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Punching shear enhancement.
8

Punching shear enhancement.

Punching shear enhancement.

(OP)
How to enhance punching shear capacity for an existing slab?

RE: Punching shear enhancement.

Increase the size of the columns, or in some cases it is possible to add steel brackets, through bolted to the columns.

RE: Punching shear enhancement.

We've used FRP to strengthen existing slabs and hollowcore planks, but it was really for increasing the bending capacity. Shear hasn't ever been an issue with any of our FRP retrofits.

RE: Punching shear enhancement.

You could add steel collars around the columns, or pour new cast-in-place concrete capitals.

RE: Punching shear enhancement.

Hilti makes a post installed punching shear reinforcement system, I don't know if it's available in the US but it's really just a bunch of bonded and anchored post installed bars.

Never used it. Seems scary but interesting. Check out Hilti HZA-P

RE: Punching shear enhancement.

Firstly, I see if strength gains in the aged concrete beyond 28d might get the job done. Next, I'd look at the methods proposed by Hokie and damsinc. Some more exotic alternatives:

1) External post-tensioning of the adjacent slab.
2) Pretensioned through bolts as shear reinforcement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Punching shear enhancement.

Same as Hokie and DamsInc, steel brackets on underside of slab through bolted to the columns. I have seen post installed concrete shear caps used as well, but have only personally done the steel brackets.

RE: Punching shear enhancement.

Google "Shear collars save a parking garage slab" by Popovic and Klein.

Here is a part extract. It is a Copyright document.

KootK will like it, hookie66 will dislike it - it uses shear-friction!



RE: Punching shear enhancement.

I might not like shear friction as a concept, but in this case don't even see how it is involved. The reinforcement shown in the photo is not in the right place, and is not developed, so how does it help with transferring the shear? Looks like to me that the load is transferred from the slab onto the column by a bearing ledge which has been broken out around the column. The ties just ensure the integrity of the capital. The article says that the circular ties restrain the concrete and provide the clamping mechanism necessary to resist the shear, but I still don't think that is shear friction. Maybe just semantics.

The radial cracking of the slab is due to the top bars being too low, and the WJE report addresses that.

RE: Punching shear enhancement.

What now? Did somebody say shear friction? Fascinating article.

I've been staring at this for a while now and see things rather like hokie does. Do we have rebar developed on both sides of the sliding planes? Sort of? I guess the hoops are technically developed on both sides of the shear friction planes even if they don't actually pass through the shear friction plane. Compared to conventional shear friction reinforcing, I'd have to think that the hoops would be a good deal less effective at restraining dilation perpendicular to the shear friction surfaces however.

I feel that the collars ought to work primarily by creating struts that fan up and out and ultimately engage the slab top steel, just as would be the case with a column capital cast with the original construction. Were I to be concerned about a shear friction plane, it would be the horizontal plane between the top of the collar and the underside of the slab.

@Bookowski: thanks for pointing us to that Hilti stuff. Neat.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Punching shear enhancement.

hokie66 see clouded area for the author/s explanation of the theory...

RE: Punching shear enhancement.

So the theory is that ring tension provides the necessary clamping force. That is quite simple, but I wonder if these collars actually work that way. Do you know if these authors actually determined by instrumentation that the bars were stressed enough to provide the clamping? As with the other type of shear friction (and crack control reinforcement in general), I fail to see that the yield strength has much to do with the clamping force, as surely you don't want the bars to yield, so it shouldn't matter what grade of reinforcement is used. Elongation is what matters, and E is the same for all.

RE: Punching shear enhancement.

It appears that they've done their due diligence with this so I'm happy to award them some KootK innovation points. When I first read the article, the first thing that occurred to me was that I would be nervous about implementing such an unconventional shear friction application without some test results. And it sounds as though they've done some testing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Punching shear enhancement.

(OP)
Thanks.
thanks

RE: Punching shear enhancement.

Hi SEAJ,

See attached... its somewhat similar to the shear collar above, but with a lot of doweling. the dowels help to control slip below the slab soffit and the column face.

I think for a larger span slab its better to use steel corbel as its easy to justify.

...and why is that collar costing 2000$ eachworm??

RE: Punching shear enhancement.

I'm late to this topic but find it interesting. And particularly interesting to me is the following statement from the article:

The radial compression resulting from tension of the circular confining reinforcement develops the normal force. The reinforcement is tensioned when the collar expands and begins to slip along a roughened surface.

What causes the collar to expand? Simply slipping wouldn't do it. I could envision it if the column were chipped away in some type of inverted cone shape whereby slipping would cause the collar to expand, but not otherwise. And I wouldn't think that that ledge would serve that purpose, at least, not without first crushing or crumbling a fair amount of concrete at the interface.

RE: Punching shear enhancement.

Archie264, the roughened interface amplitude is the 'key'.

If there was a smooth (or no amplitude to the) interface, then no normal force would be developed. For sliding (vertically downwards) of the collar to occur WITH a roughened interface it must laterally 'thrust' the collar and engage the circumferential reinforcement, that provides the clamping force.

Similar to the typical/conventional shear-friction we use with rebar developed perpendicular to an interface - for slip to occur the section tries to 'rise' over the roughened interface amplitude, which is prevented by a clamping force of the developed rebar.

