The capacity curve framework of the Eurocode EN 1993-1-6 on the strength and stability of metal shells offers a standardised algebraic relationship between the dimensionless buckling strength and the dimensionless slenderness of a shell. It is the basis of the buckling resistance assessment when used in conjunction with algebraic stress design, reference resistance design or the semi-computational LBA-MNA design. The capacity curve for a cylinder under uniform meridional compression has long been treated as the most conservative by default, recommended when no other curve can be justified. However, the most recent EN 1993-1-6, likely to be published in 2025, cautions that the most conservative choice for systems of intermediate slenderness may in fact be the curve for uniform bending. As this has come as a surprise to some in the community, this paper intends to justify the rationale behind this codified recommendation through a quantitative comparison of both systems’ computed capacity curves.
This paper illustrates that the historical perception of uniform meridional compression as the ‘most conservative’ cylindrical shell system is based solely on its more severe imperfection-sensitive elastic buckling behaviour at high slendernesses. However, at intermediate slendernesses it is the uniform bending system that exhibits a more conservative dimensionless capacity curve, owing to the mechanics of local buckling under only partial plastification of the circumference. This, as well as other interesting properties of computationally generated capacity curves, is discussed in detail herein. This paper is intended to act as a background document to the recently evolved EN 1993-1-6 and will be of significant interest to practitioners and researchers aiming to apply this new state-of-the-art Eurocode to buckling assessments.