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Timber Beam Deflection

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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?