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RE: Timber beam deflection

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I'd be interested in others explanations. The way I understand it is that
creep is a long term stretching of the wood fibers. In some ways, I would
say it's similar to a plastic deformation in that once the load is removed,
it is not expected that the beam will return to it's original shape.
Therfore, I would assume that once the load is removed, the beam will remain
deflected by the amount of creep or 0.25".
As far as reload, I would assume that the maximum creep represents a level
of equalibrium based upon the long term effect of the original load.
Therefore, I would not expect the total deflection to be greater or less
than the original after creep set in.
Since wood is a living material, I would suspect that my explanation is
lacking some consideration which others will provide.

There is one issue that I did not mention which may or may not be considered
in your "hypothetical" problem. No beam is perfectly straight. For this
reason, the contractor is expected to place the crown side of the beam at
the top. Therefore, the beam starts with a negative camber. It may be that
the original deflection of 0.25" is actually greater when you add the camber
to it.

Dennis Wish PE

-----Original Message-----
From: RoyLevy(--nospam--at)aol.com [mailto:RoyLevy(--nospam--at)aol.com]
Sent: Saturday, August 22, 1998 2:01 PM
To: seaint(--nospam--at)seaint.org
Subject: Timber beam deflection


Can someone advise on this hypothetical timber beam deflection problem?

1 .A  beam is loaded. The immediate sag is 0.25 ins. The load remains and in
time the sag creeps to 0.50 ins.

2. After that, the load is removed.  What is the sag now?

3.  The load is reapplied. What is the sag now and in the long term?

Thank you,

              Roy Levy,   P.E.