Snow Broke my FWC Hawk Roof

[pods8]

Has anyone seen this failure pattern before???

I haven't over the years but doesn't mean it can't happen. I think him concentrating the stresses over those crossmembers was a unique and unfortunate combination.

 

[pods8]

a design/material flaw in the FWC...looking at how the round strut is cut into the rectangular support just does not look right; seems that would induce a weak/failure spot...

They have been built like that for 40+ years (I can show you a 1982 roof frame right now if you'd like), also if you noted the build photos from ski3pin ATC does it too.

 

[Vic Harder]

And this is the first? Failure I’ve heard about, so chances are it’s an unfortunate fluke.

 

[Corn Husker]

I noticed in one of the pics Salmon posted on July 17 it looks like there's wood inside the broken roof bow. I wonder why fourwheel camper would be putting wood inside of them.

 

[Wallowa]

Corn,

I don't see any wood inside the roof structure. What did I miss? Longitudinal Wood strips hold the liner to the top inside and the transverse board in photo is used to push top up into place

 

[Corn Husker]

The pic of the broken roof bow where the conduit goes through. I believe you have it labeled as inside break close up. There is definitely wood inside the broken roof bow.

 

[Corn Husker]

 

[Wallowa]

Good eye! Perhaps the wood is there to provide a purchase for fasteners? Like wood strips? Lights?

 

[JaSAn]

This is as far as I could magnify it before it started to pixelate:

It looks to me like the discolored back and bottom inside of the rectangular crossbeam. What I am pointing to would be the inside bottom of the beam.
Could be wrong; my eyes don't work well at this time of night.

 

[ckent323]

That photo appears to show that a notch was cut into the square tubing and that there are no welds there. I think I see a fastener in the middle. Is that a horizontal piece of metal conduit? What is that for? Is it factory?

Notching the top of the tubing would weaken it. The screw coming in from the bottom could be a crack initiator. Downward force and the notch could allow the tube to buckle and the screw hole could lead to the metal splitting or tearing.

A closeup better resolution photograph would be informative. I would have a structural engineer look at this if you know one.

Floor joists and roof rafters in houses are not allowed to have notches because they can buckle under load.

 

[Jon R]

…

Forgive me if this is insulting anyone's intelligence or education but just to verbalize what I know about this stuff (and maybe someone can correct me if I'm wrong - my one strengths-of-materials class was very long ago): If you have a long piece of uniform material (like a virgin length of square aluminum tubing) and you apply a bending force to it, the material will start to bend into a mostly-uniform curve from end to end. The outside of the bend will all stretch and the inside of the bend will all compress and the forces will be spread over the entire length, and the material won't fail (crack, etc.) until the whole piece is very close to failure at every point along its length. However, if you modify the uniform piece in some way (drill a hole in it, weld or unevenly heat it enough to affect the metallurgy, change its shape somehow, thicken part of it, etc.), then it is no longer uniform and any bending forces will tend to concentrate somewhere. Meaning a certain point will absorb more of the overall force. Since this is generally occurring at a structurally weak point, it may end up being stressed beyond the point of failure even though the rest of the material is still well within its elastic deformation range (meaning a bend from which it will completely return to original shape).

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I know this thread is a few months old, but I wanted to correct the description of bending load distribution on a beam that was provided in the quote section above

If you have a uniform section simple beam spanning two supports with a uniform load applied along its length (like a raised FWC roof with a snow load), the greatest bending load is in the center of the span. The shear load is zero in the middle, and at its greatest just inside the beam supports at the ends. In the absence of any stress concentrations creating a weak area, you would expect the roof to fail in bending in the middle under a uniform snow load that keeps increasing until something fails. In this case it looks like that round hole for the cross pieces created a weak area.