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RE: Wood Allowable Stresses

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Jake:

I will commiserate with you on this issue.

Yes, the wood values have been declining, and yes it presents a problem
with regard to the specific issue you site. Checking existing lumber
under the Dangerous Building code (DBC) provisions, in my opinion,
technically would require the use of 150% of new building code allowable
stresses or 1.5 x 1124 psi = 1686 psi. This may present a problem
however I would question using the DBC for evaluation purposes if the
intent is to review a building which perhaps has shown no distress or
isn't being changed or altered with regard to loading (both conditions
not known).

I don't think the intent of the DBC is to ferret out "problems" unless
they are apparent or suspected. For instance, some have chosen to view
this DBC provision as applying to lateral systems for buildings which it
probably wasn't intended to address. You can see that this logic could
suggest that older buildings designed under previously acceptable
seismic and wind provisions may fail when subjected to new Building code
requirements for lateral loads even if divided by 1.5 (think URM's which
aren't presently allowed now anyway).

The need is to clarify where you fall in the "evaluation" category. Are
you looking to satisfy "improvements and additions" or to give an
opinion on whether the thing is about to fall down? This is the required
engineering judgment in my opinion.

Lastly, I do think your boss has a valid point with regard to the use of
the higher allowable stresses (i.e. 1.5 x 1898 psi) if the intent is to
show no significant danger exists. (And it's not just because I know
him.) Reason being that the DBC is a relatively static document. In
other words, its intent is to define conditions within structures which
need addressing and provide tools for Building Officials, Engineers and
others to get at and solve problems. In issuing updates I don't believe
they actually look over all the material stress changes and say, oh
yeah, wood values have gone down conservatively and we should put in an
exception to deal with it. Rather it is a guide to aide judgment and
therefore if the judgment is that the lumber values are artificially low
in this instance, then there should be some latitude. On this latter
point however, make sure your judgment is sound and not just based on
some whining by me or others.

One more picky point and I'm done. Watch your subscript letter cases. Cf
is not the same as CF and Cd doesn't exist. 

Barry H. Welliver
barrywelliver2(--nospam--at)earthlink.net
 

-----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
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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