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Re: existing wood floor joist bending stress

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In 1991 the NDS adjusted the safe bending stress of deep sawn lumber, the grading rules were not significantly changed.  This reduction in allowable bending stress was based on the In Grade Testing  program that randomly selecting 70000 specimens from mills and testing them in bending.  This exhaustive program showed something that had been suspected, that deeper 2xs to 4xs were overstressed by as much as 50%. The original misjudgment came from the extrapolation of bending stresses from tests on clear 2x2s.  Physically this can be explained by the fact that the more lowly stressed inner fibers support the highly stressed exterior fibers in shallow members.  This support becomes less effective for deeper members with a less steep stress gradient. 
This means, unfortunately, that many existing  floor and roof members are as much as 50% overstressed.  To allow more load on such members is irresponsible.  Ignorance of established facts are not a defense in a malpractice action.  The wood industry had many seminars on this subject in the early 90s.
Dick Phillips, Fellow  SEAOC