40 foot round tank cover ideas 2

[TehMightyEngineer]

Got an interesting job and I'm interested if anyone has any good concepts on how to approach this.

Client has a 40 ft diameter concrete tank with an arched steel dome over it and is looking to replace the dome. The cover has to take large snow loads (~70 PSF ground snow) plus pedestrian loads. They got a price for an aluminum dome and it's fairly high so they're shopping around for alternative support structures.

So, I'm trying to come up with clever ways to do a precast structure that can be cost effective. We have a prestressing bed for slabs and small beams but it's limited to 18" depth. Any precast structural system ideas you folks think I should look at?

This is sort of the concept I was most leaning towards:

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

http://www.americanconcrete.com

 

[jayrod12]

Your picture appears to have a column at the centre. Would this be something the client could entertain? If so then I would think you may have a fighting chance.

 

[TehMightyEngineer]

I'm trying to find that out right now. For the sake of argument lets assume both ways (with and without a center post).

Without a post, I was thinking taking tapered panels up to a peak and installing a tension ring around the tank (assuming the current dome/tank doesn't already have something similar installed).

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

http://www.americanconcrete.com

 

[jayrod12]

Interesting idea, I'd be on board with something like that. I guess my question would be how the hell would you erect it? Dual cranes? a temporary middle post?

 

[TehMightyEngineer]

In my mind I was thinking temporary middle post(s). Dual cranes seems like a bad idea and would probably price us out of the project.

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

http://www.americanconcrete.com

 

[jike]

Domes with a compression ring at the center and tension ring on the outside are erected with a shoring tower at the center.

 

[KootK]

What have you got in mind for waterproofing the lid (assuming that's required)?

Is it necessary to create a positive drainage slope within the precast?

I've sketched a few options below. Your alternative would likely be preferable with respect to shipping, erection, and ease of drainage slope creation. Pretty much everything really. That said, you indicated that you wanted some options. I take that to mean more than one.

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]

So, got a little more info on this project. It's a sewer holding tank used when they have to take a clarifier down. When not in use it stands empty or holds minor amounts of water/sewage. The top of the tank can be accessed by the public for some reason (or at least that's what it sounds like), not sure what that's all about; but I suspect snow + dead load will control. Apparently in it's life it actually was a clarifier at some point and then got converted into a holding tank. Steel framed dome roof wasn't designed for durability and is giving out.

A center column was not objected to by the client so that greatly simplifies things. It appears their only concern is cost and durability, all other concerns appear secondary. From what I gathered the cost being too high means that the project might just stall as they probably wont get funding from the city for a major project at this time.

Apparently waterproofing the tank is required; we typically use a butyl rubber sealant in a shiplap or keyed joint between precast sections. It's been accepted before for wastewater jobs for this city where it's not in direct contact with the sewage. That's a good question about the positive drainage KootK, I'll have to find out about that but I suspect we will need some small slope.

I looked at simple span slabs and it would be too expensive to ship. Your idea of a dapped beam could work, plus it doesn't all have to be precast. I could see a galvanized steel truss or something similar being acceptable down the middle and then precast panels spanning up to that. Thinking out loud though; anything that doesn't involved repetition always costs more with precast so I'd like to keep it as repetitive as possible.

Lastly, your "weird" slab panel idea looks crazy awesome. While I'd love to build it I have no idea how we would get these down the road without making every commuter in the state hate us. Though... do you have a CH-47 Chinook laying around I could borrow?

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

http://www.americanconcrete.com

 

[BUGGAR]

The weird panel scheme will erect like an Escher staircase. I would do this one just to (play) with the Contractor.

 

[JStephen]

Years back, we did a steel tank roof where a consultant had designed and detailed the entire structure. It was about double the cost of what a normal steel tank roof structure would have cost. The lesson learned is that in construction, you gain a lot of efficiency from repetition and familiarity. So you can come up with all sorts of goofy ideas, and if somebody was accustomed to doing it like that all the time, it might work great and be done at a reasonable price. But if it's the first time for somebody to build it like that, you'll pay through the nose. New and different is great if you have deep pockets. "Playing with the contractor" is expensive.

"do you have a CH-47 Chinook laying around I could borrow? "

I actually checked into this years ago. The local helicopter company had about a 5,000 lb limit on their machine. To lift anything heavier, they had to fly a helicopter down from Kansas City, and that would run around $50,000 if I remember right. But the major obstacle was that they weren't allowed to lift a load and then carry it over people, so you couldn't just pick up an arbitrary (oversized) load and fly it to the far side of the metroplex.

 

[TehMightyEngineer]

Good info Stephen and after working for a precast plant I can see exactly where you're coming from. I've seen people detail out box culverts that end up costing twice as much as they should just because they required details that were non-standard for our industry. I imagine similar is true to most (all?) products and precisely why I wanted to poll everyone here to find the path of least resistance. No sense reinventing the wheel, especially if the goal of the project is to save money.

And, yes, I was definitely kidding about "playing with the contractor". I'm all for trying new and different but only when a clear path forward exists and there is a good chance savings can be realized.

Interesting anecdote about the helicopter. I always assumed it to be some astronomical cost but I imagine if you had a low-ish weight piece and the issue was the cranes reach or lack of access, then I could definitely see a helicopter actually being a practical way to set something. Probably not precast concrete though, darn stuff is usually quite heavy.

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

http://www.americanconcrete.com

 

[KootK]

If there's just one of these things, the location isn't too remote, and durability is the name of the game, I feel like CIP concrete will be the way to go for the owner.

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]

You might be right as they could do a dome then rather than prestressed panels. Cranes are more expensive than a pump truck, too.

Then again, precast gets you better quality control and clear cover requirements. Plus the prestressing can save you a lot on weight which might become important if the walls aren't able to support a heavy concrete slab.

So, maybe you're right; maybe not. We'll submit our price and let the owner decide.

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

http://www.americanconcrete.com

 

[KootK]

Are you going with the pie slices on a central column then? Is there already something robust upon which to install the column? I would expect so.

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]

Well this is all at the preliminary, cost-estimate stage so I suspect we'll have some details ironed out down the road. For now I'm assuming I can saw-cut out a section of the bottom of the tank and cast a footing into the tank. Either that or install a footing directly on the bottom of the tank pinned into the bottom. If these aren't realistic when we get into it then this could be an issue but is probably beyond the concerns of this stage.

As for the pie slices, that appears to be the way to go right now. It's doable in precast, ship-able, cost effective, wont be the hardest thing to set, and should go very quick once we've finished fabrication. Only other thing I think that could derail this is if the tank walls/foundation can't support the panels weight.

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

http://www.americanconcrete.com

 

[KootK]

Eh, the tank walls may actually be stronger with the extra weight on them so long as you don't trigger footing issues.

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]

Very true, they're probably flexurally controlled so the compression will likely add to their capacity.

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

http://www.americanconcrete.com

 

[KootK]

Always good to come to the table with more than one option. Of course an all steel option would likely be preferable once durability has been sacrificed by introducing any steel. The proposed system could be pitched quite easily if one could sort out the tapered bearing situation.

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]

That could work. I've worked with similar setups for wood timber trusses in old mills. Haven't tried that sort of configuration in a circular pattern but no reason it couldn't work. The only downside I see is if you ever had to remove a panel or if someone overloaded it there's limited redundancy. Plus it cuts into your capacity a lot which might not be favorable.

"Very galvanized", hah! I don't know when but I'm going to get those exact words on a drawing one day.

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

http://www.americanconcrete.com

 

[JStephen]

I saw a drawing a while back that called for "stainless steel bolt holes", but I'm not sure where to get those!