Glass is being used as a prominent building material. It is typically used as transparent glazing material in the building envelope and in windows. Glass is also used for internal partitions and is a architectural feature. When used in buildings, glass is often of a safety type, which includes reinforced, toughened laminated glass.
Ordinary glass is extremely dangerous because it breaks into jagged pieces that causes serious injuries. Falling glass from high rise buildings can cause fatal injuries. it is imperative to use proper glass in our buildings.
Annealed glass is glass without internal stresses caused by heat treatment. (ie toughened or heat strengthening). Glass becomes annealed if it becomes heater above transition point then allowed to cool slowly through not quenched. Thus glass made using the float glass process of manufacture.
As a result glass is simply described or specified as ' float glass'. Annealed glass is the common glass that breaks into large, jagged shards that can cause serious injury. hence annealed glass is considered a hazard in architectural applications.
Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools.
Types of safety glass
There are different categories of safety glass:-
Toughened (T) - Toughened by heat treatment. It disintegrates into small, granular pieces which are not sharp and reduces the risk of surgery. Small pieces will still be dangerous to children and to ensure that they are kept away in the event of breakage.
Laminated (L) tow or more sheets of ordinary glass attached together by a plastic inner layer. On impact any broken glass will remain attached to the plastic layer reducing the risk of injury.
Wired glass - has a network or mesh of wires embedded in it.
Toughened glass does not break very easily, if impacted with force that it is not able to withstand it breaks into small fragments that do not inflict grave injuries. Tempering of glass is a process by which the impact strength of normal annealed glass is increased 4 to 5 times. the process involves heating up the glass near about melting point ie 650 degree Celsius and subsequent quenching. the sudden temperature difference puts the glass into a state of compression with the center core in tension. Tempered glass gets added strength from these compressed surfaces.
The use of this type of glass will help in safety. Architects and builders have already switched to tempered glass in new buildings.
laminated glass is a type of safety glass that holds together when shattered. in the event of breakage it is held in the place by a layer of poly vinayl butyle (PVB) between tow or more layers of the glass. The interlayer keeps the layers of glass bonded even when broken, and its high stenght prevents the glass from breaking into sharp pieces. this produces a characteristic ' spider web' cracking pattern when he impact is not enough to pierce the glass completely.
Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered. Shop front glazing and windshields are typically laminated glasses. The pvb layer also gives the glass a much higher sound insulation rating. It blocks 99% of the UV light. Using toughened glass on windshields would be a problem when a small stone hits the windshield at speed as it would shatter into small squares making visibility very difficult and it would blow the small squares into the passenger and the driver's eyes.
For many years now wired glass has been the common fire rated glass product specified in schools. It is no wonder it provides economical and reliable fire protection. for a long time wired glass was was the only product available. But the drawback is that it cannot hold much impact. When it breaks the wires can create nasty snags giving serious injuries.
Article is a part of Indian express authored by A.R Santhakumar.