## 5 Conclusions  
The buckling issue of cylindrical and stiffened shells, especially axially pressed cylinders, has contributed to the continuous development of the stability theory of shell structures. To explain the dispersion of axially compressed cylindrical shell experimental results and the large differences between them and the predictions of the classical linear buckling theory, the stability theory of shells has gradually evolved from the initial linear theory to a relatively well-developed nonlinear buckling and post-buckling theory. Nowadays, theoretical explanations for the sensitivity of geometrical imperfections and the post-buckling properties of the stiffened and unstiffened shell structure have been presented. With the continuous development of the shell buckling theory, the stability of the cylindrical shell with and without stiffeners has been solved from a general theoretical point of view. However, there are still the following issues in cylindrical shell structures, especially ring-stiffened shells, that need to be solved urgently:  
1. The strong dispersion of the ultimate load of the experimental results and the large differences between these results and the theoretical calculations are mainly due to the high sensitivity of the initial geometrical imperfections, whereas there are always unavoidable and unmeasurable differences between the practical shell structures and the theoretical model. Hence, an approach that can reasonably quantify geometric imperfections needs to be proposed. 2. For stiffened cylindrical shell structures used in steel structures, the simplified analytical models employed for stability analysis still need to be improved, for example, for the ring-stiffened cylindrical shell. The application of the finite strip method and the smeared stiffener theory for buckling analysis is still not well addressed. Figure 11: Construction principle of bolted tanks. 3. Since ring-stiffened shells are already insensitive to geometric imperfections, a transfer of the method of unstiffened cylinders for ring-stiffened shell structures does not seem to be justified, and numerical simulations are needed to develop a design method for circumferentially variable meridional loads that are economical and safe. 4. The investigations that have been carried out do not cover the circumferentially unevenly distributed axial load in ring-stiffened cylindrical shells. It may cause the local buckling of the ring-stiffened cylindrical shells. Throughout the history of the research into cylindrical shells with and/or without ring stiffeners for more than a century, shell buckling has been established as a complex but also crucial task for structural safety. Accompanying analytical methods and the computer analysis of shell buckling problems has been a very important area of computational mechanics. Computer analysis is actually a new research paradigm for shell buckling. For further investigation of stiffened cylindrical shell structures, computer analysis is an essential research tool because \"in fact, computer analysis is nowadays on the same footing as laboratory experiments, and computer packages play a role comparable to that of testing machines\" [59]. Conceptualization, H.P. and Z.L. ; methodology, Z.L. ; software, Z.L. ; validation, A.J. and A.D.; formal analysis, Z.L. and H.P. ; investigation, Z.L., H.P., A.J. and A.D.; resources, Z.L., H.P., A.J. and A.D.; data curation, Z.L. ; writing--original draft preparation, Z.L. and H.P. ; writing--review and editing, A.J. and A.D.; visualization, Z.L. ; supervision, H.P., A.J. and A.D.; project administration, H.P., A.J. and A.D.; funding acquisition, H.P., A.J. and A.D. All authors have read and agreed to the published version of the manuscript. * _[PERSON] et al. (2017)_ [PERSON]; [PERSON]; [PERSON] Rethinking imperfections in tanks and silos. In _Ce/Papers EUROSTEEL 2017_; Ernst & Sohn: Copenhagen, Denmark, 2017: pp. 960-969.