Moment of Resistance:

Consider a cantilever subjected to a clockwise moment M at the free end. Let us take a section XX at a distance x from the free end, as illustrated in Figure.

 Figure

As seen earlier, top fibres are subjected to elongation & bottom fibres are subjected to contraction. The resultant of tensile stresses T, shall be equal to the resultant of compressive stresses, C. Let the distance among the lines of action of T & C be a.

∴          T × a = C × a

This couple is said the moment of resistance. For equilibrium of the portion of the beam up to XX, the moment of resistance offered by the section will be equal to the bending moment M.

∴          T × a = C × a = M