Reinforced Concrete Columns and the Plastic Centroid

[Perception]

Hello everyone,

I don't understand why it is necessary to sum moments about the plastic centroid when analyzing columns with axial loads + moments. In theory shouldn't the sum of the moments about any point on the cross section produce the same results? In beam analysis this was the case.

After finding the steel and concrete forces acting on a column for a point on the interaction diagram, I tried to sum moments about multiple points along the height of the column. Each time I got a different answer and I don't understand why. I do notice that the sum of the forces add up to a non-zero value which is different than with pure flexure. Is the why the sum of the moments produce different results if you chose different points throughout the cross section?

Thanks

 

[KootK]

Try this thread: Link

Essentially, you can take the moments anywhere but, for things to remain consistent, you need to include the additional moment created by the resultant axial loads's eccentricity from the point about which moments are summed.

In my opinion, the key to it is summarized nicely by these statements from the referenced thread:

It depends how you do it.

If you add all the moments of the steel and concrete about the NA you will get a different answer to adding the moments about the centroid.

If you add the moments about the NA then add the moment due to the resultant axial force x eccentricity of NA from the centroid, you will get the same result.

If the resultant axial force is zero, then it makes no difference.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.