Ribboning platform canopies herald the end of upgrade works which, under a multitude of constraints, transform this major London station
One of the best things about going up The Shard is watching the seemingly tiny trains below snake to and from London Bridge station. This view has got even better recently as construction of the new Grimshaw-designed station has gathered steam, revealing the sinuous form of the roof, which reads not as one structure but rather as nine interconnected tapering ribbons, their metallic surfaces gleaming in the sun.
This fifth elevation was of particular interest to the planners, says Grimshaw associate principal Stuart Grahn, and central to achieving the key goal of uniting the two previously disparate elements – terminus and through station, which together form one of the UK’s biggest stations. The 31,000m² roof is now complete along with most of the station works and the project is due to be officially completed in spring. It has been a long haul – construction of the platform structures started in 2013 – with the added extreme logistical difficulties of keeping the station open throughout, a situation akin to carrying out open-heart surgery, says the practice. The result was a complex nine-phase construction, with platforms rebuilt two at a time.
The roof concept was for typically 255m long canopies over each pair of platforms in ribbons that gradually rise to express the unified central public concourse - located unusually on the floor below. The rise covers about 90m. As the roof ribbon lifts up, it admits light to the lower level through clerestory windows, with larger openings on the north side for indirect daylight. On the south side, the opening is smaller to reduce direct sun. This light is maximised by chamfering the bridge decks to the concourse.
The pre-set positions of the tracks and the 15 platforms give an irregularity to the plan, with a wider expanse used to navigate the transition between the higher through platforms and the terminus platforms. Here, two larger tapering ribbons step down to meet the lower level. From above, the different roof strands are clearly visible, breaking down its visual impact while retaining a single identity across the whole site
‘The design is really trying to tie the two disparate parts together. It’s the first time it’s been unified and that’s what makes the project more exciting,’ says Grahn. ‘I hope it feels like there’s a continuity.’
Restricted track possessions and the resulting need for speedy construction that minimised works on site informed the prefabricated design of the roof canopies.
‘The programme was driven by rail possessions which gives a very compressed construction period. So we used prefabricated modular systems as much as we could. This meant we could bring large elements to site and install them overnight when we had access over the rails,’ explains Grahn.
The solution was the use of hundreds of prefabricated, 3m wide by 2.5m deep steel cassettes topped with Kalzip aluminium roofing, which the architect had used at Reading Station. This standard length is increased where the canopies widen out to meet each other over the tracks in the central part of the platforms.
‘It’s a good material for railway use. It’s robust with a naturally anodized finish. We wanted to avoid any kind of powder-coated surface that could scratch easily and get flakey. The metallic aesthetic helped sell the concept of a heavy base with the light roof element floating above,’ says Grahn.
Prototypes were erected off-site, one to demonstrate visual ambition and the other speed of erection, quality, and design and maintenance issues. Roof cassettes are mounted on a platform canopy formed by 3.8m high Y-shaped columns spaced at 15m centres and connected by spine beams. The columns incorporate rainwater pipes and electrical services, again to reduce clutter. With a de-cluttered platform a priority to give drivers and passengers clear sightlines, the cassettes incorporate a recessed linear trough for continuous, pre-wired services such as CCTV, lighting and the Customer Information System. This sits 1250mm away from the edge of the canopy to enable safe access for maintenance without the need for track possession. Service hatches in each cassette soffit at 3m centres allow maintenance access.
Trucked to site in loads of three, individual units were installed by tower crane, with each cassette plugged into the next and spliced with mechanically-fixed, aluminium flashing.
The concourse is created with the help of new steel bridge decks that support platforms and rail tracks. Above the concourse, the canopies are elevated and angled on the steel substructure rising up gradually to a high point of 9.5m. A trafficable gutter for maintenance is incorporated alongside the clerestory windows.
Particular attention was paid to the perimeter of the station roof along St Thomas and Tooley streets and its relationship there to the surrounding environment. The curving canopy is extended out to announce the entrances in the urban realm, with its arc visible from the nearby More London development. On St Thomas Street, care was taken with the interface of the new and listed arches, with the glazed upper structure expressed as a flowing lightweight element in contrast with the heavy Victorian structure.
At the terminus, canopies on the six terminating platforms are designed to sweep underneath the existing roof, which was constructed by Network Rail as part of the Shard interface works. This interface was designed so that the new canopies do not rely on making a structural connection to the terminus.
The extreme logistical difficulties had long delayed any attempt to embark on a much-needed redesign of this station, which dates back to 1836, and the transformation finally went ahead as part of the Thameslink expansion programme. Even before its final completion, it’s clear that the station has been transformed beyond recognition, not only for those looking down on it from on high but, rather more importantly, for its many thousands of users. Hopefully, with the end now in sight, regular London Bridge passengers will feel that the many years of construction-related pain was worth it for the considerable long-term gain.