Track - bridge Interaction. Recommendations for calculations

Summary

Interaction between track and bridge, i.e. the consequences of the behaviour of one on the behaviour of the other, occurs because they are interlinked, regardless of whether the track is directly fastened or has a ballast bed.

This interaction takes the form of forces in the rails and in the deck and its bearings, as well as displacement of the various elements of the bridge and track.

If the interaction is under control, then the bridge will continue to fulfil its function, i.e. supporting the track without the track being subject to anomalies.

There are two types of anomaly: rail fractures, or disruption of the link between track and bridge such that track stability is no longer guaranteed (one notable example of instability is when the ballast loosens following considerable displacement of the bridge deck such that sufficient resistance to lateral buckling at the ends of the bridge is no longer guaranteed).

Therefore interaction - strictly speaking - must be taken into account as a serviceability limit state (SLS) as regards the bridge, as well as being an ultimate limit state (ULS) as regards the rail. Forces and displacements must therefore be calculated using the partial safety factors for the loads concerned. The relevant factors are applied to the forces according to the checks required at ultimate limit state as regards the strength of the bearings and the substructure.

This leaflet, which is a result of the work of ERRI Specialists' Committee D 213, gives methods for calculating the forces and displacements that are linked to interaction phenomena.

The acceptable limit states for the track depend on its design and state of maintenance. The permissible limit values given in the leaflet, whether for displacements or additional stress in the rail due to interaction phenomena, are the values that are widely permitted for standard track components in a good state of maintenance. If a railway for its own reasons operates outside the foreseen scope of application, that railway will still be able to use the calculation methods in this leaflet by replacing the criteria given here with new criteria based on its own experience and observations (this mainly concerns 72 N/mm2 for permissible additional compressive stress in the rail and 5 mm displacement of the deck under braking forces).

Similarly, the track strength taken into account and the temperature increase envisaged are drawn from the knowledge of the various railways. It is perfectly possible to use this leaflet but with different values, if the need were to arise.

It should also be noted that the displacements (or rotations) to be checked only concern what has to be checked to guarantee that the behaviour of the bridge cannot damage the track and alter its behaviour.

There are other checks to be made as regards displacement (and rotation) of the structure, these being concerned with problems of comfort, dynamic behaviour or simply strength. The checks with regard to interaction do not cover the other checks that are necessary.