Safety analysis - Structural Actions

Question

Hi, 
 In the safety analysis (Plaxis 2D) additional displacements that are generated are generally discounted/considered not to have a physical meaning. Is it therefore reasonable to assume that shear forces and bending moments calculated in these stages similarly do not to have a physical meaning. i.e they are not meaningful actions, and should not be considered as an ultimate/ULS structural action. 
 Thanks!

Vasileios Basas · Answer

Hi James, 
 A safety analysis reduces the soil properties to a level where a proper failure surface is formed, and the model is no longer stable. The stresses and forces are redistributed during this process, leading to a rapid increase in structural forces. However, structural elements with elastic behaviour will not fail but will continue to be loaded, which is unrealistic. 
 Of course, you can mitigate this using structural elements with elastoplastic behaviour. In this case, you introduce a maximum axial force/bending moment, which will limit that structural element's resistance. Based on the structure's capacity, this may be the correct approach because it can improve the system's response (soil - structure) to more realistic values. 
 Another option is first to run a safety analysis to specify the Factor of Safety of your model. Once you have that value and the failure mechanism is known, you can then re-run the safety analysis with a target &Sigma;Msf slightly less than the value that has been calculated. During this analysis, the program performs a full safety analysis until failure. Then it recalculates the last step before the target value of &Sigma;Msf to reach the target exactly. This will give you the deformations before failure and, therefore, more realistic structural forces. 
 Nevertheless, interpreting results from a safety analysis certainly requires careful consideration and some engineering judgement.