Timely warning on glass assumptions

It is far too tempting to back the easy option that glass just might survive the initial thermal shock and hold together, if perhaps luck could only play a role.

In practice, standard soda-lime-silica, ie “window” glass, has limited resistance to thermal exposure: it is a classic isotropic brittle material which produces complex cracking patterns caused by restrained expansion when subject to even relatively mild thermal stress.

The critical factor is the temperature differential between hot and cold glass. Our statistical evaluation shows a critical temperature differential of only 80ºC produces a 99.99% failure probability. Such a condition is likely for even relatively undeveloped fires in modern building environments. Cracking occurs within the first minute of exposure followed by integrity failure (ie gaps) by 5 minutes.

Double glazing units do not fare any better. It is a misconception that the air gap might moderate thermal shock in a fire. The inner pane in a glazed unit exposed to fire cracks and fails just the same as a monolithic piece, at which point the increased thermal shock on the second pane leads quickly to its failure.

Fire-resistant glass, on the other hand, is an entirely different question.

It is certainly justified to raise fire safety in the context of sustainability and to sound the alert on the wider implications of sustainable design. For example, new highly insulating materials based on plastics and other organic materials may have major implications for the potential fire load of a building. More attention needs to be given to the protection of property, communities and the environment against fire. We should reflect on the reality that a building that burns down is fundamentally questionable regarding its sustainability.

Mike Wood, global consultant, fire protection glass, Pilkington Group