Footing Punching Shear Reinforcement

[Galambos]

I have an EXISTING 24"sq concrete column bearing on a 10ft sq footing, 30" thick, f'c=6ksi.

i am adding several stories above this footing on top of the existing 4 story structure, but i am limited by the punching shear capacity of the footing.

i wondered if anyone has any ideas to reinforce the footing for punching shear. i think that thicknening the footing with a composite topping wont work because the bearing stresses will still be founded below the new topping.

im trying to avoid shoring up the column and collaring on a new column "wrap" to increase bo.

any ideas?

 

[apsix]

What about bolting on a steel collar to the column and bearing onto the footing, to increase the shera perimeter.

Otherwise, 'collaring on a new column "wrap"' seems feasible, but why would you have to shore up the column to do this? (BTW I don't know what 'bo' is)

 

[BAretired]

None of us know what "bo" is, so why are we responding to this thread? Reinforcing the footing for punching shear is not a problem, so what, exactly is Galambos getting at?

BA

 

[abusementpark]

None of us know what "bo" is

bo = body odor

You're welcome.

 

[BAretired]

Notwithstanding the clever remarks of abusementpark, what exactly is the problem? Reinforcing for shear is not usually a problem. Perhaps abusementpark can elucidate a bit further on what he perceives to be the problem.

BA

 

[BAretired]

Or, perhaps Galambos could expand on his initial question. I don't see the problem.

BA

 

[abusementpark]

bo is the perimeter along the critical section as defined in ACI for punching shear (it is directly in the formulas for punching shear). bo is based on the width of the column and the depth of the footing as defined in ACI.

It sounds like he needs bo to be larger to make the punching shear work.

 

[BAretired]

So "bo" is actually b₀, right?

BA

 

[Galambos]

BAretired's brash responses never cease to amaze me. Wow. Perhaps, if you dont see the problem, you have no business responding to the thread, or at least keep your opinions to yourself and act like a professional.

Kudos to abusementpark for explaining bo or b0. Collaring onto a steel column may work although I have about 1000 kips to transfer into it.

As for shoring, my intent was to relieve load on the footing to install the repair, although, I can see the collar working without shoring.

 

[JAE]

I would look at the steel bracket collar idea first. I'd also have to question whether the soil bearing pressure is OK with all those additional stories added. Wouldn't there be a limit to the bearing pressure?

One other idea that I thought of is what I did on an old steel fabrication plant once, where the spread footings were inadequate. We drilled holes through the footing vertically - around the column - and drilled in auger cast piling. The tops of the piling were mushroomed out directly beneath the existing footing to serve as a sort of in-ground capital to engage the existing footing.

For your case, the added piling would increase footing capacity (in terms of the soil) and also perhaps negate the shear issue since you could probably get the piling close enough to the column where shear wouldn't matter.

 

[ToadJones]

i am not familiar with the "collar" approach although I can imagine what it is.
Does anyone care to elaborate?

Thanks.

 

[vandede427]

i knew what he meant by bo.

 

[ToadJones]

...not everyone uses the ACI code in this forum

 

[ron9876]

I also know what bo is.

You could use gunite to increase the column size if that would help. If you are adding several stories it would seem that there would be more questions than punching shear in the footing.

 

[BAretired]

Sorry Galambos. I apologize. I should have put 'b' and 'o' together.

BA

 

[GalileoG]

I do not mean to derail the thread, but I too would like to pose a question about punching shear for a spread footing:

In the Canadian Code, if the face of the column is less than 5 x d from the edge of the slab, then that face shall not be effective in resisting punching shear.

However, for foundations, more often than not, the pier is less than 5 x d from all edges of the spread footing. Does this automatically rule out punching shear from governing or should I check for punching shear assuming all faces are effective?

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[ToadJones]

Always check punching...because it is a rather simple check, if for no other reason.

 

[Galambos]

BA, apology accepted.

Yes, punching shear is not the only issue, although I have repairs in mind for the underreinforced footing and soil bearing issue.

I suppose I would need shrink reducing agent in my mix, if I were to use a concrete collar. Also, I'm curious if the adhesive anchors would relax to much during loading that would make the collar (steel or concrete) ineffective. Perhaps, the movement is negligible.

Thoughts?

 

[BAretired]

I'm not sure about a shrink reducing agent. If the collar shrinks, it wraps more tightly around the existing column thereby improving bond.

I would think roughening the contact surface would be a good idea as well as headed studs epoxied into the column.

BA

 

[mijowe]

How much do you need to increase the bo? I would feel more comfortable with a steel collar, but transferring 1000 kips is a lot for mechanical fasteners.

Did you test for the concrete strength? 6 ksi is a lot for a foundation.