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Partially Connected Column & Punching Shear
2

Partially Connected Column & Punching Shear

Partially Connected Column & Punching Shear

(OP)
Hi all,

I'm trying to find ways of making the below detail work.

I'm using AS3600 currently, but am definitely open to other code's justification. I know we are pretty conservative when it comes to punching shear.

At this stage the Arch. is aiming for "X" to be 200mm, however I can only seem to squeeze 400kN punching shear capacity out of the configuration below.

Does anyone have any suggestions or point me towards material that can help me justify the smallest possible "X" dimension?

Thanks

"Shibby right..."

RE: Partially Connected Column & Punching Shear

What are the magnitude of spans and loadings?

My guess is your 275mm slab is based upon your shear issues, or are your spans large too that results in 275 (deflection and flexure)?

Also, be careful of using your average prestress values in your punching shear equations for edge (and corner) supported column where the P/A is not fully developed.

And 'kick' the architect in the head too...

RE: Partially Connected Column & Punching Shear

I will go further than Ingenuity and suggest that you ignore the prestress completely. You will need sufficient reinforcement developed to the back of the column to transfer the moment to the column. With 300mm wide, I doubt that you can get it in there!

I would be insisting on an edge beam to tie the whole thing together.

PS you cannot "assume" no moment transfer to the columns. It is there and the only way it will go away is if the column connection fails and that is a brittle failure so no zero column stiffness assumptions are allowed!

RE: Partially Connected Column & Punching Shear

(OP)
All fair points and duly noted.

Spans are roughly 8.5m each direction for an office environment.

"Shibby right..."

RE: Partially Connected Column & Punching Shear

Damn architect's. That connection would feel better with a 300mm x 300mm bearing area as a minimum.

Have you looked at studrails. Reid and Ancon can supply and have good enough technical data regarding how to design them.

https://www.dropbox.com/s/mflb5saeb6tj767/Studrail...

Other than that you could look at detailing a shear cage with normal reinforcement

https://www.dropbox.com/s/0k8weh7lkd1pr4y/Shear%20...

I also remember something that either rapt or hokie posted on the forums sometime ago regarding the use of bottom reinforcement for punching shear. Even though it's not explicitly written into the codes it does make the connection a lot more ductile. There's nothing like the feel of an N28 in your hands. That'll hold the crack from opening.

RE: Partially Connected Column & Punching Shear

This needs an edge beam, or a beam in the other direction. Architects can't dictate everything about our structures.

RE: Partially Connected Column & Punching Shear

(OP)
I was trying to avoid the use of shear rails, as this project isn't in Australia thus there may be problems with procurement and supervised installation.

Will definitely be adopting integrity reinforcement as detailed in the Canadian code. I would rather it hang than fully punch, if worst comes to worst!

I think at this stage I really need to either thicken the slab and increase the "X" dimension.

"Shibby right..."

RE: Partially Connected Column & Punching Shear

(OP)
Asixth, this is what the shear rail option would look like...

"Shibby right..."

RE: Partially Connected Column & Punching Shear

Looks good. Just enough room to throw some heavy reo thru that column junction.

RE: Partially Connected Column & Punching Shear

Has anyone done a comparison of AU practice against US in terms of punching shear capacity?

AU standard uses a concrete shear strength of 0.34*sqrt(fc) which is higher than the the 3*sqrt(fc) when all the conversions between SI and US units crunch out.

RE: Partially Connected Column & Punching Shear

asixth,

Haven't done it lately, but the last time I did, I remember coming to the conclusion that the overall factor of safety was intended to be about the same. In addition to the maximum shear stresses, you have to consider the different load factors and different strength reduction factors used by the two codes.

RE: Partially Connected Column & Punching Shear

I still do not see how you can fit any decent top reinforcement into the column. The gap between the column bars is a maximum of 300 - 50-50-25-25 = 150. You can probably fit 2 bars between! And they have to develop fully so they cannot be large bars. For 8.5m slabs, that is not much tension force!

