Rectangular Tank Corner Reinforcement

[NoName1]

I am working on a rectangular SBR tank for a wastewater treatment plant, and I reviewed some standard details on drawings we have by another engineering firm. For liquid containing tanks they add extra steel, same size and spacing, at the corners offset with the main reinforcing.

Is this something that is normally done? I did not see it mentioned in PCA's Rectangular Tanks.

 

[JedClampett]

Yes, this should be done. If you do the design of a rectangular tank, the edge moments are usually much higher (and in the opposite direction) than the midspan moments. And they drop off very quickly. So to use the same reinforcing for the entire wall as is required in the corners would be wasteful and might cause congestion issues. So we add interior bars with a hook at the corners. I usually try to use the same bar size as the regular horizontal reinforcing to be a little less confusing. These continue a short distance from the corner, like 4'-0".
The PCA's Book is pretty tough to use. If you can get a copy of the Bureau of Reclamation's "Moment and Reactions for Rectangular Plates" (out of print), you'll find it more useful.

 

[LobstaEata]

Jed is right on the mark. Tanks are designed as plates restrained along three edges for construction and test loading conditions and along all four edges for in-service loading conditions.

I've also seen inside corner chamfers at wall intersections as well. Diagonal corner bars are lapped with horizontal bars at corners of some of the larger tank designs.

 

[hokie66]

If opening moments at the corners are high, "normal" reinforcement configurations are not efficient in carrying the tension force around the corner. This is because a diagonal tension force must be developed across the corner, and the concrete alone is not adequate to develop the force in tension.

There was a recent discussion of this issue in this forum. I suggest having a look at the comments, as well as reviewing some of the papers quoted, a number of which are by Nilsson et al.

 

[hokie66]

I found the thread which discussed the corner reinforcing.

Thread507-228698

 

[NoName1]

Thanks for the responses. I will look at the other post.

What would be the reason that these standard details did not use the extra steel in non-liquid containing tanks?

I did find a copy of the "Moment and Reactions for Rectangular Plates" online in pdf form.

http://www.usbr.gov/pmts/hydraulics_lab/pubs/manuals_monographs.html

 

[hokie66]

You cannot always infer from the reinforcing arrangement how the structure is actually modelled. For instance, a tank can be designed with the assumption that the corners form plastic hinges, and the reinforcement in the span carries the moment left over after the hinges takes all they can.

 

[234eng]

Could someone please post a sample how to use the Moment and Reactions for rectangular Plate?
Long time ago, I used these tables to obtain moment and reaction. Now I look at this table and a little lost.

 

[NoName1]

I read the other thread and realized I was unclear in my original post.

I am designing a tank, but I was looking at the details on some old drawings we have for ideas.

I understand the larger moment in the corners and that extra steel is required for the tensile force due to the shear in adjacent wall.

The details I was looking at show the reinforcement required by analysis on the main drawing sheets. Then the details show extra steel (not required by analysis) only for liquid containing tanks.

So if by analysis I need #5 @ 12, is it common practice to use #5 @ 6 if my analysis shows I do not need that much?

 

[kslee1000]

Involved many years in hydroelectric/waste water/drink water retaining projects, never used any of those corner reinforcing details mentioned above. Our practice is to keep stress low in both concrete and the reinforcing steel (select thiker walls/higher reinforcing ratio with smaller bars in a closer spacing), maintain adequate concrete cover (often resulted in thicker walls), and keep the reinforcing configuration as simple as possible to avoid construction mistakes, and congestion that may cause construction difficulties and lead to poor quality of the final product.

At the corners, we simply provide L bars with tension splice length pass beyond the interior face of the transverse wall in both faces (fool proof), and keep all other bars straight. The location of splices can be staggered to further avoid congestion and concentration of weak plane. Fillet is used for thinner walls, or structures containg flow water with wave effect.

So far being successful.

 

[miecz]

MJA1-

Reinforcing required for water retaining structures is greater than that required for non water retaining structures, a result of the durability factor, "S", in ACI 350. So, when you say "the reinforcement required by analysis on the main drawing sheets", you need to say whether you mean required by analysis of a water retaining structure (covered by ACI350) or by analysis of a normal concrete structure (covered by ACI318). That said, if the reinforcing shown on the main drawing sheets meets the requirements for a water retaining structure (ACI350), then I see no need to add extra reinforcing on the detail sheets.

 

[NoName1]

Thanks for the responses.

Yes I do mean the reinforcement required for a water tank (ACI 350)when I said "required by analysis"

 

[miecz]

MJA1-

A question. Sounds like you are reviewing a facility where the designer has some tanks that are designed as water retaining structures and some buildings that are not. And, it sounds like he/she has economized by using the same details for both?

 

[oneintheeye]

I suppose it depends on the size what becomes economical but I would concur with kslee1000, just use L bars around corner with full laps (staggered) and keep corners simple. If you can reduce the steel in centre of further up wall then do it.

 

[vincentpa]

The ACI detailing manual has typical tank corner details. However, it doesn't offer any explanations as to why they are doing what they are doing. There isn't one reference out there that gives complete information on tanks from design to detailing. PCA leaves out the detailing completely. If one of the guys on this site with a lot of experience ever wrote a design guide for tanks, I'm sure it would be a best seller. There are so many threads on this site regarding concrete tank design. Think of all the engineers that are just winging it.

 

[dinjin]

Isn't it true that liquid containers have greater
pressure on the sides as the depth increases vs
dry containers?

 

[jike]

Be sure to consider both cases when the tank is hydrostatically tested with out backfill and then when it is backfilled and empty. This is why we provide hooked bars in both layers.