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RE: Timber Beam Deflection

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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.

Dennis Wish PE
-----Original Message-----
From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
Sent: Sunday, August 23, 1998 12:54 PM
To: seaint(--nospam--at)seaint.org
Subject: Timber Beam Deflection


As one of those that suggested that the final deflection was 0.75 inches, I
would like to explain my thinking.

When creep occurs, strain continues without increase in stress, contrary to
Hooke's Law.  When the load is released, the stresses in the beam return to
zero, the stress condition that the beam was in when it was first loaded.
When the beam is reloaded, I could see no reason that it wouldn't behave in
a
manner similar to when it was first loaded.

It is acknowledged that creep diminishes with time, becoming negligible
after
about two years (Ref.  USDA "Wood Handbook").  But that is with the load
constantly on the beam.  I don't know of any research that has been done
with
wood to measure creep due to cyclic loadings, similar to what has been done
with concrete.

I think that your hypothetical question is a good one for a research
project.

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

Roy Levy wrote:

. >  I wish to thank all the responders for their prompt replies.  They all
. > made some sense, although the  conclusions may have differed.
. >
. > This was a theoretical  problem and the assumption was that the beam
was
. > straight without initial camber.
. >
. > For those who suggested that the final deflection will be of the order
of
. > 0.75 inches,  does that imply that  sufficient repetitions of the
. > described sequence  could cause the deflections to increase without
bound?