Punching Shear For Column to Pile Head Design 2

It looks like the code specified critical plane is located quite close to the pile edge, in this case, I would assume a critical as shown on the sketch. But that is only my opinion, let's see how others think.

Another critical shear plane you need to check is as shown on the image below. You may need to enlarge the cap to ensure it encloses the failure plane.



Another check.

The picture posted implies there is no cap plate and IMO, the concern SHALL BE the punching of pile head thru the interior of hollow pile.

English is not my first language ,let me explain with other words , the concern shall be PIERCING OF pile head by square column to inside of the steel pipe...

If you provide more info, (pile wall thk, design loads and pile max. reaction, material etc) you may get better responds..

The following picture depicts the case , ( a snap from PILE DESIGN and CONSTRUCTION PRACTICE M.J.Tomlinson )

Good Luck...

I think the best option is a strut and tie model; 4No struts connecting the pile wall center to he column center could be a reasonable approximation.

@LYJLYJ the recent fib Model Code is staying away from the Eurocode punching shear critical perimeter i.e (2 x D) labeling it as unsafe and considering a narrower section 0.5 x D same as ACI. So, i think, a critical perimeter according to ACI 0.5 X D will soon be adopted by Eurocode.. i guess the Americans win...

I don't think punching shear is a problem here. Rather it's an issue of concrete crushing, and connection detailing (which is, I think, what HTURKAK was getting at).

The force comes from the column directly into the slab. It won't "punch through" the slab because the force goes directly into the pile.

Now, if the column load is much higher that what is allowed to go into the pile, then the load would have to spread out to other piles via the slab and you would have punching shear. But, not if all the axial force is expected to go directly into the pile.

The image below shows the varies failure cone of the punching shear for column continuous through the slab. It is provided for illustration purpose only, as I don't recommend the 1.5d critical distance. I think for OP's case, the minimum check shall be performed on a critical shear plane with the floor fully loaded, and the earth pressure conservatively ignored. But this is only a personal opinion.

I agree with Josh -- with the pile and column opposite each other, I don't see a traditional punching shear issue. Even with the different sizes and pile being hollow, the 1m cap seems adequate to redistribute by bearing.

If there is substantial load from the slab passing to the pile, or an unbalanced moment at the column base, those each might trigger a punching shear check.

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just call me Lo.

The OP ( Mr LYJLYJ (Structural) is too busy and hardly ever responds or , to read the posts..If the OP provides more detail we may see the probable failure scenarios..


The traditional punching shear may be an issue for the flat slab .. Regarding the crushing and piercing of the pile cap, i think this should be a concern if a thick capping plate is not provided.

In past , i provided vertical dowels ( welded inside of steel pipe ) together with a capping plate or, if the empty pipe pile driven without capping at bottom , the ground level inside the pile will be a few feet below and will be visible . Filling the empty space with concrete together with pile head and dowels provided, the crushing risk will be eliminated.

Dear All,

Many thanks for the reply, apologize for late response, this is my first time of using this forum, never thought to get the overwhelming response in such a short time and during Christmas week, very grateful!

The material used will be concrete column and concrete hollow pile with wall thickness of 180mm thk. For the hollow circular pile, there will be pile plug provided.


For the column load ULS, we are talking about 15000kN.

For the punching shear checks mentioned by r13, please correct me if I'm wrong, the checks on punching shear through slabs will come from the load from flat slab system , ie self weight of slab, finishes, live load and unbalanced moment.
Also for the mentioned critical plane by r13,

Does this mean we should only ensure concrete shear stress is not exceeded by checking the perimeter of column with column load at ULS.

My real concern is really whether the dimension of the pile head, ie width is sufficient to ensure the load transition mechanism, whether the load from column/pile will distribute 45 degree which means it will pass through the flat slab. Also, is hoop stress in the pile head a concern?

Reason being for the controlled pile head dimension is that the project is an offshore project, we are thinking to control the weight of the pile head so that we don't need massive falsework to support the pile head during concrete casting and curing. The falsework will be a steel collar wrap around the pile, acting like a nib around the pile to support the formworks, hence the controlled dimension of the pile head to reduce the cantilever length.

Again, many thanks for all the contributors.

As everyone is alluding the main failure mode of the pile-cap will be concrete crushing, punching shear isn't the issue. The conventional slab punching shear model doesn't represent what is happening In the pile cap, that is why I said a conservative strut and tie model could be the solution to evaluate the stress in the concrete and also estimate the required rebar in the pile cap.

The flat slab should be designed to support the moment due to the eccentricity of the column from the pile center, I would normally use proper ground beams tied to the pile cap on both orthogonal directions, but a slab could do it if it has sufficient stiffness.

I also think the pile itself should be checked as a column with lateral support soil springs. ICP pile could be high strength, but the crossectional area of the pile is ~70% of the 800x800 column above I think it's worth checking.


All this is form the 'B-team', if you can wait a bit after the festive season you will get a much better answer form the 'A-team', I mean the likes of @KootK's

LYJLYJ said:

For the punching shear checks mentioned by r13,...

Click to expand...

Typical slab subjects to load on top face only, but this is not the case for pile supported slab. We usually ignore the subgrade for pile design, but in punching shear check, I simply consider this is an upside down floor system, now the soil pressure causes the unbalanced shear. Be prudent on foundation design, especially shear.

r13 said:

Typical slab subjects to load on top face only, but this is not the case for pile supported slab. We usually ignore the subgrade for pile design, but in punching shear check, I simply consider this is an upside down floor system, now the soil pressure causes the unbalanced shear. Be prudent on foundation design, especially shear.

Click to expand...

Huh? This comment doesn't make much sense to me.
- For a soil-supported foundation slab, I view the punching shear checks as an upside down elevated slab. But, NOT ever for pile supported slab.
- Like I said before I don't think the COLUMN load contributes to punching shear in the slab because that column load goes directly into the pile.
- However, any load applied directly to the top side of the slab (other than load applied directly over the pile) will create shear and some unbalanced moment that could cause a punching check to be necessary.

There is unbalanced shear or not is up to your calculation, and level of conservativism.

You are talking about 15000kN ULS column load .. but you did not mention about ULS moment..What is your plan for the transfer of column moment to pile cap and to the pile ? My first respond was , referring the first picture , the pile is steel pipe..Pls provide more info. regarding the whole structure..

If the ULS column load 15000kN, together with the flat slab will be much more and the nominal stress for column fcmin=15000000/800*800)= 23 MPa . and for the circular pile fcmin= 15000000/pi(500**2-320**2)=32 MPa.. (4650 psi )..
The nominal stress should be around fck/3 for driven piles..

Pls provide more info. regarding the whole structure and the design moments for superstructure and piling to get meaningful responds.

hetgen said:

...I mean the likes of @KootK's

Click to expand...

Thanks for that, it made my afternoon. The sketch below shows my thoughts which are, in large measure, an aggregation of other people's thoughts. There is one aspect that may be of particular interest however.

Once upon a time, I was under the impression that any STM design was a safe design, without exception. Over time, I've come to believe that, while usually the case, that is not always the case. And that's unfortunate as it somewhat neuters my favorite feature of STM. Lower bound is just downright sexy. In the context of this problem, I believe that there is, in fact, a very high capacity punching shear failure mode that, in my book, is worth a quick check to keep the Ambien/Scotch consumption down.