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Allowable Stresses In Wood

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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?