How this calculator works
The shortest of the bending, shear and deflection spans governs this simplified screen. Enter final adjusted design values for the actual species, grade, moisture, temperature, duration, size, treatment and restraint. Values are not selected from species names here. Built-up plies must act as assumed and be connected appropriately. Live-load-only and total-load deflection may require separate runs.
Formula
I=bh^3/12; S=bh^2/6; Lb=sqrt(8 Fb S/w); Lv=4 Fv bh/(3w); Ld=[384 EI/(5wr)]^(1/3), for delta<=L/r. Limiting L=min(Lb,Lv,Ld).
Worked example
A 100 x 200 mm beam with E=10 GPa, Fb=10 MPa, Fv=1 MPa, w=1000 N/m and L/360 gives bending span 7.30297 m, shear span 26.6667 m and deflection span 5.21982 m. Deflection governs.
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Frequently asked questions
Can I use nominal lumber sizes?
No. Use actual surfaced dimensions and adjusted design values from the applicable standard or supplier.
Are duration and moisture factors calculated?
No. Enter values after all required adjustments. The reference text records where you obtained them.
Can I build from the limiting span?
No. Results are estimates and are not a substitute for a licensed engineer. Bearing, connections, stability, local code requirements and all applicable load combinations need separate checks.