NDS Beams and Stringers vs Posts and Timbers 1

[wikidcool]

2018 NDS 4.1.3.3 defines "Beams and Stringers" as lumber with a 5" or greater nominal thickness and a width more than 2" greater than the thickness. 4.1.3.4 defines "Posts and Timbers" as 5"x5" or greater nominal thickness with a width not more than 2" greater than thickness. That would technically put 6x8, 8x10, etc in the Post and Timber category, even when used as beams.

But 4.2.5.4 states: "When Post and Timber sizes of lumber are graded to Beam and Stringer grade requirements, design values for the applicable Beam and Stringer grades shall be used..."

Where I practice in the western United States, Douglas Fir-Larch is the prominent building lumber species. DFL #1 has Fb = 1,350 psi for B&S grading, but only 1,200 psi for P&T grading. When Joe Framer calls up the local lumber yard and orders a bunch of "6x8s", I'm wondering what grading category is typically provided. Does anyone have any experience with this?

 

[jerseyshore]

This is an interesting question. I feel like in general it's hard to get any info from a lumber yard on manufacturer unless it's an LVL or PSL with a big stamp on the side.

I've never specified anything besides regular 2x's or 4x4's or 6x6's before so I have no idea of this one. Curious to see if anyone has any knowledge of typical grading for these shapes.

 

[Lomarandil]

I don't do a lot of timber design, but my understanding is that the 4.2.5.4 clause is a rare occurrence, and that P&T is the default for larger sizes.

 

[gte447f]

I would default to assuming the lesser design values for posts and timbers unless I had definitive verification of the alternative, and I would not waste time searching for any such verification myself. It would have to come to me completely unsolicited, which probably isn't ever going to happen.

 

[wikidcool]

I would default to assuming the lesser design values for posts and timbers unless I had definitive verification of the alternative,

Of course, and that's what I've been doing for years, but that's not my question. I'm curious about what lumber yards are supplying "in the wild" for these beams. I spec a LOT of 6x8s.

 

[gte447f]

Sorry I can't be of help. I don't know what is being supplied in the wild.

 

[gte447f]

wikidcool, I did find the following excerpted from the west coast lumber inspection bureau grading rules:

Pieces 5" x 5" and larger, with the width not more than
2" greater than the thickness, may be graded under this
paragraph provided all faces are graded as narrow faces.
Pieces so graded will have a fiber stress similar to that
of the following grades. If grade stamped, they will be
stamped with stamps showing only the appropriate fiber
stress in bending “f.”

I'm not sure exactly how to interpret the above, but it seems to me like maybe the "post and timber" sizes that get graded as "beams and stringers" will be stamped with a bending stress value instead of a regular grade stamp (e.g., structural, No1, No2, etc.).

Still, I have no idea how common this practice is or whether these pieces are regularly available.

 

[SE2607]

I've always used P&T for the sizes you mentioned in your post. I doubt if many Joe Framers would mention lumber grade at all when getting quotes even though I always show the grade on the plans right with the beam size, not just in some note buried in my structural notes. Even then, I don't always get what I specified.