Also, your studs are at 154 spacing with a depth of 220. Hope someone tells the concrete the shear failure plane has to be at 30 degrees or less.

And you have ignored the applied moment from the analysis. That has to be included. Even your Mmin is wrong as it is based on a 200 long column, not 600.

I would be requiring a steel column with a full welded steel shear head.

RE: Partially Connected Column & Punching Shear

(OP)
I think I need to go back and read about column design and analysis.

My current understanding - reduce column stiffness in floor structure analysis as this will result in a conservative floor design. Then when designing columns and things such as punching model floor with 100% column stiffness to see what moment shall be transferred into the columns?

In that case - how does one achieve a pinned-pinned RC column? Obviously it's all in the connection detail. I'll review this.

I'll look into the stud rail formulae and calcs to see if the design requires the shear crack to cross all four shear studs. Maybe there is some sort of assumption worked in that explains this. But I do agree with you, seems weird it would detail them like that.



RE: Partially Connected Column & Punching Shear

(OP)
Follow up question... when would Clause 9.2.3 (AS3600) ever apply in practical problems?

RE: Partially Connected Column & Punching Shear

(OP)
"Follow up question... when would Clause 9.2.3 (AS3600) ever apply in practical problems?"

Answer: Rarely at ULS, however Vuo is important in the overall punching shear picture.

RE: Partially Connected Column & Punching Shear

Not for properly detailed insitu columns - floors but there would be other examples where it comes up. A concrete slab supported on a steel column or likewise a steel column transfer load on a concrete slab.

I'm quoting rapt again from a post many years back

Quote (rapt)

Punching Shear is a Brittle Failure condition. The moment cannot redistribute away from it without a collapse occuring. Codes do NOT allow redistribution from a brittle failue condition for very good reason, they cause collapses. There is no elastic/plastic action, it is elastic/collapse for punching shear!

thread167-280869: punshing shear and unbalanced moment

Also can you post the provisions from the Canadian Code regarding integrity reinforcement? I'm in my own jam with a punching shear problem at the moment.

RE: Partially Connected Column & Punching Shear

Trenno,

With your small connection to the column, just about the whole moment has to transfer through the front face (normally this is about 60%), so the full moment at the end has to be carried by reinforcement into the back face of the column.
.
The only way to get 0 moment is to detail it as a pin so that no moment can develop. As far as I am concerned, that Is just about impossible in this case, unless you have no top reinforcement and then you have no punching capacity - so collapse. Unless the pin is in the column below and above the slab - good luck with that.

We do not know if this is a braced or sway structure! if sway, it cannot be a pin anyway and must be designed for full moment no matter what you want to do!

Asixth, thanks for resurrecting that one, I was obviously more eloquent in my younger years!

RE: Partially Connected Column & Punching Shear

Quote (Trenno)

My current understanding - reduce column stiffness in floor structure analysis as this will result in a conservative floor design. Then when designing columns and things such as punching model floor with 100% column stiffness to see what moment shall be transferred into the columns?

This is fairly sound and fairly common. The one chink in the armour is that, rationally, you need your slab top steel to be compatible with your punching shear assumptions. The codes that I work with decouple punching shear and slab moment which makes this invisible to designers. At edge columns and unbalanced columns, I would consider designing the slab top steel for moments reflecting column fixity.

Quote (trenno)

I'll look into the stud rail formulae and calcs to see if the design requires the shear crack to cross all four shear studs. Maybe there is some sort of assumption worked in that explains this. But I do agree with you, seems weird it would detail them like that.

The crack only needs to cross one stud. I see no problem with 220 long studs spaced at 156 o/c.

Given your geometry, you might try this:

1) Abandon punching shear. You're not grabbing enough stuff.
2) Evaluate based on one way shear at the column face.
3) If shear works somehow, design your slab as pin supported.
4) Get as many top bars into your slab as will fit.
5) Design you columns for moments reflecting the capacity of the bars that you provide at over strength.
6) Use something akin to the integrity steel provisions that we use in Canada as a fail safe.

