How to Calculate Beam Bending Stress

1. Determine the bending moment

Start with the bending moment at the beam location being evaluated. Keep the sign convention consistent with the analysis that produced the moment.

2. Choose the section-property form

Use section modulus when S is already known. Use I and c when the centroidal second moment of area and the distance to the extreme fiber are known.

3. Use the elastic flexure relation

The two supported calculation forms are equivalent when S=I/c. Inputs should be converted to one consistent unit system before calculation.

4. Interpret top and bottom fibers

Under the frozen convention, positive stress means tension and negative stress means compression. Positive sagging moment gives top compression and bottom tension.

5. Keep geometry and capacity separate

Bending stress is not a capacity conclusion by itself. Material strength, stability, load combinations, and code requirements require separate design checks.

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