The maximum permissible shear stress tc max given in BIS: 456 – 1978 is based on diagonal compression failure. This means that the maximum permissible shear stress that can be applied to a concrete member is limited by the diagonal compression strength of the concrete.

Explanation:

Diagonal compression failure occurs when a concrete member is subjected to a diagonal compressive force. This type of failure is common in reinforced concrete beams and columns. The diagonal compressive force causes the concrete to fail in a diagonal plane, resulting in a shear failure.

In order to prevent diagonal compression failure, BIS: 456 – 1978 specifies a maximum permissible shear stress tc max that can be applied to a concrete member. This maximum shear stress is based on the diagonal compression strength of the concrete.

Factors affecting diagonal compression strength:

The diagonal compression strength of concrete depends on several factors, such as:

1. Concrete strength: The diagonal compression strength of concrete increases with an increase in concrete strength.

2. Shear span-to-depth ratio: The diagonal compression strength of concrete decreases with an increase in the shear span-to-depth ratio.

3. Reinforcement: The diagonal compression strength of concrete increases with an increase in the amount of reinforcement.

4. Transverse reinforcement: The diagonal compression strength of concrete increases with an increase in the amount of transverse reinforcement.

Conclusion:

In conclusion, the maximum permissible shear stress tc max given in BIS: 456 – 1978 is based on diagonal compression failure. This maximum shear stress is limited by the diagonal compression strength of the concrete, which depends on several factors such as concrete strength, shear span-to-depth ratio, reinforcement, and transverse reinforcement.