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top reinforcement anchorage over edge columns

top reinforcement anchorage over edge columns

top reinforcement anchorage over edge columns

(OP)
good day,

While designing a flat slab, the edge columns (no framing beams) are assumed as fixed. Top reinforcement is required above these edge columns. small amount of reinforcement will be developped into the columns using a standard hook and the remaing into the slab. my question is regarding the top bars, How can i check that these bars will develop the fy for tension since there is no critical section beyond which the reinf't shall extend ??
Thanks

RE: top reinforcement anchorage over edge columns

bend them down into the column to develop the fy

RE: top reinforcement anchorage over edge columns

(OP)
I meant the reinforcement adjacent to the column. how do u check that it will develop fy since no critical section is available ?

RE: top reinforcement anchorage over edge columns

Hi Structural87,

Assuming full fixity between edge column and flat slab could be unrealistic, but whatever support condition exist in reality top reinforcement adjacent to the edge column could be U shaped bars and should be placed within the effective width 'be' (see clip below)

Its also good practice to provide two bars running parallel to the slab edge between the reinforcement of an external column.

RE: top reinforcement anchorage over edge columns

(OP)
Thanks hertgen, dont u think a standard hook will do the job ??

RE: top reinforcement anchorage over edge columns

Hi structural87,

I think to effectively transfer the torsion from the slab to the column we will need a U-bar to wrap around the slab edge, a round link like beam stirrups will actually be better.

If you draw STM model with 3D truss representing the slab edge (see below) its apparent why we need to have a U-bar to transfer the torsion.

RE: top reinforcement anchorage over edge columns

(OP)
thank u hetgen for ur clear reply.
for a flat slab without edge beams, i consider the columns as fixed. Check the punching shear taking into account the unbalanced moment. after punching is checked, i release my edge columns (just the edges) and keep the interior as fixed and design my slab accordingly.

RE: top reinforcement anchorage over edge columns

There's an ACI document that deals with slab/column joints. I believe that document recommends an effective width, based on joint geometry, over which your top steel beyond the column may be considered effective.

While I understand the logic behind u-bars, I've yet to see that recommended or executed in practice. I suspect that many contractors would deem the u-bars less constructible than conventional detailing. My experience may only be valid in my market of course.

The usual model for design is not to treat the slab edge as a torsion member but, rather, as the tension flange of a pseudo T-beam with a compression block of a width equal to the column face. The relevant strut and tie model pertains to the lateral spread of the compression block force out to the tension steel adjacent to the column.

This jives with the effective width concept and crests a demand for a tie force parallel to the slab edge that could be satisfied with edge bars, as mentioned above or compression in the slab edge between columns. If the column is in close proximity to the slab edge, there is usually ample top steel available for this purpose in the column top mat.

As for the development length in the top bars, that's a bit nebulous. Technically, it would be dictated by the STM model mentioned above. In practice, however, that STM model is rarely checked explicitly. My recommendation would be to:

1) hook the bars at the slab edge.
2) extend the bars as far as those developed into the column.
3) use as small a bar size as possible.

Quote (OP)

after punching is checked, i release my edge columns (just the edges) and keep the interior as fixed and design my slab accordingly.

This worries me if I understand correctly. Punching shear capacity checks assume and require that the joint has been properly designed for flexure. If the flexure design at your edge columns is nominal, so is your punching shear capacity.

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: top reinforcement anchorage over edge columns

(OP)
Thank u for ur reply.
1- if i use a u bar, what is required length ? Is it the standard ld for top and bottom extremities ?

2- if the edge of the slab have drop beams all over the perimeter, is it ok if u design the slab as a pinned edges and therefore, release the edge columns ?
Thanks for the help.

RE: top reinforcement anchorage over edge columns

Definitely U bars around the edge. I would extend the legs a full lap length past the face of the column at least. And make sure they get corner bars in the correct position and continuous through he column reinforcement.

RE: top reinforcement anchorage over edge columns

1) whether u-bars or hooked bars, I would think that they would need to extend to your inflection point plus whatever extension is mandated by your code.

2) if you design your slab as pinned at the beams, you'll still need to design the beam - column connection for compatibility / redistribution torsion of the edge beam at minimum. At least that's how it works in North America.

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: top reinforcement anchorage over edge columns

(OP)
So releasing the column or designing an edgw column to just carry axial force is forbidden, right ?
I posted this thread because i am analysing a roof slab where the edge columns are asking too much reinforcement because of the unblanced moment transfered by the slab and not enough axial force to reduce the tension stresses. The column dimension is ok for all the floors except for the last floor where the steel ratio jumps from 1 to 6% for the reason stated above (design is according to aci 318-08)

RE: top reinforcement anchorage over edge columns

Yeah, that's a common problem. Some engineers will assume a pinned connection at the roof as you've suggested. Even if you can make the columns work, getting the roof slab/column connection to work by the book in a flat plate is difficult, especially given field QC realities at those joints.

I've known other engineers to reduce edge column stiffness by 0.5 so that edge columns attract less moment. The justification for this is usually that the lack of axial load will cause edge columns to crack early in the load history. I don't know where this procedure is formally documented however.

I don't think that releasing your columns is forbidden but it is my preference to modify their stiffness instead. As I mentioned above, punching shear provisions assume proper flexural design at the joint. The more your flexural assumptions become BS, the more questionable your punching shear capacity becomes.

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: top reinforcement anchorage over edge columns

(OP)
I agree with u kootk.
however, if u are in a flat plate with no edge beams to prevent a punching failure, reducing the modifiers of the column will result in a lower column reinforcement and since the unbalanced moment will be transfered to the column, which can not handle that moment since he is designed to attract less load then there is a question mark. Ur flexural design of the slab is designed to support the amount of unbalanced moment but not the column.
what do u think ?

