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.