4x8 vs 6x8 SYP #2

[masterdesign]

Wondering if I am missing something.

4x8 SYP #2
Fc = 1350 psi

6x8 SYP #2
Fc = 525 psi

Looking at buckling only, it appears a 4x8 is stronger than a 6x8?

The numbers don't lie and clearly the reason is because the values for visually grade timbers is so much lower than visually graded dimension lumber, but it just doesn't seem right.

Please help me feel better about it.

 

[Eng16080]

Yup, I've always thought this seemed off as well, but I think you're mostly correct here.

The timber grade assumes "Wet Service Conditions" which I assume means the wet service factor (Cm) is included in the Fc value in the table, which is not the case for the dimension lumber value. I don't know the reasoning behind this. I guess it's being assumed that Southern Pine timbers will only be used in wet service conditions. Perhaps you can use an increased value if this won't be the case. Even then, there would still be a considerable difference between stress values.

 

[Ron247]

As far as the "Wet Service, it is written directly over the SYP entries in Table 4D, and is noted as such in the Adjustment Factors prior to the table.

I think you can use the larger "Green" dimensions for the lumber size. Table 1A of chapter 3 I think shows the sizes. See Footnote 1 of the Table 4D footnotes. That will give you some more allowable.

Since you cannot use the 2" to 4" values, it kicks you into the 5x5 timbers. They rate all of them the same for some reason. A 5x5 and a 12x12 would both be 525 psi whereas typically you see a drop in most values with larger sizes. A 4x8 is 1350 psi, but a 4x12 is 1300 psi, not much deduction.

Maybe they have not researched/tested the larger sizes as much.

 

[lexpatrie]

I suspect this is beam and stringer versus post and timber.

Posts are intended to be use primarily in compression with minimal bending, and some checking and cracking isn't all that meaningful.

Beams are obviously typically bending so they are more important on the bending and the grading follows.

I would not use "wet dimensions" you're expected to use the nominal dimensions, that's written in the standard.

The lower stress on the larger element has more area, so it may counteract the lower allowable compression, and there are other effects, since the unbraced length and the "slenderness" are affected by the dimensions, so the 6x8 will have a slower drop off as a compression element than the 4x8 when they are used as columns and the 4" and 6" govern slenderness.

 

[Ron247]

I would not use "wet dimensions" you're expected to use the nominal dimensions, that's written in the standard.

I got my info from the the 2018 NDS. What code/year are you getting that from and would that apply to both columns and beams, or just beams/stringers? I have never needed to use the larger dimensions but know others who have based on the notes I cited.

 

[lexpatrie]

Source: 2018 IBC

I'm not convinced there are established "wet" dimensions for green timber, but I'd strongly suspect typical design practice is to ignore this, and I'd point at the Thompson design example as a west coast design example that uses the 'dry' net dimensions for various calculations. I'll lean back on 1604.4 now. Are there published design values for green lumber, I don't design with it, as I'm not west coast?

Side note: I said "nominal dimensions" above, what I meant was the net dimensions, so 1.5" x 3.5" for a 2x4. Nominal is technically the 2" and the 4".

 

[Ron247]

View attachment 15588
Source: 2018 IBC

Interesting. 2018 NDS shows and states you can use green dimensions (not nominal and not net). Green for an "8" is like 7.625 rather than 7.25 I think. The table outlines them. If IBC, allows NDS, then we have the classic confusion of what to apply.

The reason I would consider using them if the code does allow it, is the very low magnitude of Fc published once you get to 5x5 or larger.

 

[lexpatrie]

IBC is generally "after" the design standard so it would override the design standard. As a precedent the 2000 IBC modified a LOT of ACI 318-99 in Chapter 19.

I don't know why anybody would bother with green dimensions and checking things based on those section properties because they WON'T last. I don't think the NDS says what you think it says.


Source: 2024 NDS

OOKAYY. That's just weird.

 

[Ron247]

As far as the "Wet Service, it is written directly over the SYP entries in Table 4D, and is noted as such in the Adjustment Factors prior to the table.

I think you can use the larger "Green" dimensions for the lumber size. Table 1A of chapter 3 I think shows the sizes. See Footnote 1 of the Table 4D footnotes. That will give you some more allowable.

The things I cite here also appear to be in the 2024 NDS. Table 4D and Table 1A are in the Supplement I think, not the code.

 

[Ron247]

2304.2 says "not nominal". Green dimensions are not nominal. Nominal is the even numbers 4, 6, 8 etc. They are the actual dimensions of the green lumber "I think". Table 1A of Ch3 shows them listed next to dry and both listed as "minimum". 7.25 dry and 7.5 wet for example for a nominal 8. The nominal 8 is also in the table, next to them.

 

[Brick Jagger]

I agree with Ron. I think it is a mix of quality control issues for larger sizes and maybe also less research.

Side note, I've never looked into the strengths of southern pine before and didn't realize how hugely different they are from the typical woods I design with (DFL / Hem-fir). The drop off from 2" to 4" to 5"x5" members is much less pronounced with non-southern pine.

 

[Ron247]

I think it is a mix of quality control issues for larger sizes and maybe also less research.

I was amazed years ago when I would look into a "Don't Rule" of some kind in a design code. Many times, it was not that they looked into it and found the item to be dangerous of undesirable, it was just that it was never researched. That is why some commentaries are so valuable. If we never know why it is discouraged, we may come up with our own logic. That can be dangerous. "Don't play with matches! Why? Because dynamite is more fun"

As far as the green lumber issue, having the Fc so low because it was based on wet but allowing slightly larger dimensions would be better understood if they gave some indication of why. Without that, it really does not make much sense. One reason may be that they found that as the dimensions shrunk while drying (as pointed out by Lex), the allowed Fc went up at a slightly higher rate. At least that (if it were true) would give us some reason why and a connection to the allowance. "Your allowed to use Green dimensions. Why? Because we think anyone who would use Green dimensions is probably going to do their calculation wrong anyway, so why care."

 

[lexpatrie]

As a footnote, the 2017 Woodworks design example has 4x8 end posts for shear walls, if we go that route, the "green" dimension only applies to the 8" dimension.... which is a bit funky, if you ask me, I haven't looked too closely, so I can't say if they use the net dimensions for the post capacity calculations, but i do remember having some difficulty recreating the numbers, perhaps that little green footnote was part of it.

Five-Story Wood-Frame Structure Over Podium Slab - WoodWorks | Wood Products Council

Design example illustrating the seismic and wind design of a hotel that includes five stories of wood-frame construction over a one-story concrete podium slab and is assigned to Seismic Design Category D.

www.woodworks.org

There's a fair bit of nuance in this document regarding green lumber so it would be a worth read for those in the "green zone". I'm a bit mystified by the language in the NDS, it's not something I've noticed before and I like to think of myself as pretty proficient. Foiled by the fine print yet again. And I read that standard entirely back around 2012.

 

[Ron247]

The 4x8s along with 4x12s are completely in Table 4B whereas it only takes the narrowest dimension becoming 5 or more that kicks you into Table 4D. One reason I have never taken the Green dimension allowance, is I have only used timbers that size on new construction. If I was trying to check something existing and was barely out of allowance, I might fall back then as a solution. I don't think the item is going to truly collapse because of it.