PSA 1.4 — Bending & Torsion of Pipe: Section Modulus and Combined Loading
A pipe between supports is a beam. Bending moment M produces a linear stress distribution — zero at the neutral axis, maximum at the extreme fibre:
σ_b = M / Z Z = I / c = π (Do⁴ − Di⁴) / (32·Do) (section modulus) I = π (Do⁴ − Di⁴)/64 (moment of inertia) Feel for Z: 6″ sch 40 → Z ≈ 1.39×10⁵ mm³; 12″ STD → Z ≈ 7.19×10⁵ mm³. A 10 kN·m moment stresses the 6″ pipe to 72 MPa but the 12″ to only 14 MPa. Z grows ~D²·t — size beats grade for carrying moment,…
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