Steel joint classification (AISC)

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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 – \( \frac{S_{j,ini} L_b}{E I_b} \le 2 \)


  • 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
  • Mj,Rd – joint design moment resistance

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