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Re: WOOD - Engineered Lumber vs. Sawn Lumber

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Is there a special designation for the PSLs with camber? The last time I
tried to specify this, the contractor told me that cambering was not
possible.

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.

Regards,
Bill Allen

----------
> From: Dennis S. Wish PE <wish(--nospam--at)cyberg8t.com>
> To: 'seaoc(--nospam--at)seaoc.org'
> Subject: RE: WOOD - Engineered Lumber vs. Sawn Lumber
> Date: Tuesday, September 09, 1997 8:52 PM
> 
> I have read through a few comments and want to add a few of my 
> own.
> If economically possible I would use only manufactured wood 
> products. I find Parallams to be exceptional to GLB's. If you 
> are not aware, you can order any Parallam with a Camber by 
> specifying a Commercial grade Parallam and the appropriate 1.5DL 
> camber.
> I just finished a small Baptist Church design which required an 
> auditorium with walls exceeding 16 feet in height (2x6 @ 16" 
> o/c) with a TJL sloped top chord roof spanning 50 feet. To 
> accommodate the tall wall and insure that it is straight, I 
> specified the new Timber Strand Studs. This added about 10% to 
> the material cost, but reduced the labor and scrap rate for 
> dealing with crowned studs.
> 
> I can appreciate John Rose's comments about mixed sawn / 
> manufactured lumber products however, I think my original 
> concerns where the mix of parallam headers in conventional 2x 
> stud walls. The height differences compared to sawn lumber are 
> not that critical and easier to conform than trying to get a 
> Glu=Lam into the same width wall.
> 
> My concern for LVL's like Micro-Lams has more to do with the 
> misuse of them for top flange nailed connections. Micor-Lams are 
> great for face nailing but can delaminate if top nailed. I also 
> don't like the idea of splicing plies together in the field. It 
> seems so much easier to order a full width member in Parallams 
> which is generally carried in stock by most yards.
> 
> Finally, there is the environmental concerns that make 
> manufactured lumber much more desirable. Mfg. wood products are 
> harvested from young trees grown specifically for this purpose. 
> No trees are removed from forested lands (according to older 
> manufacture lectures) to produce TJ products. This means that 
> mfg. products are self perpetuating without endangering existing 
> mature trees.
> 
> If you haven't notice from the 1994 code, it appears as though 
> the quality of wood that we are seeing in conventional lumber is 
> not as desirable as it used to be. The grading rules have 
> relaxed and with that the stresses have reduced. I've run into 
> at least one problem with calculated beams deflecting greater 
> than calced. Crowns can appear in the field due to changes in 
> humidity and the concentration of cross-grain that is appearing. 
> I'm not well versed in this area, but have had to do a lot of 
> reading lately to understand how these problems occur when the 
> numbers don't justify it. One informative book that I found is 
> "Structural Design in Wood" by Stalnaker and Harris (Van 
> Nostrand Reinhold Press). There is an excellent chapter on how 
> defects occur in wood.
> One interesting example is that You can not take a 20 foot long 
> DF Select Structural, cut it into two beams and guarantee that 
> you have two 10 foot long DF Select Structural beams. This was 
> news to me. The reason is that the location of defects is a 
> determining factor in the visual grading process. By cutting the 
> beam in half, you my change the percentage of defects in each 
> half as well as the location of the defect. This can 
> dramatically change the grade of the lumber and it's applicable 
> stress.
> 
> If I could, I would eliminate all the guesswork and design with 
> only manufactured lumber.
> 
> My $0.50 worth.
> Dennis Wish PE
> 
> 
>