# Calculation background of edge hinges meeting at a common node

## Problem description#

On the user interface edge hinges are defined continuously along the edges of surface elements. However, in the calculation model in the case of edge joints, finite elements are connected only at the nodes, with single connecting spring elements. In the calculation model nodes where more edge joints are connected from more directions can be critical. In such cases, it is important to know the background of the operation of the edge hinges in the calculation model so that they are set correctly. Improper definition can lead to instability of the model. The following example illustrates such a problem.

### Example model and the description of the problem#

The slab is a slab panel construction and the load transfer direction is parallel to the rib.  For this reason, moments are not transferred to the rib and the walls running parallel to the rib. Therefore, edge hinges were defined between the slab and the rib and between the walls running parallel to the beam and the slab. The rib is a fully restrained beam.

### Expectation#

Torsion of the beam is transferred to the wall by the mean of the fully restrained end releases.

### Error message#

The rib can rotate freely around its own axis and the calculation stops.

## Explanation#

On the user interface edge hinges are defined continuously along the edges of surface elements. But in the calculation model in the case of edge joints, finite elements are connected only at the nodes, with single connecting spring elements. In the calculation model nodes where more edge joints are connected from more directions can be critical. The last node along the edge hinge may often be separated from the finite elements of the connecting surfaces, as was the case in this model.

The following figure helps to understand the phenomena. In the example model the node marked with yellow circle is the critical one. The black nodes show the location in the model, the blue ones show the offset in the calculation model because due to the edge hinge. Red circles show the position of the edge hinge.

Since the edge hinge was defined along the entire length of the rib, the last element of the rib should also attach to the slab by an edge joint. However, each element of the wall is also separated from the slab by an edge joint and thus from the rib defined for it. Therefore, if there is no torsional stiffness in any of the edge hinges, the last node of the rib will be unstable.

## Conclusion#

In such cases, by giving a minimum rotational stiffness about its own axis to one of the edge joints, that does not affect the results of the calculation, the program no longer shows instability and the calculation runs.