Designing an RC (Reinforced Concrete) section involves determining the required amount of reinforcement to resist the applied loads and ensure the structure's safety and stability. In the design process, there are two possibilities: under-reinforced and over-reinforced sections. An over-reinforced section refers to a design where the amount of reinforcement exceeds the required amount. This condition is considered undesirable due to several reasons, as explained below:

1. Reduced ductility and sudden failure:
- An over-reinforced section leads to reduced ductility, which is the ability of a material to undergo deformation before failure.
- When the section is subjected to high loads, the excessive amount of reinforcement prevents the concrete from yielding and undergoing plastic deformation.
- Instead, the section fails suddenly and catastrophically without warning, which is highly undesirable as it does not provide any signs of distress or time for repair or evacuation.

2. Brittle behavior:
- Over-reinforced sections tend to exhibit more brittle behavior compared to properly designed sections.
- The excessive reinforcement restricts the ability of the concrete to crack and distribute the stresses evenly.
- As a result, when the section reaches its ultimate capacity, the failure is sudden and without much deformation or warning signs.

3. Uneconomical use of materials:
- Over-reinforced sections consume more reinforcement than necessary, resulting in an uneconomical use of materials.
- Reinforcement is typically more expensive than concrete, and using excessive reinforcement increases the project's cost.
- Moreover, the additional reinforcement also leads to increased congestion, making construction more complex and time-consuming.

4. Difficulties in construction and maintenance:
- Excessive reinforcement in an RC section can make it challenging to place and compact the concrete during construction.
- The congested reinforcement can also hinder proper inspection and maintenance of the structure, leading to potential durability issues in the long run.

In summary, the design of an over-reinforced RC section is undesirable because it leads to sudden failure, reduced ductility, increased brittleness, uneconomical use of materials, and difficulties in construction and maintenance. It is essential to ensure that RC sections are designed with the appropriate amount of reinforcement to achieve the desired structural performance and safety.