In shear design of an RC beam, other than the allowable shear strength of concrete $( \tau _c)$, there is also an additional check suggested in IS $456-2000$ with respect to the maximum permissible shear stress $(\tau _{c \text{ max}})$. The check for $(\tau _{c \text{ max}})$ is required to take care of

additional shear resistance from reinforcing steel

additional shear stress that comes from accidental loading

possibility of failure of concrete by diagonal tension

possibility of crushing of concrete by diagonal compression