allowable shear stress in wood 2

[bjb]

Can someone explain why the allowable shear stress in wood greatly increased between the 1991 and the 2001 NDS? I notice that the shear stress adjustment factor is gone. In the case of SPF, the value went from 70 to 135 psi. The old shear stress factor was to account for the influence of splits, shakes, and checks. Are they saying that the presence of these does not affect the shear strength of the wood?

 

[SlideRuleEra]

The ASTM procedures used to compute shear value "double counted" the effects of splits, checks, etc. This was corrected in the ASTM document in 2000. The NDS standard was corrected in 2001. See page 2 of the following link:

http://www.awc.org/Publications/papers/NDS2001article.pdf

 

[bjb]

Thanks SlideRuleEra. As I think about the wood framing that I had designed prior to using the 2001 NDS, I seem to remember a fair number of beams that had shear stress governing the design.

 

[UcfSE]

Thank you SlideRuleEra, that's a great link. It helps clear up some confusion I had as well about the increase in allowable shear stress. I would like to add that if you use the ASD Manuals published by the NDS be careful, there are mistakes in allowable load tables (Table 5.4a in the ASD supplement), at least for moment capacity of SYP sawn lumber. It appears that the shear capacity V and EI are correct but it also looks as though the moment capacity uses old NDS values for Fb. There may be others as well but I didn't check the entire table.

 

[bjb]

I've noticed too that there seem to be many mistakes in the NDS and it's supplements

 

[boo1]

Nice link SlideRuleEra, Thanks

 

[SlideRuleEra]

Glad that I could help.