Thanks Dave,

From your explaination what I have understood is ...

If
Sb = calculated bending stress
St = Calculated torsional stress

As Max shear stress is Tmax = [ (Sb/2)^2 + St^2 ]^0.5
But we calculate 2*Tmax = [ Sb^2 + 4*St^2 ]^0.5 and take this as displacement expansion stress "SE". And this is directly compared with yield stress Sy, instead of comparing Tmax with 0.5*Sy.

Thanks for this clarification.

Regards
Zahid.