Combined Stress Analysis per ASME B31.8 & B31.4
X42=42,000 | X52=52,000 | X60=60,000 | X65=65,000 | X70=70,000 | X80=80,000
Free span for bending stress (0 = buried/fully supported)
Gas ~3-8 | Oil ~50-55 | Water ~62.4 | Empty = 0
0 = above-ground. Typical: 3-5 ft for buried pipelines.
Understand longitudinal stress components, von Mises criterion, B31.8 combined stress limits, and restrained vs. unrestrained pipeline analysis
Longitudinal stress acts along the pipe axis and consists of three components: Poisson stress from internal pressure (nu x S_H), thermal stress from temperature change (E x alpha x deltaT), and bending stress from pipe weight or soil loading (M/Z). These combine with hoop stress to determine the pipeline's structural adequacy per ASME B31.8.
ASME B31.8 uses the von Mises (octahedral shear stress) criterion: S_eq = sqrt(S_H^2 + S_L^2 - S_H*S_L). The combined equivalent stress must not exceed k x S x T x SMYS, where k is the design factor (0.72 for Class 1), S is the temperature derating factor, and T is the joint factor.
Von Mises (octahedral shear) is used by ASME B31.8 for gas pipelines: S_eq = sqrt(S_H^2 + S_L^2 - S_H*S_L). Tresca (maximum shear stress) is more conservative: S_eq = max(|S_H - S_L|, |S_H|, |S_L|). Von Mises is generally 15% less conservative than Tresca for biaxial stress states in pipes.