How do you Calculate yield moment of a concrete beam?

Yield momnet; or the moment at which the concrete beam will fail due to bending can be calculated by:

M= (W*L*L)/8

Where M= Yield Moment
W= weight on beam (Kn usually)
L= Span of Beam (M usually)

M will be in :
Kn per m
or
Kn/m

hope This helps.