Slab Depressions in Double Tee Floor Systems

[KootK]

See the somewhat fictionalized rendition of my project below. Originally, it was desired to use hollow core precast plank for this second floor transfer slab. Somewhere along the line, before it landed on my desk, it was concluded that the spans were too long for 12" HC and that a double tee system would be better. As things have progressed, the question has arisen "how do we handle slab depressions?". Without egregiously sacrificing the economy of the TT's, it seems to me that they really cannot be handled.

Before I throw in the towel on this scheme and try to steer things in another direction, does anybody know of any tricks for making a go of it with the TT's? While I haven't done a TT transfer slab before, I know that it definitely has been done before. I'd think that this would be a common problem in need of solution. The only simple idea that I can come up with is to do it all in the topping. But, then, that gets heavy, expensive, thick, and generally ridiculous.

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.

 

[EZBuilding]

Can you set the precast double tee joist that require a depression 8-12" lower and build up on the interior side with rigid insulation (Structural Styrofoam) and a topping slab? Might lead to some tricky attachment details at your supporting member and I am not sure if you have the height below....

 

[KootK]

Thanks for the suggestion EZ. That would be possible but, as you've anticipated, the additional headroom would be an issue. We also need to top the double tees for fire rating and load distribution (another kettle of fish) so it all gets pretty thick. Lastly, my real building actually has depressions all over the place. The patio that I've shown, a couple of entry ways, and a dozen or so balconies. I'd wind up dropping pretty much the entire deck to get it done.

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.

 

[Celt83]

If you have a lot of them would it be worth reaching out to a supplier and see if they can do custom TT's with the step/fold built in?

 

[briancpotter]

What's the purpose of this depression? I'd think ANY flooring system would have difficulty handling what in essence is an unframed 12' x ~50' opening, it seems unfair to blame the double tees, heh.

What's the loading on the floor? 42' is well in the ballpark of 12" HC for most uses.



Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[KootK]

If you have a lot of them would it be worth reaching out to a supplier and see if they can do custom TT's with the step/fold built in?

I'll ask it. Somewhat oddly, I'm sometimes a precast engineering. What I know of my clients' fabrication techniques leads me to believe that this would be quite difficult. I'd think that one would have to block out the flange and the tops of the stems and then come back and add the fold after the fact.

What's the purpose of this depression?

The usual business where the interior and exterior walking surfaces are at nearly the same elevation but the exterior has a bunch of building envelope stuff piled up on top of it.

I'd think ANY flooring system would have difficulty handling what in essence is an unframed 12' x ~50' opening, it seems unfair to blame the double tees, heh.

A common permutation of this is to run hollow core in in other direction, drop the edge planks has needed, and smooth out the odd bits with topping. It make sense when you've got a high aspect ration building with all of the depressions on the long sides, which is normal.

What's the loading on the floor? 42' is well in the ballpark of 12" HC for most uses.

It carries three floors of wood residential structure coming down as line loads parallel to the plank. Parallel, at least, for 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.

 

[briancpotter]

As far as solutions, I can think of a couple options (assuming "frame it in a sensible way with a beam where the depression edge is" is a no go):

1) Cast on a flange at the correct height after the double tee has been poured, prior to erection (it would have to be post-pour as I'm not aware of any double-tee forms that can accommodate a jog in height like you're showing). This would be kind of a hassle (read: $$$), and you'd likely get some gnarly stress concentrations at the edge of the depression where you have a discontinuity, but your span is low enough and your section deep enough that you could probably make it work.

3) Frame the whole thing with light gauge after the tees have been erected. The issue now is that your "tee" is spanning 12 or so feet with 12" of missing depth and no top flange, but again, it's possible this is workable with the span and tee size numbers you're working with. You could also look into combining this solution with a deeper tee at this section (you'd have to ask around for a precaster that has the forms for them), though then you run into headroom issues.

You also might look into framing this area with single tees - you run into the same problem, but single tees tend to be able to handle a lot more capacity, so might work better.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[Agent666]

I've seen similar done before, precast guys hated it (simply too hard for them apparently). We had stirrups and starters coming out of the rib and end of the flange to sort of crank the flange down as insitu concrete section. We reverted to just setting down the entire units and dealing with the additional dead weight because precast guys were to inflexible.

Our set down was only 3" from memory, yours is a lot more savage at 8-12".

I'd look at ways of reducing the set down, it's a case of the architecture needing to follow the practical engineering realities. We do our best but sometimes the architecture needs to pull its head in. Perhaps a cutoff drain at the threshold can allow for a much smaller set down for example, or a fall away from the building to allow less set down.

Lastly might just need to throw through additional beams to frame out the lower portion, or turn the floor system 90 degrees locally and span the other way if it worked for your wall loads. You could have ribs at closer centres if it was an issue.

 

[Celt83]

might be dumb but...

 

[jayrod12]

Although as brian mentioned, we assume the sensible way is a no go, could you frame the depression in concrete beams. Even maybe precast beams? It certainly would simplify things. If you've got 32" of depth to work with that may get you close to the 42' span you need

 

[KootK]

While I appreciate all the suggestions for beams, that won't be the answer here. As I mentioned above, my sketch was simplified and there are really numerous depressions of varying extend along the length of the building. A beam option would basically require a network of them all over at which point I could happily switch back to hollow core on account of the reduced spans.

Simplifying my building has not simplified my thread it seems. That's what I get for trying to game the system.

