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Glass Selection

Rules of thumb

The selection of the most appropriate type of glass for use in certain situations is not straightforward, despite the huge amount of published guidance available. These rules-of-thumb may help to point you in the right direction:

Single glazing

• If the location requires a safety glass, and the floor level is the same on both sides of the glass, i.e. a glazed screen between a lobby and a corridor, then you may use either Toughened Glass or Laminated Glass. Toughened glass is usually cheaper but cannot be worked after toughening. Read the Safety Glass page.
• If the location requires a safety glass, and the floor level is different on each side of the glass, i.e. a glass balustrade or a full-height window above ground level, then you should generally use Laminated Glass, held in such a way that the broken pane cannot become detached from its frame or fixing. Read the Glazed Balustrades page.

Insulating glass units - inner pane

• If you are using an insulating glass unit as floor-to-ceiling glazing in a curtain walling system, the inner pane should generally be Laminated Glass, held in such a way that the broken pane cannot become detached from its frame or fixing.
• If you are using an insulating glass unit in an overhead situation, the inner (lower) pane should generally be Laminated Glass, held in such a way that the broken pane cannot become detached from its frame or fixing.

Insulating glass units - outer pane

• If you are using an insulating glass unit, with high-performance glass, it will sometimes be necessary for the outer pane to be one of the strengthened glass types in order to resist the solar-induced thermal stresses in the glass. Heat Strengthened Glass has the advantage that it does not suffer from the problems associated with Nickel Sulfide inclusions but, if broken, the remnants of the pane may fall as large shards with the possibility of causing serious injury to those below. Toughened Glass has the advantage that the remnants of broken panes will generally fall as small cubes with less possibility of serious injury to those below.
• It is generally acknowledged that the breakage of Toughened Glass cannot be accurately predicted and remnants of broken panes may fall as quite large pieces, comprising a number of small cubes held together by friction, and these pieces have the potential to cause serious injury to those below.
• There now (Autumn 2006) seems to be yet another complication with some reputable specialist curtain walling contractors pushing for the use of Annealed Glass in preference to either Toughened Glass or Heat Strengthened Glass in this situation. This is despite the annealed glass generally having to be thicker (and hence heavier) than either of the other two types to achieve the same strength.
     The reasoning behind this change of approach is difficult to pin-down but is probably due to the perception that there are still continuing problems with the "spontaneous breakage" of toughened glass. Debate still rages as to the cause of these spontaneous breakages (one consultant estimates that only 10-15% are due to Nickel Sulfide inclusions whereas 85-90% are due to latent edge damage) but, whatever the cause, it is generally accepted that the problem is more prevalent with toughened glass than with annealed glass.
• The use of annealed glass, which is not a "safety" glass, is the subject of considerable debate as there is, at present, no legislation preventing its use in this situation. An argument may be put forward that annealed glass at the thicknesses required (8mm, 10mm or even thicker) will stay in place by friction and its own self-weight if broken, certainly that it will stay in place long enough for someone to make a temporary repair to ensure that the broken pane does not fall out of the frame.
• There now seems to be even less clear industry guidance on the appropriate glass type for this situation and a risk assessment will be even more crucial. In particular it is essential to check that the proposed glass, of whatever type, is strong enough to withstand the impact loads from cleaning cradles and the like - these being the most likely cause of glass breakage.

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