How to place minimum steel in a rectangular footing? 14

[pob11646]

I have got a question about the placement of minimum temperature and shrinkage steel, say for a 35" thick rectangular footing 14 feet long and 5 feet wide. Let's say, that all I need for my footing is minimum temperature and shrinkage steel.

Hence, Ast = 0.0018bh = 0.0018 * (14*12) in * 35 in = 10.58 in2.

Say, I am using #7 bars, thus, I will need 10.58 in2/0.60 in2 = 18 # 7 bars.

Method 1: Or, say I place bars in the top and bottom layers. For my bottom layer, do I need to place 14 #7 parallel to the short side, and 4 # 7 parallel to the long side. And do the same for the top layer.

Or, Method 2: I still place bars in both the top and bottom layer. For the reinforcment parallel to the do I just need to place a total of 18 #7 in both the top and bottom layers, say 9#7 in the bottom layer, and 9#7 in the top layer.

For Method 2, the Ast required parallel to the long side will be 0.0018 * (5*12) in * 35 in = 3.78 in2, or 7 # 7 bars. Do I place a total of 7#7 bars in both the top and bottom layers, say 4#7 in the bottom layer, and 3#7 in the top layer.

Method 3. Not to confuse matters, but can I place reinforcement in the bottom layer alone, say a total of 18# 7 bars, say 14#7 parallel to the short side, and 4#7 parallel to the long side.

Please advise whether Method 1 or Method 2 is more appropriate. And is Method 3 practical?

 

[oneintheeye]

no dont you put all the min tensile on tension zone and whatever is required in the top (or bottom) for T and S? Half total or more/less if thick or thin.

 

[miecz]

The lesson I take from this is that one needs to be very clear when writing, because the reader's interpretation is going to be clouded by what he/she thought before reading it.

I am impressed by the clarity of JAE's question to ACI. However, the response is disappointing. Why, if this is what they meant, couldn't they have answered question 2 with something like: Yes, 10.5.4 negates the use of 10.5.1-3 for structural slabs and footings. Case closed. How about a little discussion as to how two seemingly unrelated phenomena have the identical requirement? I wish they had written something logical, rather than authoritative.

 

[JAE]

miecz,

I feel the same disaapointment in ACI's reply - but as was mentioned earlier, these are tech questions that are answered by non-committee members. But their reference to MacGregor's book helped. MacGregor is on the 318 committee.

 

[BAretired]

In building construction the amount of such steel is usually arbitrarily chosen as 0.002 to 0.0025 of the slab area

The above is a quotation in reference to Temperature and Shrinkage Reinforcement from "Introduction to Reinforced Concrete Design" by Hale Sutherland and Raymond C. Reese (Copyright 1943...Second edition 1953. Mr. Reese was Chairman of ACI 318-63.

BA

 

[ali07]

I agreeed with BA, the top face of thicker footings shall not have without any reinfcrcement, or otherwise make the footing tapered (If not uplift) at the end or otherwise expect cracks.

But what is the limit at which footing acts as thicker?

 

[hokie66]

I don't think BA said that. There is no reason at all to place reinforcement in the top of a footing if the top is in compression. Invariably, top reinforcement in thick footings CAUSES plastic settlement cracking due to inadequate consolidation.

 

[StructGen]

JAE, as I have said again in my previous post, the answere you have got from ACI is not the official standing of ACI please do read the disclaimer on website. This is some one's interpretation of code not the opinion of ACI-318 Commitee. What you got is nothing more then a personal opinon of you me or any one else.
Have you had a chance to talk to Mr Frosch of 318-commitee member?

I am post disclaimer of ACI for you guyes

TERMS AND CONDITIONS OF USE

The technical information provided by ACI’s technical staff (“Information”) is a free service. This Information is intended only for the requester. The Information is based on the personal knowledge of the technical staff and does not represent the official position of ACI. ACI assumes that the requester has the skills and experience necessary to determine whether the Information given by ACI is appropriate for the requester’s purposes, and to use and implement the Information. ACI recommends that the requester retain the services of a qualified professional when implementing any Information. Requester agrees that any Information is used or implemented at Requester’s OWN RISK.

