Glass - Use in Architecture
Hazard Related Topics
Architectural Glass is used in buildings in various forms and at different levels. Where glass is installed overhead pedestrian, vehicular or any other traffic, there is a risk of injury to personnel from falling glass. The risks are not always obvious and should be the subject of a risk assessment. There are recent cases of injury to persons from falling or collapsing glass and several near misses.
Glass is usually manufactured in the first instance by taking molten silica and floating it on a 'sea' of liquid tin. All float glass contains impurities including Nickel Sulphide. All float glass will shatter under pressure and 'splinter' into fragments that can be lethal when falling from height.
Float glass can be 'toughened' by putting it through a heating process that very nearly melts it and then, using a jet of cold air, cools it to ambient temperature. Toughened glass requires a greater pressure to break it. The pressure required is in the order of 4 times the pressure required to break the equivalent thickness float glass. When it shatters, toughened glass fragments into small crystal like pieces. These pieces are in themselves not as nearly as dangerous as shattered float glass. However, the Nickel Sulphide impurities in the glass can form large crystalline areas, which are significantly dangerous when falling from height. In addition, these large crystals can 'join together' to form even bigger fragments approaching a quarter square metre in size.
Glass can be laminated by sandwiching a layer of transparent material between 2 or more glass sheets. The glass used can be float or toughened. It should not be assumed that toughened laminates are 'safer' than float glass laminates.
When toughened laminates shatter, they loose their intrinsic lateral 'strength'. If a glass panel is 'suspended' as a 'roofing' material, the shattered panel will naturally sag and drop out of its frame and fall as a complete, but sagging, panel. The amount of sagging will depend in part on the relative weight of the glass laminates and the strength of the laminating material. A metal matrix can be installed immediately underneath suspended glass panels (as part of the glass frame) to 'catch' the sagging material. However, this will reduce the amount of light transmitted through the panel and will do little to enhance the appearance of the glass panel that it supports.
When laminated float glass shatters, it does not normally loose its lateral strength. The laminate material can be strong enough to maintain its shape and therefore contain the glass in its framework even if suspended. Laminated float glass can, therefore, be a better solution than laminated toughened glass.
As described earlier, Nickel Sulphide is present in glass as an impurity. When glass is 'toughened', the process can force some nickel sulphide crystals to literally shrink in size due to the sudden cooling process. The sudden cooling also locks these shrunken crystals into the toughened glass sheet. Unfortunately, the shrunken crystals have a latent life of their own and will take the opportunity to expand back to their original size. The time taken for this process is variable, instances of a year have been recorded and the process may never take place. If the crystals do in fact expand, they will cause the glass to shatter and for no apparent reason and with no external intervention. This catastrophic change is only associated with toughened glass due to the toughening process.
Some specialists recommend the reprocessing of laminated glass for extended periods of time to eliminate the nickel sulphide problem. The process is, apparently, unproven and is not widely accepted as a solution.