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RE: Timber Beam Deflection

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Dennis,

As I said in my post, I think that the topic of creep in wood would be an 
interesting research project.  In concrete, the creep is related to 
compression.  On the other hand, steel is noted for "relaxation" which is 
related to tension.  Glass "flows" under its own weight, maybe not so much 
now with modern glass, but glass made in the last century was noted for its 
flowing, creating distorted images in window glass.  While we know that wood 
also exhibits continuing movement under load, I don't know whether it is due 
to compression or tension, or a combination of both.  Wood is the poor cousin 
of structural material, its been used so long it gets no respect.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Dennis Wish wrote:

. > Roger, you may have a point, but if I were guided by logic I would think
. > that creep is the effect caused by an equalization due to tension stress 
. > in the wood fibers. It would seem reasonable to assume that if the maximum
. > tensile stress is not exceeded there can only be a maximum amount of creep
. > that can occure regardless of the number of times you reload the beam. I
. > would also think that since fatigue is not an issue with wood, the maximum
. > amoung of creep would not exceed the original deflection amount unless the
. > length of time that creep occurs has not reached maturity.
. > Therefore, I still can't understand how the deflection will increase by
. > unloading and reloading the beam.
. > Here is another analogy, although it is based upon other organic and
. > inorganic materials. Guitar or piano strings (both nylon, gut and steel)
. > will stretch when first installed. In fact, Piano strings tend to last in
. > excess of 50 with retuning due to slipage of the pegs or from 
. > evviornmental changes. There is an adjustment period before the elongaton 
. > due to tension in the wires reaches equalibrium. Once that point is 
. > reached, no further elongation is expected unless there in a change in 
. > the enviornment (temperature, humidity etc.) I would find it difficult to 
. > expect a beam to act any different but the problem is an interesting one 
. > at any rate.
. > 
. > Is there any test data on this that I can learn about to understand your
. > definition better.
. >