ALL INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT. Neither ACI nor its subsidiary corporations, affiliates, principals, officers, employees, agents, and their successors (“Releasees”) shall be liable for damages of any kind, including any special, indirect, incidental, or consequential damages, including without limitation, lost revenues or lost profits, which may result from the use of any Information.

In exchange for receiving Information from ACI, the requester releases ACI and the other Releasees from any and all actions, causes of action, claims, demands, damages, charges, appeals, expenses, compensation, including compensatory, exemplary, statutory and punitive damages, attorneys fees, costs, suits, debts, sums of money, contracts, liens, controversies, and agreements whether known or unknown, that may arise from the giving or use of the Information (“Claims”). The requester also agrees to indemnify and hold the Releasees harmless from any and all Claims brought against the Releasees by third parties at any time in the future.

Do you accept the above Terms and Conditions of Use?

 

[hokie66]

We have to remember that the ACI code (like almost all codes) is the work product of a committee. Like the old saying, "a camel is a horse designed by committee".

 

[JAE]

StructGen, no - I haven't had time to contact anyone else such as Mr. Frosch.

I understand your mention of the disclaimer - but when I find in MacGregor's textbook an application of that section that concurs with the tech response, and two places in his book that confirm a tensile use of 7.12, then I'm convinced. You can believe what you want, OK?

I appreciate the debate here in this thread and on Eng-Tips in general.

 

[rowingengineer]

Must say this is a great debate and very interesting. This could go on forever I think what needs to be taken into consideration is:

1. The supporting sciences for codes do not always provide crisp answers to engineering problems, building requires the use of judgment. Judgment has soft boundaries and is influenced strongly by what is considered to be acceptable risk.

2. THE PURPOSE OF THE CODE: the main function of the Code is to keep people out of trouble, to make structures safe, to make it difficult for somebody to design an unsafe structure.

As Hokie point out, the committee that developed the code decided what was acceptable risk and this committee also decided what science to use to develop the rule.

Thus even if the committee chairperson said that "Clause 10.4 is ....". You could still debate this interpretation, based on other knowledge ect.

Based on what I consider acceptable risk for my designs, I would use definition used in MacGregor book. and sleep well.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[oneintheeye]

Stru gen you are rejecting mcgregor approach on the basis he is only one member of the commitee and so not neccessary the commitee view but still hold fast that Dr ghosh is absolutely correct despite only being one member? I would suggest going with mcgregor as also the technical help states as this is the more conservative route and rest easy. If you were in court and a lawyer asked ACI for technical help they would get that response, maybe worth remembering.

 

[miecz]

herewegothen

Problem is, in many cases, the Macgregor approach is not the conservative route. At this point, I'm left with doing it both ways, and picking the more conservative of the two. Oh, well, it's not my money...

JAE

Not only does Macgregor have it that way, but every textbook I've looked at has it the same way as Macgregor, except for PCA's "Rectangular Concrete Tanks", which follows ACI350. My wife says I can be very stubborn.

 

[BAretired]

How does ACI318 define flexural cracking moment? What area of steel is required to resist 1.1 to 1.5 times flexural cracking moment?

BA

 

[80smetalfm]

AS3600 only deal with concrete strength of up to 65 mPa. Having said that its only a matter of months before the new AS3600 issue is out which i understand caters for up to 100 mPa concrete.

Going back to footing reo,be careful with placement of tensile reo at correct face as footing with two columns require flexural steel at top face! (inverted beam).

 

[BAretired]

The 35" deep footing supporting two columns described by the OP is more of a beam than a slab. Minimum reinforcement should not be taken from the slab provisions of the code but from the beam provisions.

BA

 

[hokie66]

BA,

We would have to agree to disagree on that. A footing is a footing.

 

[canstructural]

Hokie,

See this attached example. It's copied from "Design of Concrete Structures" by Arthur H. Nilson & David Darwin (Cornell and Kansas Univ. Professors), it's a text book for engineering graduates in some of the universities.

Anyhow the idea and the way he is treating the slab as beam spanning between supports make sense.

 

[hokie66]

The Nilson and Darwin example supports BA's opinion as to which minimum reinforcement provision should be applied in the case of a strap footing. At least the example doesn't show reinforcement being used on the compression face where none is required.