No, not a muscular Belgian named Jean-Claude, but an ingenious system of glazing protection that is solving a shattering problem at Waterloo International Terminal. Andy Pearson found out how to protect 2.5 acres of failing glass
It is 10.30pm on 4 June 2001. At London's Waterloo International Terminal, a team of abseilers have assembled close to the western arc of the station's giant arched steel and glass roof; their brightly coloured climbing harnesses stand out in the glare of the platform's artificial lights. From their vantage point, the abseilers watch as the last train of the day creeps into its berth and discharges its cargo of passengers on to the deserted concourse.

As the last of the passengers disappear through the platform exit, the abseilers spring into action. They climb high into the roof, using its supporting trusses as ladders. Their mission: to remove the glass from an extensive section of the roof's glazed west wall. The team has just six hours to complete its task before the station reopens and the platforms are packed again.

The abseilers are part of a team from glass protection contractor Filmtek, the company employed by Eurostar to provide a solution to the roof's well-publicised problem of roofing panels breaking, threatening to shower the platform with fragments of shattered glass. Last summer, the Buckinghamshire-based firm began removing the toughened glass panels from the glazed western section of the station's roof. They then attached a film and retention system to each pane of glass and replaced the unit in the roof.

This was far from an easy task. The station had to remain open, so the work went on at night.

"If we delayed opening the station we would have to pay Eurostar something ridiculous like £10,000 per minute, so there was no way we were going to finish late," laughs Alan James, operations director at Filmtek. The glazed area was over 2.5 acres – Waterloo International Terminal is one of the world's longest railway stations.

Designed by architect Nicholas Grimshaw & Partners, the station's snaking, asymmetrical roof is over 400 m long; it tapers from a width of 48 m in the north to 32 m in the south. The roof is divided laterally in two: a gently pitched, metal-clad eastern half punctuated with raised strips of glazing, and a more steeply pitched western side – known as "the west wall" – that is clad entirely in overlapping scales of glass, sweeping from track level over the platforms to join the eastern half near the highest point.

Before Filmtek began work last summer, the terminal's roof looked a sorry state, with netting draped beneath its arched length. The nets had been fitted three years earlier after some of the roof's toughened glass panels had shattered without warning. "The glass dropped straight out of the frame in a number of large pieces, shattering only when it hit the ground," says James. A subsequent investigation traced the problem back to minute impurities in the glass called nickel sulphide inclusions, which had caused the glass to shatter (see "Crystal clear", overleaf).

The problem led Eurostar to sue the original construction team and the claim was settled out of court in spring 2001. However, having successfully settled the claim, Eurostar had still to resolve the problem of how to make the glazing safe. To replace every pane of glass in the roof would have been expensive. So the company turned to Arup Facade Engineering, the consultant that had provided Eurostar with technical advice for its recent court case.

Arup had worked with Filmtek on another project in which nickel sulphide had been a problem. However, the glass panels at Waterloo were much larger, and consequently heavier, than those the Filmtek glass protection system was designed to support. So Filmtek developed a heavy-duty version of the system specifically for the project. It then set about convincing Eurostar of the system's capability.

In spring 2001, Filmtek carefully removed a 3.1 × 1.6 m glass panel and its extruded aluminium frame from the station's west wall. The panel was taken to a storeroom deep in the station's bowels where the Filmtek team set to work. First, they removed the U-shaped frame that surrounds the glass panel on three sides by loosening the tie-bar joining the two legs of the U. The team then cleaned the glass, applied a sheet of the newly developed FT800 multi-ply transparent polyester film to its inner face, fitted a new gasket in the frame and replaced the glass.

Next, the film was anchored to the U-frame. Filmtek has designed the film sheets to be larger than the panes, to create excess on all sides of the glass. The anchor system simply clamps this excess to the glazing frame by trapping it beneath a high-grade stainless steel angle section, which is screwed to the glazing frame (see figure 1) and any surplus film is trimmed. If the glass shatters, the film supports its weight while the anchor system holds the film in place. "It is the least visually intrusive solution we could have used," says Anthony Smith of Arup Facades. Eurostar's requirement was that the system should be robust enough to support the glass for 24 hours to give its maintenance team time to remove the damaged panel.

