Located just outside of Heathrow, Stockley flyover is a fully welded steel bridge which spans over 5 mainline tracks going in and out of Paddington station and was commissioned as part of the Crossrail upgrade to improve the Heathrow Express service.
The bridge was constructed on a purposely built embankment which was later incorporated into the trackway, adjacent to the site where it was to be launched and has two spans positioned consecutively, supported by an east, central and west abutment. For the launch the bridge was made into a single span by attaching additional top and bottom chord sections to join the two together.
Various launch methods were looked at however the most suitable solution was to construct a track system beneath the bridge where bespoke steel bogies with flanges were positioned. These were then clamped onto the bridge structure to avoid special attachment details.
A temporary nose structure was attached to the front of the bridge to help maintain balance and to compensate for pre-camber and deflection during the launch. A temporary structure was also attached to the back to provide ballast, ensuring the bridge didn’t topple over. This also housed some of the launch equipment including; hydraulic strand jacks, generators and the control station.
The strand jacks were set up in a push pull configuration and were anchored from the east embankment some distance behind to the bridge to the east abutment. Prior to any movement of the bridge it was necessary to carry out some testing to verify that the planned procedure would actually work. A trial movement of 2.5m forwards and backwards was conducted.
The main challenge with the launch was to ensure loading on the embankment remained uniformed as it had limitations on the ability to withstand point loads from the bogies during sliding. If not controlled this could have created a local collapse of the embankment material. This was exacerbated by the bridge deflecting as it was being launched as it created uplift forces at the rear. The solution was to provide a hydraulically controlled jacking ramp at the east abutment to balance out the imposed loading.
The initial launch solution was to position static rollers on each abutment onto which the temporary structure would traverse. Concerns were raised with this approach as loads would not be redistributed to the east embankment due to the bridge pre-cambers. This resulted in the decision to install travelling bogies that moved along with the bridge during the launch. As the bridge moved the bogies were discharged at the east abutment.
In the design of the launch system it was assumed that the steelwork would remain reasonably flat on each bottom flange of the bridge, however an investigation into the tolerances implied that there could be a torsional distortion of the bottom chords of up to 16mm, which could create extremely high contact stresses with the rollers. Concerns were raised by the bridge engineers that the corners of the structure could be damaged or crushed due to the high contact forces.
The final solution was to position a Teflon coated slide bed system on each abutment. As the curvature and pre-camber of the bridge meant that the bottom chords were at different heights, the slide beds were fitted with a hydraulic system to enable the height to be adjusted to match the bridge as it was moved into place. Forces during each phase of movement were continuously monitored by an operative to ensure the launch was complying with the predetermined forces.
Once the bridge landed on the west embankment it was fully supported again, but care had to be taken to ensure there wasn’t any change in arrangement of the bridge as it could have imposed additional loading elsewhere on the temporary works. The structure was eventually lowered down onto the bearings that were slid underneath.
The bridge was then separated back into two spans, the temporary structures attached to the front and back were removed and all the contaminants from the temporary welding were cleaned off, returning the bridge to its original form. The track was opened some 5 or 6 months after the installation and during the works there was no disruption to the operational railway below.