TANK WALL DESIGN

[nader666]

Hi everyone,

I am reviewing a modification to wastewater existing tank by adding a dividing wall, the tank is 29' x 29' 18' high and

the dividing wall is 18' from the side wall. The structural engineer submitted a structural calculation based on 11.5'

water depth using PCA tables using three side fixed wall and came up with 14" wall with # 6 @ 12" rebars verticaly and

horizontaly. When the depth mistake has been discovered, the calculation went back to him to revise it, he did not

change the design calculation, he just used a software bending digram using wall supports fixed on one side and hinged on

the other side with hinged support at the bottom and because this case is not listed on PCA tables, he is using the

software and that he design would stay the same. Please advise if you think the wall be structurally sound and stable.

I think it is underdesigned that way

 

[JedClampett]

Yeah, your wall is going to fail. Either hire a different structural engineer or stay out of it's way when it's filled.

 

[JAE]

The PCA tables are OK but may be conservative and the actual 3D analysis the engineer followed up with might be more accurate.

Nader - are you "reviewing" the design to provide quality assurance? What is your role here?
If you are doing the design review and you don't know how to check a wall like this then you should get someone on board who can help you with this.
We simply cannot, here, "advise if we think the wall will be structurally stable".

 

[msquared48]

Outside of the strength issue, there is the crack control issue here. Per ACI 350-06, the minimum vertical reinforcing for these walls is .003bt, or about .51 sq. in. (#6@10 Vert)

The minimum horizontal reinforcing, depending on the concrete strength used, would be anywhere from .003bt to .006bt. I suggest you get your hands on a copy if you are analyzing these tanks.

Mike McCann
MMC Engineering

 

[JedClampett]

I'm not sure of the experience/competency of your engineer. But tanks are deadly serious designs:
thread507-299397
thread507-296127

 

[slickdeals]

You can use FEA software, but if you are doing so you should properly capture the stiffness of the end walls and the base slab. I would think that the support conditions would be closer to fixed on 3 sides. Also don't forget the minimum requirements for reinforcing and worst case scenario of one of the tanks being empty.

 

[JedClampett]

I'm not a fan of the PCA publication and don't have it. But I've run FEM on walls and compared it to the "Moments and Reactions for Rectangular Plates" by the United States Department of the Interior Bureau of Reclamation and they match almost exactly. Now they don't have crazy cases like hinged wall, fixed wall and hinged bottom, but that's because no one would ever do that.
Another point: if you design the edges as fixed, you're going to have to dowel and epoxy the reinforcing into the walls and slabs. If the reinforcing is spaced at 6 inches, you're almost certainly going to have a reduction in capacity. If they're pinned, then the shear load must be considered (actually the shear load needs to be considered in either case).
I'm not going to do a remote design (for free) for a condition I don't have all the information for. Maybe nader is not telling us the whole story. But when I hear of a design that has a fixed edge, a hinged edge and a hinged bottom, I smell a design that is being incorrectly done.

 

[briancpotter]

A quick sanity check: ballpark moment capacity for the wall is 0.9 * 0.44 in^2/ft * 60 ksi * (0.9 * 11 in) = 235 k-in/ft.

Ballpark design moment based on PCA coefficients (assuming 3-sided fixed) is 1.3 * 1.7 * 61 * (18ft * 0.070 ksf) * 18ft^2 / 1000 * 12 in/ft = 660 k-in/ft.

Even assuming the PCA coefficients are conservative (which I wouldn't do), I'd treat any FEA analysis that said such a situation was ok as extremely suspect.

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.

 

[Splitrings]

I doubt the original tank wall is designed for moment that the divider wall will induce if only one tank is full. This is if the new connection is close to being fixed. Even if the connection is a simply a shear connection, I doubt the existing wall is designed for the tensile forces according to the new ACI 350.