Embarrassing Failure leads to interesting question

2012 August 8
by Daniel Lakeland

Today I was at a neighbors house sitting in her hammock/chair thing hanging from a rope on a branch of her backyard tree. She told me that the wooden spreader cross-member which helped form the shape of the chair had been lost, and she had replaced it with some kind of alternative wooden dowel, but that it was too thin and if you pulled yourself up out of the chair by it it would probably break.

Having some background in wood structural design I told her that wood often can hold more load than we expect, that it tends to flex a lot, and will normally give some warning in the form of creaking and popping before failing. After saying this I pulled myself up on the bar a bit and the bar immediately snapped like a firecracker without any warning. This of course left me with "egg on my face" and a person with more social smarts than I do would probably not blog about this fact over the entire internet. But I couldn't help wonder what led to the disconnect between what I thought I knew about wooden structural members and what I experienced first hand with this randomly selected piece of wood. The question is, which of the following situations best explains the result:

  1. What I told her is perhaps simply false and I was misremembering what I thought I "knew" about the properties of wood as a structural material? Perhaps wood often does tends to fail in a brittle manner without warning?
  2. Perhaps what I told her is true of wood chosen for structural purposes (such as visually graded Douglas fir beams) but is not necessarily true for other types of wood (I have no idea what kind of wood this thing was made of, but it did seem rather dry and possibly a little weather-worn)?
  3. Perhaps this particular piece of wood was in fairly poor condition, and then the follow up question arises about what sort of condition variables affect the brittleness of wood?

To answer some of these questions I'd like to do some simple calculations of the stresses expected within this member and try to determine whether it should have failed as it did  if it was a structurally sound piece of wood, or maybe if it was substantially less strong than what would be expected for structural woods. Since I don't have the time right now I'll have to follow up tomorrow or the next day, so stay tuned. I hope to learn something interesting out of all of this. After all, I need some way to get this egg off my face.

 

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