According to IS: 456 - 2000, side-face reinforcement should be provided when the depth of the web of a beam exceeds 750 mm. Let's understand the reasoning behind this requirement.

- What is side-face reinforcement?
Side-face reinforcement is additional reinforcement provided in the web of a beam to control the shear stresses and prevent shear failure. It is usually placed near the supports or at locations where high shear forces are expected.

- Importance of side-face reinforcement:
Side-face reinforcement helps in increasing the shear capacity of the beam by enhancing the bond between the concrete and the steel reinforcement. It also helps in redistributing the shear forces and preventing the formation of diagonal cracks.

- Reason for the specified depth:
The depth of 750 mm is specified in IS: 456 - 2000 as a threshold value for providing side-face reinforcement. This means that when the depth of the web of a beam exceeds 750 mm, side-face reinforcement becomes necessary.

- Role of beam depth:
The depth of a beam affects its shear capacity. Deeper beams have a higher resistance to shear forces due to the increased lever arm, which helps in resisting the bending moments. However, as the depth increases, the concrete near the supports becomes less effective in resisting shear forces.

- Shear failure:
Shear failure in a beam occurs when the shear forces exceed the shear capacity of the beam's cross-section. This can lead to sudden brittle failure and collapse of the structure. Providing side-face reinforcement helps in preventing shear failure and enhancing the overall safety of the beam.

- Other factors:
Apart from the depth of the web, other factors such as the magnitude of shear forces, the type of loading, and the design requirements also influence the need for side-face reinforcement. These factors need to be considered in conjunction with the specified depth requirement.

- Conclusion:
According to IS: 456 - 2000, side-face reinforcement should be provided when the depth of the web of a beam exceeds 750 mm. This requirement is based on the need to enhance the shear capacity and prevent shear failure in deeper beams. It is important to follow the design codes and guidelines to ensure the structural integrity and safety of the beam.