Ponding of Concrete Roof
Ponding of Concrete Roof
(OP)
I am evaluating a circular concrete tank roof. It is (was) a flat slab sloped in one direction at an initial (design) slope of about 1/16" per foot. It now has 9.5 inches of sag at the center, equating to L/67.
Without putting pencil to paper I know something has to be done. However, just to have calculations backup I started researching. I can find no guidance for ponding on concrete. There's plenty on timber and steel framing, but these don't translate well since they integrate the idea of individual members.
Does anyone know a way to do this other than an incremental analysis?
Without putting pencil to paper I know something has to be done. However, just to have calculations backup I started researching. I can find no guidance for ponding on concrete. There's plenty on timber and steel framing, but these don't translate well since they integrate the idea of individual members.
Does anyone know a way to do this other than an incremental analysis?






RE: Ponding of Concrete Roof
RE: Ponding of Concrete Roof
RE: Ponding of Concrete Roof
RE: Ponding of Concrete Roof
The clients know that it is a problem and needs attention, because I told them while I was standing on it with them. They want to know at what depth of water it will fail, regardless of the fact that such information is useless.
It is 53' O.D. and thickness varies from 11" to 14". Asymmetrical reinforcing to accommodate openings and concentrated loads. Built in 1978.
RE: Ponding of Concrete Roof
I am concerned though with his question as to what depth it will fail. As I see it, there are too many unknowns to properly answer this question without putting yourself and your company in a lot of avoidable litigation.
Personally, I would recommend a temporary roof drain be installed at the lowest point of the roof, perhaps a small electrically operated sump pump, and immediate measures be undertaken to effect a permanent repair. This could work for a short time for rain water while the fix was being designed.
My other concern is ponding and freezing due to melting snow. The temporary sump pump solution would not work well here.
Some questions:
What is happening at the connection of the roof to the side walls?
What is happening at the underside of the slab inside the tank?
What is in the tank, if anything?
Should the roof fail, what would be the effects on the surrounding area?
Does the local BD jurisdiction know about this yet?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Ponding of Concrete Roof
RE: Ponding of Concrete Roof
This is an industrial client and all the client contacts are mechanical engineers. They want to know the depth at failure merely because they're curious. They were all making their guesses when I left. These were the same guys that wanted me to finish doing wind and seismic calcs to determine how to connect three existing pressure vessels, stacked vertically, to become a tower - even after they had given up and ordered one fresh from a fabricator due to discovered material flaws.
To answer your questions:
- The simple supports at the top of shell/wall have rotated significantly. Hence my recommendation to replace rather than repair.
- No clue about the condition of the underside of slab.
- Tank does stay about half full of Compound C87R or something like that. Secret and proprietary. SG = 1.20
- If the entire tank were to collapse the spill would be the worst consequence.
- The BD only wanted to ensure they'd pay for a permit when replacing.
RE: Ponding of Concrete Roof
Presuming a uniform slab load from ponding is very conservative, but it does have the advantage of simplicity (and did I mention it's conservative?).
As to checking in-field deflections, all I can do is point you at the long-term deflection calculations in ACI (i.e. creep et. al.)
True failure load? True failure deflection? Perhaps a hinge mechanism can be presumed and a failure load derived from that, with the presumed plastic hinge rotation of 2x the theta needed to get Mp in the plate? If those mech engineers are as technical as they sound and as curious as they sound, you might point them in that direction, since at large deflections the plate is no-way elastic throughout, and the elastic deflections can be largely ignored once the hinge mechanism forms. But the embedment and development and hooks would all have to be adequate to develop the full force in the bars, and so on. At that point, you'd probably get concrete crushing and eventually the bars have to snap at something like 200 ey. I imagine the extreme yielding would be limited to the bar that's exposed to air at the hinge, but you're making a fair leap from what's been researched and studied, after all. But so long as everyone's doing it as an exercise / betting pool, it's an approach. Oh, yeah, you're also neglecting any potential arch / wall action, which I believe can be significant but also, very hard to quantify in a practical sense.
RE: Ponding of Concrete Roof
Michael.
Timing has a lot to do with the outcome of a rain dance.