Phi/C Reduction analysis with structure elements

Question

When performing safety analysis to check FOS of a anchored excavation (with diaphragm wall piles for example),via Pfi/C reduction method, if we check the force or moment in structure elements its considerably high and is far beyond capacity of structure. 
 anyone has any idea is it wrong or not? should we introduce elasto-plastic structure element for safety analysis as a solution?

Stefanos Papavasileiou · Answer

Dear Ahmad, 
 As described in detail by Berk, the interpretation of such FOSs require more attention. 
 Indeed, as stated before me, the soil can be the first to fail instead of a stiff structure, in which case you would expect that when using elastic material for structures this would result in higher than normal forces applied. 
 With elasto-plastic material type for the structures, this can improve the response of the system (soil - structure) to more realistic values. 
 This falls back to the geotechnical engineer to evaluate and decide which approach will provide with the required results.

Stefanos Papavasileiou · Answer

Dear Nitha, 
 In this note, we practically hinting not to use residual strength for structural elements when running a Safety analysis. But, of course, the decision is on each user that knows their model better than anyone else. 
 If you would consider it, then the safety factor can be largely affected by the structures failing rather than the soil. Typically the soil will fail first, therefore, you can ignore the drop to residual strength for structural elements.