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Re: Timber beam deflection

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I concur with Dennis' comments.  If we assume the beam is straight when
installed, the long term deflection (creep) of 0.25" remains.  When totally
unloaded, the beam would have a permanent set of 0.25".  Whe the load is
reapplied, the beam will deflect under the dead load the expected amount, but
added to that deflection will be the 0.25" permanent set.  This is one of the
reasons why when you repair a wood roof (usually a panelized roof framing
system) you remove all dead load before you proceed with the repair.  In fact,
if you can jack against the beam and remove some of the permanent set, you can
strengthen the beam to counter some of that deflection.

If we take Dennis' last comment and look at this problem realistically, if the
contractor sets a beam with the crown down (which happens more than we would
like), the beam has a permnanent deflection before any dead load is added.  Now,
place the dead load and add the long term deflection, and you have some real
problems with roof drainage especially if the roof is sloped at code minimums.

Dennis S. Wish PE wrote:

> 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) [mailto:RoyLevy(--nospam--at)]
> Sent: Saturday, August 22, 1998 2:01 PM
> To: seaint(--nospam--at)
> 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.