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RE: WOOD - Engineered Lumber vs. Sawn Lumber

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On Wednesday, September 10, 1997 7:25 PM, Bill Allen, S.E. @ 
ALLEN DESIGNS [SMTP:BAllenSE(--nospam--at)pacbell.net] wrote:
| I'm not sure how to explain this. If you camber the beam for
| 1.0*DL
| deflection then, theoretically, when the dead loads are 
applied
| the beam
| will be level. If you camber the beam for 1.5*DL deflection,
| after the dead
| loads are applied the beam will still have a negative
| deflection (camber)
| or "crown" equal to 0.5*DL deflection. Right?
|
| Regards,
| Bill Allen
|
| ----------
| > From: Stan Johnson <hawneng(--nospam--at)wac.com>
| > To: seaoc(--nospam--at)seaoc.org
| > Subject: Re: WOOD - Engineered Lumber vs. Sawn Lumber
| > Date: Wednesday, September 10, 1997 5:50 PM
| >
| > Bill Allen, S.E. wrote:
| >
| > > I believe one should be careful about specifying
| > > a camber=1.5*DL
| > > deflection. While this is appropriate (and
| > > required by Code) for roof
| > > members, it will leave a crown (theoretically
| > > equal to 0.5 * DL deflection)
|
|
| > > on floor members.
| >
| > I don't understand why this would be the case?
| >
| >
| > Stan Johnson
| > BS, EIT, Are we talking gold crown or porcelain

Bill,
According to the AITC 3rd Edition, Table 5.9, the recommended 
Minimum Camber for GLB's is 1.5 times Dead Load Deflection. 
There are two notes attached:
"a) Roof Beams, the minimum camber of 1.5 times dead-load 
deflection will produce a nearly level member under dead load 
alone after plastic deformation has occurred. Additional camber 
is usually provided to improve appearance and/or provide 
necessary roof drainage. Roof beams should have a positive slope 
or camber equivalent to 1/4" per foot of horizontal distance 
between the level of the drain and the high point of the roof, 
in addition to the minimum camber, to avoid the ponding of 
water. In addition, on long spans, level roof beams may not be 
desirable because of the optical illusion that the ceiling sags. 
This condition may also apply to floor beams in multi-story 
buildings.

b) Floor beams - the minimum camber of 1.5 times dead-load 
deflection will produce a nearly level member under dead load 
alone after plastic deformation has occurred. For warehouse or 
similar floors where live load may remain for long periods, 
additional camber should be provided to give a level floor under 
the permanently applied load."

I believe the 0.5 time Dead Load that you mentioned has to do 
with creep. Also according to the AITC page 4-90:
"b. The inelastic deflection, creep or permanent set of glued 
laminated timber averages approximately one-half of the 
calculated dead-load deflection. This deflection can be offset 
by camber, as previously stated. The permanent set of 
un-seasoned sawn lumber often used for secondary framing is 
approximately equal to the calculated dead-load deflection. 
Therefore, the total long-time deflection is approximately 2 
times the calculated initial elastic deformation. The additional 
ponding load resulting from the permanent set of all members 
within a particular framing system should be considered in the 
design analysis."

Hope this clears it up a bit.

Dennis Wish PE