Minimum Reinforcing in Concrete Tanks

[miecz]

In light of the recent discussion of minimum reinforciment of footings according to ACI 318 (thread507-185223) I'm wondering how other tank designers are interpreting the minimum reinforciment provisions for Envorinmental Structures in ACI 350.

I'm using the method found in the examples in the PCA publication "Rectangular Concrete Tanks", 5th Edition.

According to this publication, which is referenced in ACI 350, tensile reinforcement in tank walls needs to meet Article 10.5.1, as modified by 10.5.3, and. according to Article 10.5.4, meet Article 7.12.

The difference between ACI 318 and ACI 350 is that the minimum reinforcing ratio in 7.12 for a wall with 40 foot between joints is .0050, vs .0018 for such a wall in ACI 318.

Seeing that the wording of Article 10.5 is identical in both publications, it would seem that the minimum tensile reinforcing ratio for walls with joints at 40 feet is .0050.

However, the PCA Publication (Example 1,under "e") checks Article 7.12 with half the T/S reinforcing at each face.

So, again, I'm wondering how other tank designers are handling this for tank walls.

 

[rholder98]

I was of the belief that if you satisfy your flexural requirements, you then compare the total amount of steel to your temp/shrinkage minimum, for the whole section. However, 10.5.4 of 350 seems to suggest that you should have the minimum temp/shrinkage on each face. Now, after re-reading 7.12.1 of 350, I see that it says, "The minimum reinforcement provided in any direction and on any face shall be the greater of shrinkage and temperature reinforcement or flexural reinforcement." Sounds like minimum shrinkage and temperature steel on each face.

But then, a little further on, 7.12.2.2 says the shrinkage and temperature reinforcement must be distributed so that no less than 1/3 of it is on any one face. The accompanying commentary says that the s/t reinforcement is typically divided equally to each face. This points back to my original argument, and also agrees with the PCA book.

So, after all that, I have come full circle, back to my original position. Satisfy your flexural requirements, then compare the steel in the whole section to your minimum temp/shrinkage requirement.

 

[hokie66]

I think most of the confusion of the engineers in the earlier post is due to failure to distinguish between flexural reinforcement and direct tension reinforcement. Flexural reinforcement is to provide strength, while direct tension reinforcement in things like floor slabs and tank walls is to limit crack widths.

It is perhaps unfortunate the way ACI and other codes commingle the two, but engineers only need to keep in mind the distinction, and provide for both. In walls where a strong degree of crack control is required, .0018 is certainly not enough. So I expect your .005 is what is intended. The Australian code makes this .006, placed half each face for walls greater than 200mm thick.

 

[miecz]

rholder98-

Excellent point. Both of these Articles are modified from ACI 318, and, they seem to completely change the meaning. I agree with your interpretation of Article 10.5.4 of ACI 350.

Because you didn't mention formula (10-3), do you believe that 10.5.1 states that Article 10.5.4 should be followed in lieu of formula (10-3) for a tank wall? Article 10.5.1 of ACI 350 is identical to that of ACI 318, and the upshot of the previous thread was that (10-3) didn't apply to slabs of uniforn thickness. However, the examples in Munshi's book check the provisions of 10.5.1, 10.5.3, and 10.5.4.

 

[rholder98]

I think 10.5.1 does apply, because it says "At every section of a flexural member. . .", which, for a tank, includes walls. So (10-3) applies for each face, and not less than 200/fy. Also, it need not be greater than 4/3 that required by analysis, per 10.5.3. However, the reinforcement provided on each face has to be at least half (actually, one-third) the total temperature and shrinkage steel required according to gross concrete area.

I think the same goes for slabs in this case, as well, when they are treated as structural mats.

 

[miecz]

hokie66-

I agree. Tank designers think a little differently from the rest. That's why I started a new thread: to discuss this issue as it relates to tanks.

 

[miecz]

rholder98-

OK, so we seem to agree on all counts. That is, I've been designing tanks the way you describe it.

Next, I've noticed that Chapter 13 applys to "walls reinforced for flexure in more than one direction." Do you agree that a tank wall, though free at the top, qualifies as a two way slab?

One of the posts from the previous thread argued that two way slabs may have less stringent requirements than a one way slab. Article 13.3.1 makes no mention of Article 10.5.1, but brings the designer directly to 7.12. Do you think that the minimums of Article 10.5.1 are implied by the phrase "determined from moments at critical sections?"

 

[vincentpa]

I would follow both 10.5.1 and 10.5.4 ACI 350. Whichever is more stringent would govern. ACI 350 is quite clear in 10.5.4. It seems a little much to use 0.005 on each face but ACI is the law when adopted by the specs. Munshi is only an author. His book does not hold any weight with the building code. If you go to court if something goes wrong and say you used Munshi instead of ACI 350, your insurance will be the party paying. I would use ACI 350 10.5.4 & 7.12 literally until ACI clarifies things. It's a legal world sometimes, not only an engineering one. I am now modifying an "old timers" design because he did not follow ACI 350 exactly. His design seems sound but the law is the law.

