Pile cap in Two Pours

[slickdeals]

Folks,
I have a situation with a pile cap supporting a basement slab. I have attached the proposed construction sequence.

I am concerned about punching shear especially with the two pours. I will have tie-in reinforcing between the pours and extend the reinforcing from the piles and hook into the 500mm slab.

Thoughts/Suggestions?

 

[csd72]

Not a good detail overall with regards to shrinkage restraint. The piles will act as anchors preventing any lateral movement.

If you are going to do it as 2 stages then you might as well do it as separate pile cap and ground slab with a bond breaker between and compressible material around the columns.

 

[slickdeals]

The basement slab is hydrostatic. Refer to thread507-305553

 

[slickdeals]

The columns are on a 11m x 8m grid. The columns support a 6 story building. Typical pile caps have 6 piles. The basement slab is 500mm thick to resist a 3.6m hydrostatic head. The piles will be in tension during the full hydrostatic loads. There will be continuous dewatering during construction.

 

[csd72]

Sorry Slick,

understand now.

The horizontal shear on the pour joint will be proportionate to the punching shear stress.

I would definately suggest a tamped surface to help with the friction.

 

[hokie66]

I wouldn't have a problem with the detail or be too concerned about punching shear. Design the composite pile caps by using strut and tie analysis.

What is missing from your sketch is the mud slab for installing the waterproofing. After that goes in, I would use fibre cement sheets to protect the waterproofing during construction of the slab.

 

[Teguci]

It is a weakened slip plane and it will have a small impact on the pile caps strength. I would be highly tempted (especially considering the initial tension load) to add an equivalent amount of reinforcement across the slip plane to counter the calculated shear friction loss. This should compensate for VQ/I for your bending and cracks through a compression strut for your strut tie model.

 

[BigH]

Don't forget to properly green cut the first placement to ensure good contact or the two - i.e., ensuring no laitance.

 

[rscassar]

I could be going completly down the wrong path here but I would think this is more a strut-tie issue with a compression strut crossing a cold joint.

 

[ron9876]

Why not just pour the pile cap and let the slab run over the cap. Tie the slab to the piecap with hooked dowels.

 

[bgatto]

In such a situation I would use "Sika Latex" or something similar to provide the adhesion between the "old" and the "new" concrete. Together with dowels of course.

 

[Teguci]

OK, revised answer:
Find the angle of your compression strut from horizontal = theta, and the compression force, F.
The shear in the slip plane due to the compression strut, V,req'd = F Cos Theta for each compression strut.

Provide shear friction reinforcement to compensate for this shear such that Vreqd <= phi Avf Fy mu

Note that in a perfect world these shears will cancel each other but I'd be tempted to ignore the cancelling effect in order to compensate for any local shear bending requirements ( the VQ/I requirements for instance).

 

[rscassar]

The F cos theta is going to be equivalent to the tension force that the steel in the bottom of the pile caps are designed for. So it is likekly that and equivalent amount of steel will need to be provided vertically as there is horizontally.