In Canada, the code allows a related procedure for the punching shear design of corner columns.

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: Partially Connected Column & Punching Shear

(OP)
Can someone please link me to a good information source that shows good detailing practices for beam/slab/column connections.

I want to get a feel for how exactly how much moment can be transferred through these junctions, and then design accordingly.

RE: Partially Connected Column & Punching Shear

If you are going to base it on one way shear, the whole load is going to have to transfer through the 300mm width of slab at the connection and can only include the reinforcement that is developed into the back of the column, i.e. about 2 bars. The results may actually be worse than the punching shear result with has a longer shear perimeter! And depending on the actual arrangement, there may also be torsion involved!

RE: Partially Connected Column & Punching Shear

@Rapt: I suggested one way because a) I expect it to be worse b) it can be made to jive with the limited developed top steel that will be available and c) given the geometry, I feel that it's a better representation of expected behaviour.

I would argue that the length of the one way shear failure plane could be taken as b + d at minimum.

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: Partially Connected Column & Punching Shear

(OP)
Ask and you shall receive...

Edge beam granted.

RE: Partially Connected Column & Punching Shear

Good decision, and you will be able to sleep now.

RE: Partially Connected Column & Punching Shear

I do love a happy ending.

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: Partially Connected Column & Punching Shear

(OP)
The one hand giveth; the other hand taketh away.

They are pushing for precast columns now. Which isn't the end of the world I guess...

RE: Partially Connected Column & Punching Shear

KootK,

Not sure where you get the +D from, I would be using the width of the connecting face, so b = 300. But you can still only include reinforcement that will develop to the back of the column!

And a win for common sense! As long as the precast columns stop at the bottom of the edge beam (fat chance of them agreeing to that!)!

RE: Partially Connected Column & Punching Shear

@Rapt: The +D comes from my feeling that this would be analogous to the one way shear situation where a wide beam bears on a narrower column. While North American codes don't have much to say about that, most firms have internal procedures that allow shear to be taken on a width that is often less than the full beam width but also greater than the width of the column. And that makes sense to me.

The limited space for top steel does not play as important a role in the method that I proposed, It would really be bottom steel that would be of interest.

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: Partially Connected Column & Punching Shear

KootK,

The critical shear plane is at the face of the column where the width is 300. That is what you have to design for. The code logic of allowing the first check at d or D from the face of the support does not apply if the section shape changes as it effectively does in this case.

There is negative moment there, so the top reinforcement is critical, not the bottom.

Even if you could wish away the negative moment and assume a pin support (which you cannot), the bottom reinforcement assumed in the shear calculations must develop past that point by at least D + development length! It has to develop fully into the column. You have exactly the same problem.

RE: Partially Connected Column & Punching Shear

If this is a project designed to ACI 318, how would you get two cables within the column cage in both directions?

RE: Partially Connected Column & Punching Shear

(OP)
Nope, AS3600.

RE: Partially Connected Column & Punching Shear

Quote (Rapt)

The critical shear plane is at the face of the column where the width is 300. That is what you have to design for.

Not in my neck of the woods. The sketch below illustrates how a lot of design firms in my area handle wide beam one-way shear at narrow columns. And it makes sense to me. When one-way shear failure occurs, a frustum of concrete doesn't pop out with vertical sides. Rather, the effective shearing surface will broaden a bit, consistent with the fact that the shear is strutting into the joint from multiple directions. This is why the punching shear surface is permitted to be wider than the column face after all.

Quote (Rapt)

The code logic of allowing the first check at d or D from the face of the support does not apply if the section shape changes as it effectively does in this case

I believe this to be a different animal altogether. That code provision acknowledges that loads applied within "d" of the support probably strut their way to the support directly rather than induce shearing stresses into the incoming flexural member. The "d" that is referenced is a dimension parallel to the direction of the slab/beam span. The "d" that we're talking about here is perpendicular to the beam/slab span.

