回帖：In structural engineering and desing, this problem does not exist., The analysis is an elastic analysis and the initial stress of the system is usually known. Based on the moment and shear, the reinforcement can be designed based on the ultimate limit state. The two definition of FOS will be the same as this is a linear process. In geotechnical engineering, the initial stress is not known and we analyze the ultimate limit state. The FOS will be dependent on the definition of the FOS as this is now a nonlinear process.
Both definitions are used in the world. For abutment or similar where the failure is controlled by external load, 超载法 can be suitable but the strength reduction is more appropriate for slope failure induced by water and soil self weight.
The calculation above has not recognized the nonliear nature of this problem and is hence not a correct/reasonable study.
Both definitions can be used, depends on the choice of the engineers. We must also remember that there are other definitions of FOS in the world !