Determining Metal Tubing Strength for Tree Props

NickfromWI

NickfromWI

King of Splices
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Mar 30, 2005
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So I've been having a welder make us up these little props. I think we got the design down pretty solid:

Joan prop design.png

They basically take a piece of square tubing (usually 4x4), weld a plate on the bottom to be the base, weld a plate on the top to be cap, drill a hole in the middle of the cap, weld a nut to the top of the hole, then we screw in a piece of threaded rod (usually ⅝" or ½") into the nut at the top. Bam. Done.

We finally have a tree to prop that is much larger than all the others we've done. We're at a point where I'm wondering about the strength of the metal. I asked the welder and he said he didn't know and I could look it up and for liability reasons he didn't wanna say much more than that. I called a few local metal workers that specialize in structural work and they said they can make it and certify it to a certain strength, but that they couldn't give me the strength until the contract was signed and their price was like $1,000 more than what my welder charges.

I tried googling it, and I'm learning that I can't figure it out. I know the dimensions are 4x4, wall thickness 3/16" (which my welder says is way overkill, but the cost isn't much more than ⅛" so why not hedge our bets a little....and I totally agree with this mind set). I know there are 2 factors here: 1- the actual compression strength of the metal tubing and 2- how much force the treaded rod/nut can handle.

So all that being said, do you guys have any wisdom to impart to guide me (other than pay the $1,000!)???

love
nick
 
  • Thread Starter Thread Starter
  • #4
Well you guys are operating the way I'm operating...."that oughtta hold 'er!"

But there IS a limit of what the metal can hold. You take a big tree, add some dynamic multipliers for when the wind picks up, and eventually the metal would crush. Also, we have to take into account the height of the post. Imagine a 4x4 that was 50 ft long. That seems like it would be an issue, too.

Lemme go check out this chart Raj posted..
 
  • Thread Starter Thread Starter
  • #7
I just found this little calculator thing online. You put in the tube diameter and wall thickness, it tells you the strength. Can this be right? 133,000 pounds?

steel strength.jpg
 
At 50' in length deflection is your worry not compression. For liability reasons you should consult an engineer. Anyway, that square tube is probably a36 not 4130 but don't hold me to it. That chart states "round" not "square"
 
Do you know what load the brace will see? If not, finding the strength of the steel is pointless.
 
  • Thread Starter Thread Starter
  • #11
I have estimated the load to be no more than 3,000 pounds. The wood itself based on a log chart, factor in an estimated guess for the branches and foliage, then I doubled it for force from the wind.
 
could you ask your welder friend what company that he got the steel from and talk to them? Could you consider using acro props and modify the top as the acro props are designed to support heavy buildings?
 
What is that 8x8 foundation plate on? Mineral Soil? Organic Soil? Seems like it is too small and will sink/ lose support over time unless its on a hardened surface. $.02.
 
  • Thread Starter Thread Starter
  • #17
Wood Collector said:
could you ask your welder friend what company that he got the steel from and talk to them? Could you consider using acro props and modify the top as the acro props are designed to support heavy buildings?
Click to expand...
When I did that he said, "It's called INDUSTRIAL steel. You can google it." Gee thanks. I'll ask him where he gets the metal from and ask them.

DMc said:
Nick, is that a hypothetical or an actual estimated prop length you are planning to use.
Click to expand...
That was a total hypothetical. This particular prob will be just over 6ft long.

SouthSoundTree- said:
What is that 8x8 foundation plate on? Mineral Soil? Organic Soil? Seems like it is too small and will sink/ lose support over time unless its on a hardened surface. $.02.
Click to expand...
This particular one is on concrete. I've installed smaller one (big peach trees, tea trees (leptospermum), japanese black pine just by putting the foot on the ground, but the loads are VERY small there.
 
  • Thread Starter Thread Starter
  • #18
Wood Collector said:
could you ask your welder friend what company that he got the steel from and talk to them??
Click to expand...
When I did that he said, "It's called INDUSTRIAL steel. You can google it." Gee thanks. I'll ask him where he gets the metal from and ask them.

DMc said:
Nick, is that a hypothetical or an actual estimated prop length you are planning to use.
Click to expand...
That was a total hypothetical. This particular prob will be just over 6ft long.

SouthSoundTree- said:
What is that 8x8 foundation plate on? Mineral Soil? Organic Soil? Seems like it is too small and will sink/ lose support over time unless its on a hardened surface. $.02.
Click to expand...
This particular one is on concrete. I've installed smaller one (big peach trees, tea trees (leptospermum), japanese black pine just by putting the foot on the ground, but the loads are VERY small there.

Wood Collector said:
Could you consider using acro props and modify the top...
Click to expand...

What a great question!!! I've never considered that and have never heard that term. When I googled it, it seemed like it was made almost JUST for this purpose. I'll see if any are available locally. That would be great if I could cut off the top of the acrow prop, cut threads into it, then thread on an attachment my welder made. I could have 10 of the attachments made and waiting to go, then just buy the proper sized prop on an as-needed basis and assemble on site!
 
c63b1820-435c-481c-969e-ec64cec3e55e_400.jpg



http://www.homedepot.com/p/Tiger-Brand-Super-S-Series-8-ft-4-in-Jack-Post-J-S-100/100022783

Under $50.
 
I wonder how well that top plate that mounts to the limb will handle flex? That would be my concern. Houses don't move around like limbs...at least not in GA.

CA...maybe.
 
I think the square tube you picked out will work just fine at 6' but at 50 I think something with structure like H beam would be best to lessen deflection. Or making some type of A frame would be ideal.
 
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