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TOUGH TALK AND
PROFESSIONAL JUDGMENT
Independent glazing consultant Philip Rougier considers whether toughened glass really is the perfect solution to every problem.

Glass and Glazing Products, October 1995 - "Tech Talk"
Copyright © Philip Rougier 1995

One minute it's there. A clear, rather unremarkable sheet of glass with a discreet BS 6206 'T' marking tucked in one corner. Then presto! - with a loud bang it's all gone - leaving a gaping hole, and thousands of glass fragments all over the floor. Toughened glass is a magical product, starting as annealed glass it gains up to five times its original strength by judicious heating and cooling. And usually it is slightly cheaper than laminated. Furthermore it breaks into small, relatively safe, pieces minimising risk of injury. So, it's the perfect answer? No it is not. It's a fact that toughened glass fragments can cause injury to people and damage to property. Usually the direct injuries are limited to lacerations, shock and bruising although collateral injuries can be anything at all. Damage to property usually is not too severe, but knock-on effects can be expensive. And all such situations can be dangerous to your wallet.

HOW CAN TOUGHENED GLASS BREAK ?

There are many ways. 'Spontaneous' occurrences, where the glass just appears to break on its own, can arise through the swelling of nickel-sulphide inclusions that have not had their rightful chance to expand in the cooling stage of the heat-treatment process (raw material and procedural controls, including a 'heat-soaking' treatment stage, have reduced this problem in recent years). Or through the glass being restrained in its frame without adequate tolerance, or 'pinched' by a mounting or fixing, then subjected to temperature fluctuations and resultant thermal stress. Or the pane can suffer minor damage during handling or installation, creating localised distress that marks the beginning of a failure pattern. All these possibilities can be minimised by careful manufacture, handling and installation. It would be a magnificent achievement if such failures could be avoided by sophisticated production techniques and skilled installation. The fact is that these failures and their effects can be minimised, but not eradicated. Toughened glass can also break under dynamic loads - bricks, .22 air rifle pellets, hammers, hail-stones like saucers, albatrosses dying at 9000 feet, Boeing 747 wheel-nuts. Some give more concern, and make more mess, than others - but they all break toughened glass. As Dale Carnegie said: "The only way to get the better of an argument is to avoid it." In just the same way, you can get the better of a toughened glass argument by avoiding its use in certain locations where broken fragments might cause injury or damage. There are few 'official' guidelines but the problem is real enough, and the consequences can be reasonably forecast - so it is a matter for professional judgment. As in so many areas of building specification, the duty knows no boundaries. It extends to all the specifiers - architect, surveyor, glazier planning supervisor - no matter what assumptions each might make about another's competence and awareness.

SOME EXAMPLES

Single overhead glazing is a continuing problem in shopping malls and markets, covered footways between classrooms, railway stations and so on. Shoppers, school-children and travellers showered with broken fragments of toughened glass suffer only lacerations, shock and bruising. And the rain pours in just slightly ahead of the claims. Swimming pools, which need safety glazing as a matter of course, are expected to receive their fair share of glass breakages and bare bodies. Glass fragments, particularly the smallest ones, become embedded in feet - and anywhere else, if a child falls over on a wet surface after a breakage. If pool maintenance staff have the time and skills, toughened glass fragments can be removed from the bottom by a diver with a squeegee - but it could be easier to empty the pool. And then they can move on to dismantling the water pumps and filtration equipment. Areas where food is stored or prepared are a 'no-no' for small fragments of broken glass. A classic case where the restaurant owner receives a cut lip from a customer who's suffered a cut intestine. Or where thousands of jars of baby food are removed from supermarket shelves. Glazed mezzanine floor and staircase balustrades in office areas can lead to a problem or two.

Any 'minor' personal injury aside, (the degree of an injury's severity is an inverse square of your distance from it) glass fragments trapped in keyboards, computer ventilation slots, photocopiers, laser printers and faxes can cause a workplace to shut down and a specifier to shut shop. When Toughened glass is to be used in locations where its broken fragments could cause injury or damage, measures should be taken to offer containment. For roof glazing, a combination of toughened and laminated glasses in a sealed unit, or separately, can be very effective - the lower laminated pane lends containment, the upper pane gives superior strength against wind/snow load. A separate polycarbonate sheet suspended below the toughened pane can also provide a containment solution. In theory, toughened glass can be filmed - but if the film is not carried over supports it is possible for glass fragments and film to fall out together, and wrap themselves around anything in their path.

There are many glazing products, each with benefits and disadvantages. None is perfect for every application. Toughened glass receives no condemnation here - on the contrary, like alcohol, it is a wondrous product, of great benefit when used responsibly. But toughened glass is no panacea, and informed decision-making is necessary at professional level. Astonishingly, many people (including some highly qualified technical types) simply do not believe there is a problem. "After all", they ask , "I'm getting 'unbreakable' glass aren't I" ? "It won't break, will it" ?

Glass and Glazing Products, October 1995 - "Tech Talk"
Copyright © Philip Rougier 1995