RE: top reinforcement anchorage over edge columns

I did mean that you then extend the bars as far as required for the moment at the face (top and bottom) by lapping other bars to them!

If the column is cracked relative to the slab, then you can use the reduced stiffness. But you need to check cracking in both the slab and the column. If they are both cracked to the same degree, then the relative stiffness will be the same as if they are both uncracked and full stiffness would give the same result. You cannot simply assume that the column is cracked without checking the slab as well!

RE: top reinforcement anchorage over edge columns

Quote (Structual87)

however, if u are in a flat plate with no edge beams to prevent a punching failure, reducing the modifiers of the column will result in a lower column reinforcement and since the unbalanced moment will be transfered to the column, which can not handle that moment since he is designed to attract less load then there is a question mark. Ur flexural design of the slab is designed to support the amount of unbalanced moment but not the column. what do u think ?

I disagree. Having established the stiffness of the various elements of the system, your analysis should produce a set of forces that is consistent among the various members.

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: top reinforcement anchorage over edge columns

(OP)
Ok. I generally use safe 12 for slab and horizontal elements design, and etabs for the 3d modeling. Slab is generally designed elastically without assigning stiffness modifiers while in the vertical elements design, 3d model is based on a cracked model. Therefore, distribution of forces will differ between the 2 softwares.
What is the procedure u follow ? Do u check everything on the same model ?
(Btw thank you very much)

RE: top reinforcement anchorage over edge columns

I've employed all of the methods that we've discussed at various points in my career. It ends up being a judgement call that needs to be made by the engineer of record. I do use different stiffness modifiers for my lateral and gravity load designs. One thing that's always bothered me is that most codes place a limit on how far designers can deviate from the elastic moment distribution. Usually it's on the order of 10-20%. Obviously, pinning your columns or cutting their stiffness in half would violate this in spades. Of course, we do this all the time when we consider a continuous beam or slab to be simply supported.

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: top reinforcement anchorage over edge columns

I would never assume continuous beams or slabs to be simply supported if they had column supports!

Codes always allow you to use the cracked stiffness if you want to. But you have to be consistent for all members in the design and between analysis and design.

If both slab and columns are cracked, it is always possible to use the reduced stiffness, but you have to calculate it, not assume factors like you do for sway analysis. And then the columns and framing members all have to be designed for the actions they attract.

RE: top reinforcement anchorage over edge columns

Quote (rapt)

I would never assume continuous beams or slabs to be simply supported if they had column supports!

While I disagree with the practice, some North American firms will pin their edge columns. In my experience, most North American firms will consider slabs as pinned at foundation walls and shaft walls unless said slabs are near the bottom of tall buildings with very beefy shear walls.

In addition to cracking, I've also heard the argument that dead load moments tend to dissipate as a result of creep. That is correct, if difficult to quantify.

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: top reinforcement anchorage over edge columns

There are a variety of assumptions which are safe and reasonable in regard to cracked or not. The one I would never compromise on is fixity of a flat plate for punching shear design. For flexure, yes, but not for punching shear. But even if you consider the full column stiffness for the floor design, you don't have to do that for the column.

RE: top reinforcement anchorage over edge columns

(OP)
Regarding the consistency of the results, slab analysis is done while modeling just the floor that we are interested in(assuming fixity at the bottom of the column) while columns design is performed taking into account a craked model and where all the floors are included.
therefore, there will be a non negligible difference between moment transfered to the column (edge one) in both models.
punching shear check for the slab will experience an unbalanced moment much higher than the one found in the 3d model.
So if in the reality, slab will experience the higher moment , punching shear will be ok but the column wont since he was designed to support less moment.
What i want to say is that assigning modifiers even for checking punching may not that bad for consistency. Otherwise, the column must handle a higher moment than found in the 3d model. I am thinking loudly.
any comments ?

RE: top reinforcement anchorage over edge columns

Too much modelling, and too little common sense. You need to be an engineer, not just a computer tech.

RE: top reinforcement anchorage over edge columns

I've always had philosophical issues with the fake pinning of columns, even for column design. My take is that, for the pinned column business to be kosher for column design, the axial load on the column ought to be less than the balanced failure load calculated using an overstrength rebar fy (1.25 x 60 ksi). If this condition is not satisfied, I feel that the column will be prone to brittle concrete crushing failure before the assumed flexural hinge is developed. Following through on this tends to also result in fairly large 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: top reinforcement anchorage over edge columns

(OP)
I starred ur poat hookie ! Thank u

RE: top reinforcement anchorage over edge columns

Kootk,

I would agree with using pins at relatively flexible wall supports, but there is no punching shear problem there, so all is conservative.

Never for an end column support, no matter what the building height. Yes reduced stiffness if it can be shown that the columns and slabs are cracked under the full vertical loading (so never for a lower level multi-storey column which will be in compression).

For a multi-storey, there is even an argument to use full uncracked stiffness for the columns and reduced slab stiffness, increasing the column moment, as the slab will be cracked and, at lower levels, the column will be in compression and uncracked under vertical loading.

RE: top reinforcement anchorage over edge columns

We're on the same page with regard to punching shear Rapt. I specifically referenced the design of columns and only design of columns in my last post.

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: top reinforcement anchorage over edge columns

KootK,

Not in the 1st paragraph!

"most North American firms will consider slabs as pinned at foundation walls and shaft walls unless said slabs are near the bottom of tall buildings with very beefy shear walls"

RE: top reinforcement anchorage over edge columns

I don't follow Rapt. The snippet that you quoted is from my second to last post and it deals with slabs rather than columns which, as you've pointed out, don't have punching shear issues. Where is it that we disagree?

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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