how to increase footing thickness? 3

[abdallah hamdan]

Hello everyone, I have a Footing that was poured at 55 cm thickness, 15 cm lower than required (70 cm), punching shear failure will occur, How is it possible to increase the thickness of the footing?
note that the footing was poured recently, and no work has been done on the column necks
any ideas
thanks in advance

 

[XR250]

Bigger base plates on the columns?

 

[abdallah hamdan]

the columns are concrete, making the column bigger does not solve the situation, because of one-way shear, its more than the footing capacity.

 

[Celt83]

Early enough that demolish and replace will the most cost effective solution.

 

[SE2607]

Roughen the top surface with a bush hammer, apply Sikadur Hi-Mod 32 (check specs, I could be wrong) bonding agent. It's fairly pricey stuff, but, if applied correctly, will make the bonded surface stronger than the original concrete.

 

[Eng16080]

I agree that demolish and replace is probably the best solution.

But just to brainstorm, if the only concern is shear, what if you pour the remaining depth of concrete on top of the existing footing? Then, calculate the total shear capacity as the sum of the two pieces. (I'll admit I've never done this and am possibly overlooking something.)

 

[JAE]

Then, calculate the total shear capacity as the sum of the two pieces.

I'd be wary of doing this - you run the risk of one placement failing in shear first and then the other - domino effect.

 

[StrEng007]

[highlight #FCE94F]I agree that replacing the footing is absolutely the first choice. However, I know how hard it can be to make that happen given the individual details of a project, the pace, and personalities involved.
[/highlight]

What exactly is causing your iso-footing to fail? Is it flexural shear or two way shear.

punching shear failure will occur

Punching shear is two-way shear. It is influenced by the critical perimeter of the column. In theory, increasing the column will increase the critical perimeter and provide you with a larger capacity.

So this leads me to believer your failure is NOT punching shear, which I believe you confirm in your next post.

making the column bigger does not solve the situation, because of one-way shear, its more than the footing capacity

This sounds like your flexural/one-way shear, which you're saying here is the governing failure.

Potential solution:
In either case, have you considered increasing the overall bearing area of the footing? I know this solution might get some kick-back from other participants on this forum (que the angry villagers with pitchforks and torches )

I have been able to successfully do this on projects before where foundations were not already heavily loaded... and demolishing existing footings would cause an owner/developer mutiny that would severely impact the design team. You have to weigh these things out and make your professional decision.

The idea is to pour an additional band/apron, concrete lip... whatever you want to call it, and increase the overall area of the foundation. Your location of critical one-way shear will remain the same but your demand value should decrease.

You'll have to watch out for developing the reinforcing for the new extended band. I've used drilled and epoxied dowels so developing the bar doesn't become a tail chasing exercise with trying to lap develop the existing reinforcing. You'll have to choose placement carefully so as not to foul with the existing rebar placement.

Check your shear friction and manufacturer literature for your epoxy. Also make sure your primary flexural steel at the location of critical flexure (located in your original footing) is still adequate.

 

[JAE]

I don't have a pitchfork but I do have empathy for the owner who didn't pay for a belts-suspenders repair to an incorrectly built footing.

 

[bridgebuster]

I found these drawings in my files for an abutment footing widening. I didn't have any involvement with the project. I regularly look at bid documents and save things that could be useful later on.

 

[Eng16080]

I'd be wary of doing this - you run the risk of one placement failing in shear first and then the other - domino effect.

That did occur to me, but I don't see how, for example, the top pour could fail without engaging the bottom piece.

the columns are concrete, making the column bigger does not solve the situation, because of one-way shear, its more than the footing capacity.

I don't agree with the 2nd part of this statement. If you were to make the column as big as the footing, how does that fail in shear? Making the column larger should help with both two-way (punching) and one-way shear.

 

[hokie66]

Bend over the column starter bars, put the crane hook on them, and jerk it out. Anything else will be dodgy.

 

[JAE]

That did occur to me, but I don't see how, for example, the top pour could fail without engaging the bottom piece.

Maybe I'm not following you but if the top piece fails in shear first, then you are just back to square one where the OP is now...an insufficient footing.

 

[abdallah hamdan]

demolish and replace is the first solution but we need more time and a cost-effective solution, i have multiple suggestions that you mentioned but

1-increasing the area of footing or the column to the maximum size we can does not solve the problem from the designer's perspective unless we increase the thickness
2- widening the footing making a cage of reinforcement of 20/30 cm around the footing from the sides and above, as in the photo

3-pour 15 cm concrete above the footing, normal concrete or a cement-type material that has a strong bond to the concrete I found, or the Sikadur Hi-Mod 32 as mentioned above, but I'm worried that the layer will break and not be as strong as expected and separate from the footing and fail itself, and then the footing

Has anyone tried the third solution or has any resource or detail that discusses/shows this type of solution?

 

[dik]

Sikadur Hi-Mod 32 is great stuff... I've occasionally used it in past. Check with the supplier to confirm it is suitable.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

I'd never do that, for the very reason you stipulate.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

Is the footing too small, too?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[abdallah hamdan]

The footing is not small ,2.8x2.8 m , we are forward to demolish and replace but discussing if the third solution can be done and how

 

[Greenalleycat]

I agree with the concern of adding extra thickness - I'd be dubious about getting meaningful bond to contribute to punching shear failure

I am a bit confused about the issue though
If it's an in situ concrete column and there has been no progress on the column then can you you just increase the width of the column base (tapered/flared base)
This would increase the shear area for punching and allow it to work?
The architect may not like that idea of course...

 

[Eng16080]

Maybe I'm not following you but if the top piece fails in shear first, then you are just back to square one where the OP is now...an insufficient footing.

I was merely speculating that the top would not fail in shear independent of the bottom. Both would have to fail together. But, actually, forget that! I think I have a better way to explain what I'm getting at.

Instead, suppose that the top fails in punching shear. The failure perimeter should be larger than the column perimeter and should theoretically match the critical perimeter, bo (if following ACI 318). With the top piece of concrete having failed, it would still provide a larger surface area than the column itself in terms of transferring the load to the lower piece of concrete. This would have the same effect as if the column was larger. In fact, if this is accurate, you could fix the shear issue simply by pouring a smaller "footing" on top of the existing one with the dimensions set equal to bo. It would essentially be a concrete base plate.

Like this: