Thanks Dave,
The idea you proposed works great. so basically I did place a local coordinate and rotate then I used Esel to select the required elements as Bellow:
LOCAL,12,1,0,Dc/2,pos_z,,,theta,r
CSYS, 12
ESEL, S, CENT, z, -b/r,b/r
ESEL, r, CENT, x, -b,b
Many thanks again for your valuable feedback
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