pile supported slab 1

[struct_eeyore]

Hi all,

I'm in need of direction about detailing a pile supported slab. I cannot find any provisions specifically requiring pile caps/drop caps at grade level slabs. Is there any issues with simply casting, say a 6" slab, directly on piles? My punching and moment transfer shears work fine. Would the IBC provision of minimum 3" pile embedment into concrete apply? In general, are lateral forces the only reason behind the 3" embed requirement? Can this be circumvented by projecting more rebar from pile into cap/slab for higher shear friction? Thanks!

 

[DaveAtkins]

Are you assuming the slab gets no support from the soil? If so, then I cannot believe a 6" slab will work for punching shear.

DaveAtkins

 

[struct_eeyore]

correct - no soil support. piles are only on 8' centers, so punching 2 way shears are low.

 

[jayrod12]

When you've checked your punching shear, have you accounted for the pile being poured at a higher than design elevation?

As long as you've done that, I see no issue with a 6" slab directly on the piles. Are we talking driven piles or cast-in-place? If CIP I've never even paid attention to embedment, in fact I wouldn't expect it and would think it is weird if there was.

 

[struct_eeyore]

The piles are augercast - I do not anticipate the piles and slab will be cast monolithically.

 

[DaveAtkins]

This is a two way slab. 6" thick actually does satisfy ACI. Did you include live load in your analysis?

DaveAtkins

 

[struct_eeyore]

of course. my one concern is that there is some provision somewhere that specifically states that piles must be embedded in the carried members.

 

[structSU10]

Whenever I have asked a geotech if the pile could not be embedded in the slab they look at me like I'm crazy. I ask them a minimum and they always reference the 'rule of thumb' number (at least as far as I can tell) of 4" for wood pile and 6" for steel piles. They usually say they want it to be restrained by the slab, but its not anything that is calculated.

 

[struct_eeyore]

my plan was to have 4 hooked bars extend into the slab.

 

[atrizzy]

I never understood the benefit of embedding a reinforced concrete pile into a reinforced concrete pile cap (provided that lateral shear transfer is designed for). Can anyone explain this?

 

[JAE]

For auger-cast piling, the actual pile cutoff elevation has a bit of tolerance to it.

For these and drilled piers we usually express to the contractor that the top of pile may have to be hand chipped with a chipping hammer to set the top of pile at the correct elevation.
For your case, with relatively thin, 6 inch slab, you might want to note something like that on the plans. Chances are that some piling may end up being 2 inches or more higher than expected which would have a significant effect on your punching shear capacity.



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[canwesteng]

Embedding piles is just a rule of thumb, but if you only have shear friction to transmit lateral to the piles, consider the case of uplift and shear simultaneously (most people don't it seems).

 

[jayrod12]

uplift and shear simultaneously

You have a case where the uplift exceeds 75 PSF? Holy man.

 

[elreedyman]

the pile cap shall be rigid as it transfere the axial force and also convert the momenet to adding axial and tension to the pile. The puchinh check is essential and also the stresses. But 6" is very thin it can not consider a rigid to transfere load so it needs to be address.

http://www.elreedyman.com

 

[hokie66]

I agree with JAE. You need to make sure the pile is at or below the slab soffit before casting the slab. Reinforcement rather than embedment should be used to develop the lateral force.

With a thin slab like this, you have to watch cover, and the effect that has on effective depth. Is there a membrane under the slab?

jayrod12, we don't know much about the OP's building, but uplift from hydrostatic pressure often is a problem with below ground slabs.

 

[MIStructE_IRE]

Cropped piles are line jagged mountain tops, not flat level surfaces. Be very careful with your punching shear. Intuitively I don’t like a 6 inch slab in punching..

 

[KootK]

I cannot find any provisions specifically requiring pile caps/drop caps at grade level slabs. Is there any issues with simply casting, say a 6" slab, directly on piles?

Similar systems are common in my area. There may not be any "rules" requiring drops but I'd recommend them regardless:

1) This will give the ability to accommodate top of pile elevation tolerances as mentioned by others.

2) It's nice to be able to develop, or nearly develop, you pile dowels into the slab thickness which helps with.

2a) Reliable moment transfer between slab and pile.

2b) Punching shear & moment transfer behavior that is consistent with the intent of those provisions.

2c) Shear friction, if relied upon, as that technically requires bar development on both sides of the slip plane.

3) As shown in my first sketch below, I tend to question the appropriateness of typical punching shear design methods when it's a thin slab on a wide column without the benefit of a column above. I feel like you end up with a punching line load of sorts rather than a true M/Sx + P/A mechanism. That said, this happens at elevated roof slabs a lot and I've not heard of problems.