The general derivation of the Flexural formula uses the pure bending case, where the distance from a curved section to the neutral axis is assumed to be constant even after bending, denoted by y. The Flexural formula, as we all know, is used to determine the bending stresses at a distance y from the neutral axis.
Now my question is that can we use this general Flexural formula to find the bending stresses at a distance y from the neutral axis, when a shear/transverse load is applied to it? Because I was thinking that the distance y from the neutral axis of that section (whose bending stresses I am trying to compute) will not remain the same after bending due to this transverse loading. So why do we still use it? [Because literally the derivation of the Flexural formula assumes that the distance from the neutral section to any of the section at distance y remains constant after bending in pure bending case].