[1] For a simply supported or a continuous beam the shear force is generally maximum at the support.

[2] The design shear force is generally taken at a distance d from the face of the support for cast in-situ beams.(monolithic)
( where d is the effective depth of the beam)

[3] The fundamental reason behind this is that the shear crack forms at angle of 45°.

[4] This concept is well stipulated in clause 22.6.2 of IS 456 ( Indian code of plain & reinforced concrete).

The above mentioned rule is not valid in some cases and the design shear is considered at face of support when we have beam hanging from supports above or in case of ledge beam.

This is small post on critical shear section. Any comments or suggestions are welcomed