Reinforced Concrete Tank/Wetwell 1

[sclee21]

I am working on the structural design of a 35 foot deep rectangular pump station wet well. The inside dimensions of the wet well are 19' X 16'. I am analyzing the longer wall as a fixed-hinged-fixed-free plate using PCA - Rectangular Concrete Tanks, Fifth Edition. The equivalent fluid load of the soil is 90 PCF, f'c is 4ksi. I keep coming up with a minimum wall thickness of 36-inches with the controlling factor being shear (no shear reinforcement). On first glance it seems a little thick to me but I believe I am doing the calculations properly. Any suggestions or comments?

Recommendations/options for shear reinforcement?

 

[BAretired]

It seems thick to me too. Why not incorporate minimum shear reinforcement to reduce the wall thickness?

BA

 

[JedClampett]

I usually estimate wall thicknesses for buried structures by using a minimum of 12 inches or one inch per foot depth. So 36 inches for a wetwell buried 35 ft is in the right range. But I'd suggest fixing the bottom as well as the sides. It might save you 6 or 8 inches in thickness. Another trick is to take controlling shear at a distance "d" from the support as ACI Chapter 11 allows.
And is the top really free? Usually these things have a top slab so you can model it as supported. But the top condition isn't really going to affect bottom thickness.
I'd use the Bureau of Reclamation's "Moments and Shears on Rectangular Plates" instead of that crappy PCA document. It's out of print, but someone has a copy around you.
And if the walls need to be 36 inches thick, make them 36 inches. Don't mess around with shear reinforcement. Concrete walls, once you build the formwork, are pretty much just the price of concrete. Shear reinforcement is much more expensive and unreliable than concrete thickness.

 

[hokie66]

Spoken by a man who has designed lots of stuff which is buried below ground. Listen to Jed...he knows.

 

[ishvaaag]

Just in case, available in the web.

http://www.usbr.gov/pmts/hydraulics_lab/pubs/EM/EM27.pdf

 

[minorchord2000]

Thanks for that post ishvaaag, I lost my copy and downloaded and saved the one you posted.

 

[BAretired]

Jed and hokie,

As much as I hate to be argumentative, I cannot understand why the wall thickness would be selected on the basis of only the depth of the wet well. Surely the horizontal span would have something to do with it too.

I admit to having limited experience in this type of design, but I cannot understand why a 36" wall without shear reinforcement would be more economical than, say an 18" wall with shear reinforcement. Perhaps some additional comment from the old masters would be in order.

BA

 

[JedClampett]

BA, I'm talking about estimating thicknesses, not final design. But it works out that way in a surprising number of cases.
I have two reasons to stay way from shear reinforcing in walls. One is, that there really isn't very much good guidance in either textbooks or code. So I've used single horizontal bars, tied to the vertical steel. This seems very labor intensive and I'm never sure what they look like after the wall is poured.
The second reason is that when I size the wall thickness based on shear allowables, the vertical reinforcing is more reasonable. If I used an 18 inch thick wall, I might need #11's at 6" spacing (rho=1.5%, Mc=167 ft-kip/ft). With a 36 inch thick wall, maybe #8's at 12" (rho=.4%, Mc=230 ft-kip/ft).

 

[BAretired]

Jed, if the bars are as congested as you suggest, I can see your point. I hadn't worked through to see how much steel would be required.

I agree that single horizontal bars would be labor intensive. Perhaps there are other options, but closed stirrups are going to be a pain to install.

Would you consider tapering or stepping the walls from, say 12" at the top to 36" at the bottom?

BA

 

[JedClampett]

I've tapered walls (28 ft. deep reservoir) and stepped them (55 ft. deep pump station). Just once I'd like to get some feedback from a contractor that it saved them money. But I'm dreaming because they didn't know what the alternatives were. And sometimes the concrete wall thickness helps me resist flotation.
I strongly suspect that each one costs about the same or at least close enough that they don't affect the bid price.

 

[BAretired]

Good points, Jed. Thanks.

BA

 

[sclee21]

Thanks for the link and your comments.

By what method do you estimate the shear d off the face of the support?

Subtract the soil pressure for a distance d?

 

[JedClampett]

Yes, you can take the bottom reaction and subtract the soil pressure from the bottom to "d" and use that to calculate your shear demand.
But don't do that for the internal pressure.