settlements of tank on ring wall footing, etc. 1

[abih]

I am working on a 100' dia by 25' tall ground storage tank farm project. To estimate the tank center and edge settlements, I need to assign loads to the ring wall footing and the compacted fill (underneath the tank). I remember reading a technical paper authored by Boberg, however, his calculation in an example does not explain how the total vertical load is split between the two supporting components.

My question is simply, how to split the load? Also, does anyone know where I can find some un/published settlement monitoring data at the edge and center of large, spanning several years after construction completion?

Many thanks.

 

[JedClampett]

If you have a good structural designer, he/she tries to make the loads reasonably close. So if you get the liquid height (hopefully you already have that), multiply by its density (62.4 PCF for water), that should give the loads for both the interior and the ring beam. There will likely be interior columns supporting the roof, and these will have a little more load, but their settlement is not that important.
If someone there has the Gaylord and Gaylord Structural Engineering Handbook, it gives the method of designing the ring beam.

 

[cvg]

the loads on the ring beam are mostly from the tank shell and partly from the roof. unless you have no roof columns in which case all the roof load bears on the ring. load in the center is uniform due to the weight of the fluid. you can't make the "loads" close, but you can make the "bearing pressures" close. you do that by setting the width of the ring beam.

 

[fel3]

If you don't mind old data, here are some ball-park numbers you can use for comparison. The data comes from a brochure titled "Welded Steel Water Storage Tanks" (see attached) that I received in 1983 [sic] from USS Fabrication in Long Beach, CA. In my experience, having designed and later reviewed shop drawings for about a dozen welded steel tanks over the years, these numbers are reasonably conservative. In fact, that's one reason I kept this brochure: it has provided good preliminary data for my ringwall designs.

The closest size listed in the brochure to your tank is 100' dia x 32' tall. It gives the following gross weights: bottom = 90,600 lb, roof = 68,800 lb, roof structural = 43,000 lb, and shell = 178,000 lb. With respect to loads on the ringwall, you can ignore the bottom plate because 1/4" thick steel plate is only about 10 psf and this is tiny compared to the other numbers. For the roof and roof structural, you only need to include the portion of the roof that is supported by the shell. The interior columns will support the rest. I think a 100' dia tank will need a center column and one ring of columns at 0.5r. That means that the shell must support that part of the roof and roof structural between 0.75r and 1.0r, which is about 56% of the roof area and *very* roughly the same percentage of the roof structural supports. This results in the shell supporting 62,900 lb for the roof and roof structural or 200 lb/ft onto the shell. Because the shell for a 32' tall tank will be proportionately a little thicker overall than the shell for a 25' tall tank, it would be conservative to simply prorate the weights using (25/32)*178,000 lb = 139,100 lb = 443 lb/ft. However, this is a good upper bound and will suffice here.

So far, then, we have about 200 lb/ft (roof and roof structural) + 443 lb/ft (shell) = 643 lb/ft tank weight on the ringwall. To this we need to add the column of water over the ringwall, which is typically 1/2 of the ringwall width times the height. For example, for an 18" wide ringwall and a maximum water level of 22', the maximum static water load is (0.75 ft)(22 ft)(62.4 pcf) = 1,030 lb/ft. Finally, add the weight of the concrete ringwall (e.g. an 18" wide x 30" tall ringwall = 563 lb/ft). The total static load for this example is thus 200 lb/ft + 443 lb/ft + 1,030 lb/ft + 563 lb/ft = 2,236 lb/ft or 1,491 psf on the soil.

There is an additional seismic loading on the ringwall to consider, but I don't have any examples at hand.


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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[fattdad]

"Differential Settlements in Steel Tanks," D'Orazio and Duncan.

You'll find what you need in that publication.

f-d

¡papá gordo ain’t no madre flaca!

 

[abih]

Thank you all, in particular fel3 for breaking down the numbers.