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Lateral Loaded Pile with Pile Cap & Slab-on-ground

J

Jfet

Structural
Joined
Jun 24, 2025
Messages
17
Hi Gents,

I have a quick question regarding the laterally loaded piles.

I'm working on an apartment project (soil: sand) where piled footings are used to support the superstructure, including both walls and columns.
I understand that lateral loads can be resisted through friction under the footing (and possibly the slab-on-ground) as well as passive soil pressure acting on the pile cap.

Given this, I'm wondering:
  • If these other mechanisms (friction and passive resistance) are sufficient, does that mean the piles don't need to be designed for lateral loads at all?
  • Or, if the resistance is shared between the piles and the surrounding soil/structure, how do we determine the proportion of lateral load taken by each considering the difficulty in quantifying the stiffness of the frictional and passive resistance components?? And how can we identify the point of fixity for the piles in this case?
My current references only cover laterally loaded piles with given lateral load applied to them, with either free or fixed heads, which doesn’t provide much insight into this kind of shared-resistance situation. How do you normally design pile in this case?

Does anyone have experience with this or know of any resources that deal with lateral load distribution involving friction (& passive resistance) from pile caps and ground slabs?

Thanks in advance!
 
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What type of piles are you using? Timber?

Are your pile caps isolated or are they tied together with a grade beam/ structural slab at the grade level?
 
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jerseyshore said:
What type of piles are you using? Timber?

Are your pile caps isolated or are they tied together with a grade beam/ structural slab at the grade level?
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Concrete pile. No tie beams. Just pile cap under slab-on-ground. To transfer the lateral load, I am thinking of using dowels to connect the pile cap and slab (might be an issue for slab since the pile cap has much larger axial load?)
 
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I believe the friction at the underside of the slab or pile cap will result in minimal lateral displacement and the lateral loads transmitted to the piles are likely negligible. However, I currently lack data to confirm this assumption.
 
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We typically don't include friction under a pile cap, since we consider any bearing load on the soil to be questionable. Passive resistance of the soil against the sides of the footing is sometimes used to reduce the lateral load on the piles, if the movement needed for mobilization doesn't cause issues with the structure. There is usually some lateral load on the piles, even with the passive resistance of the soil, due to the movement needed for mobilization of the soil resistance.
 
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BridgeSmith said:
We typically don't include friction under a pile cap, since we consider any bearing load on the soil to be questionable. Passive resistance of the soil against the sides of the footing is sometimes used to reduce the lateral load on the piles, if the movement needed for mobilization doesn't cause issues with the structure. There is usually some lateral load on the piles, even with the passive resistance of the soil, due to the movement needed for mobilization of the soil resistance.
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Yeah agree with the pile cap and the lateral displacement required for pssive resistance. But what about the entire slab-on-ground? It seems overly conservative to disregard the frictional resistance provided by the slab-on-ground.
 
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Jfet said:
Yeah agree with the pile cap and the lateral displacement required for pssive resistance. But what about the entire slab-on-ground? It seems overly conservative to disregard the frictional resistance provided by the slab-on-ground.
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Most slabs on ground aren't considered as part of the structural system of a building.

Some structural systems do use the slab but then you do need to think carefully about how to suitably engage the slab. Eg 'hairpins' in slabs.
 
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human909 said:
Most slabs on ground aren't considered as part of the structural system of a building.

Some structural systems do use the slab but then you do need to think carefully about how to suitably engage the slab. Eg 'hairpins' in slabs.
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By 'hairpins', I assume you're referring to the different settlements caused by axial loading?

Yeah, there's usually no structural connection between pile caps and slab-on-ground.
It is quite common to have pile foundations for buildings yet I haven't personally come across detailed calculations from other engineers addressing laterally loaded piles in this context (except retaining walls), particularly regarding the point of fixity. Geotechnical reports also often lack information on lateral soil springs. How do you typically approach the lateral design of these piles for buildings?
 
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Jfet said:
Yeah agree with the pile cap and the lateral displacement required for pssive resistance. But what about the entire slab-on-ground? It seems overly conservative to disregard the frictional resistance provided by the slab-on-ground.
Click to expand...
I was specifically addressing the case where the concrete component is supported by piles. For a slab on grade not supported on a deep foundation component, you could use the friction, although you'd only get to count the minimum permanent load on the slab, which typically wouldn't provide much resistance.
 
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Bingo... Whatever permanent and lateral load are transferred to the slab can be resisted by friction.

But because the SOG is often isolated from the remainder of the structural system (to mitigate shrinkage cracking), the majority of the structure's load will first look to be resisted by pile lateral movement. If that movement is enough to engage the slab, you can kick some load back into your frictional resistance.

In non-seismic regions and good soils, soil springs aren't strictly needed -- lateral pile analysis can be done by Brom's method or nomographs. If you have seismic or poor soils close to grade, you might need to go back to the geotech to get those springs or Lpile parameters.
 
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Lomarandil said:
Bingo... Whatever permanent and lateral load are transferred to the slab can be resisted by friction.

But because the SOG is often isolated from the remainder of the structural system (to mitigate shrinkage cracking), the majority of the structure's load will first look to be resisted by pile lateral movement. If that movement is enough to engage the slab, you can kick some load back into your frictional resistance.

In non-seismic regions and good soils, soil springs aren't strictly needed -- lateral pile analysis can be done by Brom's method or nomographs. If you have seismic or poor soils close to grade, you might need to go back to the geotech to get those springs or Lpile parameters.
Click to expand...

Yes, it's definitely easier with clay. I also used Broms’ method with the 9B assumption. For determining the point of fixity, I typically refer to Davisson & Robinson or AASHTO guidelines.

That said, my site has quite poor soil conditions—mainly sand—and the geotechnical engineer has been hesitant to provide spring values or related parameters.

What’s confusing to me is that piled foundations are very common in building construction, yet I rarely see them designed as laterally loaded piles for wind / earthquake in this context. This design approach seems more typical for retaining wall piles or for bridge and marine structures.
 
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yet I rarely see them designed as laterally loaded piles for wind / earthquake in this context
Click to expand...

That's an interesting comment. I always see building piles designed for lateral loads. Not necessarily all the piles under the building, but those supporting the core walls and/or shear walls.
 
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We only see timber or helical piles around here for the most part, but we'll usually resolve significant lateral loads with battered piles.
 
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Commonsense would imply that piles are used extensively to resist lateral loads especially when it comes to high rise. (I have zero experience in highrise.)

However when searching for guidance on laterally loaded piles (or piles to resist moment even harder) I have found that guidance from available texts is much harder to find. (Especially if you are relying on Mr Google.)

So in that context Jfet's comments make some sense.

Clearly piles to resist lateral loads are not uncommon. Though the analysis is not especially trivial hence the lack of easy available resources (in my experience).
 
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