For reinforced concrete columns under axial load with inevitable eccentricity, the design eccentricity shall be taken as the distance from the assumed load application point to the centroid of the section, subject to a minimum of how many millimetres?

Introduction / Context:
Columns are rarely loaded with perfectly concentric axial forces due to construction tolerances, load transfer paths, and second-order effects. Design standards therefore enforce a minimum eccentricity to avoid unconservative assessments of strength and to capture realistic bending moments even when axial load is intended to be concentric.

Given Data / Assumptions:

• Member: reinforced concrete column.
• Quantity: minimum design eccentricity to be assumed.
• Definition: distance from the assumed load application point to the centroid of the cross-section.

Concept / Approach:
Minimum eccentricity protects against idealized zero-moment assumptions. Codes commonly prescribe e(min) based on overall dimensions (e.g., l/500 + D/30) but not less than a fixed floor value to account for practical misalignments. The fixed floor is widely taken as 20 mm to ensure a minimum bending moment is always present in design checks, enhancing safety against brittle compression failures.

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