Joints are classified according to joint stiffness to:

- Rigid – joints with insignificant change of original angles between members,
- Semirigid – joints which are assumed to have the capacity to furnish a dependable and known degree of flexural restraint,
- Simple – joints which do not develop bending moments.

Australian standard AS 4100, Cl. 4.2 does not provide exact boundaries so the joints are classified according to the commentary in AISC 360-16, Cl. B3.4.

- Rigid – \( \frac{S_{j,ini} L_b}{E I_b} \ge 20 \)
- Semirigid – \( 2 < \frac{S_{j,ini} L_b}{E I_b} < 20 \)
- Simple – \( \frac{S_{j,ini} L_b}{E I_b} \le 2 \)

where:

*S*_{j,ini}– initial stiffness of the joint; the joint stiffness is assumed linear up to the 2/3 of*M*_{j,Rd}*L*_{b}– theoretical length of the analyzed member*E*– Young's modulus of elasticity*I*_{b}– moment of inertia of the analyzed member*M*_{j,Rd}– joint design moment resistance