Question

As per IS:456, the columns are designed for a minimum eccentricity of

• A. 15 mm
• B. 20 mm
• C. 25 mm
• D. 40 mm

Hint:

The minimum eccentricity formula is a key provision in column design to ensure a minimum level of bending capacity.
Remember the formula: $e_{min} = \max \left( \frac{L}{500} + \frac{D}{30}, \quad 20 \text{ mm} \right)$.
In many exam questions, simply knowing the absolute minimum of 20 mm is sufficient.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the minimum eccentricity that must be considered in the design of an axially loaded column, as specified by IS 456.

Step 2: Key Formula or Approach:
In reality, it is impossible to apply a load with perfect concentricity on a column. To account for construction imperfections and other accidental eccentricities, IS 456:2000 (Clause 25.4) mandates that all columns, even those theoretically under pure axial load, must be designed for a minimum eccentricity, $e_{min}$.
The formula for minimum eccentricity is given as the greater of the following two values:
\[ e_{min} = \max \left( \frac{L}{500} + \frac{D}{30}, \quad 20 \text{ mm} \right) \] where:
$L$ = Unsupported length of the column
$D$ = Lateral dimension of the column in the direction of bending

Step 3: Detailed Explanation:
The formula specifies two conditions. The calculated eccentricity must not be less than a value calculated based on the column's dimensions, AND it must not be less than an absolute minimum value. This absolute minimum value is specified as

20 mm.
Therefore, regardless of the column's size or length, the eccentricity used for design can never be less than 20 mm.

Step 4: Final Answer:
As per IS:456, the columns are designed for a minimum eccentricity of 20 mm (or $L/500 + D/30$, whichever is greater). The absolute minimum specified is 20 mm.