Biaxial bending check for columns: why two uniaxial checks are not enough

1Why columns carry biaxial bending

Seismic forces can come from any direction, and the load combinations in frame analysis include effects in both horizontal directions; this is more pronounced for corner and edge columns. The design load for a column is therefore usually an axial force with bending in two directions (Pu, Mux, Muy).

2The risk of checking only two uniaxial cases

A uniaxial P–M curve describes strength only when the moment lies in the direction of one principal axis. When moments act in both directions at once, the resultant moment a section can resist is generally smaller than a simple combination of the two uniaxial strengths, so separate checks can both pass while the section is in fact inadequate.

3The interaction surface

Putting the P–M curves for all directions together forms the three-dimensional P–Mx–My interaction surface. A load that falls inside the surface is within the strength of the section; a load that falls outside it is not.

4How to read the demand/capacity ratio

The demand/capacity ratio (D/C) reduces "how much of the strength this load uses" to one number: below 1 means the check passes, and the closer to 1, the less margin remains. StructFlow provides a continuous, stable D/C that does not jump with the sampling angle, making it easy to compare load combinations and design alternatives.

5Checking many load combinations at once

Frame analysis easily produces thousands of load cases. In StructFlow you can paste the ETABS Column Forces table or Excel columns directly, check tens of thousands of cases at once, sort by D/C, and look only at the few that fail.

Further reading