Shell buckling design and assessment and the LBA-MNA methodology

The European Standard for Shell Strength and Stability has many provisions for the global analysis of shell structures using finite element analysis. Three alternative treatments are permitted: a simple linear analysis (LA) giving stress resultants to be compared with simple algebraic hand calculation strength assessment rules, a fully fledged nonlinear analysis, with material and geometric nonlinearities as well as properly characterised imperfections (GMNIA), and an intermediate treatment that requires only two strength assessments: linear bifurcation (LBA) and plastic limit (MNA). Because of the sensitivity of shell buckling to different forms and different amplitudes of imperfection, which depend strongly on the stress state, the GMNIA procedure is very onerous. This paper gives a brief outline of the design process and the relationship between algebraic and computational strength assessments. It then discusses the challenges facing the user of the LBA-MNA procedure and resolves some of the uncertainties surrounding it.