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Douglas Fir vs. Hem Fir

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While allowable bending stresses are comparable, shear stresses and Modulus 
of Elasticity are significantly different.

While 90 percent of the time, DF-L, Hem-Fir and DF(S) can be used 
interchangeably, there is always the 10 percent of the time that they can't.

I try to avoid specifying No. 1 grade dimension lumber as that has to be hand 
selected from shipments of No. 2 and better.  Unless you need an awfull lot 
of No. 1 dimension lumber on a job so that a mill order would be made, the 
lumber yards will get any No. 1 (and Select Structural) mixed in a shipment 
of No. 2 and better.

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

>>I have typically specified Douglas Fir no. 1 for beams and headers and no. 
2 for studs and interior partition walls.

Recently, I understand Hem Fir is being increasingly substituted.  It appears 
that the properties are pretty close.  I also know that P.T. mud sill 
specified on the job invariably is Hem fir.

Is the substitution acceptable for walls and floor / roof framing?  Are there 
any items to watch out for?

Anand Nene, S. E.
Consulting Structural Engineers
1624 Franklin Street, Suite 1200
Oakland, CA 94612-2824<<