Douglas Fir-Larch, No. 1 - Design Values
Douglas Fir-Larch, No. 1 is a dimensional lumber grade with reference design values to NDS 2018 Supplement Table 4A. Its reference bending value Fb is 1,000 psi, compression parallel Fc is 1,500 psi, and modulus of elasticity E is 1,700,000 psi (NDS). Values are shown before adjustment factors; apply the relevant NDS C-factors or EC5 k_mod and γ_M for your load case.
| Fb | Bending | 1,000 psi |
| Ft | Tension parallel | 675 psi |
| Fv | Shear parallel | 180 psi |
| Fc⊥ | Compression perpendicular | 625 psi |
| Fc | Compression parallel | 1,500 psi |
| E | Modulus of elasticity | 1,700,000 psi |
| Emin | Modulus for stability | 620,000 psi |
| G | Specific gravity | 0.5 |
| Nominal | Actual (in) | Metric (mm) |
|---|---|---|
| 2x4 | 1.5 x 3.5 | 38 x 89 |
| 2x6 | 1.5 x 5.5 | 38 x 140 |
| 2x8 | 1.5 x 7.25 | 38 x 184 |
| 2x10 | 1.5 x 9.25 | 38 x 235 |
| 2x12 | 1.5 x 11.25 | 38 x 286 |
Source: NDS 2018 Supplement Table 4A. Reference values are shown before adjustment factors (NDS C-factors / EC5 k_mod, γ_M) - apply these per your load case in design.
Understanding these values
For a beam in Douglas Fir-Larch, No. 1, bending at Fb 1,000 psi normally sets the size, but the check that actually governs depends on the span. Short heavily loaded members are often limited by shear at Fv 180 psi, and bearing at a support is limited by compression perpendicular to grain at Fc perp 625 psi, which is the lowest of the strength values and the one most often overlooked. Used as a post or stud, the governing value is compression parallel at Fc 1,500 psi.
Stiffness for Douglas Fir-Larch, No. 1 is E 1,700,000 psi, with the reduced modulus Emin 620,000 psi used for stability calculations such as the beam stability factor CL and the column stability factor CP. On typical floor spans, deflection rather than strength decides the section, so compare E before Fb when the member is a joist. Its specific gravity of 0.5 is the value connection design works from, not the strength figures.
Within the dimensional lumber entries in this catalogue, Douglas Fir-Larch, No. 1 ranks 7 of 13 on bending at 1,000 psi; the strongest listed is Southern Pine, Select Structural at 2,550 psi. Grade choice is usually a trade of cost against section depth, so it is worth checking whether a lower grade in a deeper section is cheaper than a higher grade at the depth you started with.
The closest alternative in the same family is Hem-Fir, No. 1 at 975 psi in bending, 3% below this grade. If availability drives the choice rather than capacity, that is the substitution to price first - but re-run the deflection check as well, because stiffness and strength do not always move together.