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Is safety glass really safe?


PROBLEMS WITH glazing on the Jubilee Line's new stations seem to come down to a lack of understanding within the design community about the materials it is using.

The industry has spent decades testing, designing with and codifying materials like steel and concrete. But it seems that glass is often specified with little thought.

'Glass is a material much sought after by architects but until now essential structural design data has been hard to find,' says Ove Arup director Chris Jofeh, author of the Institution of Structural Engineers' new design guide on the structural use of glass in buildings.

While it is clear that research has been carried out into the behaviour of the various types of structural glass, not enough of this knowledge has filtered down to the designers who specify the material. The IStructE guide attempts to bridge this gap.

Jofeh says that ensuring that overhead glass panels do not fall on people below has always been a hot topic. But he points out that there are already several guidance documents and manufacturers' advice notes on how properly to design and use glazing systems.

'Often there is a tendency to label all problems with glazing as a glass disease,' he warns. 'In reality there is a whole range of things that can go wrong.'

There has been a catalogue of different problems associated with large expanses of glazing. Safety netting was commonplace in UK shopping centres in the early 1990s as so-called 'nickel sulphide inclusions' caused panes to shatter without warning.

Manufacturers responded with new heat soaking techniques which caused defective panes to shatter during the manufacturing process. They also came up with stronger types of glass to reassure specifiers.

Working out safe methods of fixing glass panels to modern buildings also continues to test designers. Waterloo International station's glazed canopy has been fraught with problems and continues to sport huge nets to protect passengers below.

Meanwhile, there is a major legal battle over who was responsible for detailing the fixings which are now thought to be over stressing the glass.

More recently, just outside Heathrow Airport at the new Stockley Park estate, Ove Arup engineers were caught out by detailing which allowed glass panels to rattle in the windand fail.

Dewhurst MacFarlane senior associate Jonathan Sakula says such detailing problems are not exclusive to glass facades but tend to be more apparent when they occur. 'Glass can be very strong but it is also very brittle - and so very unforgiving. The design, machining and installation all have to be very precise with attention to detail and cleanliness at every stage.'

Sakula points out that specialist installers are often left to produce detailed designs for glass facades. 'They will choose the glass, the thickness and often design the details,' he says, adding that these will usually be subject to review by the architect.

But all this leaves us with the problems now being tackled by London Underground. When a pain of glass shattered and fell from the roof of Stratford station, alarm bells started ringing at the Health & Safety Executive. The trouble appears to stem not from design or manufacturing problems but simply from confusion over the precise definition of 'safety glass'.

Standard glass is annealed to remove localised stress build up across the depth of the panel. While strong in compression, small tensile loads will cause cracks in the glass to spread rapidly until the glass breaks.

To counter this effect, laminating and heat toughening techniques have been developed to create so-called safety glass.

Heat toughening introduces compresses stresses in the surface so the glass is able to withstand higher tension forces before cracking. Of course, if this surface tension is broken, the forces rapidly redistribute across the sheet and the glass shatters into small fragments.

Laminated glass, on the other hand, is strengthened simply by bonding two or more sheets together with special resin. The sandwich construction allows cracks to propagate through one sheet while the others hold the entire pane together and introduces some redundancy for the system.

A combination of the two techniques can also be used.

But what should designers specify in their buildings?

'In overhead locations you must use safety glass. But under current legislation in the UK this can mean either toughened or laminated,' says Sakula. 'In the US and much of Europe you have to use laminated - I think we will eventually have to follow.'

As Sakula points out, stopping glass from breaking requires good design. Stopping it from falling on people's heads if it does break is vital. British Standard 6206 covers safety glass and goes into some detail about the quality and type of glass.

The designers on the JLE are not thought to have breached any legislation. But they still have to convince the HSE - and the public - that it is safe for people to stand under the JLE's numerous glazed canopies. Using safety glass is one thing. Designing the appropriate restraints to support it is another: this is an area which is not widely codified.

Eurocodes are on the way but until then more knowledge and clearer guidance are needed if glass is to be used safely. Better understanding of the issues might have saved London Underground from its present dilemma.

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