Punching shear enhancement.
Punching shear enhancement.
(OP)
How to enhance punching shear capacity for an existing slab?
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Punching shear enhancement.
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Punching shear enhancement.Punching shear enhancement.(OP)
How to enhance punching shear capacity for an existing slab?
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RE: Punching shear enhancement.
RE: Punching shear enhancement.
https://www.youtube.com/watch?v=UQ6ZMHTKiNU
RE: Punching shear enhancement.
RE: Punching shear enhancement.
RE: Punching shear enhancement.
Never used it. Seems scary but interesting. Check out Hilti HZA-P
RE: Punching shear enhancement.
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.
RE: Punching shear enhancement.
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.
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.
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.
RE: Punching shear enhancement.
RE: Punching shear enhancement.
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.
RE: Punching shear enhancement.
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$ each
RE: Punching shear enhancement.
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.
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.
I see the collar as haunch strut on a portal frame not bolted to the portal frame but just tied with a tension rope. (see attached sketch).
RE: Punching shear enhancement.
RE: Punching shear enhancement.
Sorry, I respectively don't see the analogy.
RE: Punching shear enhancement.
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
mya 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.
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.
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.
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.
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'
RE: Punching shear enhancement.
RE: Punching shear enhancement.
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.
I agree that 'active' prestressing forces would enhance the shear-friction model.
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.
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.