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.
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:
- Sj,ini – initial stiffness of the joint; the joint stiffness is assumed linear up to the 2/3 of Mj,Rd
- Lb – theoretical length of the analyzed member
- E – Young's modulus of elasticity
- Ib – moment of inertia of the analyzed member