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# Bent Plates and Minimum Bend Radius

## Bent Plates and Minimum Bend Radius

(OP)
I don't understand the "minium bend radius" concept. In the ASD, page 4-174, a 1/2 in A36 plate has a minimum bend radius of 1.5 X thickness.

Let's say I'm designing a skewed connection and I need a bent plate, does the minimum bend radius mean I HAVE to bend the plate 1/2 in?

Halp!

### RE: Bent Plates and Minimum Bend Radius

vooter,

It is just the radius of the bend.  Not the angle or offset.  Lets say you bent the plate 90 degrees.  Any connection would need to be held back 3/4" from the inside to allow for the bend radius.  Kind of like the fillet of an angle or other hot rolled section.  Hopes this helps.

### RE: Bent Plates and Minimum Bend Radius

I work on aircraft structure and not civil type structures.  I like to view this site to try to gleam some knowledge from the very experienced folks here.

Aluminum in aircraft has minimum bend radius of about 3 to 4 times the thickness (depends on alloy and heat treat).  Bend radius is the inside radius after bending.  Bending is typically done with a brake that has a tool to form the specified radii. If the sheet is bent to a tighter radius is likely to crack.  You can actually bend the sheet to a larger radius to ensure you do not crack the metal.  Steel can be bent to a tighter radii.

Hope I shed some light on the issue.

### RE: Bent Plates and Minimum Bend Radius

When bending steel plates, you to bend perpendicular to the grain not parallel. This will mitigate development of cracks in the steel plate.

Regards,
Lutfi

### RE: Bent Plates and Minimum Bend Radius

You might ponder that last statement in connection with square and rectangular structural tubing...

### RE: Bent Plates and Minimum Bend Radius

back to the OP ... 1.5*0.5" = 0.75"

i think a big factor in minimum bend radius is the temper (which may be evident in "A36", so excuse my ignorance).  if you bend it in the annealled condition (similar to O condition in Aluminium) and then heat treat to the required strength, you may be able to bend more tightly.

i guess really you can bend as tightly as you like, so long as you don't get cracks forming (and perform a crack check inspection).

### RE: Bent Plates and Minimum Bend Radius

If you do the math, the strain corresponding to the allowable AISC and AASHTO bending radii can be well beyond typical rupture strains.

Kinda scary.

(And yet we don't hear of bent plates fracturing all over the place in service.)

Hg

Eng-Tips policies:  FAQ731-376

### RE: Bent Plates and Minimum Bend Radius

(OP)
Thanks for the insights, etc.

I did some deep "web mining" for anything to do with bending sheet metal. I first went into my cold-formed steel books but found little (they're about using the fabricated pieces, not about fabricating the actual pieces). Then I started thinking about sheet metal fabrication and such. The best info I found was at http://www.massey.ac.nz/~odiegel/bendworks/. I downloaded their paper "Bending." It gave a formula that related a "bending allowance" to bending radius. It explained everything to me.

Basically, that 4 in x 4 in bend plate is going to be a little bit longer than 8 in. Intuitively I'd say "of course," I'd just never given the issue much thought.

### RE: Bent Plates and Minimum Bend Radius

The bend radius given in the AISC manual is for cold bending.  As mentioned earlier, the strain on the outer fiber is going to be large (it has to yield the material anyway) and may be near the accepted rupture limit.  I would recommend you consider this when allowing cold bending.  If you need tighter radii, specify hot bending.  I would recommend hot bending for some situations when cold bending might be allowed by the formula.  Good Luck.

### RE: Bent Plates and Minimum Bend Radius

Specify hot bending?

What exactly is the minimum temperature for specified "hot bending"?

I've gone looking for it and haven't found it.  The closest I got was a metallurgist saying that nothing below the transition temperature could be considered hot bending--but you can't go near that transition temperature and still be sure that you had the same grade of steel you started with.  I've taken to calling it "heat-assisted cold bending" instead, not that that really solves anything.

The other thing to worry about is bending the steel while it's in the "blue brittle" range (roughly 400-700F), which not only increases the likelihood that the steel will crack during bending but also can leave the steel embrittled afterward.

Hg

Eng-Tips policies:  FAQ731-376

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