Two weeks after the sample had been prepared, representatives from Eurostar, Arup and Pilkington (the glazing manufacturer) descended on the storeroom to witness the tests. It was a tense time for Filmtek: if the test failed, so would its chance of a lucrative contract. The effect of a nickel sulphide inclusion shattering the glass was simulated by striking a centre punch with a hammer on the 10 mm thick edge of the pane. Fortunately for Filmtek, the shattered panel remained in place in the frame. One witness was so impressed that he spontaneously walked across the broken panel, which supported his added weight without tearing – much to Filmtek's relief.

With the system proven, Filmtek was awarded the contract and set out to apply the system to all 1120 glazing panels forming the roofs transparent west wall – which is where the team of abseilers came into action. Only able to work at night when the concourse was clear of passengers, Filmtek divided the programme into two phases: first to be tackled was the vertical glazing, then the horizontal.

Using a team of 17 abseilers and operatives on cherry-pickers, the team set about removing the glazed sections. On average 32 panes of glass, varying from 0.9 × 1.0 m up to 1.6 × 4.0 m complete with its frame, were removed every night. It was no easy task given that the panels weighed up to 220 kg. The glass was taken to a workshop set up in the station's basement, three-quarters of a mile away. Here, during the day, the film and anchoring systems were applied. The following night the glass would be returned and the next group of 32 panes removed.

The teams worked throughout the summer and autumn. "There were some evenings when we could not start work because the last train was late arriving," explains James. The work was further delayed by the events of 11 September. "We had to be escorted around the station by security guards," says James. Despite late trains and security problems, the project was completed ahead of schedule.

With the roof now free of its adornment of nets, the station looks just as stunning as it did when it was first unveiled back in 1993. And with the new system in place, the threat of glass fragments showering the concourse has gone too. And the Filmtek team? The company is working with the government, developing a new system to hold shattered windows in place after bomb blasts.

Crystal clear: What are nickel sulphide inclusions?

Minute crystals of nickel sulphide can become trapped inside a pane of glass during the manufacturing process. These crystals can cause the glass to shatter without warning – often with disastrous consequences. The problem only occurs in toughened glass. This was used at Waterloo because it is up to five times stronger than ordinary glass. It’s also safer as it shatters into thousands of tiny fragments when it breaks – which are unlikely to cause serious injury – unlike ordinary annealed glass, which can fall as razor-sharp shards. Toughened glass is manufactured by heating a pane of annealed glass to around 650°C. Jets of cooled air are then blown over the glass. Glass is a poor conductor of heat, so whereas this rapidly cools the surface of the glass, causing it to contract, the centre stays hot. The pane is then left to cool. As the interior cools, it also contracts, pulling the outer surfaces with it, and this puts the outer surfaces under permanent compression while the inside remains under tension. These conflicting forces strengthen the glass. When nickel and sulphur are present as impurities in the raw materials used to make glass, problems can occur. When these raw materials are heated, the impurities can react to form tiny nickel sulphide crystals. These crystals are stable when the glass is at room temperature, but when they are heated the second time to toughen the glass, they change state to form an unstable crystal. Then, when the glass is cooled rapidly, these crystals become trapped in the glass in their unstable form. The crystals’ recovery to its low temperature state may then take several years. As they revert to their original state, they grow by up to 4%. If the crystals are trapped in the core of the pane – which is under tension – the increase in size can create stresses in the glass, which build up over time and cause the pane to shatter suddenly. The Centre for Window and Cladding Technology says one manufacturer estimates one critical inclusion occurs for every 13 tonnes of glass they produce. A process called heat soaking can reduce the risk of nickel sulphide inclusions occurring in toughened glass. Panes of glass are heated in an oven at 280°C for up to eight hours. This process accelerates the phase change in the nickel sulphide crystals, causing any panes containing the crystals to shatter in the oven. The remaining panes of glass are less likely to break in service.

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