RE: Punching shear enhancement.

Ok, I guess I can see that...on a theoretical basis, at least. It seems the surfaces would grind away at each other but then I suppose the grit would contribute as well. I guess that all goes into the mechanism of shear friction, the discussion of the veracity of which I'll leave to my betters.tiphat

RE: Punching shear enhancement.

Quote (hetgen)

I see the collar as haunch strut on a portal frame not bolted to the portal frame but just tied with a tension rope

Sorry, I respectively don't see the analogy.

RE: Punching shear enhancement.

Whoops, I'm one response behind; my prior post was in response to Ingenuity.

Hetgen, I do see the analogy, your sketch does help. Thanks.

Gentlemen, it seems that while one of you (Ingenuity) is saying that the collar expands while its roughened surface tries to expand over the adjacent roughened surface, the other of you (Hetgen) is saying it expands as the system tries to "jump the ledge" kind of like pulling a tight sock up over an ankle or tight pants up past my a beer belly. Is that about right? In any case, I think I now understand the concept; thanks. I'll still leave the shear friction debate to others, though.

RE: Punching shear enhancement.

Why ? what else is happening in the collar?

on my analogy i'm representing the concrete struts that fan up and out by haunch strut, the ring reinforcement by tension rope and the friction + bearing ledge around the column with a side plate. What would you add Ingenuity?

RE: Punching shear enhancement.

hetgen,

If I understand your 'portal frame haunch strut/collar' analogy correctly (although I have never actually seen one detailed with tension rope and a strut NOT welded to beam or column): 1) the strut is supported by the bearing ledge around the column (at the base of the haunch), 2) the strut will be inclined to as to develop tension into the 'tension strap', and 3) this is happening in 2 dimensions.

Compare this to the shear-friction collar: 1) there is NO direct bearing ledge, 2) the tension forces developed in the circumferential collar reinforcement are developed due to lateral thrust when the collar wants to slip vertically down the column through a roughened interface, and 3) this is happening in 3 dimensions.

I can sort of 'see' you analogy, but it is indirect, IMO.

RE: Punching shear enhancement.

Ingenuity,
I think in the picture of the actual column, there is a bearing ledge, albeit maybe not a very good one. And I think that is how the load would be carried.

RE: Punching shear enhancement.

hokie66,

Agree, in so far that any provided bearing ledge would make a secondary load-path, but the authors primary load-path is based upon no bearing ledge, just roughened surfaces.

The authors also repaired some column base/pile cap shear collars too, where there was only a roughened interface with no bearing ledge.

But I get it - redundancy is good - 'belts and suspenders' winky smile

RE: Punching shear enhancement.

In that picture, again I think the primary load path would be in bearing, where they have chipped away some of the base of the column. Without prestressing, I just don't buy the circumferential clamping and roughening as reliable.

RE: Punching shear enhancement.

Hi Ingenuity,

If i understand you correctly what you would have added to the analogue is roughened haunch strut end-plate and column flange with large enough amplitude which will engage the tension rope when it slip downward.

And you think the vertical shear force is handed by pure friction on the same slip surface. you could be right but I don't see this collar slipping vertically down, as hokie stated above and on his previous post there is a bearing ledge around the column ( see clip below) and in addition to that what about the resistance from the existing slab?





RE: Punching shear enhancement.

hokie66, In the column base/pile cap example the concept is the same as in slab-column junction strengthening, EXCEPT that the loads are acting in opposite direction, so the bearing-ledge would have to be at the top of the collar (clouded areas in the sketch below) to achieve what you refer. The chipped base area (circled and noted area in the sketch below) will not provide a bearing-ledge for the direction of loading in this case.



I agree that 'active' prestressing forces would enhance the shear-friction model.

Quote (hetgen)

And you think the vertical shear force is handed by pure friction on the same slip surface. you could be right but I don't see this collar slipping vertically down, as hokie stated above and on his previous post there is a bearing ledge around the column ( see clip below) and in addition to that what about the resistance from the existing slab?

If there is a bearing-ledge, then I agree that what was once a primarily shear-friction concept (as calculated-designed) becomes a predominantly strut/bearing ledge, BUT the authors concept is NOT founded on a bearing-ledge basis.

RE: Punching shear enhancement.

Ingenuity,
I don't see why not, as the chipped area bears on the new concrete. It seems to me that the hoop reinforcement would be better placed down there.

Hetgen,
In providing a bracket to resist punching shear, the shear perimeter is moved outward. If you mean by "what about the resistance from the existing slab" that you would add this in to the resistance from the new perimeter, you can't do that.

RE: Punching shear enhancement.


Hokie,

Yes i agree that the shear perimeter will be moved outward when a bracket is provided, but in this specific case they are claiming that the bracket / collar has to slip vertically down and if this happen i think the existing slab will try resist the slip around the column.


Ingenuity,

With regards to the column base repair, I think if we have a void below the existing deteriorated column as shown below and rely only on skin friction between the new collar and existing column we could say no bearing.

Thanks ingenuity, i think the column base example is easy to understand, and if this hoop tension is formed as they claim and enough shear friction contact area is provided it should work.




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