One of the most challenging projects I have been involved in was the total removal of London Bridge train shed roof. The roof architecture consisted of 3 bays and was made of 1300 tonnes of metal work, various types of cladding and patent glazing. As a heritage grade 1 listed structure, the process was more a matter of dismantling rather than demolition as the recovery of wrought iron material was paramount for the approval of English Heritage. As with all these types of projects, the primary objective was to provide a safe scaffolding access platform for operatives working within the roof space, whilst the station below remained fully operational.
Consultations and design review meetings were held with Network Rail, Networks Rail’s main contractor and their engineers, to agree on the process under which the train shed roof could be dismantled. It was quite a complex arrangement because the north and south walls of the train shed were stabilised by the roof itself. The access and protection scaffolding was therefore designed to reinstate the lateral restraint and as such maintain the stability of the station walls. The installation of the scaffolding was due to be carried out during evening, weekend and abnormal possessions when the railway was closed down at Easter and Christmas. The overall project period was 18 months which was quite a short window but was deemed to be feasible given the number of work phases that could be achieved simultaneously.
Previous experiences of working on large major station projects informed the need for an east and west gantry. This was built to span across the full width of the station, enabling elements of the access and protection scaffolding to be pre-assembled and then launched into position. The design load for the gantries was set at 10kN/sqm to accommodate the live load and the self-weight of the protection deck.
The initial concern that is usually encountered at the outset of these types of projects is whether the existing station structure has the capability of sustaining the temporary works loading. For London Bridge, it was found that the main area of deficiency related to the station platforms which had to be filled with foam concrete to provide enough bearing capacity for the support structure.
The load limits on the station presented challenges within the design process. Even with foam filled concrete there were limits to what the station platform could sustain. It was decided that the north wall would provide support on a cantilever system using RMD gallows brackets. The first line of columns had a series of steel trestles which supported runway beams servicing the north bay of the station roof and provided support for one tracking beam for the central bay. Moving across to the next line of support columns, another line of trestles were built, supporting the other tracking beam for the central bay and one tracking beam for the south bay. On the inside of the south elevation an independent scaffold was built, providing the tracking beam for the south bay. Another independent scaffold was erected and anchored to the exterior of the south elevation, providing residual access for the removal of the south wall.
The process for putting the scaffolding protection deck in place was to pre-construct elements of the deck on the east and west gantries. Built using aluminium bridge sections provided in modular forms, the section which amounted to 12.5m in width, spanned across the respective bays in the train shed. Protection for the deck was provided with corrugated metal sheeting, overlain with plywood. As the system was rolled out into place, sections were placed together and the metal decking was lapped using flashing pieces to provide a continuous system.
The load limit for the suspended scaffold system was set at 1.5kN/sqm, which allowed general access and storage of light materials throughout. Localised loading bays established within each of the station roof bays had a load capacity of 5kN/sqm. This allowance enabled concentrated loads of materials to be stored for transport to and from the east and west gantries.
The east gantry, positioned at the country end, went under construction first allowing the majority of materials to be brought in and certain existing roof materials to be taken out. A small area on the south-west side of the station, which had a road access ramp, was also used for the removal of small elements of material.
The workforce had to gain access into the roof itself to start removing cladding and metal work. This was achieved by constructing independent birdcage scaffolds which were positioned between each roof truss, creating additional loads. As each section of the roof truss was dismantled, the scaffold itself was dismantled and moved over to the next bay. By this means the roof structure was progressively dismantled. Work started on the east gable at the country end first, moving inwards towards the passenger overbridge in the centre of the station. Shortly after work began on the west gable at the town end and equally moved forwards towards the passenger overbridge.
Tests were done initially for the dismantling of the roof using cutters, nibblers, gas-axes and other manual machinery to establish the best process for removing the metal work, as it was painted heavily with lead-laden paint. It was decided that cutting with gas was the most suitable and full PPE protection was provided for the operatives.
The passenger overbridge in the centre of the station provided public access from one platform to the other and out of the station onto Tooley Street. This had to be maintained at all times because it was part of the public thoroughfare. The position of the overbridge interrupted the scaffolding deck, so a separate scaffold structure was constructed to allow operatives access to that section of the roof.
A specific concern of this project that differed from others were the possibility of large objects and materials falling due to the roof being dismantled rather than refurbished. Network Rail had previously experienced problems on other stations where materials had penetrated through the protection deck, narrowly missing passengers. To allay any fears as materials were substantially bigger than normal, a series of tests were carried out using the same assembly that was proposed for the protection deck. Materials such as 152 universal column sections were dropped from heights of 6ms to determine just how far, if at all, it would penetrate the deck. These tests were conducted on a variety of different materials, including those that had previously penetrated Network Rail’s other protection decks. In all cases, the design was proven to be quite workable and satisfactory with only indentations and very small tears created.
As well as concerns for falling debris and material, there were concerns for the ingress of rainwater into the station. A comprehensive gutter system was designed to fall towards the respective columns, channelling the rainwater falling onto the roof, down into the existing drainage system. As expected the existing drainage system was found to be inadequate and had to be cleaned to try and increase the flow of water to prevent any flooding.
Scaffold materials had to be fire treated to mitigate against any fire hazards resulting from hot works being undertaken. The station fire officers were involved in early discussions regarding access routes off the deck and the management of the public should there be a fire. The scaffolding designs at station platform level were modelled around pedestrian flow characteristics to ensure that the construction didn’t create passenger bottlenecks.
The dismantling process took approximately 12 months and there were no incidents of any significance. Where there were minor incidents of tools slipping onto the safe working area, mitigations were put in place whereby tethered tools were introduced. The process was probably one of the most successful operations that I’ve been involved in when you consider that the whole of the train shed, walls and columns were removed without any impact on the railway and without any public knowledge.