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Re: Wood Allowable Stresses[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Wood Allowable Stresses
- From: Chris Willcox <cwillcox(--nospam--at)earthlink.net>
- Date: Sun, 28 Jul 2002 15:54:14 -0700
I'll add a couple of thoughts. First, Albert is right on about the "factor of ignorance." FEMA 273/356 recommends 2.8 * NDS ASD allowable to determine the ultimate strength of wood. Even if the difference in allowable stresses is entirely due to previous overestimation (and if that was the case I think we'd see a lot more distressed buildings), there should still be around 50% reserve capacity.
Second, I worked at a summer camp in inland Mendocino County (coastal No. CA) from 1986 to 1991 and in those six years I noticed a dramatic decrease in the size of the Douglas fir logs on the skipjacks driving down Hghway 101. That observation has led me to, rightly or wrongly, to believe that the decreases in allowable stresses are due largely to quality of lumber issues. (Not least because the development of the 91 NDS was more or less concurrent with my observation.) You've probably noticed that the decrease is larger for deeper members (x10's and x12's) due to the size factor. I believe that is partly due to the size of the logs they are being cut from. Before the 2x10's and 2x12's would have been cut from the areas around, but not including, the heart and well away from the sapwood. With smaller logs, the edges of these members are more likely to include sapwood, and they are more prone to have flaws.
Finally, I agree that there aren't any signs of distress, the dnagerous buildings code doesn't seem like the right choice for the analysis. As long as the design loads are unchanged, I think that approach I would take would be to pick a couple of members and check them against the code in effect at the time of construction. As long as they were close (say less than 15% over), I'd call it good.
Chris Willcox, SE
I vented on this a couple of weeks ago, but now would like some input if
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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