In a report into the incident, TfL found the 1,300-tonne GE19 bridge dropped 200 mm after subcontractors made final adjustments before placing the bridge on its permanent fixtures.
They wrongly positioned a polytetrafluoroethylene (PTFE) plate between the base of the bridge and the temporary supports.
The main contractor – a Balfour Beatty/Carillion joint venture – had employed Fairfield Mabey for the manufacture and installation of the bridge steelwork. Fairfield Mabey employed specialist steel erection firm MJ Hughes to assist with the installation.
It is believed as the 80 mlong bridge cooled down in the evening it contracted by two or three millimetres. This led to a movement, which caused the temporary supports to be ejected from their position, resulting in the east end of the bridge dropping onto its permanent fixtures.
The incident caused five pieces of concrete decking to fall onto rail lines below. Loss of the decking allowed water that had accumulated on the surface of the bridge to pour through the bridge deck onto overhead cables located directly beneath the bridge.
Some of the debris was hit by an approaching train and the incident resulted in services into Liverpool Street station being suspended overnight.
Work preceding the fall had not been cleared with the main contractor or TfL project engineers and Fairfield Maybe’s senior site manager was not present at the time of the incident.
TfL head of safety for London Rail Martin Brown said: “The positioning of the Teflon pad was a human error but the procedures we have should have stopped this error occurring. There is a collective responsibility for ourselves and our contractors to ensure these procedures work.”
The Balfour Beatty/Carillion team has upgraded its system of checking its subcontractors while TfL has stepped up its overall supervisory role to prevent a similar incident in future.
Analysis: Designers need to consider plan jacking methods
By Martin Brown
Bridge GE19 is a Warren girder truss bridge. In its final position it sits on four bearings, allowing small temperature change movement.
At the east end the bearings permit longitudinal and 360 deg (horizontal plane) movement. At the west end one bearing is fixed, while the other permits lateral movement. The bridge was launched onto temporary packing, which sat on the top of the bearings. To achieve good contact, the top packing plates had a 3.3 deg taper.
To locate the bridge in its final position, jacking was needed in both vertical and horizontal planes.
A horizontal (plan jack) movement of about 38 mm was needed. This slow and controlled process requires a technique which significantly reduces friction.
For GE19, reduced friction was achieved using PTFEcovered metal plates, inserted between the temporary packing and the bridge.
The final bearings were not secured to prevent movement. The PTFE pates were inserted directly beneath the bridge and on the top of the tapered plates.
When the bridge cooled slightly, the small movement translated through the PTFE and bearing, squeezing the packing out.
Bridge and temporary works designers should consider how they avoid placing PTFE plates on top of packing plates when plan jacking, and how they secure final bearings when jacking.
Martin Brown is TfL head of safety for London RailTo download the main report, a flow chart of the root causes and photos of the bridge collapse click on the resource box on the right hand side of the page