Circular Concrete Tank Roof - Steel Both Faces?

[briancpotter]

I'm in the process of designing some circular concrete tanks for a wastewater treatment plant. The tanks have separately cast concrete roofs with pin connections at the walls. I've designed several WWTP in the past, but this is our firm's first tank with a roof, so we're using a few existing sets of drawings as a guide.

On the sets of drawings I have access to, the concrete roof detailing all seem to show identical amounts of steel in the top and bottom face, in both radial and tangential directions, over the entire roof, even though analysis (and my intuition) says there should be almost no negative moment. Can anyone with concrete tank experience shed some light as to why you'd need/want identical steel placement on both faces? It seems like a lot of unnecessary steel.

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.

 

[jayrod12]

could it be that the crack control rebar ended up being enough for strength? Rebar is cheap, replacing a cast-in-place roof on a WWTP tank is not.

 

[CELinOttawa]

Or that the strength rebar was replicated for the low stress side to double up as temperature and shrinkage steel while eliminating the chbace for the layers to get reversed...

 

[jayrod12]

Good point CEL. Sometimes specifications are made for constructibility. Remember the guys tying rebar a lot of the time are your most inexperienced.

 

[wannabeSE]

I have no WWTP experience and very little concrete tank experience. But, the top longitudinal (radial) reinforcing could reduce long term deflection (creep).

 

[CELinOttawa]

Quite right W.SE... Just remember that the top bars also decrease single-flexural ductility...

 

[JedClampett]

As someone who has a bunch of these under their belt, here are some possible reasons:
*We like to be conservative. So should you.
*Temperature/shrinkage etc.; Cracks that are a nuisance to a normal slab, could let in bad gasses that would corrode the reinforcing.
*Sometimes the tanks experience internal pressure loads that could reverse the moments in the slab. Note that once you design a tank, what the owner changes it to is out of your control. It might be used for digestion or some other nasty process.
*Some of us are lazy and just put two layers of reinforcing without thinking about it.
Some other things to think about. Slope the roof. The slab is likely to sag, so you want to make extra sure it drains. If you have it get thicker in the middle, you can take advantage of that in your bending calculations. Make sure your waterstop around the edge is PVC.

 

[briancpotter]

Thanks for the advice. Most were things I'd considered, and I have no problem being conservative (especially for a WWTP, which will actually see the high loads we design for). I'm mostly preparing for the inevitable round of VE where someone asks "why is there so much steel?" and I need to justify my design.

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.