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# RE: Wood Allowable Stresses

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Wood Allowable Stresses
• From: "John S" <johns(--nospam--at)alliedengineering.com>
• Date: Thu, 25 Jul 2002 12:10:05 -0600

```Try 1997 Uniform Code for Building Conservation, Section 505.1,
Exception 1.   Use the design values for the wood material at the time
the building was built.

-----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)utahisp.com]
Sent: Thursday, July 25, 2002 7:03 AM
To: SEAINT
Subject: Wood Allowable Stresses

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
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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