Providing safe working zones on inner city railway stations to carry out major station refurbishments is always a challenge, especially when the station is to remain fully operational throughout the whole project.
The refurbishment of Kings Cross station’s roof was no exception. Space was available offsite to the west of the suburban train shed, where welfare, office facilities and materials storage was established, however there was no useful access into the station or onto the station roof.
The temporary works solution was a continuous steel gantry that spanned across the full 5 bays. The gantry was supported off trestles positioned on the station platforms, comprised of RMD Slimshor and/or structural steelwork. The support trestles were assembled off site into small sections and delivered to the station by rail for erection using a small mobile crane.
The gantry provided access from the site establishment, over the live station and onto the train shed roof, whilst also providing support for the construction of access scaffolding for the refurbishment of the north gable glazing, providing a deck that facilitated the construction of scaffolding materials for the roof access system that were to be preassembled and also provided materials storage for items used on a daily basis. Further design parameters included providing protection against falling materials and to ensure the structure was water tight. Gutters and drainage systems were installed adhering to OLE clearances.
The trestles on platforms 8/9 had to negotiate a Grade 1 listed canopy which was to remain in place after the refurbishment had been completed. A full heritage survey of the canopy was undertaken to record the original structure to ensure it would be restored to original design once the station works had been completed. The trestles were supported on piled foundations, comprised of RMD Slimshor soldiers and structural steelwork.
The trestles on platform 6/7 were of similar arrangement to platforms 8/9, however due to the narrowness of the station platform, these trestles had to be specifically shaped to avoid restrictions to Pedflow and to avoid any obstructions to signal sighting.
The station spine wall was positioned on platforms 4/5 and extended out further than the rest of the train shed, obstructing approximately 60% of the depth of the gantry. The spine wall had insufficient capacity to accommodate any additional lateral loading and any structures placed at platform level would have hampered signal sighting. The solution was to suspend a bespoke frame at gantry level from a cradle structure positioned on top of the wall.
Support to platforms 2/3 presented significant issues with signal sighting due to the location of the track, so a single steel column trestle was incorporated into the support structure, aligned to follow the curvature of the platform.
Support on platform 1 had a temporary stairway incorporated for access to and from the gantry. The trestles were founded on short bored piles to avoid any effect on the Piccadilly tube line tunnel below.
The erection of the gantry was carried out in 4 hour possessions, per the rules of the route for the station. Taking into account that it would be impossible to erect the full gantry in one possession, special consideration of the design and erection ensured the structure would remain 100% stable between possessions.
Within the concept design phase the erection of the gantry presented a particular challenge. Crane access from outside the station was not readily available or feasible. An investigation into the availability of rail mounted cranes and their SWL limits, resulted in changes to the original modularization of the gantry frames to reduce their unit weight and size.
Limits imposed to the length of the gantry sections due to the crane restrictions were exacerbated when it became apparent that the fabrication of the gantry bridge sections had to be completed before the support trestles had been installed. Fabrication of the bridge sections required the incorporation of +/- 500mm tolerance on the length of the bridge units before the trestle locations had been determined.
The life span of the gantry was 3 to 4 years and performed successfully responding to the imposed loading as per the design brief. Once the refurbishment was complete, the gantry was incrementally dismantled whilst the railway remained fully operational. The dismantling strategy required as much consideration as the erection, as it was likely that the gantry was to be removed by different contractors to those who erected it.