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Re: Wood Allowable Stresses

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You probably want to talk with Buddy Showalter with AFPA (I think I got that correct).  The reduced wood values I believe are a result of changes in test methods to some extent, and learning more about wood behavior (and possibly younger growth wood).  Just because we had higher design values previously doesn't mean it was correct.

Example: At one time the allowable tensile stress for Douglas Fir was the same as bending stress.  When they were doing the machine stress rated lumber studies in the late 60's, early 70's they realized that there was a problem with the test grips (or something like that) and the actual allowable tensile stress for Douglas Fir was reduced by about 40% from what had been used.  If you consider wood bowstring trusses built in the 40's, 50's, 60's the bottom chords are greatly overstressed using the new values.  They haven't failed all that often because the 12 psf tributary live load we design for on the roofs trusses is not there, but under just dead load the trusses are at about 100% of allowable if not overstressed.  Typically we are seeing a lot of older trusses that need to be strengthened/repaired.

I would use the current allowable design values, and not attempt to use the older values.  As was pointed out, in the small clear specimens originally tested had high values, and the smaller lumber pieces today still have relatively high values if I am not mistaken.  I believe the size  affect (increase in lumber size) has a significant impact on allowable stresses, since as the wood member gets larger, there's a greater likelihood of natural flaws being present in the individual wood members that degrade its design stress values.  If you take that same piece of wood, and cut it in half lengthwise, you may be able to re-grade the lumber since there are now possibly fewer visible flaws in one or both pieces.

Hopefully Buddy Showalter can jump in on this.  Hope this helps

Mike Cochran S.E.

In a message dated 7/25/2002 6:13:52 AM Pacific Daylight Time, jwatson(--nospam--at) writes:

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