NDS Section 3.4.3

[msquared48]

How many use the provisions of this section to reduce the design shear stress, similar to the ACI provision for concrete? Just wondering. I have never used it just to be conservative, especially with the allowable shear stresses having been doubled in recent years. Seems like the envelope is being pushed here.

Mike McCann, PE, SE (WA)

 

[manstrom]

It used to be Cv=2.0 if the wood is free from splits at the ends. NDS 97 or so. fv values were very low.

Then they doubled the shear value and basically said Cv = 2.0 everywhere. I assume this was due to testing and the fact that shear failures aren't common.

If you have a big point load near the support of a beam, crushing will likely control before shear.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Triangled]

I'll use it when I need it. I've thought of 3.4.3 and horizontal shear being two different mechanisms.
I talked to an AFPA(? The little green lumber grading book) guy about the split length multipliers for various split lengths back after the 1994 quake. He said, which I can still hardly believe, was that the horizontal shear of the day for solid sawn, 90psi for DF type if I recall right, assumed split the full length of the board, I dare say a slight bit conservative.
Well of course, same species DF glulam allowed 165 psi shear, almost double, at the same time. Same species.
Now I understand it's where it should be. But observed splits in the field for modern framing in heavy shear may not be OK.

 

[Archie264]

I've used it on occasion. Specifically, I've used the provision for reduced shear stress within a distance, d, of a fully supported member. Usually this was with repetitive members, which also provided a level of comfort in that if one member were overloaded adjacent members would carry some of the load.

I've also used the provisions for notched-end members but feel less comfortable with that since the effects of poor workmanship could be more problematic in that circumstance, in my opinion.

Also, I would look at everything more carefully now that they've almost tripled the allowable shear stresses.

 

[woodman88]

Per the attached link, page 42

"Review of ASTM procedures used to establish allowable shear stresses revealed that shear values were being reduced by two separate factors for the effects of splits, checks and/or shakes. One of these adjustments was made to the base value, the other was an adjustment to design values for grade effects. In 2000, ASTM standard 245 was revised to remove one of these adjustments which resulted in an increase of nearly 2 for allowable shear design values; however, grade effect adjustments were eliminated.
In the 2001 NDS Supplement, shear design values for sawn lumber are approximately 1.95 times higher than values printed in the 1997 edition due to a change in the basis of the values established in ASTM standards. With this change in basis, shear related provisions in the NDS were re-evaluated and modified where necessary to provide appropriate designs. These changes showed up prominently in Chapter 3."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Archie264]

Oh, that's right, the fv value for Southern Yellow Pine went from 90 psi to 175 psi, a 1.95% increase. For some reason I was thinking it had gone to 250 psi. My mistake.

 

[msquared48]

Manstrom:

Yea, I remember that factor.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

I've never used it but wouldn't be opposed to it for many of the reasons people above indicated. I've had very few times where I actually needed it and typically it was something that was a one-off item where adding a little more thickness or another ply wasn't going to cost the contractor more than a few bucks. Also, it's useful as one of the back-pocket items that if you get up against a wall you can pull it out and salvage something (like contractors notching beams that shouldn't be notched).

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)