## 4 Conclusions and Discussions  
In this paper, a simple and practical approach for the rational strength reinforcement of thin-walled structures was studied. Conclusions were drawn as follows:  
1. By performing limit load analysis, the yield zone or weak-strength zone together with the shape can be easily found and thus, reinforcing ribs can be effectively placed.
2. For the region away from the structural discontinuity, if the yield zone under the limit load is quasi-rectangular or elliptical, the rational arrangements of the reinforcing ribs should be along the short side of the yield zone.
3. In the discontinuity area, the reinforcing rib with a triangular plate is more effective to reinforce the strength of the structure.
4. The application of the reinforcement to improve the strength of an engine test cabin found that, after the redesign of the rib plate, the engine test cabin can reach 98% of the limit load of the original test cabin, while the weight of reinforcing ribs is only 62% of the weight of the original ones, implying that the reinforcement design approach based on limit load analysis and proposed rib forms presented in this paper is effective and feasible for the lightweight design of thin-walled structures.  
It is noted that compared with the stress classification method, the limit load method is relatively simple and reliable. As the reinforced structures are generally complicated, it is difficult to achieve a lightweight design using the stress classification method. However, the approach proposed in this paper is feasible, and the design efficiency is high. By finding the yield zone or the weak-strength zone, reinforcement ribs can be put in the most effective places so that the whole structure can be lightweight, which is hard to achieve using other methods.  
In addition, the reinforcement design method proposed in this paper is useful for improving the strength of structures. In engineering, more reinforced structures are subjected to external pressure, and the reinforcing ribs increase the rigidity of the structures. For the  
Figure 14: Contours of the von Mises stress distribution of the test cabin under the limit load after the T-ribs are applied.  
Figure 13: Illustration of the test cabin reinforced with longitudinal and transverse ribs.  
problem of external pressure instability, the authors believe that the same approach can be adopted, but the limit load analysis should be changed to stability analysis, the limit load should be changed to the critical load, and the design of reinforcement should be related to the instability modes.  
Conceptualization, C.Q.; methodology, C.Q.; software, Q.W. and Z.W.; validation, Q.W., and Z.W.; formal analysis, Q.W.; investigation, Q.W.; resources, C.Q.; data curation, Q.W.; writing--original draft preparation, Q.W.; writing--review and editing, C.Q. and Z.W.; visualization, C.Q.; supervision, C.Q.; project administration, C.Q.; funding acquisition, C.Q. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data available on request.  
The authors declare no conflict of interest.