ACI 318 states that field bending of rebar is not permitted unless approved by the engineer. Research has shown that (cold) field bending a bar less than a #7 usually doesn't have any negative effects to the integrity of the bar. If there is no significant yielding, cracking, or, or kinking of the bar, I would expect that it's not a problem. Does anyone else have any thoughts or experience on this matter?
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Field bending of rebar 1
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I have seen rebar bent in the field where an undersize dolly was used. The bar showed severe cracking. I would have thought that the requirement is partly to make sure that the minimum bending radius is maintained.
Stephen Argles
Land & Marine
Stephen Argles
Land & Marine
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boffintech
Civil/Environmental
The problem with field bending of bars is that the minimum bending radius is not maintained. The people bending these bars have no idea that a minimum bending radius requirement even exist.
Vertical dowels at footings and horizontal dowels at wall construction joints are often bent over at 90 to facilitate foot and/or machine traffic. "L" dowels at the tops of walls to catch intersecting slabs are often bent over at 90 to facilitate backfill placement. Never a good idea.
Vertical dowels at footings and horizontal dowels at wall construction joints are often bent over at 90 to facilitate foot and/or machine traffic. "L" dowels at the tops of walls to catch intersecting slabs are often bent over at 90 to facilitate backfill placement. Never a good idea.
I would recommend field bending when you have small bars such as #5 or #6 embedded in concrete and I beleive that the ACI code is favorable to this also.
General field bending on all bars not embedded in concrete should be done with a proper mandrel and template which most rod-bende companies have even if they don't know it!
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General field bending on all bars not embedded in concrete should be done with a proper mandrel and template which most rod-bende companies have even if they don't know it!
Regards,
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Field bending of bars up to #6, by skilled steel workers, is a common practice at many countries, especially for small jobs and field changes. For bars above #6, the workers will have difficulties to handle it, and shall be avoided.
Field bending is a common practice and is used to accomadte work on the site. Generally the designer does not take into account the work that is needed to install the conrete and the bars are bent to accomadate the work Generally 6's and down are bent. 7's and up get tough. If designers don't want bars bent, they should consider common constrution methods in their design. Most bars never get stressed to their design capacity, so it generally is not an issue.
Wait until you find out what they bend with. If the bar is already cast, such as a dowel, it won't get bent, it will get "persuaded". A sledge hammer is pretty effective at bending bars, when you don't snap off because of it.
Of course the bars always yield when bent; they are undergoing significant plastic deformation. I don't see any problem as long as you get the minimum radius. There are bar bending tools that bend the bar over a cylinder that meets the minimum radius upto a given size bar. The ones that pull the bar across a wheel as it is bent cause less strain in the bar.
Of course the bars always yield when bent; they are undergoing significant plastic deformation. I don't see any problem as long as you get the minimum radius. There are bar bending tools that bend the bar over a cylinder that meets the minimum radius upto a given size bar. The ones that pull the bar across a wheel as it is bent cause less strain in the bar.
Is there a rule of thumb for a minimum radius? Some multiple of the bar diameter perhaps? We are bending a load of #7 bars in the field, I looked at the outside of the bend and did not see any visible stretch marks or cracks, but I'd like to measure the radius they are using.
boffintech
Civil/Environmental
TABLE 7.2—MINIMUM DIAMETERS OF BEND
Bar size Minimum diameter
No. 3 through No. 8 = 6db
No. 9, No. 10, and No. 11 = 8db
No. 14 and No. 18 = 10db
Bar size Minimum diameter
No. 3 through No. 8 = 6db
No. 9, No. 10, and No. 11 = 8db
No. 14 and No. 18 = 10db
In my organization, we have a policy not to specify field bending for bars greater than #5. Often to bend a bar, a contractor has to put a piece of pipe on the end and pull on that. If the inspector doesn't know much about structural work, some contractors will try and heat them with a torch. At that point they are only about as good as 40 ksi material, maybe not that good.
I have been thinking about this problem before. Remember two things. First, for a bar to hold its shape, it has to be bent well past the yield point, otherwise it will just snap back into shape. Second, as it turns out, the bars are bent well past 10x the yield strain. Therefore, it is difficult to say "don't bend them too much," because I can't find someone who will define clearly what too much is. Just as an exercise, I recommend you get out a piece of paper and compute the strain in a typical bar bent into a standard hook. Compare that to the yield strain. It will open your eyes. It should also give you comfort that plastic bending can really occur for a long time before fracture. Good Luck & Merry Christmas.
I have been thinking about this problem before. Remember two things. First, for a bar to hold its shape, it has to be bent well past the yield point, otherwise it will just snap back into shape. Second, as it turns out, the bars are bent well past 10x the yield strain. Therefore, it is difficult to say "don't bend them too much," because I can't find someone who will define clearly what too much is. Just as an exercise, I recommend you get out a piece of paper and compute the strain in a typical bar bent into a standard hook. Compare that to the yield strain. It will open your eyes. It should also give you comfort that plastic bending can really occur for a long time before fracture. Good Luck & Merry Christmas.
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