Glass in balustrades and balconies vs. plastics

Toughened Glass

Toughened glass should be used in low level glazing, glazing in and around doors, overhead and in large areas of glazing to prevent injury following accidental glass breakage. It is up to five times as strong as ordinary glass which means that it has to be hit much harder in order to break; also when it does break it is into lots of small pieces which are much less dangerous.

When used for a balcony or balustrade, calculations are required to determine the correct specification for the application concerned. These calculations are based on British Standard BS6399. Consideration of the area to be covered will then determine a suitable thickness and glass type. Your local glazier or balustrade suppliers would be able to advise on this very important aspect.

A sleek, glamorous alternative

Balustrades made of glass are a sleek, glamorous alternative to balustrades of more traditional materials, such as wood or stone. Glass balustrades can provide a light, airy feel to balconies, staircases, and even outdoor walkways. Glass balustrade construction must meet relevant code requirements, whether national or regional.

U.K. codes require that glass balustrading be constructed of "toughened" glass sometimes known as "tempered" glass.

Layers within the glass

The technique used to create toughened glass involves heating glass objects to beyond the annealing point of 600 degrees centigrade (1112 degrees Fahrenheit). How far beyond determines the varying grades of "toughening" -- toughened glass can range from twice as strong as annealed glass to as much as six times the tensile strength. Once the glass is heated, the outside is rapidly and artificially cooled, usually by jets of cool air. This method solidifies only the outside, leaving the interior molten and fluid. That's what creates the various layers within the glass.

Resistant to impact

Because of its layering, the surface of toughened glass is more resistant to impact. The same thrown object that would create a single hole in a pane of annealed glass would likely bounce off a comparative pane of toughened glass. The downside is that because toughened glass has more unity in its tensile strength, it would be more likely to explode completely if enough force was applied. This is sometimes a security concern, since once a toughened glass window is breached; there are no glass shards to discourage entry. Toughened glass is also more costly to produce, because of the extra step needed to cool the exterior. Curiously, it also has a softer surface that is more prone to scratches.

 

Physical Properties of toughened glass

Toughened glass is physically and thermally stronger than regular glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. For glass to be considered toughened, this compressive stress on the surface of the glass should be a minimum of 69 MPa. It is this compressive stress that gives the toughened glass increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin. However, the toughened glass surface is not as hard as annealed glass and is therefore somewhat more susceptible to scratching. To prevent this, toughened glass manufacturers may apply various coatings and/or laminates to the surface of the glass.          

Microscopic cracks

The practical tensile strength of glass is about 62 Mpa. However, glass can withstand extremely high compressive stresses. Therefore, most glass breakage is due to tensile strength failure. The reason that glass is weak in tensile strength is that it is normally covered in microscopic cracks which generate local stress concentrations.

Improving the strength of the glass

Glass produced with no surface flaws has strength values approaching the theoretical tensile strength of 6, 5 GPa and has been produced using very fine fibres of glass.  It does not possess mechanisms for reducing the resulting high localised stresses and so it is subject to rapid brittle fracture. There are two methods of reducing /eliminating this problem:

 

• Treating the glass thermally or chemically such that the outer surfaces are compressively stressed at relatively high levels, the middle region between the surfaces being under tensile stress.  The cracks are therefore "held closed by the continuous residual stress.          
  
                        
• Ensuring that the surfaces have no cracks and that the glass in use is not in mechanical contact with anything which could scratch the surface. Glass produced with no surface flaws has strength values approaching the theoretical tensile strength values of 6, 5GPa.

This is tempered/ toughened glass and the strength of the glass can be improved by a factor of up to 10 using these two methods.

GPa =Tensile Modulus (Mpa = megapascals) =Ultimate Tensile Strength

Polycarbonate

Polycarbonate sheeting is a type of building material used to replace glass in a variety of applications. Made from thermoplastic composites, polycarbonate sheeting typically offers greater flexibility and shatter-resistance than acrylics or plexiglass. It comes in sheets that can vary in thickness from less than an inch to several inches thick.

This material may be used as an alternative to glass in doors and windows. It is also popular for shower and pool enclosures, as well as on balconies and in greenhouses. Polycarbonate sheets may be found in sporting areas, where it is used to protect spectators from flying balls or hockey pucks and also used to make compact discs and eyeglass lenses.

Uses

Polycarbonate sheeting provides a great deal of options to installers and designers in terms of how it can be used. It can be held in place using screws, and may be used with speciality support poles designed to hold larger sheets. It can also be joined to other materials using epoxy or silicone, and can even be welded. Silicone sealant is often used to seal or caulk polycarbonate sheeting along its seams or joints.

Primary advantages

One of the primary advantages of this type of sheeting is its high strength to weight ratio. Compared to glass, polycarbonate sheeting is very light, but also highly resistant to shattering and damage. This shatter resistance makes this material much safer than glass. Polycarbonate sheeting also acts as a sound barrier, and may help control UV-light infiltration. It also offers a high degree of flexibility, making it easy to form each sheet into the desired shape.

Visibility and light transmittance

Compared to some other composite plastic sheeting products, polycarbonate sheeting tends to be fairly expensive. It may also be susceptible to damage or failure if exposed to high temperatures. Like other plastic sheet goods, polycarbonate sheeting may suffer surface scratches due to a variety of factors. These scratches can be removed using special polishes or cleaning products, though some may be permanent. Over time, excess scratching can cause the surface to appear dull, and may reduce visibility and light transmittance.

For use in applications exposed to weathering or UV-radiation, a special surface treatment is needed. This either can be a coating (e.g. for improved abrasion resistance), or a coextrusion for enhanced weathering resistance. Co-extrusion is the extrusion of multiple layers of material simultaneously. But continuous exposure to sun and weather environment makes it less suitable for architectural usage in buildings, balconies and balustrading due to the fact of clouding, susceptibility to scratching and general weathering in time. These effects cannot be rectified and the only solution is replacement.

Cost

An additional factor when considering using polycarbonate as a balustrade infill panel is the cost. On average 10mm polycarbonate will cost three times as much as 10mm toughened glass, this factor is probably the most influential on choice of material. However when comparing this to 10mm curved toughened glass the costs can even out as polycarbonate can be manually “bent” on site to most radii and will not need to undergo a specific bending process.

Compared to toughened glass, the Tensile Strength is approx. 64 Mpa which is similar. 

Whereas the Tensile Modulus is 2.3 GPa compare to toughened glass at 6.4 GPa.

 

Summary

Polycarbonates are extensively used in bottling, medical, eyeglass applications and in the environment it poses huge problems when bottles are discarded randomly and the material not being bio-degradable thus creating problems of litter and general untidiness in towns and cities. Although it is used as a substitute for glass there are many aspects that make it unpopular and for balconies and balustrading is not a substitute for toughened glass.

Toughened glass is an ideal material for use in decorative and practical architecture on interior and exterior applications. It is easy to clean, hard wearing, long lasting and fulfils the need for giving a house or building a light bright airy feeling which is in demand today.

GPa =Tensile Modulus                                       

(Mpa = megapascals) = Ultimate Tensile Strength