Piping analysis according to NB-3600 treats bending and torsion loads as equivalent. Piping sections are generally subjected to loading that includes a combination of bending and torsion. Such loading results in a biaxial stress state in the pipe wall. Although NB-3600 would still assign a single bounding stress index to the resultant moment vector in bends and elbows, detailed analysis shows that bending moments of different orientations and torsion loading have a different effect on the calculated stresses.
This paper shows the effect of applied moment direction on the bounding bend limit load that would be evaluated in a detailed (e.g. NB-3200) piping primary stress analysis. This gives the analyst some guidance regarding the potential benefit of a detailed analysis. Further, it is shown that pure bending is not necessarily the worst loading mode in an elbow or bend. The addition of a torsion component could increase the bend stress and lead to a more severe loading condition even if the magnitude of the resultant applied moment vector remains the same.