Punching of PileCap when the critical perimeter lies outside the Cap. 3

[NewbieInSE]

Dear Engineers,
What happens to Punching Shear Calculation when the punching perimeter gets outside the pile cap? Please let me know of any helpful docs in this regard.
Thanks.

 

[KootK]

1) This very much depends on the geometry and how the loads enter and exist the cap. A to-scale sketch of your situation and loads may get you much better responses.

2) By the sound of it, I would guess that the compactness of your cap will make strut and tie a more suitable design approach than sectional methods like punching shear checks.

3) Rationally, one might consider the adjusted model below. It's not without some problems requiring the application of engineering judgment however.

 

[HTURKAK]

Dear Newbie, In general the depth of a pile cap is not less than 60 cm ( even for small size driven piles ) and selected around 2 to 2.5 times of diameter of piles. That is, punching shear usually will not govern..
However, the designer is expected to check the one way and two way 8 punching ) shear considering the probable failure scenarios.The punching perimeter is calculated with column or pedestal + d/2 all the way around and the critical section for one-way shear shear is calculated at a distance (d) from the column /pedestal face.

I just write ( footing shear punching ..) and search the web.. One of the outcomes;

Then write ( pile cap shear punching ..) and search the web.. One of the outcomes;

Does this respond answers to your thread ? If not ,tell your specific problem with some descriptive sketch etc and applicable code to get better responds.

 

[MSUK90]

3) Rationally, one might consider the adjusted model below

What will be the solution for the model you have shown?
Doesn't that picture seems to work like punching shear calculations for an edge column??
HTURKAK, I will give an example of what the OP is trying to ask.
Let's say there is a pile cap supporting a single column and over a single pile(concentric). Depth=600mm, size of pile cap=1m X 1m. Diameter of Pile=500mm.
Now, the punching perimeter will fall outside the pile cap. What will you do in such case?
My take: In such case, the column will transfer the load through pile cap to pile by direct bearing?
I would like to get KootK's and your's opinion.

 

[KootK]

Yes to edge column if it's an asymmetrical case with enough meat on the one side. I actually had that typed up as a second response but had to take a phone call and never hit submit. Thanks for mentioning the edge column thing now.

Also yes to direct bearing for your one pile, one column case with some attention to lateral bursting containment.

 

[HTURKAK]

Yes ... in this case the column will transfer the load through pile cap to pile by direct bearing.. But in this case you may delete the pile cap if pile type and column size are suitable.

 

[NewbieInSE]

HTURKAK, Thanks. I know about the procedure of finding out punching perimter and sectional capacity which you have mentioned.
Kootk and MSUK90, Thanks. You are right in approximating my question. I'm not much familiar with the Strut Tie model yet, If it works then what's the procedure of finding capacity.

I'll explain a little what's troubling me.

 

[NewbieInSE]

*Typing error
Case 1: Are only two sides going to give resistance to punching this time?

 

[NewbieInSE]

Kootk, you have probably shown punching for pile.

 

[NewbieInSE]

For the case 2, which i have mentioned about cancelling out of forces, in the figure i shown the resultant shear will become zero, assume a pile cap where it won'r become zero.

 

[KootK]

Oh no, the resultant shear absolutely will not become zero. We were imagining a different situation previously.

When using a strut and tie model in a situation like this, it will not be the rebar that resists the shear but, rather, the diagonal struts. Naturally, it all has to come together as a complete system. It's helpful to think of strut and tie models as behaving like trusses in many respects. In many trusses, it is the webs that rest the shear of course.

If you wish to do sectional method checking of your pile cap, a good model is the one shown below. It will be different than standard code checks since the shear crack angles will be steeper than 45 degrees etc. The CRSI design guide on pile caps has a shear formulation designed to address exactly this.

 

[WARose]

Seems like the last time I did a cap like that I use a deep beam type method.

 

[slickdeals]

I would personally design this is a deep beam. This is no different in concept than a transfer girder.