Using ACI 318 in tanks design 3

[nader666]

Hi everyone,

I am a reviewing a tank design calculation submitted by a NYS Structural Engineer.

I do not agree with the calculation and I want to get your opinion about some of the assumptions in the structural

calculation report.

1. The report considered ACI 350 just a recommendation and based on that ACI 318 has been used to eliminate the

sanitary factor (1.3) and reduce the design moment on the tank walls.

2. The calculation is based on PCA tables but it did not follow the listed modification in the book calculation.

3. The calculation used minimum area steel equations listed in ACI 350 (As, min = ((3 x square root(f’c))/fy) x bw x d

≥ (200/fy) x bw x d) and then claimed that even the horizontal reinforcement is not meeting those equations, it is ok

because ACI 350 is only recommendation and the horizontal rebars area steel satisfies the minimum temperature &

shrinkage area steel listed in ACI 318!, As far as I know, the minimum area steel equation listed in ACI 350 is

adopted and listed in ACI 318 should be applied using both code. The horizontal reinforcement of the walls is

# 4 @ 12" .


I would appreciate your opinion.


Thank You
Nader Soliman, P.E.

 

[JedClampett]

You didn't say whether the tank was round or rectangular, nor how deep it is. But when I see #4's at 12 inches for any water bearing structure, my Spidey sense is alerted.
As a summary:
Eliminating the 1.3 sanitary factor-BAD IDEA
Not using minimum bending steel-BAD IDEA
PCA Tables-OK, if all the errata are considered. Bureau of Reclamation has better tools.
This sounds like a rookie design. A tank is no place to economize. Us hairy eared engineers know that.
I'd make my comments, note them as important and keep track of the project. There will be a failure and when they blame the contractor, your notes will be valuable to their lawyers. See below:

http://www.pressconnects.com/articl...eport-Sewage-plant-wall-flawed?nclick_check=1

http://www.knoxnews.com/news/2011/oct/27/deficient-wall-construction-cited-in-gatlinburg/

 

[msquared48]

According to ACI 350-06, the minimum vertical steel is .003bt and the minimum horizontal is ,003 to .005bt depending on the strength of the concrete used. .005 for an 8" wall gives #5@7, while .003 gives about #5@12 or #4@8. The minimum is at least 50% more than you show.



Mike McCann
MMC Engineering

 

[hokie66]

Jed has it. ACI318 has little application in the design of water retaining structures. My own minimum for horizontal steel in these structures is 0.6% Ag.

 

[miecz]

My only experience is with rectangular tanks. If this tank is rectangular, it will leak like a sieve. At worst, it will fail outright. If you don't have the authority to stop this design, then you need to get it to someone who does. Failing that, make sure you are not present the day they test the tank.

 

[JAE]

miecz - I guess I've seen multiple uses of rectangular tanks for waste-water treatment plants and water treatment facilities. Just needs to be designed to the loads.

I would say that it's not proper to refer to ACI 350 as a recommendation but not sure how things like that are enforced. Most city/entities don't specifically call out in their building codes a requirement for 350 as they usually deal with buildings. That said, the use of 350 in tanks is so typical and prevalent in the US that NOT using 350 sort of takes that NY engineer out of the "reasonable standard" that we engineers are judged by. If the tanks do leak, as miecz suggests, then that NY engineer really has no way to testify as to why they didn't use it (when questioned by the complainant's attorney).

 

[hokie66]

IMHO, we should consider all codes and standards just as recommendations and guidelines for our work, and minimum recommendations at that, regardless of whether a code has the weight of law or not. If a failure occurs, the code won't give you much comfort.

 

[miecz]

I remember the first SBR tank I designed on my own. I designed for the loads, and followed 350, except for the spacing of wall construction joints. I believe I had them at 60 ft whereas 350 recommended 40 ft. Tank leaked...a lot. Expensive epoxy injection to stop all the leaks. Unhappy client. Next tank had the joints at 40 ft. No leaks. ACI 350 has much more than minimum shrinkage/temperature steel, and much of it has to do with preventing leaks that 318 need not be concerned with. I believe the recommendations are the result of past failures, specifically, leaky tanks. I suppose there are some engineers that have enough experience to ignore recommendations such as ACI350. I suppose the first step is to determine if this designer is really smart, or whether he/she slept at a holiday inn before doing the design.

 

[Splitrings]

What does the tank hold? Water, human waste, agricultural manure, hazardous waste...? I work for an agency under USDA that works with agricultural landowners nationwide. We have designed and installed a huge number of circular and rectangular tanks that were not designed to ACI 350. These tanks have and continue to perform as they were intended. I would say we have seen far more issues with rectangular tanks than circular tanks. This is due to the under estimating of moments due to restraints at corners and such. As for circular tanks I am not aware of a single tank that had significant cracking. This may be due to the loadings we use or maybe that our tanks are typically backfilled partially. I think the design needs to address the risk that is present. Obviously a tank to store hazardous waste needs to be constructed differently than one that holds cow manure. ACI 318 10.6.5 does state that 10.6.4 is not sufficient for watertight structures. Our agency doesn't assume our agricultural waste storage structure are "water tight". There is an assumed seepage rate for concrete structures.

Regardless of which shape the tank is, I can't recall a single design where analysis didn't require more steel than the minimum required in the horizontal dimension. If this tank is rectangular there will be significant moments in the horizontal direction and if it is circular there will significant tensile hoop forces. Something doesn't seem correct.

 

[briancpotter]

Regardless of whether ACI 350 needs to be used, it's certainly inappropriate to design based on ACI 318. Section 1.1.10 states "This Code does not govern design and construction of tanks and reservoirs".

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[JAE]

briancpotter - nice post. Good point.

 

[Splitrings]

The commentary to section 1.1.10 states, "This gives material, design and construction recommendations for concrete tanks, reservoirs, and other structures commonly
used in water and waste treatment works where dense, impermeable concrete with high resistance to chemical attack is required."

NADER666 hasn't told us what this tank is designed to hold or any serviceability requirements, such as being water tight. I think most just assumed this was a water or watewater tank.

I would agree that any water or wastewater treatment should be designed using ACI 350. The intent of these structures is that they will be watertight.

 

[msquared48]

You know, I can see the need for tighter reinforcing for buried tanks containing environmentally damaging material, but have a problem in using a blind sweep if the hand to apply the same standard to underground concrete tanks containing storm water only, especially ones completely buried. This is what ACI350-06 does. It is well to note that storm water detention tanks are not specifically cited in this code, but only alluded to through the t=water containment clause.

A few thoughts here...

1. In the Northwest, and I do realize other regions have different water qualities, we do not have serious fertilizer or caustic chemical concerns (the tank is not designed to remove them anyway), and other than sediment, the only effluents we commonly worry about are oil and gasoline.

2. I also have a problem with the time of water detention in the tanks, which is commonly only 24 hours, as opposed to a water tank or digester that is continuous. Other than the bottom of the tank, any cracks of which eventually become clogged with sediment, the water has little time to escape through the walls. So what if some does and the walls are not subject to freeze-thaw?

3. Moreover, buried tanks/walls/slabs are not subject to the same freeze-thaw conditions as non buried or partly exposed wall, receiving latent heat from the ground they retain. I can see possibly applying the minimums to exposed walls. but, to me, even that is a stretch.

Personally, I think that ACI 350-06 needs to give the design engineer more latitude based on the local site conditions. It already gives some, but needs more to be realistic in its application.

Mike McCann
MMC Engineering