Biaxial Bending to AS 4100: What the Check Involves

by | Nov 11, 2026 | Steel to AS 4100

AS 4100 treats bending about both axes (biaxial bending) as a combined action. You can't check each axis alone and stop. The two moments must be combined with an interaction check, using the capacities and factors set by the standard.

What the check involves (principle only):

  • Find the design moments about both axes, M_x^* and M_y^*, plus any axial force, from the factored load combinations.
  • Find the section capacity for each axis. This is about local yielding and depends on the section's compactness.
  • Find the member capacity for each axis. For the major axis this includes lateral-torsional buckling, which depends on the unbraced length, the restraint conditions and the moment distribution. For the minor axis it depends on the slenderness.
  • Combine the two ratios using the interaction expression in the standard's combined-actions section. It is checked once for the section and again for the member.

Quick elastic screening check (not an AS 4100 check): \sigma = (M_x)/(Z_x) + (M_y)/(Z_y), \qquad utilisation = (\sigma)/(f_y) The stresses add at the extreme corner of the section. This is only a first look. AS 4100 uses capacity factors, not a single factor of safety.

What a failed check means: a utilisation above 1 means yielding (permanent deformation) or, for slender members, buckling. It does not mean the member snaps. Fix it with a larger or stiffer section, closer restraints, a shorter span or a lower load.

This page does not reproduce the interaction expression or the capacity values. Get them from AS 4100 itself.

For the wider picture, read our guide to AS 4100: what it is and what it covers.

Working

Elastic screening only: \sigma = (M_x)/(Z_x) + (M_y)/(Z_y)
Utilisation = \sigma / f_y (stress divided by yield)
Units: N and mm, so M in N mm, Z in mm³, \sigma in MPa.

References

  • AS 4100 Steel structures (check current edition, 2020 at time of writing) – Combined actions section (biaxial bending, section and member checks); bending members section for LTB. Check the relevant clause.
  • AS/NZS 1170.0 Structural design actions – General principles – Load combinations; check the relevant clause

General information only. This is not engineering advice, design approval or certification. Have safety-critical designs checked by a qualified engineer (RPEQ in Queensland).