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A question about evaluating an existing

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Scott Maxwell wrote:

>>I am going to show my ignorance in wood construction, but...

We have an existing wood structure that the architect wants to change the
roofing on.  We need to evaluate the existing wood members for the new
loading.  The roof framing is 2x8 @ 16 oc with the longest span of about
13'-6".  The existing drawings don't specify the wood grade or species.  So
the questions is how would go about "selecting" an allowable stress?  Would
you try to "back calculate" what the wood species and grade should be based
upon "existing" loads?  Do material testing to try and determine wood
species and grade...and if yes, what testing would that be?  I am guessing
that this framing was not orginially "engineered".  Any suggestions or
input would be greatly appreciated.

Thanks much,



The wood, unless it is *very* old (prior to 1960), should have grade stamps 
on it showing what grade and species the lumber is.  Grade stamps are 
supposed to be stamped within 2 feet of the end of the piece as it comes out 
of the mill.  While the grade stamped end may be cut off of some pieces, I 
have rarely found it to be cut off *all* pieces.  If a grade stamp cannot be 
found, samples can be sent to the Forest Products Lab in Madison, WI for 
species identification.  It is my understanding that FPL will evaluate up to 
5 specimens at no cost.  (Someone correct me if this is not correct.)

Prior to 1970, the grade of lumber, regardless of species, that typically was 
used was Construction Grade, which is comparable to No. 1 grade currently.  
Because design values then were based on tests of small, clear specimens, and 
1980 in-grade, full size tests showed that design values based on small, 
clear specimens were not necessarily conservative, I would suggest using the 
current allowable stresses for No. 1 grade for the species, or species group, 
of your rafters.

Hope this helps.

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