), if i consider the column base as pin?

 

[ATSE]

The code assumes your top of pile lateral restraint is not rigid.
Pile group and pile bracing are not for moment fixity, but instead to provide lateral stability and account for eccentricity of the applied load and the resisting pile centroid, which will not be perfectly aligned. That is, code-required bracing gives you a "pin" at the top of your pile, and prevents a mechanism.

 

[EZBuilding]

I agree with ATSE's understanding.

In my local practice Auger-Cast Piles are the Deep Foundation element of choice. It is standard practice for the Structural Engineer to account for a 3" tolerance between the As Designed location and the As Placed location in the foundation design. Without multiple piles, or bracing elements such as grade beams, this placement tolerance could allow for a condition which does not have a reasonable stabilizing mechanism.

I have worked on a project which utilized large diameter drilled shafts for the support of a building columns. These elements did not meet the provision of 1810.2.2 explicitly - and this is the common application for these elements in my area. I justified their use by coordinating a moment demand on the drilled shafts with the geotechnical engineer equal to the 3" tolerance times the column load.

 

[NewbieInSE]

Thanks EZBuilding and ATSE. I understood the matter.

 

[HTURKAK]

I agree with EZBuilding (Structural). But still single pile under each column could be an option for non building structures.. Two examples ;

- High tension pylons may be supported with individual pile for each leg ,
- Some industrial non building structures e.g.pipe racks. In order to avoid the underground facilities, it may not be possible to connect the piles with a tie beam , cap etc.

If the pile cap does not hinder anything, still single pile for each column could be an option with grid capping beams.

 

[dik]

Is this a seismic thing? I've often used pile caps on single piles. It allows the contractor to come back later to install anchor rods, etc. Just a matter of extending pile reinforcing into the cap and adding concrete later. Pile cap same size as the pile or maybe a couple of inches larger diameter.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[jayrod12]

I'm with Dik, in non-seismic country we use columns on stand-alone piles all the time.

 

[Lomarandil]

Using a single pile is acceptable when considered correctly. (I also use a pile cap to allow for tolerances and make the transition to the column simpler.)

However, without an at-grade element connecting the piles or columns, any lateral load on the pile top will result in lateral movement. Depending on the soil profile, the point of lateral restraint could be several pile diameters down into the soil. And for some structures, that will increase column effective length/flexibility (e.g. negative effects on structural stability) significantly.

(In addition to the pile location tolerances and resulting instability mechanism mentioned before).

----
just call me Lo.

 

[BAretired]

In my location (Alberta), single piles are frequently used when the soil is considered adequate to provide lateral resistance to the pile. The upper two feet of pile is poured after the piling crew leave the site to allow elevation adjustment. It could be considered a pile cap, but I consider it simply an extension of the pile.

In only one instance in my memory, the soil was considered unable to provide lateral resistance by the geotech, and special measures were taken to provide appropriate ties.

BA

 

[canwesteng]

Similar experience to BA in western Canada - we usually check the amount of lateral movement, and make sure we include eccentricity between cap and pile in the calculation. Although for substantial structures, all the single pile caps are connected with grade beams, which meets the IBC requirement.

 

[BridgeSmith]

I originally thought the OP was talking about a steel pile, but apparently we're discussing drilled concrete 'piles', AKA drilled shafts? In that case, we do those all the time for bridge bents (multicolumn piers).

We use the length from the cap to the fixity depth (as determined by P-y curve analysis) for the effective column length for slenderness/buckling calculations. Since our bridge substructures are exposed, we neglect the lateral resistance of the top 5' of soil, to account for the loosening effect of frost in the soil.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[MSUK90]

Extending the questions from OP’s post, I have few more questions with the same topic and all your views will be valuable:
1) Can we consider the base as fixed in case of a column resting on a single pile(cast in place-Concrete)?
How the moments will be resisted by piles in such case?? By Lateral soil restraint?
2) What lateral forces can be developed at column-pile interaction due to which the tying becomes necessary??
Both questions above assume the design is done for a building with seismic and wind analysis..

 

[dik]

The column pile interface is the same as if it was monolythic. Moments can be transferred to the pile and to the soil by active and passive soil pressure... there are numerous means of determining the effective length for moment transfer. As far as tying, I'm not sure, my first approach would be to cast a couple of horizontal bars at right angles through the pilecap and embed them into the SOG... for some seismic conditions, you may require grade beams.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enhineyero]

Codes don't explicitly say 'thou shall not use single piles' cause there are ways around it and they do give a couple of conditions when using single piles, as mentioned above.

I have no issues using single pile/pile cap provided that the pile cap is restrained by a ground beam on all orthogonal directions.

Without the ground beams, the pile cap would rotate (when eccentricity bet. col and pile exist), creating a few issues. First, how to effectively develop the bar resisting tension on the pile cap and tie it to the pile without turning the whole thing into a metal ball. Second, check if the pile is long enough to resist the moment. I can calculate, come-up with solution/detail, and justify the structural adequacy but I prefer not to....i rather have a simple life (by adding ground beams) and move on.

 

[MSUK90]

dik, I believe my understanding is correct here.. The moments from column is directly transferred to the pile like moment that transfers from column to column..
The pile then transfers the loads to soil through lateral soil-pile interaction(p-y curves are developed for that I believe)..
However, I was looking for others reviews as the seniors here said that a single pile or two piles cannot be used for a column(in any seismically active area)..
Minimum three piles should be used with a pile cap for any column as per them!!

 

[BridgeSmith]

1) Can we consider the base as fixed in case of a column resting on a single pile(cast in place-Concrete)?

It's not considered fixed at the top of the shaft or the ground surface. The point of fixity will be somewhere below that. There is apparently some disagreement of what that point should be. Some consider the moment reversal point; others where the deflection goes to zero and reverses, or the zero shear point. Complicating matters, is that AASHTO makes a distinction between the fixity point and the point where the shaft is considered braced. It at least gives guidance on where the braced point is located.

How the moments will be resisted by piles in such case?? By Lateral soil restraint?

Yes, lateral soil restraint is what mitigates the moments induced in the shaft by the loads on the column. It's similar to a soldier pile.

However, I was looking for others reviews as the seniors here said that a single pile or two piles cannot be used for a column(in any seismically active area)..

I don't know what the codes for building have to say about that, but there's no prohibition in the bridge design specs for the use of a single shaft.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

For moments applied to piles, there are usually two considerations... laterally restrained at the top and non-restrained. The lateral restraint comes from the slab. For some seismic conditions, rather than being tied by gradebeams, I'd consider the slab top of the pile tied by the slab, with reinforcing, too.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik