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Minimum width #7 hairpin 1

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kfc510

kfc510

Structural
May 14, 2021
9
bar_ce4n0p.jpg


Rebar fabricator is saying I need a minimum inside dimension of 10 1/2" (2x the corner bend diameter) for a set of #7 hairpins with 90 degree bends at the belly. I'm trying to understand why they're saying that.

Any pointers? Similar bars shown in pic.

thanks
kevin
 
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The fabricator is right. You need 12 bar diameters from the centerline of one bend radius to the next for a #7 rebar. I don't have my CRSI manual handy but will check it to provide reference in a day or so unless someone else here can provide quicker.

 
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For the why, bending rebar forces it beyond its yield limit and causes permanent, plastic deformation of the steel. If you continue bending it to a tighter and tighter radius, you'll start distorting the shape of the bar in such a way that small cracks will begin to form in the material. They likely won't be visible (unless you really bend it tight), but they'll result in stress concentrations that can cause the bar to fracture suddenly in tension.
 
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I understand the minimum diameter (5 1/4" in this case) pin for the corner bends, I just don't understand the 12d minimum width across.

I'm conceptually struggling with the idea that the bend from one corner is somehow continuing along the "belly", or short piece, on the end past the radii. For instance, if I wanted an interior width of 9" (I do!) there'd be ~3 3/4" of straight bar to play with there? Bend one corner at pin position 1, scooch it over to pin position two and bend again?
What am I missing? Something obvious, probably.
pins_dywnbr.jpg
 
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Wouldn't a standard hook be in order? or do you need to make a 'cage'? To me, a hairpin is a single bend.

image_ycjpp2.png


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Dik, it's meant to fully surround a moment column embedded in a grade beam.
u_bar_double_bend_around_column_sca3ss.jpg


Full disclosure- I'm not the engineer, I'm the ditch-digger.
 
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The last time this came up the fabricator wanted to do a 180 with a 9" pin, which worked fine in that case as the column was far away from the existing footing. When the column is located very close to the existing footing the extra protrusion of the single bend U exacerbates a loud, jarring,dusty problem

single_pin_u_bars_behind_column_cutdc7.jpg
 
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Probably a physical limitation of their bending equipment, then. Once the first bend has been made, the piece doesn't fit in the machine at the 9" spacing you're looking for.
 
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PhamENG, that's my guess as well, but it's just a guess. They're a legit operation, I should probably just listen to them.

Here's a couple of their monster bending tables
mcr_benders_jtrzhg.jpg
 
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Yep. Looks like they may have done it a time or two before today.

Good luck. I usually try to detail bent bars such that the minimum bend diameters don't overlap for just that reason. I've never had a call like this so I'm guessing its worked out so far.
 
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The weird thing is last october they delivered the pair in the first pic, with two 90 degree corners and ~9" spread. Either they cheated on the corner diameter that time (I didn't check, and those pieces are buried like Jimmy Hoffa now), or they found a trick, or something.

Now in 2021 they're saying "it can't be done!" And I'm like, "but, you have done it before."

I guess I should just accept my fate and order the single pin 180 bend and chip out the existing footing for the extra ~3" the U protrudes. Not like I haven't done it before. The crystalline silica gets old though.
footing_chipping_detail_for_u_bar_2_ndt7kg.jpg
 
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That looks a lot like a single 180 to me.

Just think of it as preserving your lungs for posterity.
 
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The last pic is a single 180, hence the Michaelangelo work at the footing...

The first pic is twin 90s. It's also the same bar wrapped snugly around the blue column.
 
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Got it. I see it now. Don't know. Maybe the machine they used on that one broke and the replacement can't do it? Or maybe somebody got creative last time and they're not with the company? Could be anything.
 
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If I'm understanding the sum of the comments, it either has to be a single 180 degree bend, or if it's 2 separate bends, they have to be 12d apart, due to the constraints of the equipment. Is that right?

Rod Smith, P.E., The artist formerly known as HotRod10
 
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That's the current story from the fabricator, BridgeSmith, though it is contradicted by the previously delivered U bar shown in the first pic which was #7 with a 9" inside-to-inside dimension.

Also, Ron posted above that the minimum 12d was a crsi requirement, but I haven't seen the citation for that.
 
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Here's the tag from the delivery last October, for S's and G's. The 11" is outside-to-outside, the manager at the fabricator always says the true bar diameter is 1/8" greater than nominal diameter.

Screenshot_20210513-130019_z5fwad.jpg
 
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For the purposes of making it possible for all of their member fabricators to be able to complete the work, trade organizations often give guidance based on the most limited capabilities of their members. There's obviously some rebar fabricators that can make bends much closer than the 12d, but I suspect a few CRSI members who still don't have the equipment to do closer bends.

Rod Smith, P.E., The artist formerly known as HotRod10
 
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