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# Wood Allowable Stresses

• To: "SEAINT" <seaint(--nospam--at)seaint.org>
• Subject: Wood Allowable Stresses
• From: "Jake Watson" <jwatson(--nospam--at)utahisp.com>
• Date: Thu, 25 Jul 2002 07:02:55 -0600

```I vented on this a couple of weeks ago, but now would like some input if
possible.

Wood allowable stresses have dropped dramatically in the last two NDS
cycles.  For example prior to 1991 F`b for a 2x12 roof joist with snow load
was: (Assuming DF#2 North)

F`b = 1.15*(1650 psi) = 1898 psi
Cd   Fb (with repetitive increase)

Under '97 NDS it is:

F`b = 1.15*1.15*1.0*(850 psi) = 1124 psi
Cd   Cr   Cf   Fb

I am evaluating a building that was built in the early 1970's.  My boss
wants to use the higher design stress as the base stress to determine
dangerous conditions.  Per the dangerous building code, hazardous is
basically when the ratio of demand to capacity is greater than 150%.  I
can't find a section in the code that allows me to use the older stresses as
the base.  That said, the new stresses appear to be incredibly conservative.

Now the question, where would you define dangerous? 150% of 1898 psi, 150%
of 1124 psi, or use engineering judgment and be somewhere in between?

Thanks for the thoughts,

Jake Watson, P.E.
Salt Lake City, UT

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