1. Biaxial Bending AISC Chapter H
Bending about both Major and Minor axis of section

2. Biaxial Bending - Shear Center
That point through which loads must act if there is to be no twisting or torsion

3. Biaxial Bending - Shear Center
That point through which loads must act if there is to be no twisting or torsion

4. Case I: Loads Through Shear Centeror

5. LRFD - Loads Through Shear Center
Bending about y-only
Bending about x-only
Interaction Formula
Allows the designer to allocate to one direction what has not been “used up” in the other direction

6. ASD - Loads Through Shear Center
Bending about y-only
Bending about x-only
Interaction Formula
Allows the designer to allocate to one direction what has not been “used up” in the other direction

7. Example
W21x68
A992
12 ft
MDx= 48 ft-kips
MDy= 6 ft-kips
MLx= 144 ft-kips
MLy= 18 ft-kips
All moments uniform over length of beam

8. Members Under Combined Forces
Most beams and columns are subjected to some degree of both bending and axial load
e.g. Statically Indeterminate Structures P1 P2 C E A D F B

9. Interaction Formulas for Combined Forces
e.g. LRFD
If more than one resistance is involved consider interaction

10. Basis for Interaction Formulas
Tension/Compression & Single Axis Bending
Tension/Compression & Biaxial Bending
Quite conservative when compared to actual ultimate strengths
especially for wide flange shapes with bending about minor axis

11. AISC Interaction Formula – CHAPTER H
AISC Curve
r = required strength
c = available strength

12. Members Under Combined Forces
Most beams and columns are subjected to some degree of both bending and axial load
e.g. Statically Indeterminate Structures P1 P2 C E A D F B
What is the effective length factor?