The Axial force N is the axial force along the element axis.

Question

I was wondering what is the meaning of Element axis in PLAXIS 2D. The subject statement was in the Manual. 
 So my question comes from a tunnel evaluation. 
 For my tunnel, forces are generated. 
 1) The hoop stress is in out of plane direction. So which direction is Axial? If that is also in Z, should I just add hoop and Axial? I have included a snap of local axis incase that is needed. 
 
 2) If axial is not in Z, is it tangential? 
 
 Following are questions for future reference 
 If Axial is in Z, then if I model a horizontal slab (Plate element), is the element axis still Z?

Stefanos Papavasileiou · Answer

Dear Paras, 
 I will comment on both questions: 
 1) The hoop stress in a plane strain analysis is the out-of-plane stress. The Axial, in that case, is acting on the circumference. We should not mix local axes with Cartesian axes. For the axial force, in principle the local axis 1 is considered but in a circular geometry it is difficult to show this. 
 2) Axial is acting on the circumference, so local axis 1. 
 When you model a horizontal slab with a plate element, you can again check the local axes defined to understand on which local axis the axial force is considered. Typically this is in the direction of the created line.