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.

 

[jayrod12]

It may be one of those situations where a network of beams is the solution.

Slab depressions on double tees just seem like a super scary proposition.

Is it possible the other depressions are less onerous than this one?

 

[KootK]

They are less onerous. Small entries and unit balconies is all. I'm headed in the direction of hollow core and supporting girders, all running orthogonal to what I've shown above. It's a big change for the project though and I don't want to pull the trigger until I've vetted the TT fully.

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.

 

[TehMightyEngineer]

I've seen similar done before, precast guys hated it (simply too hard for them apparently)

You could say it's too hard, but I'd say it's just too unique. Precast plants operate best when we can churn out the same sections over and over. Allows us to keep a steady shop schedule, bid projects easier, and keep the money train flowing steadily. As soon as you do really unique custom pieces you now need extra time to setup the forms and fabricate the atypical rebar. You need to ensure it can be lifted and handled without cracking (often with offset centers of gravity). The QA/QC folks need to spend a lot more time making sure all the unique details are accounted for. Then the estimators pricing the job out need to account for all the extra hours and unique materials. Then the production managers need to make sure they account for the extra time this empty form has to sit in the shop while the custom rebar is installed and the bonded out areas are made. Often times the unique pieces will have the plant engineer (me) have to design and/or stamp something for submittal to the EOR. And so on. All this then needs to be done at a low enough cost to keep the contractor happy. Finally, 90% of the time, if you screw up a job it's going to be on a unique piece and not a "stock piece"; so the risk on these jobs goes up.

Overall it's just far easier money to avoid the more difficult projects. Many times we will pass on a job bid just because we'd rather save the shop space and churn out septic tanks or manholes, because in the long run we'll make the same amount of money for less risk. That said, if you can get us a unique job at a slow time in our year we'll be all over that. The difficulty isn't the issue, it's just what makes us the most consistent money in the long run. It's good to find a precaster to work with early on so that they can guide you away from any details that makes the job too risky or time consuming to be feasible. Often times it can be little things, like making all the stirrups the same spacing or what not. Definitely a case where "least weight" does not equal "least cost". Remember that precasters are more concerned with getting concrete into and out of forms ASAP and then getting the forms out of the shop to make way for new forms. Anything that gets in the way of that costs money.

A little bit of a tangent, sorry KootK. I'll get a proper response to your question in a minute.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

KootK, what about this for an option? Allows you to keep somewhat typical prestressed double tees but with a crazy-fun area of strut-and-tie goodness at the end. Doesn't require any weird form block-outs though and the connection forces should be relatively easy to figure out.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[DETstru]

Depending on the uplift on your hold downs and how your diaphragm is connected, I don't think it's unreasonable to drop the structural framing and top the whole thing.

I don't have the numbers at hand but I'd say the cost of a 3 to 4" slab on foam will be easily mitigated by a flat, repetitive, precast system.

 

[KootK]

KootK, what about this for an option?

Never. That's kind of voodoo that I recommend for other folks but would never have the balls to execute myself. I'm not sure that I could go on living knowing that three stories of building is depending on that weld plate in tension not ripping out. Curious, would you be willing to do it? As a precast specialist, I weight your opinion quite heavily in these matters. As a solution, it's clever and innovative, no doubt.

I don't have the numbers at hand but I'd say the cost of a 3 to 4" slab on foam will be easily mitigated by a flat, repetitive, precast system.

Thanks for sharing your perspective DETstru. I'll mull it over. Part of what's bothering me here is how deep this floor system is getting. 32" tee + 3" topping + 8" build up = ...43"? I get that precast goes up fast but at what point do we say enough's enough and switch to a 14" post-tensioned slab with a fold in it?


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.

 

[briancpotter]

KootK-

I've done a fair share of tees, and I don't think fabrication will really be your issue here. Contra TME, I think casting these would actually be fairly trivial. As you say, all they have to do is block out the flange and a chunk of the stems, which (shouldn't) be a problem. All they'll do is put a bit of styrofoam in the forms, no goofy splicing required. It's not the prettiest thing in the world, but I'd be willing to bet your precaster has done uglier pieces.

Your issue is going to be what kind of capacity you're left with in just the two reduced-height stems, but that should be a fairly easy check (and these tees are deep enough that it's not obviously a no-go).

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[KootK]

Thanks Brian. I'll try and call some precasters tomorrow. The block out is one thing, installing a low slab after casting is another. Unless I can expect that bit to be formed and cast in the field? Have you done many TT transfer slabs? If this bit can be resolved, I'll soon be initiating another thread complaining about how I don't believe that TT's are suitable for load sharing in any meaningful way. I could use some advice from someone intimate with these systems.

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.

 

[briancpotter]

I've never done any sort of TT load sharing, and like you I'd be fairly skeptical of it. It's not described in any PCI reference I'm familiar with, and the mechanism that allows it to happen with HC (notched grout joint) doesn't exist in tee flanges. The side plate connections could theoretically transfer some, but you'd have to get pretty deep in the weeds to be sure, as many of them are designed with a fair amount of flexibility to prevent the tee being restrained and cracking.

Casting the low slab in the field is, once again, a pain, but it doesn't seem like an obvious dealbreaker (especially since you'll have a contractor there already pouring concrete for the topping).

I think if the capacity is there, this could all be ironed out, and this would be the "weird portion of the building" that all precast jobs seem to have.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.