A beam carries loads across a span and transfers them to supports. Sizing one means choosing a steel section that can handle the load over the span without overstressing or deflecting too much.
The concepts that drive it
- Load — both the structure’s own weight (dead load) and what it carries (live load), in the worst realistic combination.
- Span — the distance between supports; longer spans need much stiffer beams.
- Bending strength — the beam’s section modulus must give a bending stress safely below the steel’s allowable stress.
- Deflection — even a strong beam must not sag too much; codes limit deflection to a fraction of span (e.g., span/360).
Engineers use these inputs with the relevant steel design code to select a section (like a wide-flange “W” shape), applying the appropriate safety factors. The right answer depends on details — load paths, lateral bracing, connections — that go well beyond a simple formula.
Frequently asked questions
Can I size a beam myself? No — load-bearing beams must be designed/verified by a licensed engineer per code.
Strength or deflection? Often deflection on long spans; both must be checked.
What is section modulus? A cross-section property that sets bending strength; bigger resists bending better.