The falling glass at Stratford Jubilee Line station is just the latest in a line of glazing failures that have rattled the public. What is causing these problems, and how can the industry convince clients and users that glass is safe?
Just before midday on Sunday 6 february, the head of the Health and Safety Executive’s HM Railways Inspectorate arrived at Stratford Jubilee Line station to observe a testing exercise. Twenty minutes later, a sheet of overhead glass broke and crashed down on to the concourse.

The station was closed to the public at the time. But the alarmed inspector, Stan Hart, ordered London Underground to conduct an urgent investigation not just into the incident, but into the specification of all the glazing used in stations on the £3.8bn Jubilee Line Extension. It transpired that panes had cracked at Canary Wharf, and there was speculation that large volumes of glass throughout the new line would have to be replaced. The HSE gave London Underground until last Friday to present proposals to guarantee the safety of all overhead glazing on the JLE.

The scares at the stations, revealed in Building on 11 February, were the latest in a series of glazing failures that have shocked the public. The list includes many of the highest-profile projects in the world. The £120m Eurostar Terminal at Waterloo, the £12bn Chek Lap Kok Airport in Hong Kong, the £40m law courts in Bordeaux, a new circular building at Stockley Park near Heathrow Airport, Eva Jiricna’s Prague Orangery and, ironically, Sunderland’s National Glass Centre have all had problems with glazing.

The result has been a crisis of confidence in overhead glazing among building clients and users. As Graham Dodd, designer at Arup Facades, says: “Breaking glass is a very emotive subject. The public perception is that it’s a very dangerous thing.” The scares, seized on by the national media, have also damaged the reputations of architects, contractors and manufacturers. The general feeling was: if top architects can’t get it right on major public buildings, God knew what accidents were waiting to happen in my office atrium or shopping mall.

Architects and engineers, however, are sanguine. The modernist vogue for glass, which lapsed in the late 1970s and early 1980s, is back in force, and in the last decade, it has been used structurally. So, of the millions of square kilometres of glass specified every year, the law of averages says there are bound to be a few cases of failure. The consultants argue that usually glass damage can be traced to faulty manufacturing or installation, or to vandalism.

Architects know that large, accessible expanses of glass, as at the National Glass Centre and Canary Wharf, will attract vandals. Yet it is a calculated risk they have been prepared to take. As for the manufacturing and installation faults, are they somehow inevitable, or could the supply chain sort them out once it becomes more familiar with the use of glass on this scale? Here is the evidence from some of the projects that have run into the problem.

Case one: Eurostar Terminal

The most talked-about glazing failure, at Waterloo International Terminal, is still highly visible in the protective netting Eurostar has slung under the whole roof. It took the precaution when cracks appeared last year in eight out of 3000 glass panels.

This turned into a dispute between Eurostar and architect Nicholas Grimshaw & Partners, construction manager Bovis and cladding contractor Briggs Amasco Curtainwall (a former Tarmac subsidiary). Last month, Eurostar issued a multimillion-pound High Court writ against them.

This dispute is instructive because it shows the kind of intricate trouble all concerned can get into when glass cracks. Sources close to the Eurostar investigation say specialists examined every piece of the broken panes under an electron microscope and found nickel sulphide inclusions, as well as some impact damage.

So, if the impurities were to blame, what can manufacturers do to prevent them arising? And what can contractors do to avoid buying them? The short answer, right now, is not much. The inclusions are an inevitable result of the manufacturing process, although their severity can be reduced by putting the panels in another kiln and raising the temperature to 300 ºC. According to Stefan Behling of Foster and Partners, this should make those with the most impurities break, but the test is not infallible. However, Behling adds that European manufacturers are now offering to supply glass that is both heat-soaked and has been heat-stored in an oven for 80 hours – “a really interesting development”, he says.

Case two: Bordeaux law courts

At Richard Rogers Partnerships’ Bordeaux law courts, it was the building’s structural glass “fins” that cracked. The failure is the subject of a French Ministry of Justice investigation, which is due to be published in April. The ministry asked Ove Arup & Partners, whose Arup Facades division carried out an independent technical analysis of the failure in 1998, and Richard Rogers to produce an alternative design in which the fins are replaced with steel. The architect complied, hoping that it would be an interim measure, but it now seems that the client has lost confidence in glass – it is installing steel fins regardless of the effect on the transparency and lightness of the facade.

A source at Richard Rogers says: “As a state body in a building full of lawyers, they had to be seen to be taking remedial action, but all the other parties involved question the client’s decision … The irony is that, in the worst storms in France in 500 years, the building suffered no damage. So, the idea that glass equals fragility is just not true.”

The source added that the architect had rigorously tested the installation process to satisfy insurers that it would work. Richard Rogers is confident that the judicial inquiry will corroborate Arup’s finding that the problem was caused by the imperfect machining of aluminium plates used to clamp the glass fins, and by the use of hard aluminium washers, instead of soft ones. This meant that the aluminium plates were not positioned properly, which exerted an uneven pressure on the glass.

