The location of the stress transition zone is not only limited to the extremities of a continuous welded rail (CWR) track, the case presented in a previous article – CWR stress transition zone. A stress transition zone may also be present between two fixed zones, inside the CWR.
These internal stress transition zones are shorter than the ones formed at the end of CWR and can be generated by several factors which can be categorised into two main groups:
1. Stress transition zone caused by rail temperature changes. These can be encountered in areas of CWR track where there is a consistent rail temperature difference between adjacent CWR sections. For example:
- Between two CWR sections, each installed with different stress-free temperatures (SFT).
- CWR at tunnel portals. In tunnels the rail is not exposed to sun light and, for sufficiently long tunnels, the rail temperature is significantly different than that outside the tunnel.
- Transition between sections of track with significant changes in track sunlight exposure – for example the transition from cutting to embankment.
- Transition from white-painted to un-painted rails. The painted rail has a temperature of around 6°C lower than the un-painted one. Hence, a short stress transition zone will develop where this change in rail temperature occurs.
- Transition from exposed to embedded rails (on longer level crossings or embedded rail slab track).
- Passage over a river (on a bridge) – the condensing water and air currents will reduce the rail temperature compared to the track on the abutting sections. The bridge itself also has an influence that is included in the second category of stress transition zones.
2. Stress transition zones generated by track structure changes. The potential for variation/transition in rail thermal force may be created wherever the track structure changes. For example:
- Track across bridge – the movement bearing of a bridge allows thermal expansion of the structure due to temperature variation. If the track is continuous over a movement bearing, the bridge will tend to move longitudinally the track relative to the adjacent (often embankment) section. This action can significantly change the distribution of the rail axial forces, especially if the rail is directly fixed onto the bridge deck. For short ballasted bridge deck the impact is minimised, however, for long structures, the effect remains significant.
A bridge also subjects the track to variable bending and other actions which are influencing the variation of the rail longitudinal (thermal) forces (UIC Code 774-3 – 2001). To reduce the impact caused by the bridge induced forces, the CWR track can be interrupted by the placement of adjustment switches over the movement bearing.
Adjustment switches on the renewed TEN-T railway corridor in Prague (source of the image: www.dtvm.cz)
- Transition from ballasted to slab track or other higher fixity track systems (including glued ballast). In this section changes in the track longitudinal resistance create the potential for generating a stress transition zone.
- Change of rail section – the change in the rail section area changes the value of the thermal force even when all the other track parameters are the same. The rail thermal force in a CEN60 rail is 6.7% higher than that in a CEN56 rail. This generates a short stress transition zone (around 3 m for a 20°C variation from SFT) at the rail type change location.
- Presence of guard rails increase the thermal forces in the track. To reduce their influence, the guard rails are usually installed without being fastened to all sleepers and with fishplated joints to minimise the additional thermal forces caused by their presence (sometimes with bolts not tightened to the full torque required by a running rail joint).
- Stress transition over Switches and Crossings (S&C). For a single turnout abutted by plain line CWR there is a complex thermal force loading – two rails (two forces) at the switch toe and four rails (four forces) at the heel of the crossing. The entire S&C layout creates a complex stress transition zone with potentially significant thermal force variations compared to plain line.
In addition to these two main groups, a dynamic stress transition zone is created below a moving train – influenced by the vehicle loading and by the acceleration and deceleration of the vehicle. The longitudinal resistance is increased under the wheels and decreased in front/behind the vehicle and between bogies.
(obviously, the animation above is just illustrative and not to scale)
Slewing and lifting the track during track maintenance works may also affect the track thermal forces generating irregular stress transfer zones, where the track lateral or vertical movements are significant.