ANS:D - all the above.

In reinforced concrete construction, the number of main steel bars provided in columns depends on various factors such as the size, shape, loading conditions, and structural requirements. Different column shapes may have different minimum requirements for the number of main steel bars to ensure structural integrity and adequate strength. Let's address each type of column mentioned:

1. Rectangular Columns: In rectangular columns, the minimum number of main steel bars is typically 4. These bars are placed vertically to provide reinforcement against axial loads and bending moments.
2. Circular Columns: For circular columns, the minimum number of main steel bars is generally 6. The bars are distributed evenly around the circumference of the column to provide adequate reinforcement against axial loads and to resist torsional forces.
3. Octagonal Columns: Octagonal columns, having eight sides, require additional reinforcement compared to rectangular or circular columns due to their complex geometry. Therefore, the minimum number of main steel bars provided in octagonal columns is usually 8.

• 4 for rectangular columns
• 6 for circular columns
• 8 for octagonal columns