Biaxial Bending
Column bent about both principal axes simultaneously plus axial load
Biaxial bending occurs when a column carries axial load together with bending moments about both of its principal axes simultaneously (Mux and Muy). It is the normal condition for corner and edge columns of a building frame, columns with eccentric or asymmetric loading, and columns under combined gravity + lateral (wind/seismic) actions from two directions.
Design cannot simply add the two uniaxial designs. IS 456 Cl. 39.6 uses Bresler's load-contour interaction expression: (Mux/Mux1)^αn + (Muy/Muy1)^αn ≤ 1.0, where Mux1, Muy1 are the uniaxial moment capacities at the actual axial load Pu (read from interaction diagrams/SP 16) and the exponent αn varies from 1.0 to 2.0 with the axial-load ratio Pu/Puz. Reinforcement is arranged (usually symmetrically on all faces) and iterated until the expression is satisfied.
- Corner + edge column design in frames
- Columns under bidirectional wind/seismic
- Eccentrically + asymmetrically loaded columns
- Bridge piers under combined longitudinal/transverse load
- Capacity check of irregularly loaded existing columns