Consider a rectangular cross sectional solid beam, fixed at one end and a uniform shear force applied to the other end.
Below shows the side view for this beam.
Now, I change the support area as shown below. How should the force distribution be now?
In the first image, above the neutral axis we already know that the structure will be in tension while below the neutral axis, the structure will be in compression. The net upwards tensile force and net bottom compressive forces are equal, and when combined together, the resultant is a reaction moment (with no net reaction force along the longitudinal axis, and only net reaction force to counter the shear).
Now, for the second image, again the reaction moment will exist. Since no external force is applied along the longitudinal axis, there won't be any reaction force along that axis. The only reaction force that will exist is the force to counter the shear. So how would the tension and compression forces be distributed (on the cross section marked with blue line) so that they will return a total reaction moment only?