The correct answer is A. 6 legged.
The nominal stirrups provided in a combined footing if shear stress exceeds 5 kg/cm2 are 6 legged. This is because the shear stress in a combined footing is usually high, and 6 legged stirrups are able to provide the necessary shear strength.
Stirrups are a type of reinforcement that is used in concrete to resist shear forces. They are usually made of steel and are placed perpendicular to the main reinforcement. The number of legs in a stirrup refers to the number of bars that are bent around the main reinforcement.
The size and spacing of stirrups are determined by the shear stress in the concrete. The higher the shear stress, the larger and more closely spaced the stirrups need to be.
In a combined footing, the shear stress is usually high because the footing is supporting a large load. This is why 6 legged stirrups are required in a combined footing if shear stress exceeds 5 kg/cm2.
Here are some additional details about the other options:
- Option B: 8 legged stirrups are not required in a combined footing unless the shear stress is very high.
- Option C: 10 legged stirrups are rarely used in a combined footing because they are more expensive than 6 legged stirrups.
- Option D: 12 legged stirrups are never used in a combined footing because they are too expensive and provide no additional benefit.