CROSS Safety Report
Temporary bridge jacking
This report is over 2 years old
Transport for London (London Rail) reported publically on an incident concerning temporary bridge jacking and permission to reproduce an extract is gratefully acknowledged by CROSS.
Key Learning Outcomes
For the construction team:
Be aware that any structure which has a sliding mechanism should have the means to control and stop the movement
Having a competent temporary works designer/adviser in place to supply an engineered solution can ensure all temporary works are carefully considered and planned
Verification of temporary works erection by a competent person who can oversee and coordinate the whole process can also ensure the works are installed correctly
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Transport for London (London Rail) reported publicly on an incident concerning temporary bridge jacking and permission to reproduce an extract is gratefully acknowledged by CROSS. In early May 2008 Bridge GE19 was successfully moved into position above Network Rail tracks just outside Liverpool Street station, and a few weeks later the bridge was in the process of being 'jacked down' into its final resting place.
After work had finished for the day the temporary supports for the bridge failed at the east end. This failure resulted in the bridge dropping approximately 200mm off the temporary support plates onto the permanent bearings. Five concrete planks on the bridge deck were dislodged by this movement and fell onto the Network Rail tracks below. An approaching train struck the concrete planks at slow speed.
Following the incident TfL set up an Inquiry into the incident. This Inquiry Panel has concluded that the key contributory cause of this incident related to the incorrect positioning of slipper pads (metal plates covered in Polytetrafluoroethylene (PTFE)) between the base of the bridge and the top of the sloping surface of a set of taper plates, which themselves had been placed on the top of the temporary support arrangement.
This incorrect positioning allowed an unplanned movement to occur. The PTFE pads provide a low friction surface and were being used to make small adjustments to the horizontal positioning of the bridge. There are a number of contributory factors that are associated with this incident, and these are also discussed in detail in the report.
A CROSS reporter praises the open and prompt release of the TfL report and adds similar experiences of his own.
Some years ago I witnessed the aftermath of the failure of a bridge that was under construction – a braced-pair of welded plate girders had been erected on piers and the deck slab was being cast sequentially at one abutment and jacked along the girders with (I believe) graphite at the girder top flange/deck slab interface. There was a longitudinal fall on the bridge, the casting abutment being at the high end of the bridge. The deck slab just carried on sliding along the girders and, as it gained momentum it slide sideways, precipitating instability and collapse of the girders and deck slab.
About the same time I was working on a 1km viaduct with three 150m balanced-cantilever sections over a river. This used two travelling gantries to sequentially cast each pair of cantilever sections. The gantry falsework was supported on PTFE bearings running on steel beams and moved forwards by means of hydraulic rams but, as far as I recollect, with no means of preventing the gantry travelling forwards of its own volition - there was a very small longitudinal fall on the bridge.
The reporter believes that there should be advice available on the more general subject of temporary works involving low friction bearings. In particular it seems to him to be essential to highlight the need for vertical supports involving parts undergoing relative horizontal translation to have articulation surfaces that are horizontal as a norm or, where this is not possible (usually an exception), provision is made for any resulting non-vertical reactions/displacements. Additionally, in his view, even where surfaces are horizontal, there should be provision to allow for tolerances in such surfaces that could give rise to uncontrolled responses.
Expert Panel Comments
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This report illustrates the benefits that might have been achieved if the reporter’s experiences had been more widely publicised at the time. The important lesson is that any structure which has a sliding mechanism should have the means to control and stop the movement. In 1999 four men died when the maintenance gantry on which they were working on the M5 Avonmouth Bridge was caught by a gust of wind and blown along the beams from which it was suspended. The trolleys holding the gantry dislodged temporary beam clamps which were meant to prevent them moving and fell through a gap where a beam had been removed but not replaced. The contractors involved were fined £500,000 for breaching the 1974 Health & Safety at Work Act and ordered to pay £525,000 costs.
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