Training 7: Worked calculation, wood
A built-up header over a 12 ft opening. New calculation, Sawn lumber beam.
Inputs:
| Input | Value |
|---|---|
| Species, grade | Douglas Fir-Larch, No. 2 |
| Size, plies | 2x12, 2 plies |
| Fastening between plies | Nails |
| Length, supports | 12 ft, pin / roller at both ends |
| Line loads | D = 150 plf, L = 300 plf |
Everything else stays at its default: LRFD, dry service, normal temperature, 3 in. bearing, span / 360 and span / 240.
Two plies #
Not adequate. Bending stress is 2,286 psi against 1,788 psi, a ratio of 1.279, under 1.2D + 1.6L. Everything else passes.
Three plies #
Change plies to 3.
| Check | Clause | Required | Available | Ratio |
|---|---|---|---|---|
| Bending stress | NDS 2024 3.3 | 1,535 psi | 1,788 psi | 0.859 |
| Shear stress | NDS 2024 3.4 | 99 psi | 311 psi | 0.317 |
| Live load deflection | IBC 2021 Table 1604.3 (limit entered); NDS 2024 3.5 | 0.164 in | 0.4 in | 0.41 |
| Total load deflection | IBC 2021 Table 1604.3 (limit entered); NDS 2024 3.5 | 0.297 in | 0.6 in | 0.495 |
| Fastening between plies | NDS 2024 12.3, Table 11.3.1 | 440 plf | 486 plf | 0.906 |
Read it. The governing check is now the Fastening between plies at 0.906, not bending. A third ply adds a joint: that is why the nailing check exists and why you should open it. The largest reaction is 4,049 lb.
A word on the reference values. The NDS 2024 Supplement is not on file: the wood tables here are 2018 values marked for review, as the chapter says. Enter your own if yours differ. The inputs are in the editor.