The infill roof structure is effectively a rectangular box
The upgrade of London Waterloo station has included constructing a steel infill roof to link the former international terminal with the main concourse. CM reports on a tricky engineering challenge.
Britain’s busiest railway station, London Waterloo, has been undergoing a £400m upgrade to increase capacity. In 2014/15, the terminus was used by 99 million passengers, a 70% increase since 1998, despite little change to its infrastructure for 30 years. The number of journeys on the South West Main Line, which runs into the station, is forecast to rise by 40% in the next quarter century.
The upgrade includes bringing the former Waterloo International Terminal (WIT), on the west side of the station, back into use for domestic services. This part of the project includes a tricky piece of engineering; constructing a new steelwork roof to bridge the 14m gap between the Victorian-era main station concourse and the futuristic WIT.
The scheme is being delivered by the Wessex Capacity Alliance, which was appointed in January 2015, and comprises client Network Rail, consultants Mott MacDonald and Aecom, main contractor Skanska and rail contractor Colas Rail. The logistics of the work, including the roof connection, is a considerable challenge, says architecture design director Erik Behrens.
“Remodelling Waterloo Station and bringing the international terminal back into use is like carrying out open-heart surgery: we need to keep the station fully operational throughout this transformation,” he explains.
Waterloo station before the upgrade and the location of the infill roof
WIT was the UK terminus for Eurostar trains using the Channel Tunnel from 1994, but in 2007, with the completion of High Speed 1, services moved to St Pancras. WIT’s five platforms, its snaking blue tubular steel roof and other facilities were mothballed and are being brought back to life as part of the current project.
This began during Christmas 2016, when extensive enabling work was carried out to the WIT infrastructure.
“Cables and wires were removed, replaced and rerouted, parts of the concourse demolished, and significant structural work carried out to connect platforms one to four and the Underground,” explains Network Rail senior sponsor Paula Haustead.
This phase also involved installation of tension cables at five locations to support the tubular steel WIT arches. These arches are restrained by the 50m wide concrete slab, which also supports the WIT platforms and track, spanning between their springing points. Converting the WIT for domestic services required new stairs and lifts to be built through this slab, but creating these openings would undermine the slab’s structural continuity. Until the openings were properly framed out, the slab’s restraining function had to be transferred to the tension cables.
“These openings were made by removing concrete in a controlled way, using hydro-demolition,” explains Alliance contracts engineering manager Chris Kitching. “This means that the reinforcement remains intact and can be manipulated to frame the opening with additional reinforcement and concrete.”
Weatherproof envelope
When WIT was built, a covered check-in area, a storey lower than the main station concourse and platforms, marked the transition between the old station and the new. Under the upgrade plan, the old check-in roof would form part of the concourse for the newly repurposed domestic platforms, but a new weatherproof envelope was needed.
Installation of tension cables
Loads from this new roof would have to be supported by existing structures and foundations due to Tube tunnels and stations below ground. An early idea was to replicate and continue the WIT roof to meet the mainline station’s roof – but this came in at an unfeasibly high cost, due to its many bespoke features. Instead, a simpler 53m-span steel-trussed roof was proposed, which would oversail both the WIT and shed roofs. Louvres in the new roof would assist in cooling the space and provide ventilation for the station’s fire strategy.
Initially, the loads at each end of the roof exceeded the bearing capacity of the structure below, so two intermediate columns, supported off existing structures, were introduced. This led to a complete redesign of the roof to echo the ridgeline of the shed roof over the existing domestic platforms one to 19.
“Columns have been located so that the load from the infill roof is supported by existing assets to achieve as near-perfect load balancing as possible,” says Kitching. “We carried out a time-history analysis to understand the impact of demolition and construction on the structure.
“Around £4m has been saved by not having to install new foundations and £2m to 3m has been saved by reducing the scope of monitoring of the existing structure.”
The infill roof is effectively a rectangular steel box, 53m wide, 26m high, and 18m across where it closes the gap between the old WIT structure and the Victorian station. Some 300 tonnes of steel were used for the structure, installed by fabricator Bourne Steel.
The new roof was completed earlier this year, and platforms 20 to 22 – three of the five old WIT platforms – were reopened to the public before Christmas. Further works on the Waterloo upgrade project will continue through 2019, with the last two WIT platforms – 23 and 24 – due to open in May.
