Minimum design eccentricity for reinforced concrete columns: Columns shall be designed for a minimum eccentricity equal to 1/30 of the unsupported length, subject to a minimum of how much (whichever is greater)?

Introduction / Context:
In reinforced concrete column design, codes prescribe a minimum design eccentricity to account for inevitable construction tolerances, misalignments, and second-order effects. This ensures that columns are not designed for unrealistically perfect concentric compression.

Given Data / Assumptions:

• Minimum eccentricity e_min = L_u / 30, where L_u is the unsupported length of the column.
• A floor minimum absolute value is also prescribed.

Concept / Approach:
The design must consider the larger of the length-based eccentricity and a fixed minimum value. This prevents e from becoming too small for short columns, where L_u/30 can be less than a practical lower bound.

Step-by-Step Solution:
Step 1: Compute e_1 = L_u / 30.Step 2: Compare e_1 with the absolute minimum recommended by code.Step 3: Use e = max(L_u / 30, 20 mm). Therefore, the minimum is 20 mm.

Verification / Alternative check:
Design handbooks and IS-based provisions consistently adopt 20 mm as the absolute minimum eccentricity floor for RC columns.

Why Other Options Are Wrong:
5 mm/10 mm/15 mm: Too small; do not satisfy the prescribed absolute minimum.
25 mm: Exceeds the standard minimum and would be unnecessarily conservative for many cases.

Common Pitfalls:

• Forgetting to take the maximum of L_u/30 and the absolute minimum.
• Using center-to-center story height instead of unsupported length.

Final Answer:
20 mm