Rebar in arch 1

[WARose]

I have to do a reinforced concrete arch. In the past, I've noted on the drawings to permanently bend the rebar to match the geometry....and ergo reduced the available strength based on the cold working (i.e. as per AISI). However in this case, I doubt that is feasible.....so I am kind of left wondering: what do I do about the stresses in the bars from matching this geometry? They are pretty high (about half of the yield strength). Maybe just reduce the available yield strength based on this?

 

[dhengr]

WARose:
Do you reduce the strength of the bars, or the allowable forces in them, when they are bent or hooked in normal concrete design and work? Of course, you can bend them tight enough to cause cracking or failure. You should, of course, account for the change of forces in them due to the geometry change. What is the radius of this arch? If the bars are bent to conform to a tight arch radius, they will have been bent beyond their yield strength or they will not maintain their new curved shape, so they have been cold worked to some extent. What does AISI say about this, I’m not sure what you are referring to on this account. On most of the arches I’ve worked on, you essentially laid the rebar in the forms, and it more or less deflected to match the form shape, maybe some tying down was needed, so you must be dealing with a fairly tight arch radius. In forming the rebar to a large radius, you are not working it very far above its yield strength Fy; so in unloading it in the rolling machine, it essentially goes back to zero stress, along a like (same general, parallel) slope, a stress/strain slope of ‘E’, from the slightly higher stress point (F1) on the stress/strain curve, with some residual stress in the other layers of bar, that’s called cold working. When you reload the bars in the conc., they move back up the new (same general) ‘E’ slope curve, with primary stresses going up from mostly zero to point F1, and then they continue up the original stress/strain curve, where you will now be working at a slightly higher bar stress, but still well below ultimate. You should always be working in the lower portion of the strain hardening range of the material, and you will stay below the tensile strength of the material.

 

[KootK]

My approach to do this would be to give it no account at all. I believe that a ductile material such as rebar can "pull through" it's initial state of stress such that you wind up with the same [As x fy] that you'd normally have. I don't doubt that there as complexities, however, such as:

1) Your bar stiffness curve up to full yield would look a bit different.

2) Moments locked into the bars initially would be transferred to the concrete somehow.

That said, I don't feel that either of these effects would be appreciable. And they make my head hurt.

 

[WARose]

[blue](Kootk)[/blue]

That said, I don't feel that either of these effects would be appreciable. And they make my head hurt.

I had a look at the forces that it takes to bend 'em where you want them.....and I don't think it's going to be an issue. So I definitely agree with your statement (as far as #2 goes).

It's not that the stresses are a deal breaker.....just something that nags at me that I feel I should account for.

As always, thanks for the feedback.

 

[IDS]

I frequently work with arches with a radius of the order of 5-10 m, and I never reduce the design yield stress. When placed in a reinforcement cage there is no external longitudinal force, and any locked in bending stresses would be much less than for corner bends. Also shrinkage and differential temperature effects would be greater, and these are also ignored for ULS design.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JAE]

Hey we bend reinforcing bars to tiny (relatively) bend radii for hooks all the time.

A paper on some research on grade 60 and higher bars:

https://www.acifoundation.org/Portals/12/Files/PDFs/CRC-91-BarBending-Final-Report.pdf

A simple Q&A on the question:

https://engineering.stackexchange.c...ties-does-bending-reduce-iron-rebars-strength

An article on the effects of bending and straightening:

https://www.academia.edu/9618292/Ef...ng_to_the_Strength_of_Reinforcement_Steel_Bar

I didn't find anything on ACI's site nor CRSI's because, well, their search functions suck.


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[WARose]

Thanks JAE. I'm not talking about permanently bending bars. (If that was the case, I wouldn't give it a second thought.) It's forcing such a situation where stress is permanently superimposed that makes me second guess myself here.