ANS:B - 30 cm

In reinforced concrete footings on piles, the thickness at the edges is often limited to ensure proper structural performance, economy, and practical construction considerations. Here's why the thickness is typically kept less than 30 cm:

1. Load Distribution: Footings transfer the load from the superstructure to the underlying soil or piles. The thickness of the footing at its edges is designed to distribute the load effectively to the supporting piles or soil without creating excessive stresses. With proper design, a thickness of less than 30 cm can adequately distribute the load while minimizing material usage and cost.
2. Structural Efficiency: Keeping the thickness of the footing at its edges less than 30 cm helps maintain structural efficiency. It ensures that the footing is adequately stiff to resist bending and shear forces while avoiding unnecessary excess material that would add weight and cost to the foundation.
3. Construction Practicality: Thicker footings require more concrete and may necessitate more complex formwork, handling, and placement during construction. By limiting the thickness to less than 30 cm, construction becomes more practical and efficient, reducing labor, time, and material costs.
4. Soil-Pile Interaction: In cases where footings are supported by piles, the interaction between the footing and piles is critical for load transfer. By keeping the thickness of the footing at its edges less than 30 cm, engineers can ensure proper interaction between the footing and piles, optimizing the load distribution and overall performance of the foundation system.

Overall, limiting the thickness of reinforced concrete footings on piles at their edges to less than 30 cm is a practical and economical approach that balances structural requirements with construction feasibility. It allows for efficient load distribution, structural efficiency, and cost-effectiveness while ensuring the stability and performance of the foundation system.