What's the difference between point load and UDL?
A point load (concentrated load) is a single force applied at one location — like a column resting on a beam or a piece of equipment. A UDL (uniformly distributed load) is force per unit length spread evenly along the beam — like a floor live load or self-weight. UDL produces a smooth parabolic moment curve; point loads produce sharp peaks at the load location.
Why does deflection often govern over stress?
In residential design, especially with wood, a beam might have plenty of strength to resist the moment but still flex more than code allows. A bouncy floor or a beam that visibly sags is uncomfortable and can crack finishes. Modern wood spans (especially I-joists) are often deflection-limited at typical residential loads. Stress checks are critical but often less restrictive than serviceability.
What does "section modulus" mean?
Section modulus S = I/c where I is moment of inertia and c is distance from neutral axis to extreme fiber. It's a single property that combines section size and shape for bending stress calculations: σ = M/S. For a rectangle, S = bh²/6. Higher S means the section can resist more moment for the same stress. Tables of standard sections (W-shapes, channels) list S values for quick lookup.
What does L/240 mean?
It's a deflection limit expressed as span divided by 240. For a 10 ft (120 inch) span, L/240 = 120/240 = 0.5 inches maximum allowed deflection. L/360 is stricter (0.33" for same span); L/180 is looser (0.67"). The denominator depends on application: L/240 is standard for floor live load, L/360 if plaster or rigid finishes below, L/180 for roofs without ceilings.
Why is height (h) more important than width (b)?
For a rectangular beam, I = bh³/12 — height enters cubed, width only linearly. Doubling the width doubles I; doubling the height multiplies I by 8. That's why beams are deeper than wide. A 2×8 (1.5"×7.25") has roughly 8× the strength of a 2×4 (1.5"×3.5") even though only the height changed. This is also why I-beams put material at top and bottom (max distance from neutral axis).
What about the beam's self-weight?
For most short-span residential applications, self-weight is small relative to live load and often ignored. For longer spans (15+ ft) or heavy beams (steel W-shapes, LVLs), include self-weight as additional UDL. Wood ~30-40 lb/ft³, steel ~490 lb/ft³, concrete ~150 lb/ft³. Multiply density by cross-section area to get weight per length.
Why does point load at center produce more moment than off-center?
For a simply supported beam with point load P at distance a from left, max moment = Pab/L where b = L-a. This is maximized when a = b = L/2 (load at center), giving M = PL/4. As the load moves toward either support, the moment decreases. Same for deflection. Center loading is the worst case — design for it unless load location is fixed.
What's the difference between bending stress and shear stress?
Bending stress is normal stress from the moment (σ = M/S), maximum at top and bottom fibers. Shear stress is parallel to the cross-section from shear force (τ = VQ/Ib), maximum at the neutral axis. For long thin beams, bending governs. For short stocky beams or beams with high loads near supports, shear can govern — especially in wood (which has low shear strength parallel to grain).
How do I size a beam for a deck?
For decks, typical live load is 40 psf (residential) plus 10 psf dead. Multiply total psf by tributary width (half the joist spacing) to get load per linear foot. Use the calculator to check moment, stress, and deflection. Common spans: 2×8 @ 16" o.c. = 10-11 ft; 2×10 = 13-14 ft; 2×12 = 16-17 ft. Always check local code; many jurisdictions require permits for decks above 30 inches.
Should I include a safety factor?
Yes. Allowable stresses already include some safety factor, but for real designs apply additional load factors per code: dead load × 1.4, live load × 1.6 (LRFD steel/concrete) or use ASD with combined load × 1.0. Wood NDS has duration-of-load factors (short loads OK higher than sustained). This calculator shows theoretical results — apply appropriate safety factors per the governing code for your application.
Why are I-beams "I" shaped?
For bending, material far from the neutral axis (top and bottom of beam) carries most of the stress. The web (middle) carries shear but doesn't help much with bending. An I-shape (or wide-flange "W") puts material where it's most efficient — high I per pound of material. A solid rectangle with same depth would be much heavier for the same moment capacity. This is why structural steel beams are almost always I or W shapes.
Can I use this for engineered I-joists or LVLs?
Not directly — the calculator uses rectangular section properties. For engineered products, look up the manufacturer's allowable moment, shear, and deflection capacities from their span tables (e.g., TJI from Weyerhaeuser, Microllam LVL). These typically come with their own design software. Use this calculator for solid lumber, dimensional wood, and steel/aluminum rectangles. For LVL, you can approximate using rectangular properties with the LVL's published E and stress values.
