ANS:C - 42 diameters from the outer edge of the column

1. Load Distribution: The primary function of these bottom bars is to distribute the load from the column to the footing slab. By extending the bars into the slab, the load from the column is dispersed over a wider area, reducing the stress on any individual point and preventing localized failure.
2. Enhanced Structural Integrity: Extending the bars increases the bond length between the concrete and the reinforcement. This enhanced bond length improves the structural integrity of the footing-slab-column connection, making it more resistant to shear and torsional forces.
3. Increased Bearing Capacity: The extended bars help in increasing the bearing capacity of the footing by spreading the load over a larger area of the soil beneath. This reduces the pressure exerted on the soil and prevents excessive settlement or uneven foundation settlement.
4. Crack Control: The extended reinforcement helps in controlling cracks that may develop due to shrinkage, temperature changes, or differential settlement. By providing additional reinforcement in the critical regions, the likelihood and width of cracks are reduced, enhancing the durability and aesthetics of the structure.
5. Resisting Punching Shear: Punching shear is a critical failure mode in footings, especially around columns where high concentrated loads are applied. Extending the bottom bars helps in resisting punching shear by providing additional reinforcement to distribute the load effectively and prevent shear failure around the column base.
6. Compliance with Design Codes: Many structural design codes and standards, such as the ACI (American Concrete Institute) and Eurocodes, specify minimum requirements for reinforcement detailing, including the extension of bottom bars into footings. Adhering to these standards ensures that the structure meets safety and performance criteria.