## 3 Application of Rationalized Design of Reinforcing Ribs Based on Limit Analysis  
In this section, an engine test cabin is taken as an example to show a rational design of reinforcing ribs. The engine test cabin comes from a real-world source and was originally reinforced with ribs based on experience. The reinforcing ribs of the cabin were redesigned using the reinforcement approach proposed in the previous section and compared with the original ones with the same load-bearing capacity, i.e., limit load. The geometric model of the original design of the engine test cabin is shown in Figure 7. The length and inner diameter of the engine test cabin are 12.9 m and 5.705 m, respectively. The thickness of the test cabin is 22.5 mm,. The material of all components is low alloy carbon steel Q345R with Young's modulus being 2.0 \(\times\) 10\({}^{5}\) MPa and the yield strength being 345 MPa. Under working conditions, acting loads include the pressure load, equivalent pressure load and pipeline thrust load on tubes. Figure 8 shows the load application. The pressure load and the equivalent pressure load on the tubes are applied in the same proportion when performing limit load analysis [24]. Firstly, the finite element limit load analysis was carried out for the test cabin shell without reinforcement. It turned out that the limit load was 0.135 MPa, and the von Mises stress distribution under the limit load is shown in Figure 9. It was seen that the yield zones appeared at the large tube, the structural discontinuity region between the shell  
Figure 8: Illustration of load application: (**a**) equivalent pressure load and pipeline thrust load on tubes; (**b**) pressure load of the test cabin and tubes. Figure 7: Illustration of the test cabin with the original reinforcing ribs. and the large tube, and the region of the shell near the large tube at the top. Similarly, by performing limit load analysis on the originally reinforced test cabin as shown in Figure 7, it was found that the limit load was 0.296 MPa. In Figure 9, the yield zone at the connection area between the large tube and the test cabin shell is elongated, and an elongated yield zone is also found on the test cabin shell away from the connection region. Therefore, according to the approach for setting up the ribs in the previous section, the most effective ribs should be rib form 2 and arranged circumferentially, as shown in Figure 10, and the dimensions of the ribs are shown in Figure 10. By performing limit load analysis, the limit load with a comparison of that of the original test cabin is listed in Table 3. It can be seen that by applying the rib design approach in this paper, the limit load is close to 86% of the limit load of the original design by using only half the weight of ribs. Figure 11 shows the von Mises stress distribution of the test cabin under the limit load where only the circumferential ribs of rib form 2 are applied. Figure 10: Reinforcing ribs and the reinforced test cabin: (**a**) reinforcing ribs; (**b**) arrangement of reinforcing ribs; (**c**) reinforced test cabin. Figure 9: Contours of the von Mises stress distribution of the test cabin without reinforcement ribs under limit load. As seen in Figure 11, the yield zone was separated by the circumferential reinforcing ribs, although it was still concentrated in the upper region of the cabin. At this time, in order to increase the load-carrying capacity, axial (or transverse) reinforcing ribs should be chosen for local strengthening. Similarly, by performing limit analysis, it was found that T-ribs are more effective, as shown in Figure 12. Figure 13 shows the test cabin reinforced by both the longitudinal and transverse ribs.