Basis of the Load Duration Factor for Wood: Cd

[petrosoft]

Does anyone know why wood is stronger with a shorter load? I imagine it has something to do with fibers breaking one by one under load, but I haven't found any references to explain the behavior.

Also, it seems that wood fatigues in deflection. That is, a bend will increase over time with a constant load. Why isn't E also time rated?

 

[Ron]

The phenomenon is not fatigue, it's "creep". Wood is very susceptible to creep and long term loading is detrimental to creep in bending, axial loading and shear. Load orientation relative to the grain also has an influence. For instance, compression perpendicular to the grain is usually more detrimental than parallel to the grain.

 

[petrosoft]

Thanks Ron,
I'm curious about the actual internal mechanism for creep. What is happenning in the wood itself which causes it to creep? Also, is that creep still in the elastic range? That is, will the wood member recover fully when the long term load is removed?

 

[Ron]

Creep results from internal cellular structure changes in the wood under constant load. It can be made worse by higher moisture content in the wood, even below the 19% threshold. It is generally not an elastic condition, so recovery is not likely.

 

[JAE]

The little wood fibers get tired after a while and lose their grip.

 

[msquared48]

Sounds like more than little wood fibers are losing their grip around here...

Mike McCann
MMC Engineering

 

[Ron]

Dang JAE...wish I'd said that!

Mike...I lose my grip daily!! It's almost 5:00 on Friday afternoon...I'm ready for a beer.