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 *S _{j,ini}* with the classification boundaries given in

**Eurocode 1993-1-8**

**§5.2.2.5**.

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.

### 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 S _{j} corresponding to the bending moment M_{j,Ed} should generally be used in the analysis. If M_{j,Ed} does not exceed 2/3 M_{j,Rd}, the initial rotational stiffness S_{j,ini} may be taken in the global analysis.*

*(4) As a simplification to (3), the rotational stiffness may be taken as S _{j,ini}/η for the analysis for all values of M_{j,Ed}, where η is the stiffness modification coefficient. *

This means for the rotational stiffness:

- you should always use
*S*_{j} - but if
*M*, you_{j_Ed}< 2/3M_{j,Rd}*may*use*S*_{j,ini}