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.

 

[TehMightyEngineer]

Curious, would you be willing to do it?

Not unless I could overbuild the hell out of it and have a quality welder. I'd also want 4 weld plates per stem for redundancy.

Now that I've slept on it I've decided I don't like my own detail as it could have a big issue if any corrosion occurred and would be difficult to repair. Maybe it could be improved if the dropped area was a secondary pour with rebar extending continuously from the double tee into the dropped area?

Anyway, scratch my idea. I'm sure it could work but cheaper and likely better to just have a blocked out area of a double tee and go from there.

Contra TME, I think casting these would actually be fairly trivial.

Oh, I agree that the forming and casting would be trivial. In my previous comment I was more discussing generally what the rational Agent666s precasters may have for giving push-back on custom elements; mostly that it could be the precaster was just too busy to want to do a more complicated custom piece. For KootK in particular I expect the only pushback will be on the extra reinforcement required at the notch and perhaps the prestressing calculations to ensure no cracking occurs at transfer. Also, whoever is responsible for designing the beams strength will have fun working out the capacity with the secondary slab being effectively a non-presstressed topping. None of these would be show stoppers in my book, simply difficulties to overcome.

The block out is one thing, installing a low slab after casting is another

This wouldn't be too bad. If I had to do it we could have 180° rebar hoops extend from the blocked out web area of the double tee, flip the double tee over and form up a secondary pour on a raised platform in the shop and cast the low slab. Flip back over and ship. This would probably double the cost of the double tee but wouldn't be a terribly difficult pour.

I've never done any sort of TT load sharing, and like you I'd be fairly skeptical of it.

Thirded for typical double tees. The precaster could cast a grouted shear key profile in the flanges and this would get you some decent load sharing in my mind but obviously isn't typical outside of bridge construction. If it's typical double tee construction with just a thin topping slab then I'd only count on minor load sharing and nothing like full shear transfer.

Part of what's bothering me here is how deep this floor system is getting.

I'm more just throwing ideas out to see what sticks so this may work just as well as my weld plate idea; but what about a hybrid composite steel beam mixed with the typical double tee framing?

Don't have time to sketch it up but use a steel beam that matches the higher and lower profile you have. Have the beam cast into either a precast slab section or a cast-in-place slab to get the profile you need and give composite action for the beam. Design the beam to have similar deflection response as the double tees (matching the camber of the double tees will be interesting). Install adjacent to the typical double tees an go from there.

No idea if this will be something that meets the project goals/cost/etc. Like I said, just throwing out ideas.

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

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

 

[dik]

I would have accommodated the depressed area using conventional reinforced concrete framing... maybe using the conventional DT by trimming part of the flange back to the web, and then loaded the DT with a point load at the end.

Dik

 

[DETstru]

...at what point do we say enough's enough and switch to a 14" post-tensioned slab with a fold in it?

Right now!

 

[Brad805]

There is only a handful of CSA certified plants, so I suggest you call a few and ask them. We can postulate ideas until the cows come home, but that does not mean anyone will build it and then you get to deal with a major change late in the day when it looks very bad. Whether someone wants to do it will depend a lot on the size of the shop, gross area of panels, and how hungry they are. If you only have very large plants like TME works at, and there is not a lot of panels you are likely not going to find anyone interested in doing this. Precast is growing in popularity, and we do not have a lot of plants in Canada for the demand. The small scale plant I work for is booking projects 8months away. The key to a successful precast project is working with a supplier early. Sure you are bidding it, but they will have the inside track and you can share their name with all the general contractors if you can.

The success of a welded stub would depend on a lot of things in my mind. Rust would be a concern for sure, but with proper details it could be protected. Something along a cazaly hanger might work if it could be concealed well. It is easy to be very conservative with something like that since you can spec a lot of extra weld and use much larger bars without adding much cost. The weld quality issue is likely not a huge concern since the welding would likely be done in the shop by the plant welders.

Blocking out the TT top is also not outside the realm of possibility either if you have a plant that does a lot of insulated wall panels. Any company that does a lot of insulated panels works with crazy ideas all the time and forming odd things with foam is pretty routine.