 

[rholder98]

vincentpa, I disagree that 10.5.4 is "quite clear". It says to use the minimum required by 7.12, but again, 7.12.2.2 says to distribute the temperature and shrinkage reinforcement to each face, so I don't think it's telling us to use the entire 0.005 on each face.

Take a 12" wall, d=9". 10.5.1 says you need at least (200/60k)(12)(9)=0.36 in2/ft on each face. I interpret 7.12 to say you need at least (.005)(144)=0.72 in2/ft total. If you placed this on each face, you would double the required steel. That does not make sense to me.

Also, Munshi is on the voting subcommittee of ACI 350, so I would hope we could trust his interpretation of the intent. I agree ACI could use some clarification.

jmiec-
A tank wall, free at the top, does experience flexure in two directions, if the plan geometry agrees. That is, a really long tank would behave more as a cantilever, until it reaches the corner.

I do believe 10.5.1, .3, and .4 are implied in 13.3.1. Here again, it refers to the minimum "in each direction" stated in 7.12, which I believe is total, not each face.

 

[miecz]

vincentpa-

I've come around to the thinking that tank walls are two way slab systems, covered by chapter 13.

Article 13.3.1 simply states that "Area of reinforcement in each direction for two way slab systems shall be determined from moments at critical sections, but not less than required by 7.12.

I read that to mean total reinforcement, not tensile reinforcement.

I agree that some authors misinterpret the code. However, Munshi's book is published by the Portland Cement Association, so it carries a lot of weight with me.

 

[miecz]

rholder98-

I think Article 13.3.1 implies the use of 10.5.1 and 10.5.3, but the wording of 13.3.1 overrides 10.5.4, since, Chapter 13 specifically addresses tank walls (13.1.1), while Chapter 10 seems geared towards one way slabs.

 

[vincentpa]

To All,
I understand what you are saying but 10.5.4 refers to 7.12 and thus 7.12.1 states that the reinforcement is on each face. That is "quite clear". 13.3.1 states the minimum to be 7.12 in each direction. 7.12.1 states each direction to have the minimum temperature and shrinkage steel. I think that there should be a clarification by ACI because our "guts" tell us it's wrong. But, I think the text is clear. Maybe they are unaware there needs to be a clarification or the way in which the section 7.12.1 is worded is not correct. Maybe that was their intention to be conservative. I don't know.

For two way design, as you state, the minimum temperature and shrinkage steel is normally 0.0018 for a slab in 318 and that is for one direction at that face. If you read the commentary and PCA Notes, you will see that the minimum refering to T&S steel is only arbitrary and is not meant for both faces but for only one face to ensure that a catastrophic failure is avoided. That is, if your minimum bottom steel is less than 0.0018, you would you 0.0018 for the bottom steel only, not top and bottom steel. This minimum only ensures an unreinforced section is not created. This is one of the biggest confusing subjects of ACI 318, referrring to the minimum steel as the same as T&S steel in 7.12. ACI SHOULD NOT DO THIS. The minimum should be referenced and called something else since it has NOTHING to do with temperature and shrinkage. It only causes confusion among us engineers.

As for Munshi being on the committee, remember that it is a committee. I have been to the ACI seminars taught by members of the "committee" and I have stumped them with questions about THEIR "committee." The problem is sometimes the committee itself. In grad school, I sat two committees drafting documents for the state. What a mess!

I have been tempted to contact Munshi. I tracked down his current firm once, but I lost it. He is easy to find with a search on Google. I am facing this problem right now with the minimum steel, but I have a rush deadline so I am going the conservative route. If anyone contacts him and gets his take, post it.

I am interested in JedClampet's take. He is a tank designer with a lot of experience. He may have a good take on this matter.

 

[miecz]

vincentpa-

Thanks for sharing your thoughts. I originally intended to simply find out how others interpreted the code, and to avoid a debate on who's got it right. Sorry if I got off track. Short of a letter from the code commitee, I'm not sure this will ever get resolved.

For this discussion, I would like to avoid reference to ACI 318 and PCA Notes for 318, since, as others have pointed out, Articles 10.5 and 7.12 are completely different from ACI 350. Besides that, there was a run on aspirin at the 318 pharmacy.

Yes, I was hoping to hear from JedClampett as well. Perhaps he's at the pharmacy, too.

 

[vincentpa]

I'm relatively new to environmental structures and tanks but I have done a lot of concrete design over the years. ACI 318 leaves a lot to be desired in some cases. ACI 350-01 was the first attempt at making a legal document like ACI 318 to used as a code and specification. I have the new ACI 350-06 but I haven't cracked it yet. Things may change some day. But, since life with ACI is a SNAFU, I doubt. There will probably be opening an ACI 350 pharmacy near you soon.