Double Tee - suggestions? 1

[Structures33]

I have a wastewater treatment plant structure near Charleston, SC with basins on 2/3 of length and 2-story building on other 1/3 (common wall between the two). The blower and pump room is in the bottom level of the building. I want to use concrete for as much of the building design as possible given the corrosive environment. The building 2nd floor framing span is 34' clear and I am planning to use precast double tees in lieu of CIP beams/slab. I have not used double tees before and think it may be a good solution for this. I am looking for any guidelines, suggestions, tips on using double tees for this application. For example camber considerations, connections at the joints, topping slab (do I get the precast sections with a topping or do I specify the topping be poured), floor finish considerations for the 2nd floor, etc. The double tees will be framing into 12" thick concrete walls on the exterior of the building.

Thanks in advance for any direction you can provide.

 

[cooperDBM]

PCI (ww.pci.org) is you best source of general information. For specific details it's best to call some of your local precasters. You can find out who is near you from PCI.

 

[JAE]

One thing you might do is get a meeting together with a local precaster. Many times they are interested in upcoming projects and you can visit with them on preferable details they like to see and suggestions.

 

[JedClampett]

Double tees are good for high wind areas. I've never priced them out for other regions, but they're pretty slick for several reasons.
-Once they're set, the contractor has a safe working surface.
-Their mass is good for resisting uplift loads.
-They're pretty cheap per square foot.
-There's a lot less inspection/observations for them, as they're factory items. No welding, high strength bolting, concrete cylinders, reinforcing submittals, etc.
Some considerations you need to worry about is use of the double tees as a shear diaphragm. The double tees need to be connected to each other and at the ends to be used as a diaphragm. Alternatively, you can pour a reinforced topping slab on them, tie that to your walls and use that for your diaphragm, but that inceases price, construction time and loading. The topping is field installed. You never get it with the double tees. My rule is if the floor is habitable, I put on a topping. If it's a roof, no topping. That's because the double tees are usually poorly finished. You'd never want to put carpeting or other flooring on them directly. Hence the topping.
As far as camber, the double tee designer usually handles that, unless you have special requirements.
For a 34 foot span, hollowcore panels are also very good. They use up a lot less headroom.
You need to get the PCI Design handbook. It's a great reference in general, and will have a lot of the details you have questions on.

 

[hokie66]

Precast concrete is inherently more resistant to corrosion than cast in place, due to the better quality control on compaction and curing available in the factory. But this is only true with adequate concrete cover to reinforcement, so make sure to specify your minimum cover requirements, which for a wastewater treatment plant would be in excess of that provided by "standard" double tees.

 

[Structures33]

Thanks for this excellent information. I will contact a precaster but have one more question. How do I attach a CMU wall bearing on top of the double tees? It will be parallel with the tees. Does it make sense to install a CIP beam under the wall and have the tees run along side?

 

[JedClampett]

As far as the CMU wall, you don't want to support that from the double tees. The precasters like to make all their double tees identical, except for length and supporting a CMU wall requiresa a heavier (unique) load for one double tee. Plus tying the wall into the double tee would require drilling and epoxying in the very thin double tee flange.
Add a concrete beam and tie the double tees to that.

 

[dik]

For 34', what are your loads? Can you use 10" or 12" hollowcore?

Dik

 

[Structures33]

Yes, I have decided to go with 12" hollowcore panels. They seem to be a better solution for the span becasuse double tees seemed to be overkill when i started looking at capacities. So I've got 12" hollowcore panels with 2" topping. The CMU wall end laps of the hollowcore panels will sit on a concrete beam.

 

[dik]

Good guess... have fun!

Dik

 

[ishvaaag]

Then to avoid too much "fun", reading this advisory won't do harm ... please note the photo is from a test.

http://www.dbh.govt.nz/UserFiles/Fi...ance-information/pdf/practice-advisory-05.pdf

 

[dik]

The use of any material is subject to proper construction. For 34' spans, 12" HC will have little deflection, either up or down and with a bonded topping if each second joint is tooled, there should be no issues with the topping. I though that HC was likely the least costly precast for that sort of span and I guess 10" may not be available in some locales.

The attached pub is a really good read, thanks,

Dik