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Re: Allowable Stresses In Wood

[Subject Prev][Subject Next][Thread Prev][Thread Next] I would recommend using the current wood allowable design values for
evaluating the existing wood structures.  I imagine you wouldn't be checking
it, unless you were adding addtional load.

My understanding is that there have been changes in testing methods over the
years besides the "in-grade" testing and size effects done in the 1980's as
well.  One particular area is "Tension Parallel To Grain" values which were
decreased around 45% between the 1967 and 1970 UBC's.  Previously "Tension
Parallel to Grain" design values were the same as for "Bending".  The change
I believe was a result of the original testing program to determine allowable
design values for "Machine Stress Rated Lumber" in the Late 1960's which
indicated a problem in the test method for determing "Tension Parallel to
Grain", thus the revised design values in the 1970 UBC.  Mr Showalter  with
AFPA may be able to clarify this some more.

Michael Cochran S.E.


In a message dated 7/2/2001 5:20:18 PM Pacific Daylight Time,
frankgla(--nospam--at)pacbell.net writes:


Michael,

My understanding is that the change is due to the fact that the older
design values were extrapolated from testing of smaller specimens (2x2,
etc.) These specimens were relatively defect free and the original
testing results perhaps did not accurately reflect performance of a
normal graded member (with normal knots, checks etc associated with the
grade) whereas the 'in-grade' testing program does. It sounds reasonable
that the decline in timber quality may also have something to do with
this change, but I'm not positive of that. (Try buying a 2 x 6 today
that doesn't have exhibit wane on diametrically opposing corners....)

For existing buildings, it seems reasonable that you could check the
framing members using the code that was in effect at the time of
construction. (Unless you are adding new loads to the structure, then I
would use the current code). Whenever possible, I always like to discuss
design decisions such as this with a knowledgeable building official
prior to doing the work.

Frank

Michael Bryson wrote:

>  I wonder if someone has some knowledge about the change in allowable
> bending stresses in wood?The fb values seem to have come down abruptly
> with the UBC94 code presumably because of large scale testing done.
> For example 2x12 Doug. Fir #2 went from 1250psi to 875psi. Now can
> this change be attributed to the size effect factor because the new
> tests were done on large scale specimens? Or because of new growth
> material i.e. all the older & stronger wood had been used up and we
> are now using newer & poorer second growth wood. This is relevant
> because when assessing existing buildings which fb do you use? Old or
> new values?