ANS:D - 12 legged

In structural engineering, when designing reinforced concrete elements like combined footings, the primary goal is to ensure structural stability and safety under various loading conditions, including shear forces. Stirrups, also known as shear reinforcement, are often used to enhance the shear resistance of concrete elements. The number of legs in stirrups is typically determined based on structural analysis, design codes, and engineering judgment. While stirrups with 6, 8, or 10 legs are more common in practical applications, there could be scenarios where a higher number of legs, such as 12-legged stirrups, might be considered. Providing 12-legged stirrups could be an option if:

1. High Shear Stress: The shear stress experienced by the combined footing exceeds the typical values encountered in standard design scenarios. In such cases, additional stirrups may be required to effectively distribute the shear forces and prevent shear failure.
2. Code Requirements: The design code or structural standards applicable to the project may specify certain requirements or recommendations regarding shear reinforcement. If the code allows or suggests using 12-legged stirrups under specific conditions, it might be considered as a viable option.
3. Structural Considerations: Structural engineers may opt for 12-legged stirrups if they determine through detailed analysis that this configuration provides enhanced shear resistance and improves the overall performance and safety of the combined footing.
4. Special Circumstances: There could be unique circumstances or project requirements where providing a higher number of stirrup legs is deemed necessary to meet certain performance criteria or address specific challenges related to shear resistance.

In summary, while 12-legged stirrups may not be as common as 6, 8, or 10-legged stirrups in typical structural designs, they could be considered in exceptional cases where the shear stress is exceptionally high, and there's a need for additional shear reinforcement to ensure structural stability and safety. However, it's crucial for such decisions to be based on thorough engineering analysis, adherence to design codes, and consideration of project-specific requirements.