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