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Re: Allowable Stresses In Wood

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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?