Beam Calculator

Beam capacity depends on: Material (wood species, steel grade), dimensions (depth most critical - doubling depth quadruples strength), span length (longer = weaker), load type (point vs distributed), and support conditions (simple, continuous, cantilever). Formula: Maximum moment = (w × L²) / 8 for uniform distributed load on simple span, where w = load per foot, L = span. Required section modulus S = M / Fb, where Fb = allowable bending stress. Always use engineered calculations and building codes - this calculator provides estimates only. Consult structural engineer for actual construction.

Critical distinction for structural design: Dead load: Permanent, static weight - building materials, roof, walls, floor, fixtures. Predictable and constant. Example: 10 psf for floor materials. Live load: Temporary, variable weight - people, furniture, snow, storage. Code-specified minimums. Example: 40 psf residential floor, 30 psf bedroom, 20 psf roof in warm climate, 40+ psf snow load. Total load = dead + live + safety factor. Beams must support both simultaneously. Residential floors typically designed for 50 psf total (10 dead + 40 live).

20' span requires engineered beam - rule of thumb sizing: Wood (Douglas Fir, #2): Triple 2x12 (~3600 lb capacity, 40 psf load). LVL (Laminated Veneer Lumber): 3.5 inches × 14 inches (~4000 lb capacity, better than wood). Steel I-beam: W8×15 (~5000 lb capacity, strongest). Glulam: 5.125 inches × 15 inches (~4500 lb capacity, architectural grade). Actual size depends on: Tributary width (8' vs 16' makes huge difference), floor vs roof (different loads), span type (simple vs continuous). Never guess - 20' span failures catastrophic. Hire engineer, get permit, use professional installation.

Deflection = downward sag under load. Critical for: Structural limits: L/360 for floors with plaster (20' span = 0.67 inches max), L/240 for floors with drywall (20' = 1 inches), L/180 for roofs (20' = 1.33 inches). Exceeded limits cause: Cracked walls/ceilings, bouncy floors, door/window binding, visible sag, structural damage. Calculated: Deflection = (5 × w × L⁴) / (384 × E × I), where E = modulus of elasticity, I = moment of inertia. Depth controls deflection - deeper beam = less sag. Often deflection governs beam size, not strength. Feels unsafe even if structurally adequate if too bouncy.

Combining beams (built-up beams): Triple 2×12 can replace single larger beam for same capacity. Advantages: Cheaper (standard lumber vs engineered), easier to handle/install, readily available. Disadvantages: Must be properly fastened (bolts or nails per code schedule), not as stiff (more deflection), installation labor intensive. Fastening critical: 3-2×12 nailed together ≠ full strength. Need: Bolts every 12-16 inches staggered, or structural screws, or specified nailing pattern. Use plywood spacers if needed. Engineered beams (LVL, PSL) typically better for long spans - stronger, stiffer, straighter, one piece. Cost similar when including labor.

Safety factors protect against: Material defects, construction errors, unexpected loads, degradation over time. Building codes include: Load factors: 1.2× dead load + 1.6× live load for strength design. Or 2.0-2.5× overall for allowable stress design. Material reduction factors: Account for wood defects, knots, moisture. Code values conservative. Span tables pre-incorporate safety factors. Never reduce safety margins - catastrophic failure risk. Professional design includes: Multiple safety checks, worst-case scenarios, deflection limits, shear capacity, bearing capacity. Beam failure sudden and dangerous - always overdesign slightly, use engineer for critical applications.

Tributary width = area of floor/roof that loads the beam. Critical for sizing: Beam supporting 8' on each side = 16' tributary width. Beam at wall supporting 8' one side = 8' tributary. Load calculation: Total load = tributary width × span length × load per sq ft. Example: 20' span, 12' tributary, 50 psf = 20 × 12 × 50 = 12,000 lbs total (600 lbs per linear foot). Wider tributary needs bigger beam. Center beams carry most load (tributary both sides), edge beams less (one side). Joist direction determines tributary - perpendicular joists load beam, parallel don't.

Material selection guide: Dimensional lumber (2x, 4x): Cheapest, readily available, limited spans (<14'), allows for built-up beams. Use: Short spans, residential, budget projects. LVL (Laminated Veneer): Strong, consistent, no crown, long spans (20'+), more expensive. Use: Long spans, headers, commercial. Glulam: Beautiful, architectural, very strong, expensive, custom sizes. Use: Exposed beams, commercial, wide spans. Steel I-beam: Strongest, longest spans (30'+), expensive, needs equipment, rust issues. Use: Heavy loads, commercial, extreme spans. PSL (Parallel Strand): Like LVL but stronger, very expensive. Use: Heavy point loads, extreme applications. Consider: Cost, availability, span, load, appearance, installation capability.