Beam calculator
Three-check beam analysis: bending stress, deflection, and horizontal shear per NDS reference values.
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A beam must pass three independent checks to be structurally adequate. Failing any one means the beam is undersized for that condition, even if it passes the other two.
The Three Checks
1. Bending (Fiberstress) — The beam must resist the bending moment without exceeding the allowable bending stress (Fb) for the species and grade. This check compares the required section modulus against what the beam provides.
2. Deflection — The beam must not deflect more than L/360 for floors or L/240 for roofs. Excessive deflection causes bouncy floors, cracked finishes, and visible sag even if the beam is structurally safe.
3. Horizontal Shear — Wood beams can fail along the grain near the supports. The actual shear stress (fv = 3V/2bd) must not exceed the allowable shear stress (Fv).
What load should I enter?
Total load = dead load + live load, expressed in pounds per linear foot of beam. For a typical residential floor beam: dead load ~10-15 psf, live load 40 psf, multiplied by the tributary width. A beam carrying 8 feet of floor at 50 psf total would have 400 lbs/ft total load.
FAQ
How do I size a beam for a given span?
Determine the tributary load area, calculate the total load in pounds per linear foot, then check that the beam's allowable bending stress, shear capacity, and deflection limits all pass. This calculator handles all three checks — enter your span, load, and lumber species to see if the beam works.
What does L/360 deflection mean?
L/360 means the maximum allowable deflection is the span length divided by 360. For a 12-foot beam, that's 0.4 inches. Floor beams typically require L/360 for live load to prevent bouncy floors and cracked finishes.
When should I use an LVL instead of dimensional lumber?
LVL (laminated veneer lumber) beams are stronger and more consistent than sawn lumber. Use LVL when dimensional lumber can't span the required distance, when you need a shallower beam depth, or when spans exceed 12-14 feet.