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RE: Doug. Fir Lumber Grades

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Larry,
 
Last year I built a project using No.1 and Better and Select Structural DF beams and rafters to fit an historic structure. At the lumber yard I seem to recall the "No.1 and Better" had a stamp on it that said just that. My 1991 code lists a Dense No.1 that had a better stress value than No.1. But you can't use that value today for checking (E) lumber. The old grading rules might explain what the differences were. But you're still limited to using today's lumber stress values, from the current code.
 
Deflection may govern in a long purlin and the better lumber is only going to have a slightly better E value. Ponding of old panelized roofs is pretty common due to long-term deflection. An over-deflected purlin will also suffer from vibrations. The old lumber just won't work for new added equipment loads.
 
For a typical panelized roof with new equipment they're probably going to need to replace the purlins or reinforce them with sistered joists. Shear may also be a factor at the ends. The metal hangers need to be verified. Can you tell what type they are? If not can you analyze a custom hanger?
 
One solution may be to add new larger purlins in between the existing purlins at the equipment perimeter and leave the roof intact.
 
 
 

Dave Gaines, P.E.

Structural Project Engineer
HDR ONE COMPANY | Many Solutions
251 S. Lake Ave, Suite 1000
Pasadena, CA 91101
T: 626.584.4960
F: 626.584.1750
email: david.gaines(--nospam--at)hdrinc.com

 


From: Larry Hauer [mailto:lhauer(--nospam--at)live.com]
Sent: Friday, September 26, 2008 2:06 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Doug. Fir Lumber Grades

Thanks to Gerald and Dave for their input. Yes, I've gone through this before with panelized roofs and know the problems. I gues my new question is: To use the values for "No.1 and Better", would the piece of lumber have to be stamped as such? As far as I know, back in the '70's there was no such grade, so I imagine lumber of that era couldn't be designed as such. Do lumber pieces used today have a stamp that actually says "No. 1 and Better", if that is what is called for in the wood specs, or is it something like a mixture of No. 1 and Sel. Str.?
 
Thanks again,
 
Larry




Subject: RE: Doug. Fir Lumber Grades
Date: Fri, 26 Sep 2008 12:53:26 -0500
From: David.Gaines(--nospam--at)hdrinc.com
To: seaint(--nospam--at)seaint.org


Larry,
 
For a panelized roof system the owners and contractors probably did not use "No.1 and Better" grade of purlins and subpurlins. You may need to get up on a lift and find a few grade stamps to confirm the contractor used what was shown on the plans. Another way to confirm (E) lumber grading is to compare the actual size and position of knots to the WWPA or WCLIB lumber grading rules, but that requires a bit more skill and research.
 
Gerard has made a good point. You won't be able to make the (E) 1970's joists or purlins calc out for new loads using today's allowable stress values. You'll have to reinforce them or replace them for new loads. Check the beams and girders too. You may find elevated stress levels. Good luck.
 
Dave Gaines, P.E.
(626) 410-3631 cell
(626) 794-4117 home
(626) 584-4960 office direct line

gainesengr(--nospam--at)earthlink.net

From: Larry Hauer [mailto:lhauer(--nospam--at)live.com]
Sent: Friday, September 26, 2008 6:57 AM
To: Struct. Eng. Assoc.
Subject: Doug. Fir Lumber Grades

I am checking an existing "panelized" roof system, (cira 1970's), for the addition of equipment and screening on the roof. Ever since, I believe, the '94 UBC there have been stress values for doug. fir 2x and 4x for "No. 1" and "No. 1 and Better", and I have never understood when it is appropriate to use the "No. 1 and Better" values. Can anyone shed some light on this?
 
Thanks in advance,
 
Larry Hauer, S.E.


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