Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

[LD1]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

Diameter being equal, solid will be harder to bend. But not much more so. Its the extreme most fibers that are doing the most work.

That said, I wouldn't want rebar anyway if the goal is to have the targets swing. Get a solid rod.

 

[dragoneggs]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

Stiffness is a cube function. Twice the dia will be 8 times as stiff.

Yes, basically... and not to be a nerd here, but properly, Stiffness is generally linear at least when you are working in the elastic zone. Force (lb) / deflection (in) = Stiffness. And yes, deflection is largely driven by the moment of inertia (a cubed function) but you must also factor in the modulus of elasticity of the bending member. For instance, a square tube with the same material and height equal to diameter of a round tube with same wall thickness, is more stiff. And a steel member the same diameter (geometry) as an aluminum member is much more stiff. So... you have the shape to consider but also material. I will go back to my hole now. :ashamed:

 

[FTG-05]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

Diameter being equal, solid will be harder to bend. But not much more so. Its the extreme most fibers that are doing the most work.

That said, I wouldn't want rebar anyway if the goal is to have the targets swing. Get a solid rod.

Probably won't use it either. Lowe's didn't have either 5/8" or 3/4" rebar or 3/8" black iron pipe. So the next time I go to my steel supplier, I'll get some round rod instead of rebar.

I'll probably also see if I can bend those legs a bit more to make them more vertical. Didn't have time to do it right, so I guess I have time to do it again.......

 

[Sodo]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

Rebar is bent all day, every day all over the world. Nobody grinds or uses heat (to my knowledge,,,, but I've been wrong before). But you certainly could do that (if there are no construction workers watching),

Not sure what you guys are getting at. FTG's project doesn't have any bends where breaking is a remote possibility.

FTG you don't have to go to the store. just straightening up those legs more vertical (where they don't add bending to the support rod) will help a lot. Try just setting that cross support rod on a couple 2x4 stakes (wood) you'll see what I mean.

 

[GLyford]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

I put rebar in some sonotubes over the weekend, 4 of the holes used those square plastic footer forms at the bottom. So I needed a handful of 2' long pieces for bottom bars and for extensions on the top.

I had a bunch of 4' pieces on hand, some older, some newer. The older stuff bent into j's easily, and I could cut it 1/4 way through with a hacksaw, and it would bend there and then fatigue crack. The newer stuff would not bend at all by hand without a cheater, was twice as hard to score with a hacksaw. I had to score two sides to get it to break, and then it snapped like glass, no bend at all before breaking.

So while both batches met the minimum strength rating for rebar, don't count on all of it behaving the same as what you are used to.

 

[Sodo]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

GL, good point - there are different grades of rebar and the stronger grades are for tension, not to be bent in a tight radius. And would crack when bent in a tight radius. I don't anything about the grade never had the need to study it. But I bet you can only get the lowest grade at "the store", and I'd expect you should be able to bend it in almost zero radius without cracking.

 

[SPYDERLK]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

Rebar is bent all day, every day all over the world. Nobody grinds or uses heat (to my knowledge,,,, but I've been wrong before). But you certainly could do that (if there are no construction workers watching),

Not sure what you guys are getting at. FTG's project doesn't have any bends where breaking is a remote possibility.

FTG you don't have to go to the store. just straightening up those legs more vertical (where they don't add bending to the support rod) will help a lot. Try just setting that cross support rod on a couple 2x4 stakes (wood) you'll see what I mean.

I've had problems bending 1/2" rebar with it snapping, so I didn't try to bend those legs too much and having it do the same; otherwise they would have been a little more vertical. If 3/4" rebar doesn't work, I may have to add a vertical rebar piece to each one, we'll see.

Definitely not one of my usual "way overkill" jobs.

ETA: thanks for the suggestions on the washers, I'll try that!

It will depend on bend radius and degree to diameter. The cold working that forms the ribs hardens the surface of rebar somewhat. Additionally the ribs present a thicker section inhibiting bending there and presenting a stress riser immediately next to each. Ideal to initiate a CRACK.

 

[SPIKER]

Re: Which will bend less or be more rigid: 3/4" rebar or 3/8" black iron pipe?

never had much issue bending rebar, usually just 1/2 and 3/8" and rare anything heavier but remember cutting a LOT as a kid & bending a lot into J hooks as well. last summer I rolled and cut/welded up a bunch of 3/8" into a LARGE 14' wide arch for a front yard trellis. Rebar has been made form about any scrap available so it does have a lot of defects and batches can be worse than others.

on my page 1 pics where I made my target I used 1/2" washers welded to the ends and framework to make it collapsible target holder the 1/2" pipe nipples slide easy on the 3/8 rebar and have not rusted into place in 3 years setting outside. The washers between the pipes help reduce friction between pipe sections. Not sure where the materials used in OP came from but the stuff I built has about 40 lbs total and nothing bends... seems to me maybe way it was welded up heavily caused the warping.?


Mark