Steel joint classification (AISC)

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:

• 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