Rethinking How to Specify Lumber on Construction Plans 3

[FrankWoeste]

Due to additional grade-marked products entering the U.S. from other countries over the past 20 years, a simple specification such as No.2 KD-19 XYZ-species may no longer clearly communicate to other parties what products and design values were assumed and used by the RDP in their design work and thus required for the project. For example, see 2024 NDS Supplement Table 4G starting on p. 63 for lumber made in the U.S. and around the World at the AWC website:

https://awc.org/pdf-viewer/?idp=1413&idf=4

The authors suggest a more robust specification as published in the Structure Magazine:

https://www.structuremag.org/?p=23897

Your comments are valuable and greatly appreciated.

 

[lexpatrie]

I usually specify minimum allowable stress values on the drawings. Since I do a fair mix of new/old fix/retrofit, I usually disclose the design values used for the old wood as well.

Please excuse all the excessive semicolons. It's something by cad program has decided to start adding every time I touch the drawing.

Glad so see you here. Your publications have been quite useful to me over the years. Guardrails, ledgers, deck loads all spring to mind as being foundational works in the field. Also appreciations to Dolan, Bender, Parsons and your other frequent collaborators whose names I have regrettably not remembered to mention. Osterberger. There. That's one more.

Regards,
Brian

 

[Eng16080]

Agree with lexpatrie. My notes are similar:

I have similar notes for LVL and PSL material. For LVLs in particular, I always design for the lowest values that a builder might realistically find locally. For example, I think a Parallam LVL has an Fb of 3100 psi. This is great and all, but I find that when engineers design based on this value, they'll often find a Microllam LVL installed on-site with an Fb of 2650 psi. In general, I think it's wise to very explicitly state what the material properties are expected to be.

 

[XR250]

Honestly, for residential, I have found the best practice is to just design conservatively. No one is reading your specifications.

 

[lexpatrie]

Not sure what I'm supposed to understand from that comment XR250. My design does not contemplate willful disregard for the supplied drawings on the part of the contractor. I rely on the building department to notice when "bamboo, couch stuffing, and twigs" are substituted for actual grade stamped lumber. At which point it's hourly to "fix" it. And my work will start once I receive a down-payment on the expected expense of reworking their nonsense.

 

[XR250]

Not sure what I'm supposed to understand from that comment XR250. My design does not contemplate willful disregard for the supplied drawings on the part of the contractor. I rely on the building department to notice when "bamboo, couch stuffing, and twigs" are substituted for actual grade stamped lumber. At which point it's hourly to "fix" it. And my work will start once I receive a down-payment on the expected expense of reworking their nonsense.

Honestly, I have never had to "fix" a lumber grading issue in 30 years. Everyone around here is using #2 for joists, STUD grade for 8 ft. studs and #2 for longer studs. And I can count one hand how many failures I have seen that were due to a grading issue. Mostly in old joists caused by knots being on the edge of the tension side.
I generally call out joist grades on the plans (#2) and stud grades when it is important but my plans are very sparce when it comes to specs.

 

[Eng16080]

There will always be a broad spectrum of what builders might do with a set of structural plans. I've personally encountered builders who followed my (very detailed) plans almost exactly, and then others, who disregarded the plans to such an extent that I questioned if they even used them.

For me, I don't really care to cater to the latter. Just because some builders won't read my notes or look at my details doesn't mean I shouldn't have those notes or details. I like to think that the engineering actually matters and that at the end of the project, between my (hopefully) well thought-out plans and a competent, detail-oriented builder executing them, the end result will far exceed what might have been without an engineer and without a decent builder. Maybe I'm full of crap, I don't know. I certainly don't think it's foolish to explicitly state the materials that I expect to be used on the job. If there were an issue later, I generally like to have things clearly documented.

And at the same time, to XR250's point, I still try to provide designs where I'm not pushing the limits of what a particular material is capable of. That way, I have a bit of a buffer if there is a mistake.

 

[XR250]

Honestly, I have never had to "fix" a lumber grading issue in 30 years.

Actually, I take that back. The local lumber supplier had me help them out during the supply chain issues where they were selling European spruce lumber. The local AHJ flagged the job during the framing inspection. I had to decipher the grading stamp for them. Turns out it met code - but just barely for density.

 

[Alistair_Heaton]

Out of interest what was the European grading issued?

Since Russia closed what's available has changed alot.

 

[phamENG]

but just barely for density

That's my biggest concern. And it's prompted me to restrict lumber used to domestically sourced materials. What passes for 'Southern Pine' in the non-SPIB tables in the NDS have laughably low G values.

 

[Jdub Engineer]

It seems like the summary of the article is to spec the Fb, Fv, Ft, Fc\\, Fcperp, E, and G of the material you're using, instead of just the species and grade so that it is easier for lumber yards to substitute stuff that is equivalent or better.

Am I getting that right?

 

[bones206]

We had a multifamily project recently where the plywood sheathing the GC used was from South America. It had a PS-1 stamp, but we did some digging and apparently there was a big lawsuit by American manufacturers and the APA that showed the plywood failed at levels well below PS-1 specs. We read about a federal court injunction in 2022 but yet somehow the GC was able to buy it and build a 5 story building out of it in 2024.

https://www.nahb.org/blog/2022/08/b...dered-to-revoke-certificates-and-grade-stamps

 

[Eng16080]

It had a PS-1 stamp, but we did some digging and apparently there was a big lawsuit by American manufacturers and the APA that showed the plywood failed at levels well below PS-1 specs.

This is a good example why some of us like to explicitly show design values on our plans.

 

[bones206]

As far as the GC knew, it was meeting the specs. But in reality the grading stamp was not a legitimate reflection of the true grade.

 

[Lomarandil]

And I think that's the rub.

I work with pretty sophisticated heavy contractors... and even when (as sometimes required by spec) I do list all the expected design values, the best they're going to do is ask their sales guy at the lumber yard. That sales guy likely does not have the qualifications or inherent motivation to do anything beyond reading the grade stamp.

This is a grading agency issue, not an engineering specification issue.

 

[bones206]

Agreed. And the worst part is these shoddy wood products are also accelerating the deforestation of the Amazon.

 

[Jdub Engineer]

I agree and disagree about the rub. some lumber yards are more sophisticated, and they can/will figure out how to substitute lumber correctly and save the customer money by doing that. The less sophisticated lumberyards will have to follow suit to keep up. If we do our part by providing a better spec, then the industry can start to follow us. it may take a few years or decades, but at least we're not part of the problem.

 

[phamENG]

The less sophisticated lumberyards will have to follow suit to keep up.

You'd think, right? But sadly I think you'd be wrong. There will always be a demand for fast and cheap regardless of the consequences.