Beam bending stress
How bending moment and section modulus provide a preliminary elastic stress screening value.
ENGINEERING PRINCIPLE
In elastic bending, normal stress varies linearly from the neutral axis and reaches its maximum at the outer fibre.
σ = M/ZVariables and units
- σ = nominal bending stress (MPa)
- M = bending moment (N·mm)
- Z = elastic section modulus (mm³)
Worked example
A 25 kN·m moment on a section with Z = 250,000 mm³ gives σ = 25 × 10⁶ / 250,000 = 100 MPa.
Calculation method
- Obtain the governing moment from a load and support model.
- Convert moment to N·mm.
- Divide by the elastic section modulus about the bending axis.
Practical applications
- Machine frames
- Support beams
- Lifting members and brackets
Review boundary and reference
- Shear, deflection, lateral stability, connections and load combinations are separate checks.
- Section properties must match the actual orientation and geometry.
Formula and displayed units independently checked for the stated conditions. This is not a code-compliance design method.
Source: Elastic flexure formula from standard strength-of-materials references.