Back in 2002, we started on a journey that saw the total refurbishment of the 27,000sqm roof. The roof hadn’t been maintained for probably 30 to 40 years and so the glass was falling out, it was leaking and desperately needed cleaning. As with all these types of projects, it was essential that the station remain fully operational at all times. This meant trying to devise a suspended access and protection system to allow workmen to carry out the required task throughout the day. This is a far more efficient working process, as opposed to trying to shoehorn the work into evening possessions, which are very inefficient and expensive.
The initial stages of the design process required that we conduct a dimensional survey of the station roof and a structural assessment of the capacity of the roof in its capability of carrying the suspended access and protection deck. The feedback from the assessments informed that the roof might not be strong enough to carry a significant load. This put the entire scheme in jeopardy, as the work that would be required to strengthen the roof to enable it to carry the load, would outstrip the benefit of refurbishing and repairing it.
After detailed discussions with our client, their consulting engineers and the scaffold contractor, we managed to arrive at a loading pattern which reduced the imposed loading on the roof to substantially lower levels. We were then able to move forward with the design which allowed for the access and protection deck to be suspended from the existing structure.
The construction of the deck still needed to have the lowest weight as possible. To do this we designed the system in an aluminium space frame structure, overlain with a lightweight metal deck with a composite sheet, which provided weather protection, sound isolation, fire protection, impact load protection and a solid platform for the workmen.
It was essential that the deck didn’t actually attach to the existing structure in a way that would impose constraints and restrict access to the steelwork or roof, so a secondary suspension system was developed. This provided duplicate support locations on the existing roof structure, allowing the load to be transferred whilst work was carried out at the primary support location. Once the work was complete the load was transferred back to the primary suspension deck, allowing the secondary point to be removed.
The entire deck was suspended beneath the bottom corner of the roof truss by about 500mm. The roof itself overall had a depth of 4.6m, requiring the need for secondary access to be constructed to allow workmen access to the glass, purlins and gutters. The solution was to accommodate rolling platforms and rigs on the suspended deck, to traverse up and down each bay of the roof structure. Once the glazing, purlins and gutter were removed, it was decided that the roof trusses should also be removed due to the poor condition. This meant at one point approximately 50% of the station roof had been removed and the only protection to the railway and passengers below, was the suspended deck which was then acting as a full secondary roof.
To get the materials off the suspended deck and out of the station was a logistical nightmare due to the restraints and tightness of the site and the space available at road level. It was decided that a steel gantry was to be constructed over Cab Way, resting on bespoke foundations in the footpath and on the side of the wall. To avoid overloading the brickwork a series of roller joints were designed to allow vertical loads to be carried without imparting any horizontal forces. A mini-city was built on the gantry with 20 cabins providing access for all the sub-contractors, a clean room, toilets, all the messing facilities, meeting rooms and also provided space for a bespoke, mobile rolling crane to be sited. The crane was used to pick up materials from the suspended deck onto the gantry and then either to a goods passenger hoist or directly to a crane to be loaded into wagons.
The erection of all the access systems and suspended scaffold took place at night. Special methods were developed to get large areas of the refurbished roof built, so that work on top of the deck could continue in the day, subsequently reducing the lifespan of the project.
The process worked particularly well. During the project, there were a few torrential downpours with most of the rain being collected by the deck, although there were times when the station platforms became flooded when the existing drainage system couldn’t cope with the additional water flow.
At one point there was a fire when the paint store blew up on the deck. I was sat at home worrying as messages started coming in saying the station was on fire, however when I got to the station I found that it was only this area and that there was no damage to the station or the protection deck.
As the work was being completed it was agreed to sequentially remove the access and protection deck from the far west corner of the station and draw it back to the gantry. As the station roof was revealed, the public was able to see the structure had been cleaned, painted and re-glazed. The dismantlement of the deck took approximately 4 months and the final removal of the gantry, with the reinstatement of the footpath and roadways, took another 3 to 4 months.
The refurbishment was so successful that so much sunlight was let in that the station became too bright for the train drivers, who were not used to working in that environment. It was decided that nettings were to be attached to the refurbished roof and shrouds were built on the main concourse were the CIS screens were located. To date, there has been no feedback of any difficulties as a result and to all accounts, it has been one of the best successes of a major station refurbishment in the London area.