by Dwaine Charbonneau, P.E. at RedBuilt
The good news is that I have another story I can tell my grandkids someday: "I remember when a number two Southern Pine two-by-four was good for 1500 psi. Now that was a stout piece of lumber!" The bad news, of course – aside from the alarming realization that I really need some better stories to tell my grandkids – is that an SP #2 2x4 isn't as strong as it used to be, as of June 1.
This is the sort of news that doesn't exactly go viral, much less pique the curiosity of toddlers, but if you're into the design of wood structures, it's likely you've heard about it. And since you're perusing the RedBuilt™ blog, you may be wondering how the drop in certain design values for certain sizes and grades of Southern Pine (SP) affects RedBuilt™ engineered wood products. We, in turn, are curious how this might affect you and others in the structural design community.
Before I tell you what this means to RedBuilt™, allow me to point to a few aspects of this story that are sometimes neglected. These significant reductions in SP design values affect more than just the SP #2 2x4. The decreases involve seven grades and six sizes of lumber, from No. 2 Dense through Utility, from 2x2 through 4x4. The affected design values include those that are related to axial stress, such as bending (about a 30% loss), tension (20%), and compression (35%), but not those that are associated with specific gravity, such as shear or compression perpendicular to grain. In all, you'll find 50 revised numbers in Table 4B of the NDS Supplement: Design Values for Wood Construction.
Given the section sizes of lumber involved, we're not looking at a typical floor joist or roof rafter, except for light loads or short spans. More common uses for 2x4 and 4x4 members that call upon bending and axial strength include:
Another use that may come to mind when thinking about combined bending and axial forces is truss chords. Indeed, for those who build trusses with visual grades of Southern Pine, the decline in these values may force truss designers to consider a number of alternatives. For instance, you might throw in another web to shorten the top chord panel. Or you might bump the grade to No. 1, or the size to 2x6. Or you might eschew the visual grades altogether and go with MSR or MEL mechanical grades. For example, 1500f-1.6E MSR is a good substitute for the former SP #2 2x4, if you can get it.
The effect of the SP design value reductions on RedBuilt™ products is minimal precisely because we use mechanical grades in our trusses. A 1500f-1.6E piece of MSR is always good for 1500 psi in bending – not to mention that we use nothing less than 2100f-1.8E MSR in our trusses. The RedLam LVL we use in our I-joist chords is also qualified and controlled to strength, rather than visual standards.
RedBuilt™ may provide visual grades in accessories, such as blocking, where axial stresses are nonexistent. A minor change in our bridging for 4-foot-on-center truss systems is possible once the other shoe drops, so we're keeping a close eye on design value revisions that are in the works. Reductions for more sizes and grades of Southern Pine and Mixed Southern Pine – potentially anything in NDS Table 4B – may follow the ALSC Board of Review meeting in October. Exactly when or by how much, nobody knows.
So what does this mean to you? Have the reductions in design values for these sizes and grades of Southern Pine affected your work? Has it forced a change in grade, size, or species, or no changes at all? Have you been pro-active with the adjustments, or did you wait until June 1? Does this affect your confidence in visual grades, or cause you to re-assess your use of visual versus mechanical grades in your designs?
It's an interesting topic in many ways, so please don't hesitate to share your thoughts.
Here are a few related links. A good Q&A article from Coastal Contractor:
An in-depth slideshow provided by Woodworks:
Amended design values and span tables for the NDS, IBC, and IRC:
- Studs and posts: combined bending and axial
- Braces: tension and compression
- Top plates: diaphragm chord axial forces, bending between studs
- Connections: net section and group tear-out