Robust knockdown factors for the design of cylindrical shells under axial compression: Analysis and modeling of stiffened and unstiffened cylinders

For the design of thin-walled cylindrical shells under axial compression empirical knockdown factors are applied. These knockdown factors are based on experimental results from the beginning of the 20th century and have been shown to be very conservative for modern shell structures. In order to determine less conservative and physically based knockdown factors for the design of axially loaded shells, different analytical and numerical design approaches have been developed. In this paper common as well as new shell design approaches are presented in detail and evaluated regarding the lower-bound buckling load. Among these design approaches are the EN 1993 1-6, the reduced energy method, linear buckling eigenmode imperfections, perturbation approaches and the new threshold knockdown factors. Important analysis and modeling details of each design approach are described and test examples are given and validated. Advantages and disadvantages of each approach are listed and design recommendations are given. A comparison of deterministic design approaches with modern probabilistic design methods is shown and the range of application of both design philosophies is discussed. Orthogrid stiffened cylinders with weld lands from NASAs Shell Buckling Knockdown Factor Project (SBKF) are modeled, analyzed and lower-bound buckling load calculations for improved knockdown factors are shown.