Quote (Rapt)

Even if you could wish away the negative moment and assume a pin support (which you cannot)

The method that I proposed does not ignore negative moment or wish it away. Rather, it explicitly accounts for the over-strength top steel moment that can be developed within the joint and prescribes that it be included in the design of elements where that action is important.

Quote (Rapt)

There is negative moment there, so the top reinforcement is critical, not the bottom...the bottom reinforcement assumed in the shear calculations must develop past that point by at least D + development length! It has to develop fully into the column. You have exactly the same problem.

Yes, there would be negative moment. But not much for the reasons that you've stated above (limited space etc). So, for the sake of one way shear assessment, my method would assume a true pin at the connection and reinforce accordingly. In my country's code, that means that designers must develop enough bottom steel into the support such that it it provides an effective tie for the final compression strut that dives from the beam/slab compression block down into the support. It generally take very little steel. And that steel doesn't have to be developed for fy; it's strength can be prorated based on available development length if necessary. Failing that, you get a pass if you can simply provide a hook or head inboard of the support face. The support reaction provides substantial confinement in this situation which make developing the ties less critical than in other strut and tie applications.

In a situation like this, Canadian integrity steel requirements would lead to more than enough developed bottom steel for the one-way shear check. Integrity steel would be two or three 20/25M bars which I would develop within the column using headed bar anchors.

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: Partially Connected Column & Punching Shear

Kootk,

Considering the concrete edge is near the front face of the column, not well past the back face as you have shown, I would still use 300 at the face of the column! Some sensible codes tell you to check to the face of the support if that is a worse case, and that the D from the support rule is only where you have constant conditions into the support.

If there is negative moment at the critical section, then the shear calculations must be based on the top steel in tension, even in your code. If there are different loading conditions that cause situations where there are both negative and positive moment then both must be checked. But you cannot base it on bottom reinforcement if the moment is negative.

You can get 2 or possibly 3 bars into the column. Do you really think that 3 bars will be sufficient Ast in the shear calculations with a slab width of 300 or 600 or whatever you want to assume to provide a shear capacity to carry about 30m2 of 275 thick slab plus the added loads? If the bars are only partly developed, you can only use that percentage of their area in the calculations.

And the horizontal bars need provide the tension force to the back of the column, not just inboard of the support face.

RE: Partially Connected Column & Punching Shear

@Rapt: I've reached my three volley maximum. We'll have to just agree to disagree on this one. Thanks for the debate.

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: Partially Connected Column & Punching Shear

(OP)
I think will have to agree with Rapt on this one, I think the logic is more concrete.

However, the real problem is now the connection between the precast column and the edge beam.

I'm thinking recess the top portion of the column such that it creates a nice bearing surface for the edge beam to sit on.

RE: Partially Connected Column & Punching Shear

That's a very common method for parking structure column to spandrel connections in my area. I've seen it go up with six story precast columns. Pretty efficient.
 

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: Partially Connected Column & Punching Shear

Yes, you can just use polystyrene to block out the part of the column where the beam penetrates. Allow the bars to go through. Requires a bit of cleanup before casting the beam.

RE: Partially Connected Column & Punching Shear

(OP)
Yep, sounds good. Thanks.

Now I'm trying to get my head about what punching shear with a spandrel looks like (see sketch below). I'm wondering if you need to reduce the extends of the critical shear perimeter from dom/2 to something more aligns with the crack angle shown. However I can see in Figure 9.2.1 (B) AS3600, that the shear perimeter for my situation does in fact extend past the width of the beam.

Now I'm also thinking that by introducing a spandrel beam, we are now looking at mostly just a one way beam shear problem. As load will be attracted to the edge beams, then through the beams to the columns?

RE: Partially Connected Column & Punching Shear

(OP)
I've also checked the bearing surface for crushing and it is fine ^^^

RE: Partially Connected Column & Punching Shear

I hope you have some really good column ties over the depth of the beam connection, and especially over the slab depth, to transfer the forces to the column, just so that it does not tear itself apart!

