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Fw: structures & ethics

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Ned,
 
According to Wood Handbook, USDA Forest Service, Table 4-3b, the as-tested complete  failure stresses in Douglas-Fur Coast, @12% moisture: compression parallel to grain - 7,230 PSI, modulus of rupture - 12,400 PSI.
 
Compare it to the allowable values with all possible factors - and you will get a safety factor of 6 or more against the ultimate failure of an ideal specimen.
 
I would think that even with the obvious difference between the ideal and reality, the assumption of the safety factor of 4 appears valid. 
 
At the same time, I most certainly agree that the engineered wood products have a much lower safety factor. 
 
Steve Gordin, SE
Irvine CA
 
 
----- Original Message -----
From: Waltz, Ned
Sent: Monday, November 22, 2004 11:57 AM
Subject: RE: structures & ethics

I don?t have much to add to your discussion other than to caution you that your assertion that wood has ?safety factors? in the order of 4-6 is in error.   I cannot think of any wood material property that has a safety factor in that range.   

 

A typical ?safety factor? in wood is in the order of 1.2-1.3 (i.e. bending and tension) with 5% of the material expected to have less than that?..some of which may even approach 1.0  Some have even argued that there really is no safety factor with wood.

 

If you have a material with a large COV, you might see a 3-4 difference?ON AVERAGE?between a short term test and a long term design value.  But most of that falls away when you account for load duration effects and material variability.  Comparing apples to apples with respect to load duration?you can expect 5% of your material to have a strength less than 1.2-1.3x the allowable stress.

 

Ned Waltz, PE

Corporate Engineer

Trus Joist, A Weyerhaeuser Business

Boise, ID