# Elastic modulus about the geometric or principal axis

QUESTION
The elastic sectional modulus is S= Ix/ymax.

For symmetric cross sections, the geometric (centroidal) axis = principal axis, and the calculation is trivial.
However, for asymmetric sections, the principal axis can be rotated with respect to the geometric axis.
I’ve read that asymmetric sections (e.g. angles) bend about their principal axis.

So, that begs the question: What values do you use to determine the elastic modulus then? and would the section neutral axis coincide with that of the principal axis?

I think in the case of asymetric sections, I would use Imin/ymax as a conservative value for the elastic modulus of bending.

REPLIES

@denial
Determine your principal axes and their orientations. (They will be perpendicular to each other.)
Decompose your applied moment into components about each of the two principal axes.
For any point of interest on your cross-section calculate the stresses resulting from each of the components, then sum them.
Note that in general the plane in which your beam will deflect will not be the plane in which your moment acts.

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