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Re: 1991 NDS Values for Lumber

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Dennis Wish wrote:

>>The problem is that the 91 standards is based upon the results of a twenty 
year study on the quality of wood. Although the quality of the wood may be 
the same after the '91 NDS was published as it was in 1980, the old stress 
values were allowed by code prior to adoption of the '91 NDS.
In my opinion, there is sufficient room in the factor of safety (as well as 
the fact that this is a statistical average for wood) to accept the prior 
code stress when recalculating the member for revised loading or remodeling 
occuring after the adoption of the '91 NDS. <<

Dennis,

I would like to take exception to that statement.  I have witnessed a 
bowstring truss (101' span) that had (2) 3 X 12's as the bottom chord.  Out 
of 9 trusses, 7 bottom chords had failed in tension (from memory).  When 
analyzed based on allowable tension stresses in effect when constructed, the 
trusses were completely adequate under dead and live loads.  *But* when 
analyzed based on the allowable tension stress after 1980, the bottom chord 
was seriously overstressed under dead load alone.  That the roof did not 
collapse was, in my opinion, due to the roof sheathing (diagonal boards) 
causing the roof to act as a barrel shell rather than as a truss supported 
roof.

I would recommend that *all* analysis of older wood structures be based on 
the allowable stresses that resulted from the full size, in-grade tests, 
i.e., the values in the 1991 NDS and later.

As for equivalent grades, Construction Grade was the commonly used grade 
prior to 1970 and is equivalent to today's No. 1 Grade.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona