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RE: Douglas Fir Larch Strength

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The 1988 WWPA values originated over 60 years ago when wood samples were taken from short clear sections. Apprximately ten years ago or so, a new method was adopted to choose sections that represented the materials most used in construction. After a ten year study (I'm giving this to you from memory so the time spans may be incorrect) it was discovered that the values used did not represent the quality of the material used in the filed.
In addition the grading proceedures varied by the length of the member and the number and placment of deficiencies within that length of material. For example, if an 8' length was visually graded as a Number 2 grade DF and then when it was cut to 6', the location of knots at the extreme fiber moved away from center or was removed, the member may have been graded differently.
The result was that the grading of most members reduced the allowable bending stress to represent a more realistic condition in the field. Remember that the quality of wood we find today is not nearly as consistent or abundant as it was ten years ago. This combined with the results of the study resulted in reduced stresses.
Now, also remember that the current NDS requires the use of multipliers based upon depth of the member, width to depth ratios and more. Once the mulitplies are taken into consideration the difference in stress in not nearly as great as it appears from the base stress values printed in the code. Refer to the UBC and you will find separate tables for 5.5 wide beams and 2x joists that have already been increased by appropriate mulitpliers (including repetitive member increases) to help simplify the selection. I think you will see that the stresses have degraded but not that much.
The NDS represents the results of the study. These revised values have found their way into the 1994 UBC as well.
Hope this helps.
Dennis Wish PE
 
-----Original Message-----
From: Jie Lu [mailto:jie(--nospam--at)ibrahim.biz1.net]
Sent: Tuesday, September 15, 1998 8:16 AM
To: seaint(--nospam--at)seaint.org
Subject: Douglas Fir Larch Strength

Is there someone can help to explain the following question?
 
The "fb" for Douglas Fir Larch No. 2 is 1250 psi for single memebr based on WWPA's 1988 Manual  "STANDARD GRADING RULES FOR WESTERN LUMBER".
 
However, the NDS Supplement 1991's Table indicates the "fb" value is only at 875 psi (see page 20).
 
Thanks.
 
J. L.