The general relationship for determining the tensile or compressive stress in a beam due to bending is- S = (Mc)/I, where: M = moment, joules, I = moment of inertia of the cross section, and c = distance from the neutral axis to the outermost fiber. The stress at any other point than the outer fiber is proportional to its distance from the center and would equal-- S = (y/c)(Mc/I) = (My)/I.

The values of c and I can be determined from the geometry of the beam. The moment, M, depends upon the loading of the beam. As can be deduced, the maximum stress will occur at the location of the greatest moment (for a beam of constant cross section).

The transverse shear stress at any point is equal to the magnitude of the shearing force divided by the cross-sectional area at that point. It is seldom possible to determine by a glance just where the maximum shear and the maximum moment will occur in a loaded beam, so it is usually desirable to construct both the shear diagram and the moment diagram for the beam under consideration; then the points of maximum shear and maximum moment will be readily apparent.