Restraining pile cap top with ground slab

[Sandychan]

Dear All,
I am an engineer practising in Sweden. I am working with pile foundation design for a rebuilt a motor showroom now.
The idea for this project is to keep the existing vast raft foundation (this will only serve as a showroom floor at the end), re build new steel super structure (much heavier column loads than the old system),
and use piles to support new columns.

The existing raft slab will be removed locally where pile caps have to be placed. The slab is 250 mm-thick and reinforced with net of bar diameter 10 mm spaces 150 mm c-c top and bottom.
As my client would like to save the number of piles as many as possible, my boss suggests me to use pile caps with 2 bored piles in multiple locations but we both feel insecure about rotation at he pile cap top.

In case both 2 piles are eccentric to the column center on the same side, is the a way to check the slab capacity against moment and how? The maximum load on each pile is
720 kN and the maximum tolerance for the pile is 30 mm. I hope I don't bore you all with a lot of text and thanks in advance for any suggestions.
SandyChan

 

[RPMG]

Sandychan,

I'm from the US, and to repeat what you have said (in US Customary!), there is an existing mat slab 10" thick reinf w/ #3 @ 6" oc top & bott used to park showroom cars. It is proposed that a steel superstructure be added, but the steel columns cannot be placed on the mat. Therefore, the concept is to reinforce the mat foundation with piles, and place a number of columns anywhere on the existing mat, so long as the centroid's of gravity line up (320 kips total). The goal here is an analysis procedure that provides the applied moment at the existing slab.

In general, I would use a shell element as the mat with springs, with varying stiffnesses based on a geotechnical report, and the dishing behavior of mat foundations. The column/pile loads can be applied, and moment checked at the existing mat slab.

However, I think that there are practical issues with this concept including differential settlement between the mat and pile, loss of bearing at the mat, an that the pile may not improve the bearing capacity at the column.

But ultimately, this is a unique problem in which I have no experience. I would drill piers at every column location and provide isolation joints at the mat.

 

[KootK]

I'll assume that the top of cap will be at the underside of the replaced raft slab when all is said and done.

For interior situations, you shouldn't have any difficulty. You'll need to connect the cap and raft slab with vertical dowels capable of transferring the shear delivered to the slab by the pile caps. From there, the load will become tension/compression in the raft slab and ultimately friction between the raft slab and the the soil.

At edge conditions, the detailing is a bit more crucial as you have to rely completely on a tension connection to the raft slab.

What type of piles will be used? Most types of piles can resolve small load eccentricities without lateral support at the top. The eccentricity just becomes a bit of pile lateral loading that gets resisted by horizontal soil pressure. I've done similar things with single pile per column solutions.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Sandychan]

Thanks RPMG and Kootk for your reply. I first checked the raft slab as a beam subjected to moment = 21.6 kN.m (720 kN*0.03m) and the section is good for that. Also, I extended the same amount of slab reinforcement into the pile cap with proper development lenght. Do you think I could say that now the cap top is restrained by the slab? I also should try the shell element with spring. Loss of bearing at the mat and differential are something I should discuss further. I only think that the slab will take care of only its own weight and live load instead; as a result, less loads to carry and it might be happy.


KootK, the pile cap top will be placed at the middle of the slab thickness (125 mm under the slab top surface). I use C-shape rebar (the open part of C is on the top) for my pile cap bottom reinforcment and bent its ends around the slab's top reinforcement. I don't know if that are suitable either. Steel borred piles are used for this project and the supplier is called Rukki (Finish company).When you mentioned about shear transferred to slab, did you mean pile reactions? Now I might have troubles with edge pile caps. If eccenticities are toward the inside of the building, there is no slab to help out.

 

[KootK]

When you mentioned about shear transferred to slab, did you mean pile reactions?

Yes. As I see it, the best way to resolve pile eccentricity is to introduce a lateral reaction at the pile caps to introduce a counterbalancing moment. That lateral reaction would be provided by the slab. I don't recommend trying to convert pile eccentricity into slab moment for the following reasons:

1) Slab flexural stiffness will not be very substantial relatively speaking. That won't be where the eccentric load "wants" to go.
2) Detailing a moment transfer connection between your pile, pile cap, and slab might be tricky.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.