Flitch Beam Calculator
Check a flitch beam (2x lumber bolted to steel plates) for your span and load. Pick the lumber, plies and plate, enter the span and loads, and see whether it works and the longest span it can carry.
Works: a 2-2x12 + 1/2 in plate over 16 ft uses 93% of its capacity, limited by bending.
| Check | Used | |
|---|---|---|
| Bending | 93% | OK |
| Shear | 27% | OK |
| Live load deflection (span/360) | 44% | OK |
| Total load deflection (span/240) | 45% | OK |
Longest span for this load: 16′ 7″. The beam weighs 27 lb/ft, which the checks add to the load.
Douglas Fir-Larch No. 2 lumber and ASTM A36 plates, simply supported, with allowable stress design (ASD) under the National Design Specification (NDS) for wood. Loads are per foot of beam, without the beam's own weight. The button opens this beam in the free beam designer, where you can add point loads, more spans, other wood species and the bolt layout.
What load to enter
A beam under floor joists carries half the joist span on each side. Multiply that width by the floor load: a house floor is about 15 psf dead load (the floor itself) and 40 psf live load (people and furniture), so each foot of width adds 15 lb/ft dead and 40 lb/ft live. Add any wall or roof that bears on the beam.
For example, joists spanning 12 ft on each side put 12 ft of floor on the beam: 180 lb/ft dead and 480 lb/ft live. Over a 14 ft span, a 3-2x12 + 2 × 3/8 in plates works, using 63% of its capacity in bending. Open this example in the beam designer.
What the calculator checks
- Bending in both the wood and the steel. The plate is about 18 times stiffer than the wood, so it's turned into the wood that would do the same job (n = Esteel / Ewood = 18.13), and each material's stress is checked against its allowable stress: Fb′ = 900 psi × the size factor (1.0 for a 2x12) for the wood and 0.6 Fy = 21,600 psi for the steel.
- Shear, split between the wood and the plate by their stiffness.
- Deflection: live load to span/360 and total load, including the beam's weight, to span/240.
The plates add a lot: three 2x12s alone have an allowable moment of 7,119 ft-lb, and with two 3/8 in plates they have 27,223 ft-lb. Bending usually governs on short spans and deflection on long ones.
The flitch beam design guide works through these checks by hand. The calculator assumes the plies and plates are bolted so they bend together, and that the joists brace the top of the beam; it doesn't size the bolts or check bearing at the supports. The beam designer does the full design.
Flitch beam calculator questions
How far can a flitch beam span?
The calculator's starting beam, a 2-2x12 + 1/2 in plate, spans 16′ 7″ under 500 lb/ft. The flitch beam span table gives the longest span of 2x8, 2x10 and 2x12 flitch beams for loads of 200 to 1,000 lb/ft.
Can I use a different wood species or grade?
Yes, in the beam designer: open your beam with the button above, then change the material. Stiffer wood carries more of the load itself, so the plate's share drops.
When is a steel beam better?
On long spans or heavy loads, where a flitch beam needs thick plates and gets heavy. Compare with the steel beam span chart.