The correct answer is: A. Upper flange of top tier.
A grillage foundation is a type of foundation that is made up of a series of beams that are connected together to form a grid. The beams are typically made of concrete or steel, and they are often reinforced with steel bars. Grillage foundations are used to support heavy loads, such as those from buildings or bridges.
The minimum cover provided on the upper flange of the top tier in a grillage foundation is 15 cm. This is to ensure that the concrete is protected from corrosion and other damage. The cover is also necessary to provide a safe working environment for workers who are installing or repairing the foundation.
The lower beam of the lower tier does not require a minimum cover, as it is not exposed to the same level of wear and tear as the upper flange. The ends of external beams also do not require a minimum cover, as they are not as critical to the structural integrity of the foundation.
Here are some additional details about each option:
- Option A: The upper flange of the top tier is the most exposed part of the foundation, and it is therefore the most vulnerable to damage. The minimum cover of 15 cm helps to protect the concrete from corrosion and other damage.
- Option B: The lower beam of the lower tier is not as exposed to the elements as the upper flange, and it is therefore not as vulnerable to damage. A minimum cover is not required for this beam.
- Option C: The ends of external beams are not as critical to the structural integrity of the foundation as the other parts of the foundation. A minimum cover is not required for these beams.
- Option D: None of these options are correct. The correct answer is A.