Steel Column Connection to Concrete Slab - Recessed Pocket 4

[KootK]

I'm getting pushed pretty hard to use the detail below. The reason is pretty straight forward: compared to a column welded to a cast in plate, plumbing and erection is much easier with this detail. It definitely can be done with the welded detail however.

The photo at the bottom was taken from a related thread on this forum and shows a steel column that is supported directly above a concrete column below. While a consensus has not been reached there, my opinion is that a similar, recessed pocket detail is to blame for the cracking shown.

There are other details that I could use here but that's not what I'm seeking. What I'd like to get feedback on is a method for evaluating whether or not the detail shown below works. What needs to be checked here in order to give this a passing grade?

My specific concerns and opinions are as follows:

1) The recess forces one to either drop the top reinforcing mat or push the bars out to the side of the pocket.
2) With the mat dropped, punching shear is a beast and significant cracking can be expected as shown in the photo.
3) With the top steel moved to the side of the recess, it's tough to meet the code rules on the amount of steel that may be placed over and near the column.
4) With the top steel moved to the side of the recess, I again wonder about punching shear capacity. See the hypothetical cracks in my sketch below. My concern is exacerbated by the interrelated nature of punching shear and flexural capacity (Link) at column/slab joints. If I shift my moment capacity away from the column, does that mess with my punching shear capacity?
5) I have renovated buildings from the 70's and 80's that had similar details but with drop panels. They seemed to be performing well.

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.

 

[hokie66]

KootK,
I thought you were the designer. By the time the column is wrapped, you would have enough room for my concrete column. Problem solved.

 

[MJB315]

I'll just chime in and also suggest the concrete pier. It seems like it'll do the trick.

Out of curiosity -- what FEM program was that?

"We shape our buildings, thereafter they shape us." -WSC

 

[KootK]

My firm is the designer. Just not me personally. I mostly just hover and dream up creative ways to make people's work more difficult than it needs to be.

We've got a few situations. Exposed columns, wrapped columns, columns in walls. Uplift and no uplift... The concrete column idea may well work for a few. It's just a little weird looking in my market. In general, a steel deck is supported by steel columns in my neck of the woods. Of course, nothing is written in stone.



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.

 

[Brad805]

MJB315, that was done in Diana FEA (

http://tnodiana.com/).

I do not think the recess is saving much, but you are a smart guy, so I will leave it at that.

 

[KootK]

I do not think the recess is saving much, but you are a smart guy, so I will leave it at that.

As in saving money? I don't really know what it saves. I just know that I get a lot of contractor requests for it when I show columns welded to cast in plates. Apparently, the name of the game is erection and plumbing which are tough for me 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.

 

[Brad805]

I fully understand why they request you delete cast in plates. Think about what one has to tie to so the plate does not float. More importantly, think about which trade installs the embed plate. If it is your steel guy, then he will come up with a good method to keep it fixed if they have done them before. If it is someone other than the steel guy, it is a total crap shoot how level the plate will be after the slab is cast. They are always out of level and are notorious for leading to extra time on site field fitting columns. We have tried collars, but those are not simple either, plus they add extra parts. Even if the plates are perfect, it takes more time on site to weld than in the shop.

I do not follow why the plumbers would object to the cast plate. Regardless if cast in place or a traditional base plate, he still has to route his stack around a plate.

The recess is not very easy to form either, but if it were me, I would cast long anchor bolts with the column so I have something rigid to fix the anchor bolts too. We have thousands of feet of recess's (for precast walls) on a prison project and they are all over the place as far as level is concerned.

It is all about the dollar with the steel guys we deal with. We had one extra us for a 1 3/8" diameter drill bit for field drilling 16 holes.

 

[KootK]

I do not follow why the plumbers would object to the cast plate.

Here, plumbing = making columns plumb/vertical.

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.

 

[hokie66]

One way of setting a plate level is to provide 2 plates, one on the top of the concrete column, the other at floor level, joined by something like a small square hollow section. This allows the reinforcement to pass through as normal.

 

[Brad805]

Got it.

 

[Brad805]

Hokie, that is a good idea, but then they claim we are crazy for spec'ing all this steel. You cannot win sometimes.

I like many of the steel trades we work with but somedays,

Just a joke of course. No disrespect intended to any steel guys.

 

[hokie66]

I have used that detail and similar many times, not necessarily on top of a column, but somewhere in a floor. Just set the fitment on the form, nail it down, and Bob's your uncle. Rather than trying to keep bolts in place, the site workers love it.

 

[KootK]

One way of setting a plate level is to provide 2 plates, one on the top of the concrete column, the other at floor level, joined by something like a small square hollow section. This allows the reinforcement to pass through

If I understand correctly, this would solve the plate levelness issue but not the column plumbing issue. Also, would the two plate assembly be set on the column with anchor bolts and levelling nuts Of it's own? The top of the column would be quite rough.

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.

 

[JAE]

When someone proposes a cost savings measure, I hate to veto it based on gut feel alone.

KootK - I can certainly appreciate your view here. I just know that my past mentors from years ago used to tell me that if a structural assembly doesn't look right (in the gut) then it most likely isn't right - check it again with extreme bias.

So in evaluating a slab pocket like this I'd start out with a high level of prejudice against it...and continue that skepticism throughout my checks.
Just sort of a built-in safety factor coming from my attitude.

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[hokie66]

KootK,
No bolts or levelling nuts, just anchorage by the bottom plate. Admittedly, when I have used the detail it has been usually placed on a plywood form and nailed down, so providing a good surface on the concrete would require a little bit of effort, but am sure those good Canadian concreters could manage, particularly if you emphasize how much work you are saving them in not having to form the setdown.

What column plumbing issue? Don't allow pipes and conduits in your columns.

 

[KootK]

I just know that my past mentors from years ago used to tell me that if a structural assembly doesn't look right (in the gut) then it most likely isn't right - check it again with extreme bias.

That's sound advice JAE, thank you. My "structural vision" was hard earned. No sense setting it to the side now.

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.

 

[JAE]

Heh - sort of like a voice inside you saying "Use the Force, KootK, use the Force. Close your eyes and trust your gut."



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