Glass Types Key Points
- Heat strengthened glass has lower surface compression than temper toughened.
- Heat soaking is an additional test measure, not a different manufacture process.
What types of glass balustrades are there?
Regulations and standards for glass used in buildings can be considered to fall within three categories. Each of the glass types of barrier requires differing design loads and therefore, differing glass specifications:
- Free standing glass protective barriers
- Barriers with glass infill panels
- Full height glass barriers (not usually applicable to balcony design)
The forces the glass must be designed for are: Line load (Horizontal UDL) kN/m2, uniformly distributed load (UDL) kN/ m2, concentrated load kN and Impact Force. With a framed balustrade containing glass infill panels the balustrade frame, rather than the glass, needs to meet the line load.
Toughened, tempered, heat strengthened or heat soaked?
Tempered glass, or toughened as it is typically called, tends to be the most popular choices for balustrades whether framed or frameless. Tempered glass and heat strengthened glass are relatively similar in the way they are processed. The main differences are typically:
- Impact resistance Tempered glass has an accelerated cooling process which creates a higher surface compression. This makes the surface around four or five times as strong as untreated glass. The heat strengthening process is slower, meaning compression strength is lower and making it twice as strong as untreated glass.
- Manufacture Because of the slower cooling process of heat strengthened glass, it requires more control and is therefore typically harder for glass processors to get right than tempered (toughened) glass.
- Fragmentation Typically the higher the surface compression, the more pieces the glass will fragment into (if it is laminated this can significantly change the fragmentation pattern). Therefore temper toughened glass will fragment into smaller particles whereas heat strengthened glass will fragment more like annealed/untreated glass.
- Vulnerability to NiS breakage Nickel Sulphide can occur in both tempered toughened glass or heat strengthened glass. However, because the surface compression of heat strengthened glass is much lower than temper toughened glass it makes NiS much less likely to cause a spontaneous fracture in heat strengthened glass.
Heat soaking is not an alternative process, but rather an additional test method to reduce the likelihood of Nickel Sulphide inclusion. The process is a heat test to destructively uncover NiS inclusions present. Some glass manufacturers suggest that this testing may risk slightly de-tempering the glass. Selecting the correct product from the glass types available can seem worryingly difficult but is actually quite straight forward.
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