## 3 Stiffened Shell Structures
### Development and Progress of the Research in the Areas of Ring Stiffened Cylinder  
The buckling of stiffened cylindrical shells is one of the main considerations in the design of stiffened shell structures. In the 1930s, [PERSON] [6] made the first findings on stiffened shells. One of the first findings was that the stiffened arrangement has an influence on the load-bearing behavior of the shell. [PERSON] [28] presented a review of experimental and theoretical studies on the buckling of stiffened cylindrical shells. After that, [PERSON] and [PERSON] et al. [29; 30; 31] investigated the influences of eccentric loads, geometrical imperfections, and boundary conditions on the buckling loads of stiffened cylindrical shells, respectively. An extensive compilation of critically selected tests was published in 1969 by [PERSON] in [32]. Since intensive research in the 1960s and 1970s, in the twentieth century, research almost came to a standstill, and only sporadic further research results were published that dealt with experiments around the topic of ring-strengthened cylinder shells under axial pressure. In this context, [PERSON] and [PERSON] [33] studied the buckling behavior of ring-stiffened cylindrical shells under axial pressure and transverse load at the upper edge of the specimens. Other publications, such as [34], highlighted the larger differences between welded test specimens and carefully machined cylinders. In the late 1980s and early 1990s, [PERSON] [35; 36] investigated theoretically and experimentally the post-buckling of cylindrical ring-stiffened shells and analyzed the sensitivity of the shell buckling under external pressure. However, in many early papers, the parameters that determined the buckling behavior, such as geometrical imperfections associated with the ideal cylindrical shape and residual stresses, were often not adequately listed in experimental publications. The work by [PERSON] [37] is one of the few publications that provide extensively documented measurements of the initial geometrical imperfections. They investigated the load-carrying capacity of eccentrically loaded cylindrical shells with variable ring stiffener spacing and then subsequently published the test results of the cylindrical shells with stiffeners. [PERSON] et al. [38] investigated the buckling behavior of stiffened cylindrical shells under combined loading based on reliability analysis. [PERSON] and [PERSON] [39] presented a post-buckling analysis of stiffened cylindrical shells under combined external and axial pressure loading. [PERSON] [40] provides a comprehensive summary of the current research state and validates the cylindrical stiffened shell on the basis of various design codes. The results show that none of the current steel structure design codes specify cylinder loading with axial pressure and ring stiffness. A comparison of the test results using buckling reduction curves according to DIN 18800-4 shows that the provisions are significantly conservative with respect to the ring-stiffened cylindrical shell structures. [PERSON] [41] focused more marginally on stiffened ring shells when he performed tests on small-scale spiral-folded cylinder specimens under axial pressure and checked the suitability of Eurocode 3 for the design of the cylindrical shells. [PERSON] and [PERSON] et al. [42] investigated the influence of ring-stiffeners on the bearing capacity of axially compressed cylindrical shells using a numerical approach. The potential material savings are suggested, and the beneficial influence of the ring stiffeners on the buckling load capacity is discussed. In his dissertation [3], [PERSON] proposed suggestions to address gaps in design standards. More recently, [PERSON] and [PERSON] et al. [43; 44; 45] presented an investigation of cylindrical ring-stiffened shells based on axial buckling tests and numerical simulations.