Rolling out of the access and protection deck at London Bridge Station

There are many projects where an access and protection deck have been rolled into place.  These are mainly projects with large clear spans that have activity beneath that cannot be disrupted.  In most cases the area that is being covered is rectangular with relatively straight parallel sides.   London Bridge however was unique as the roof consisted of three barrels that were curved and tapered at one end.  Rolling out an access and protection deck in this case was seen to be very challenging but was the only way in which Network Rail would allow the work to be carried out.  More conventional methods would have increased the time frame by 3 to 4 months and would have required the station to be completely empty of trains and other infrastructure.

The curvature and tapering of the roof meant that the access and protection deck could not be rolled into position via any motorised system or Turfers, so operatives were required to move it into position manually. Suspended scaffold platforms positioned alongside the tracking system that provided support, also provided somewhere for the operatives to stand on.  Each section spanned 12½m and was designed to expand in size individually, similar to that of a trombone, allowing the sections to expand from 22m to 27½m as they slid down from the tapered end.

A trial element was assembled off-site prior to the installation.   This made sure that the practicalities of the sliding trombone arrangement would work without any deterioration in the capacity or strength of the structure as it was sliding in and out.

Scaffolding design trial element
scaffolding design trial element

Gantries were constructed earlier in the scheme using standard scaffolding systems and RMD formwork frames.  These were positioned at the town end and country end and provided platforms for the operatives to build the sections and begin the rollout.

The first section was assembled at the country end with a sheet of metal decking pre-attached to the top.  This provided somewhere for the operatives to stand when it came time to join one section to another.  The section was then pushed out to the passenger overbridge in the centre of the station.  A further section was built and rolled down to the outward edge of the previously launched section.  Once in position, the trombone sliding element was locked off and bolted through using a clamp device that had been designed to be operable from on top of the deck.  The wheels on which it was rolled were jacked down so the weight was no longer transferred through the wheels but directly into the support structure alongside the track.  Metal decking was then overlapped at the joint, allowing the unit to become one solid working area.  In total there were 10 sections rolled out from the country end and the process was repeated at the town end.  The beauty of carrying out this sort of arrangement was the reduced amount of work carried out at height.  It also the reduced number of physical component parts that would be used to fix the joining sections.

 
Access and protection scaffolding design in operation
 

After the entire deck was assembled a gutter was installed to collect any surface water runoff that was encountered once the roof had been dismantled.  Marine plywood was installed over the top of the metal decking and further protection was installed around openings which occurred around the columns due to various construction reasons.

Once the project was complete the access and protection deck was removed in the reverse process to the assembly.  The plywood was walked along the deck, back to the gantries at either end and then discarded.  The joints that were made in the metal decking were disconnected and the sections were incrementally rolled back, dismantled and taken away from site using the loading out facilities that were installed earlier in the scheme.  The whole process of erecting the access and protection deck took 5 weeks and the dismantlement took 4 weeks.  A time frame which would have been impossible to achieve by any other means.