No Pile cap needed below column

[sybie99]

Hi Guys

Simple question:

I have 1000diameter piles supporting a 300mm suspended ground slab and 350square columns directly ontop of pile.


Am I right to think that no pile cap is needed if the pile will not punch through the slab?

Do I treat the pile basically as a column and check direct/face shear and punching shear. If not a problem to thickening needed. So basically whether there is a column on top of the slab or not makes no difference to whether I need a thickening or pile cap, its all goverened by the possibility of the pile punching through the slab.

Of course if the pile is of a small diameter, there is a good chance I might need a thickening.

Cheers

 

[slickdeals]

I think this is no different than the requirements for the transfer of a load from a column above to column below through a floor slab. See requirements in 10.15 Transmission of column loads through floor systems.

 

[BAretired]

You do not need a pile cap if the peripheral shear stress is acceptable around the pile periphery.

BA

 

[DaveAtkins]

Also check the following:
* bending in the slab (this assumes multiple piles are being used to support the column, which may not be the case)
* one way shear in the slab (probably won't control)

DaveAtkins

 

[ishvaaag]

And as a foundation (other than the mechanical requirements) it is usual not to require a pile cap for piles directly supporting coaxial columns as long they are braced in X and Y directions; and yours are by your slab (even normally this would be thought to happen at ground level, and you have not THERE the lateral support). Normally, anyway would be good enough if the pile cantilevers vertically just little till the slab.

So the question would be refered to the ensured general stabilty of the building-structure-soil interaction, to be analyzed in the usual ways, i.e., for your problem, would not having some pile cap, or some pilecap restriction to displacement mean something that makes the foundation not work? If everything works, you know you have a sound foundation.

 

[slickdeals]

Ishvaag,
You bring up a valid point. If the column is not lined up with the centroid of the pile, then the induced moment will find its way into the slab based on joint stiffness. The slab will then have to be designed to take its share of moment.

I am not sure if you are in seismic country. If you are then you are required to tie your foundations together.

 

[Teguci]

Assume that the pile will be eccentric to the column above due to construction tolerances and have a design ready to deal with this additional moment in the slab for both directions. Essentially, your slab is your pile cap.

 

[paddingtongreen]

Is there a design moment in the column base? if so, will any additional moment caused by the eccentricity be significant? How was the reaction to that design moment to be distributed?


Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[apsix]

With a 350 sq column on a 1000 dia pile it is likely that any base moment will go straight into the pile.
The presence or not of an isolated cap on that pile will hardly make any difference, as long as slab punching is not an issue.

 

[hokie66]

Agree with apsix. The pile will be much stiffer than the slab.