How to Calculate Punching Shear of a Hollow Concrete Column on a Reinforced Concrete Slab?

[drucurl]

So I'm basically asking what is supposed to be my "loaded perimeter"

The Problem: Using Micro Steel piles embedded in concrete cap.
I want to ensure that the (upward) reaction force that the piles would have won't puncture the concrete.
So that's a punching shear calculation. The only problem is that most of the design guides deal with solid columns on concrete.
Thanks much in advance.

 

[KootK]

Do your piles have steel cap plates on them?

What's the typical ple diameter?.

 

[NedGan76]

Even if the section is hollow, you have to consider only the outer perimeter as for a solid section. If it fails, the column will punch through the slab anyway.

Cheers!
Ned Ganchovski

A better software for your calculation notes, free and open source:

https://github.com/Proektsoftbg/Calcpad

 

[JAE]

Agree with NedGan76

 

[drucurl]

@KootK
the diameter is 102mm and I'm not sure if the piles have steel caps. This is why I'm asking because if they did, I wouldn't consider them "hollow"

@NedGan 76 I'm not sure that's a correct analysis. The overall area of contact is now much less than if it was solid. Are you sure about that? Have you seen a similar example anywhere? I looked at punching shear for steel pipes joined and tried to substitute the pile cap as one of the pipes and the pile as the other pipe.

 

[JAE]

It's not the overall area of contact but the perimeter shear plane that would fail. The center part of the concrete (within the diameter of the pile) will simply go along for the ride with the pile as it punches through.

 

[structSU10]

a micro pile is filled with grout.

 

[NedGan76]

@drucurl. I am pretty confident that you cannot take the inner perimeter. But I see your point. What you can legally do is to take out the UDL that acts on the area enclosed by the punching perimeter and reduce the total punching force. As one can notice, this load does not contribute to the stresses along the perimeter of interest. So your reduced punching load will be:

P[sub]1[/sub] = P - p *A,
where P is the column reaction, p is the distributed load in the same load combination, A is the area inside the perimeter.

Cheers!
Ned Ganchovski

A better software for your calculation notes, free and open source:

https://github.com/Proektsoftbg/Calcpad

 

[milkshakelake]

@ned That would only apply for a very large diameter hollow section with a small thickness slab, neither of which is the case.

I would check the concrete bearing stress using the area of the hollow section, assuming no cap plate and no grout (which is unusual).

 

[NedGan76]

I do not know what dimensions do you have in your case. It would be much easier if you post some picture of your structure.

I am not sure this applies only for thin plates. What do you mean with "That would only apply for a very large diameter hollow section with a small thickness slab, neither of which is the case". You can subtract the load inside the perimeter for any column and any slab thickness. Actually, in Eurocode this is the recommended practice for foundations.

Cheers!
Ned Ganchovski

A better software for your calculation notes, free and open source:

https://github.com/Proektsoftbg/Calcpad

 

[milkshakelake]

Oh sorry, I misunderstood what you were saying. Yes, you do that. I was only looking at the sketch, envisioning an inner and outer punching shear perimeter.

 

[JAE]

If the whole slab failed and was punched down over the column (i.e. a punching shear failure per your question) the part of the slab over the inner portion of the pile would just still be sitting there on top of the pile with the surrounding slab dropped down around the pile.

There's usually very little load on the small little area on top of the pile to push down. The major load is on the surrounding slab areas.

The shear failure is a circular area around the pile - a band equal to the thickness of the slab.