Why do we use minimum eccentricity in column design?

When we design an axially loaded column, it is very important to consider some moments along with the axial load. As in practical scenarios, it is not possible to neglect the moment completely.

According to IS 456:2000, in order to ensure the safety of the column we should consider a minimum value of eccentricity. Also, we further reduce the design strengths of concrete and steel for a good design of columns.

Some key points according to IS 456: 2000 are as follows:

• Design strength: The design strength of concrete is reduced to 0.4 fck and Consider 0.67 fy for design strength of steel

• Minimum eccentricity : It should be , ex min = greater of (l/500 + D/30) or 20 mm and ey min = greater of (l/500 + b/30) or 20 mm

• Note: ex min and ey min is the minimum eccentricity in x and y direction and l, b and D are the length, width and depth of the column respectively.

Column with an axial load "P" and moment in x direction "Mx "

So yes even if you are designing the column as a axially loaded column, you should consider the eccentricity in both directions. Thus the moment corresponding to the minimum eccentricity is considered for the design of reinforced concrete column.

Conclusions

In conclusion, the use of minimum eccentricity in column design is essential for ensuring the stability and safety of structures.

Some key learnings are:

• Failure mode : When the load is applied at an eccentric location, the column is prone to both compressive and bending stresses, which can lead to failure.

• Min. eccentricity: When we ensure this condition, then only compressive stress is present in the column, which is better for concrete.

• Column design: The use of minimum eccentricity in column design is also important to ensure that the structure is able to resist seismic forces and other types of loading. So yes for the design of columns, engineers must take into account the eccentricity of the load.