In the context of RCC column design as per IS 456, which of the following factors most significantly influences the effective length of a column in a frame that is unbraced in one direction and braced in the other? 

Explanation:

Effective Length of RCC Columns

In the design of reinforced concrete (RCC) columns as per IS 456, the effective length of a column is a crucial parameter. It is influenced by various factors, among which the relative stiffness of the beam and column plays a significant role. This is because the effective length of a column is determined by the boundary conditions and the stiffness of the connecting members, which in turn affects the column's buckling behavior.

Analyzing the Given Options

1. "The grade of concrete used in the column." (NOT the most significant factor)

   • While the grade of concrete affects the strength and stiffness of the column, it does not directly determine the effective length, which is more influenced by boundary conditions and the relative stiffness of the members.

2. "The relative stiffness of the beam and column." (Most significant factor)

   • The relative stiffness of the beam and column significantly affects the effective length. Columns in a frame depend on the stiffness of the connecting beams to determine their effective length, which influences their buckling strength.

3. "The length of the column." (A factor, but not the most significant)

   • The length of the column does affect the effective length, but it is the boundary conditions and relative stiffness that are more critical in determining the effective length for design purposes.

4. "The cross-sectional dimensions of the column." (A factor, but not the most significant)

   • The cross-sectional dimensions affect the column's strength and stiffness, but the effective length is primarily influenced by the relative stiffness of the frame members and boundary conditions.