Help Understanding NELMA Grading Standards

Woodland Dave · February 09, 2024, 10:28:29 AM

I'm trying to asses my stack of white ash beams I have for a future timber frame build and have read though the forum here to learn how grading works. This brought me to the NELMA grade rule book. I'm having some trouble understanding it. All my beams are box hearted so I'm mostly worried about checking and splitting. I think I understand all the other categories (knots, slope of grain, etc.) but the checking section confuses me. In section 6: Beams and Stringers, posts and timbers, 25.1 states "Seasoning checks, single or opposite each other with a sum total equal to 1/4 the thickness." This is for select structural grade. It's the same for No. 1 grade but for No. 2 grade it just says "seasoning checks". I take this to mean the checks can only go through the wood one quarter of the the width for structural and No. 1 grade. So for No.2 they can go all the way to the pith, as some of my timbers do? What it says about splits I don't understand either.

What folks are talking about in this post has me worried about my timbers which is why I'm trying to understand all this. My situation isn't as bad as the original poster but I want to learn more.
https://forestryforum.com/board/index.php?topic=95937.0

Thanks!

Don P · February 09, 2024, 07:50:10 PM

Checks, I believe your interpretation is correct, notice in 25.3 it says "CHARACTERISTICS and limiting provisions" then under checks it says seasoning checks, as in yup that is one of the characteristics of #2 beams and stringers, it'll have seasoning checks. I do not see limiting provisions only a characteristic.

I pulled out the NGR grading manual, kind of the model manual for the grading agencies like NeLma to develop their rules under. I think you can access this free from the cwc website. I'm just going to quote the glossary and see if that sheds enough light without me muddying things.

704 Checks: A separation of the wood normally occurring across or through the rings of annual growth and usually as a result of seasoning.

a) A surface check occurs on a face of a piece
b) A through check extends from one surface of a piece to the opposite or adjoining surface.
c)-f) don't apply enough to type right now. Well, if you see roller check anywhere, that is a machine made check when a cupped piece goes through a feed roller (planer) and the cup gets smashed flat and cracks the piece. Usually lumber rather than a heavy timber.

742 Splits: A separation of the wood through the piece to the opposite surface or to an adjoining surface due to the tearing apart of the wood cells.
a) A very short split is equal in length to 1/2 the width of the piece (me, width=wide face)
b) A short split is equal to the width of the piece but in no case exceeds 1/6 of the length.
c) A medium split is equal in length to twice the width of the piece and in no case exceeds 1/6 the length.
d) A long split exceeds a medium split. (me, long split=firewood)

Reboot if that didn't get it. Also, A friend asked about getting some ash stair treads. I have no live ash left and the standing ones are not structural at all.

When grading mentally be an uninvolved grader, the independent third party. Never let your need guide that judgement, be ruthless on the sawyer, he can make more. Despite what I've said, I've had more than connections fail under me.

Checking and splits relate to horizontal shear. Notice in most beams that is far from being the control. If it is getting high on say a short, heavily loaded beam, say that church tower that collapsed a couple of weeks ago, pay more attention to anything that would contribute to a shear failure.

edit, see also 802.0 in your NeLma book

Woodland Dave · February 10, 2024, 01:03:39 PM

@Don P

Thanks for the thorough reply. Good tip on the NGR grading manual. I did find it online. I think I now get how to asses the beams without thinking the grade rules will give me some sort of clear pass or fail.

I also think I understand how you rationalize all this. For beams that have large shear loads, the timbers should be free of checks and splits because its these defects that contribute to failure under large shear loads. So it comes down to understanding the stresses on each member and using a timber that can obviously perform for the task. There isn't some special grade rule equation that will scale the timbers capacities down by accommodating for it's defects. Better to have something free of defects in high stress locations.

This all reinforces my desire to learn more structural engineering as I talked about in another post. Seems like at least the basics would be massively helpful.

Don P · February 10, 2024, 07:30:42 PM

Quote from: Woodland Dave on February 10, 2024, 01:03:39 PMSo it comes down to understanding the stresses on each member and using a timber that can obviously perform for the task. There isn't some special grade rule equation that will scale the timbers capacities down by accommodating for it's defects. Better to have something free of defects in high stress locations.

I'm going to tear that apart a little.
The first sentence is correct. The more you understand the conditions of use the better you can pick your timber.

The second sentence I disagree with. The purpose of grading is to group timbers of similar capacity together and assign them safe working stresses. You grade then look up the allowable strengths in the supplement for that species and grade. The engineering tells you the strength required.

However, wood is a natural material, it is not uniform like steel. Within a grade there is a bell curve of actual strength variation between timbers. That is where knowing how to see and assess the controlling defects in any given timber and knowing the locations in the structure needing the best timber let's you better sort what to use where. Which I think is your point in the last sentence.

The building safety folks want you to be a parts installer. If you pick from the #2 pile where the plans call for a #2 you will not be wrong if the grading is "on". But for instance, I know where the kitchen and bath are, barring any other high loads I'm going to sort joists in favor of better ones in those heavier rooms. Then at the cutting bench I can sometimes bump the grade by removing the controlling defect. And I can slip and slide around on the layout to make sure I don't further weaken a timber by working it in a defect zone, clear wood for connections, look at their design criteria and what further "damage" of notching, nailing or bolting would do to a defect zone. They want you to assemble a uniform part some other knowledgeable person stamped. That's pretty much bs with a natural material, you really need to be able to look at and understand the natural thing in front of you to make the best assembly. Happily it isn't really that hard.

Woodland Dave · February 11, 2024, 12:02:14 PM

This really helped my perspective on all this. Thanks Don! I appreciate you "tearing apart" my poorly worded post. I realized that my second sentence you corrected didn't really convey my meaning right but it doesn't matter because you gave me a much better way of looking at things. I know what I need to start studying up on now.