**Introduction**

A mechanical hinge is a type of connection used in structural systems that allows rotation between two members. It is commonly used in steel structures and is designed to provide flexibility and redistribute forces within the structure. When subjected to external loads, a mechanical hinge undergoes both elastic and plastic deformation. The plastic hinge is a critical component in the structural response and is often analyzed to determine the capacity and behavior of the structure.

**Plastic Hinge**

A plastic hinge is a section of a structural member that has undergone plastic deformation. In structural analysis, it is assumed that the plastic hinge forms at a specific location in the member, where the moment capacity is fully realized. The moment capacity at the plastic hinge is often denoted as the plastic moment.

**Treatment of Mechanical Hinge**

When analyzing a mechanical hinge, it is common to treat it as a plastic hinge for simplicity. This assumption allows engineers to determine the behavior and capacity of the structure without considering the complex nonlinear behavior of the hinge itself. The plastic hinge assumption implies that the moment capacity at the hinge is fully realized, leading to certain behaviors.

**Possible Values of Full Plastic Moment**

1. **Equal to zero**: If the full plastic moment at the mechanical hinge is assumed to be zero, it means that the hinge does not contribute to the moment capacity of the structure. This assumption implies that the hinge is fully restrained and does not allow any rotation or deformation. However, in most practical cases, mechanical hinges are designed to provide flexibility and allow rotation. Therefore, assuming a full plastic moment of zero is not realistic.

2. **Equal to unity**: Assuming a full plastic moment equal to unity implies that the hinge contributes fully to the moment capacity of the structure. In this case, the hinge allows rotation and deformation, and its contribution to the overall behavior of the structure is significant. This assumption is often used in the analysis of steel structures, where the plastic hinge formation is well understood and predictable.

3. **More than unity**: Assuming a full plastic moment greater than unity implies that the hinge has an enhanced moment capacity compared to its actual behavior. This assumption is not commonly used in structural analysis as it would overestimate the capacity of the structure and lead to unrealistic results.

4. **Less than unity**: Assuming a full plastic moment less than unity implies that the hinge has a reduced moment capacity compared to its actual behavior. This assumption is also not commonly used as it would underestimate the capacity of the structure and lead to conservative designs.

**Conclusion**

In conclusion, when treating a mechanical hinge as a plastic hinge, it is common to assume a full plastic moment equal to unity. This assumption allows engineers to simplify the analysis and determine the behavior and capacity of the structure. Assuming a full plastic moment of zero, more than unity, or less than unity would not accurately represent the behavior of the hinge and may lead to unrealistic or conservative results. It is important for engineers to consider the actual behavior of the hinge and its moment capacity when designing and analyzing structural systems.