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.