Mixing rebar sizes 4

[Iasonasx]

If I can design a concrete beam and the area of steel I need is best suited by mixing two different sizes of rebar (let's say #10 & #11), which would be recommended to be set on the outer sides and which in the inner part of the beam?

 

[BigH]

I wouldn't - although not a structural type - you might consider that the closeness of sizes might make it a problem should the workers pick up bars from the wrong stockpile. Should it happen - NO. Could it happen - Yes. There was a good article in an Indian concrete journal about bar sizing.

 

[BigH]

Meant to say that I wouldn't mix bar sizes . . .

 

[SlideRuleEra]

Speaking as a former bridge contractor, take BigH's advice - don't mix rebar sizes - they will get mixed up on the jobsite.

Also, consider two different sized bars (say #10 and #11) mixed together for the same purpose. Under load, both will stretch the same amount in the beam, but the stress will not be proportionally divided between the two sizes in the intended manner. The #11 will be more heavily loaded than expected. Edit: My statement in this paragraph is wrong, sorry. SRE

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[Iasonasx]

"the stress will not be proportionally divided between the two sizes in the intended manner. The #11 will be more heavily loaded than expected"

That I was not aware of by anything official or published. Would you be kind enough to give me any references to that? I am very interested in this. I have to confess that I suspected that the size difference could make either the smaller or the larger behave differently than expected. Any literature/reference on this matter will be very appreciated.

I was expecting actually an answer in case I do proceed with the two different sizes and I had thoughts such as:

A) Put the smaller sizes outside because smaller will better integrate with the concrete and will have less chance of cracking when close to two surfaces instead of one surface.
B) Put the larger bars outside because in case of torsion they will perform better than the smaller rebars
And irrelevant to the above mentioned, ... 'Don't mix them because someone will place them in the wrong order" which I did receive.

 

[dhengr]

Iasonasx:
I think SlideRuleEra meant to say... The two bars will stretch the same, strain the same, so the stress in the two bars should be essentially the same. But, the forces in the two bars will be different in proportion to their areas. The primary thought trying to be conveyed is that to save a few bucks on steel, you should not do something (mixing bar sizes) which increases the potential of an error (rebar placement error) in the field. On the other hand, if the two bar sizes are significantly different in length or bending details, there is some less chance of confusing the two sizes during placement.

 

[KootK]

At ultimate, I'd expect the force in all of the bars to be As x Fy, regardless of bar size. While I wouldn't mix and match normally, I'd go with larger bars to the outside. I like the torsion argument and I also like to keep as much tension steel as close to the stirrups as possible. You may well have internal stirrups though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

Iasonasx - I apologize for the confusion I created. We are talking about "dissimilar sizes" and I was thinking "dissimilar materials".

dhengr - Thank you for "bailing me out".

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[Iasonasx]

Thank you everyone. Excellent feedback. Very appreciated.

 

[TehMightyEngineer]

I'll add a slightly different opinion. I would only mix bars if it was a precast job. If this is something that our company is building over and over then it's worth it to try to maximize the savings on rebar and labor tying bars. If I can save a good amount of steel or make it easier for the guys in the shop to tie then I'll mix bars. Once I get the guys in the shop used to doing it one way or another it's quite easy to get them used to a particular rebar layout. However, for one off things or something that I have no idea who's building it I wouldn't mix bars in the same layer. The simpler the design is to understand the more likely that there wont be errors during placing the bars. Even if it saves money on paper if they mess up the rebar and have to delay a pour then you've eaten up all your savings and then some.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[JedClampett]

I'd do it and not worry so much. Let's say I have a 35 foot long beam. I'd put the #11's on the outside, and the #10's in the inside. But to minimize the confusion, the #10's would only be 20 ft. long centered in the span, just in the area of maximum moment.

 

[IDS]

In my opinion mixed bar sizes are likely to receive closer attention both during placement and inspection, and if the wrong bar sizes have been used it will be more obvious, so I would say that if anything the mixed size arrangement is less likely to have an undetected error in the completed structure.

As for placement of the larger bar, I agree with Jed that it will normally make sense to have additional bars shorter, in the area of maximum moment, and obviously the full length bars should go in the corner, but there will be times when it makes sense to use the smaller bar for full length, and the larger diameter for the additional shorter bars.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BadgerPE]

When I use multiple sizes in retaining wall designs, I always opt to skip a size. So I would use #4/#6 or #5/#7 etc. It makes it easier in the field to verify quickly.