RE: Partially Connected Column & Punching Shear

When they do this in all precast systems in my area, the "structural column" is the narrower bit throughout the entire length and the ledge bit is basically treated as a full length corbel.

Regarding your new punching shear case, this is pretty much exactly the case that I presented above: wide beam on narrow column. This is an excellent paper on the subject Link.

You've got a few choices:

1) Punching shear
2) The sketch that I posted above.
3) Run your stirrups through the joint and deliver some of the shear around the side of the beam. This is basically a poor mans STM model and does require a tie into your column or some moment in the slab to resist torsion in the beam.

The third option is the one that I see the most. Note that it is really an alternate means of handling two way shear even if it presents like a one way solution. I can provide some details if you're interested in pursuing this.

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: Partially Connected Column & Punching Shear

I see no reason to consider punching shear as you are incorporating a beam to carry the load in one way action. I would prefer more bearing into the column. It was 200 before, why the change? And I agree with rapt, the whole thing needs to be well tied together, which is a problem with most precast elements.

RE: Partially Connected Column & Punching Shear

I would use U bar into the column rather than cogged top and bottom bars. Much better development.

I doubt with a connection of 140 that you can get all of the reinforcing bars in where you need them. There will have to be corner bars for the slab bars and they will have to be inside the column bars, while the column ties are outside the column bars providing an effective development between them.

50 cover + 24mm column bar + 24mm corner + 24mm slab bar = 122 + at least 24mm and preferably 40 cover to the slab bars + tolerance for placement of corner bars in the bends does not add up.

Go back to 200mm connection!

RE: Partially Connected Column & Punching Shear

(OP)
I've been asked by my manager to design this connection using STM. Having very little experience in this method of design, please bear with me.

Below is my first attempt at the strut/tie configuration. I have a vertical shear and a clockwise moment. I'm assuming the vertical shear will be at the beam's centreline and the strut forming from the slab, which is just the resolved couple force from the moment.

Is STM the best way to push forward for this connection?

Feedback is most welcome - I'm sure KootK would be chomping at the bit to help out.

RE: Partially Connected Column & Punching Shear

(OP)
And yes, it's all RC now. No more precast (for the time being...).

RE: Partially Connected Column & Punching Shear

Sweet! Unfortunately -- but awesomely -- I'm in Jamaica right now which will slow me down some due to a lack of scanner access and a decent data plan. But we shall persevere!

1) Will you have a roof version of this? That would be the worst case and is more or less what you've presented in your STM. You'd have some other options at a floor only joint.

2) At the joint, I believe that the moment should be represented differently as it will come into the beam continuously along the beams length.

3) I think that STM a good approach. Simpler methods of checking bearing and development length would just be incomplete versions of the same thing.

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: Partially Connected Column & Punching Shear

I drew this as though the beam concrete wasn't there but, obviously, it would be. The model assumes that the column ties would continue full depth throughout the beam depth. It simplifies the model but I'm not sure how practical it is.

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: Partially Connected Column & Punching Shear

KootK, common on, you are in Jamaica...what is with the red marker pen and white paper sketch. A photo of a sketch done on the beach in the sand with piece of drift wood is more appropriate smile

Enjoy Jamaica. I do recall my wife and I in 1990 (before kids) doing the snow-bird trip to Jamaica to get away from the winter blues of Toronto.

Trenno - sorry to hijack your thread. Back to normal programming.

RE: Partially Connected Column & Punching Shear

Yes, I can only imagine how many of my resort cohorts were also doing STM models at the bar between scuba lessons. I am what I am.

The only non-canned thing that I'm doing this trip is that I've hired a guy to drive me from Momtego Bay to the mean streets of Kingston and back. I can go pretty much any place that I want in between in a 12 hr period. I'm game for suggestions if you've got 'em Ingenuity.

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.

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