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When to use Sj or Sj,ini

In case of a semi-rigid joint, the effects of the behaviour of the joints on the distribution of internal forces and moments within a structure, (and on the overall deformations), should be taken into account. When these effects are sufficiently small they may be neglected, this is the case when you have a pinned or rigid joint.

Whether a joint is pinned, semi-rigid or rigid can be determined by comparing its initial rotational stiffness Sj,ini with the classification boundaries given in Eurocode 1993-1-8 §

The effect of the joint behaviour can be represented in a moment-rotation diagram or by a rotational stiffness value.

Possibilities to model the joint’s stiffness

Moment rotation diagram

Eurocode 1993-1-8 §5.1.1 (4) allows you to use any appropriate curve, including a linearised approximation, such as a bi-linear or tri-linear diagram. This last one is used in PowerConnect.  If you are working with the Diamonds <> PowerConnect link, this tri-linear diagram is transferred to Diamonds automatically, ready to be used in the global analysis.

Example of the tri-linear moment-rotation diagram in PowerConnect

Rotational stiffness value Sj or Sj,ini

From Eurocode 1993-1-8 §5.1.2 (3) and (4) we learn that

(3) In the case of a semi rigid joint, the rotational stiffness Sj corresponding to the bending moment Mj,Ed should generally be used in the analysis. If Mj,Ed does not exceed 2/3 Mj,Rd, the initial rotational stiffness Sj,ini may be taken in the global analysis.

(4) As a simplification to (3), the rotational stiffness may be taken as Sj,ini/η for the analysis for all values of Mj,Ed, where η is the stiffness modification coefficient.

This means for the rotational stiffness:

  • you should always use Sj
  • but if Mj_Ed < 2/3Mj,Rd, you may use Sj,ini

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