The source said it had been a “sobering” experience but insisted that the practice would not hesitate to use glass in the future.

Eva Jiricna had similar trouble on the Prague Orangery, when steelworkers misaligned the pinpoint fixings with the holes at the edge of several glass panels. “If the glass is slightly off position, they wiggle the glass panels in through the holes. When the building settled, the fixings exerted uneven pressure on the glass panels causing them to crack,” said Duncan Webster, associate director on the project.

Case three: Chek Lap Kok

When about one-third of the 13 800 glass panels on Foster and Partners’ £1bn Chek Lap Kok Airport in Hong Kong developed unsightly bubbles and discolouration, schadenfreude drowned out the fanfare that had greeted the spectacular, bird-shaped structure.

The size and scale of the damage was sensational –the 80 000 m2 of high-performance glass had been manufacturer Pilkington’s largest-ever order, and replacing the defective panels was going to cost it – or someone – £13.5m. A spokesman for the Hong Kong Airport Authority said: “It has not posed any safety or performance problems, but it is not what we ordered.”

The story emerging is that the problem was caused by Flacheglas-Pilkington’s failure to control the temperature and speed at which the panels were heat-soaked during toughening. The problem was thought to be exacerbated by the low-emissivity coating on the glass.

As one consultant on the project puts it: “You are trying to toughen a low-emissivity glass that is designed to reject heat. You put it in an oven and it starts to distort more than clear glass would. Its surfaces cool more rapidly than the core, so they end up with small indentations. When you come to laminate them (by sticking two panes to a plastic interlayer), they do not stick perfectly.”

But surely the problem could have been predicted or detected before damage on this scale occurred? This is difficult to say for this airport, but it seems that it will be easier on future projects. One project source says: “The failures were a big learning curve for Pilkington and the whole industry. Specifiers and consultants need to understand the subtleties of the process, from when glass is manufactured on the float line to when it lands on the building.”

A source close to the project adds that a new generation of ovens that can better control the toughening of glass has now come on the market. “The more modern and controlled the oven conditions, the better the end-product,” he adds. Pilkington declined to comment.

What is to be done?

Foster and Partners’ Behling argues that the publicity given to these high-profile failures is misleading. “Less than 1% of court cases are related to glass, compared with 60% to waterproofing problems and 40% to stone or lightweight cladding,” he says.

Be that as it may, the cases have triggered a rush to formulate new codes of practice. A code for glass specification is to be drawn up in consultation with the British Standards Institution by September. The European standards body, CEN, is also producing a code that will include recommendations on heat soaking glass.

Architects expect that new industry standards will require the use of toughened laminated glass in overhead installations, which Arup Facades’ Dodd describes as the “belt and braces” approach to safety in glazing.

The cost of using glass will rise – toughened laminated glass is twice the price of toughened glass. But increased demand for laminated glass should ensure that manufacturers compete to supply it and to refine their production processes.

Above all, say architects, there is no need to panic. As one Richard Rogers Partnership designer says: “The greatest tragedy is that everyone is more cautious about using glass, when we now have better knowledge about the constraints, benefits and conditions of using it.”

The three common architectural glazing types

Toughened glass  This is made by heating the glass in an oven then cooling it very rapidly. This compresses the surface and tenses the core. This built-in stress makes the glass four times stronger than the annealed glass in a domestic window. When it breaks, it is with a loud smash and a frosting effect; it then shatters into small pieces. In a vertical setting, it tends to stay in place after it breaks, but it can fall in lumps the size of a fist. Nickel sulphide inclusions are a source of spontaneous breakage. Toughened laminated glass  This costs about twice as much as toughened glass. Prompted by the row over the JLE glass, the HSE’s HM Railways Inspectorate is set to insist on its use in overhead situations in stations. It is possible to laminate any kind of glass with any other, but the safest kind is toughened laminated. This is two panes of toughened glass stuck to a PVB or strong polyester interlayer. Bulkier than toughened glass, it is unsuitable for use in translucent, lightweight structures and pinpoint fixed structures. When it breaks, it does not fall but sticks to the plastic interlayer and stays in its frame. Rafael Viñoly’s canopy over Yuracucha underground station in Tokyo, Nabil Fanous Architects’ glass cube reading room in Riyadh, Saudi Arabia, and Kohn Pedersen Fox’s Samsung headquarters in Seoul are among the few buildings where it is used structurally. Laminated heat-strengthened glass This has twice the strength of annealed glass, but is lighter than toughened glass. It is heat-treated, but cooled slowly. It is not a safety glass – it breaks as easily as annealed glass, but does not fall away. It can withstand higher temperature differentials than annealed glass.