30' dia x 19' tall water tank with roof
30' dia x 19' tall water tank with roof
(OP)
The company I work for (design/build construction company) will be bidding the design, fabrication & construction of a 30' diameter x 19' tall water tank (100,000 gallons) for a coal plant. Most likely, since I specialize in structural steel & reinforced concrete design, I will be recommending that we seek a third party designer who specializes in water tank design. However, management is asking that I at least consider handling the engineering internally. I'm not opposed to it, as long as I have a good grasp on some reference material.
The Gaylord Structural Handbook has some references for tank design, and it's essentially based on hoop tension. This is a pretty simple concept, so designing the walls for the hoop tension seems pretty basic. However, there are two other issues that I'm concerned about:
1. This tank has a roof (with a post in the middle I assume), which will impose a compressive load onto the tank walls, so buckling of the wall is a concern. I'm pretty sure that Troitsky's Tubular Steel Structures has some information on that, but I'll look.
2. I would think that seismic loads would be a HUGE consideration for something like this. 100,000 gallons weighs about 833,000 lbs, so even if we're only dealing with a 5% seismic load, that's about 42,000 lbs! How do you account for such a large lateral force within a thin-shelled circular tank?
I'm sure there are various design codes for water tanks of this nature, but it's not really in my wheelhouse and I don't know which organizations handle this topic.
I will still be recommending that we seek a third party designer, but I'd like to at least be able to talk intelligently about the topic. Any help?
The Gaylord Structural Handbook has some references for tank design, and it's essentially based on hoop tension. This is a pretty simple concept, so designing the walls for the hoop tension seems pretty basic. However, there are two other issues that I'm concerned about:
1. This tank has a roof (with a post in the middle I assume), which will impose a compressive load onto the tank walls, so buckling of the wall is a concern. I'm pretty sure that Troitsky's Tubular Steel Structures has some information on that, but I'll look.
2. I would think that seismic loads would be a HUGE consideration for something like this. 100,000 gallons weighs about 833,000 lbs, so even if we're only dealing with a 5% seismic load, that's about 42,000 lbs! How do you account for such a large lateral force within a thin-shelled circular tank?
I'm sure there are various design codes for water tanks of this nature, but it's not really in my wheelhouse and I don't know which organizations handle this topic.
I will still be recommending that we seek a third party designer, but I'd like to at least be able to talk intelligently about the topic. Any help?






RE: 30' dia x 19' tall water tank with roof
A tank that diameter could have a rafter-supported roof with center column or a self-supported cone roof. If a flatter roof is specifically required for access or other reasons, go with the rafters, otherwise, make it self-supporting to eliminate some corrosion issues.
Usually, tank owners will issue specifications themselves or through a consulting engineer, and final tank design will be by the tank company. In some cases, the foundation will be by the owner/consultant, in some cases by the tank company.
RE: 30' dia x 19' tall water tank with roof
RE: 30' dia x 19' tall water tank with roof
I would definitely specify this out. You'll want a steel tank manufacturer fabricating it. Many of them have their own in-house engineers as well. They'll design something that's the most economical for them to fabricate and it will take them a fraction of the time to design it. It would be a big chunk of work to properly design a steel tank without having ever even seen an example calc before.
You'll have to choose between welded & bolted construction, as well as a good coating system. Periodic maintenance of steel tanks is a big part of their lifecycle cost.
RE: 30' dia x 19' tall water tank with roof
What's going to happen is they're going to throw an conservative price at this tank, and if by some miracle we get it, then we'll hopefully seek a third party designer/fabricator. We just need to make sure we're high enough on the bid. So, I need to have a rough idea on the tank dimensions for bidding purposes only.
We've settled on AWWA D100-11. For a 34' diameter x 15' high tank, I think we can get away with 3/16" shell plate (minimum shell thickness for this size tank, per AWWA). As a conservative estimate, they're probably going with 1/4" shell plate and bottom plate in the quote. The shell plate and base will have full penetration butt weld seams. The stuff that we're going to struggle with are the items such as - two manholes on the bottom shell plate, access hatch on top, external ladder, overflow pipe, vent, roof framing (with center post), etc.
The overwhelming cost is going to be the field labor due to the amount of welding. If anyone has any good references on typical roof/wall details or center post details, by all means, point me in the right direction. Thanks........
RE: 30' dia x 19' tall water tank with roof
The problem with a round tank + rolled (dish) bottom + round walls is the complex multi-curved fitting and setup by the fab shop: Like a ship's hull: All the welded joints are easy to draw. Very, very hard to roll up and get accurately fitted and welded.
Reinforcements under (or around) the intersections between legs and the lower floor to prevent buckling or erosion or collapse) of the tank are equally difficult.
RE: 30' dia x 19' tall water tank with roof
The bottom would normally be lapwelded from the top side only.
RE: 30' dia x 19' tall water tank with roof
JStephen - I seen where it's discussed about the bottom construction....either butt weld with a backer, or lap welded. With the lap weld, how would you handle the uneven surface where the wall meets the bottom? You'd almost have to have the walls sit on a common ring-plate, then fill in the rest of the bottom with lapped plates?
RE: 30' dia x 19' tall water tank with roof
If you butt-weld with a backup bar, you have exactly the same issue.
Normally, there would be 1/2" padding or grout under the tank shell. But if neither is there, it still isn't a problem.
RE: 30' dia x 19' tall water tank with roof
OK, so a flat bottom makes things simpler.
Try this then to do all three: reinforce the bottom joint, make the bottom joint (wall-to-bottom) easier to weld, porvide a backing plate where the columns intersect the tank. Roll a 4x4x3/8 (or similar) angle to the diameter of the tank.
Fillet weld (inside and outside) tank walls to the angle iron, and tank bottom plates to the angle iron. NDE (dye penetrant) inside and outside welds to verify no pinhole leaks.
Add reinforcement plates under the vertical support tubes, with ribs at each junction. Rolling even a large angle iron is simple, and provides a backing plate and doubler plate that avoids the more tricky, more expensive butt joint. A double fillet takes more welding inches, but is a more forgiving weld.
RE: 30' dia x 19' tall water tank with roof
The rolled angle may have been used with riveted tanks, I'm not sure how they made that connection.
RE: 30' dia x 19' tall water tank with roof
RE: 30' dia x 19' tall water tank with roof
RE: 30' dia x 19' tall water tank with roof