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DETAILED GLASS INFORMATION IS AN INVALUABLE RESOURCE

TECHNICAL & PRACTICE - SHORTCUTS

This is the first NBS Shortcut in a series which will appear 10 times a year in the AJ. This piece focuses on the main classifications for glass and glazing and on BS 953-1:1995 'Glass for glazing', which lists more than 20 main varieties of glass, with many more subsets representing composites, coated glass or other manufacturing processes that give the glass a range of properties suited to particular situations.

Some British Standards are fascinating, well researched, crucial and informative for an understanding of a subject, product or practice? and then there's BS 6262-1: 2005. Admittedly, 'Glazing for Buildings - General Methodology for the Selection of Glazing' is the first part of a seven-part document, but as it contains nuggets such as 'in the case of internal doors, the wind loading is negligible' and 'security glazing is used in situations where a high degree of protection? is required', surely few would weep if it disappeared.

Fortunately, the remaining six parts provide an invaluable resource - detailed information on glass and glazing-related topics, from energy considerations to frame design. This Shortcut presents a snapshot of available glazing types suited to different functions, locations and manufacturing processes.

FLOAT GLASS Henry Bessemer, more famous perhaps for his iron-to-steel converter, patented float glass in 1848, although it was not until the turn of the 20th century that Pilkington made it commercially viable to produce it in significant quantities.

Manufactured by pouring 1,000infinityC glass on to a bath of molten tin so that it floats, it spreads out to an even thickness of around 7mm. Thinner sheets (down to 0.4mm) are produced by removing it slowly, thus allowing it to settle; thicker sheets (up to 25mm) by removing it from the bath more quickly, containing the spread. Other methods of glass production still exist, but this is generally accepted to be the more cost-effective. Plate glass, for example, a common method of sheet-glass manufacture prior to the much cheaper float glass process, involved pouring molten glass on to two metal plates where it would cool, be lifted off and ground and polished in an attempt to make the two surfaces even.

The industrialised efficiency heralded by the early manufacture of float glass, which became fine-tuned after the Second World War, has now resulted in around 260 float-glass plants worldwide producing around 800,000 tonnes of glass per week between them.

The other main production process is passing molten glass through rollers of various apertures to control thickness, producing 'sheet glass'. This tends to have uneven surfaces and has effectively been superseded by float glass. But whether float or rolled, glass that has been subjected to controlled cooling (which helps reduce residual stresses in the material) is known as 'annealed glass' or 'untreated glass'. BS 953-1:1995 describes this as 'ordinary glass' and it forms the base for most wired, coloured or patterned glass.

Glass is remarkable for its ability to not weaken over time.

Windows subject to flexing under wind loads, for example, do not weaken and fail due to the fatigue of repetitive stresses.

The molecular structure of glass is such that it doesn't degrade, but under excessive strain it will fail suddenly and dramatically.

Ordinary annealed glass is not very strong and a range of treatments moderates the tendency for oat glass to break into dangerous fragments.

Some of the key techniques to improve the strength of glass include toughening, heat strengthening and laminating.

It was early in the mid-17th century that heat-treated glass was discovered to have significantly improved strength qualities, but it took another two centuries to be able to explain it scientifically.

Essentially, rapid cooling causes the exterior of the glass to solidify very quickly while the core takes longer, contracting to a greater extent than the surface condition. This sets up tensile stresses in the middle and compressive strength at the surface and it is the latter that has to be overcome before the 'weakness' of the internal condition can be exposed. Two main types of heat-treated glass are commercially available: toughened and heat-strengthened.

TOUGHENED GLASS Toughened glass (also known as tempered glass) has a surface compressive strength of up to 100N/mm 2, suitable for safety glass specifications, and can be as high as 150N/mm 2, enabling it to withstand mechanical forces and thermal shock respectively four and six times that of annealed glass. Unfortunately, toughened glass is no more elastic than annealed glass and its deection characteristics are limited. Its BS 6206 'safety glass' label is premised on the fact that on breaking, the release of pent-up internal energy shatters the glass into small, relatively harmless blunt particles. Such are the internal forces that pane edges need polishing to remove irregularities that would otherwise reduce the stress resistance. This glass should not be considered as security glazing and it cannot be cut or drilled after tempering, so manifestation or other decorative treatments should be applied rather than etched, although the latter is possible with care.

HEAT-STRENGTHENED GLASS Heat-strengthened glass is simply a halfway house between annealed and toughened glass - meaning it is cooled more slowly than toughened glass, producing panels with a surface compressive strength of 25-60N/mm 2. It has a resistance to mechanical forces and thermal shock respectively more than 1.5 times and two times that of ordinary glass, although when it breaks it fractures into shards like annealed glass.

WIRED GLASS Wired glass, commonly known as Georgian wired glass, is annealed glass that has 12.5mm welded wire mesh embedded in it during the rolling process (which is used to create the desired patterning) and outside the UK it is made by pressing the wire into the molten glass. Cast wired (patterned) glass is 7mm thick, and the polished wired glass variety is 6mm, the latter losing 0.5mm each side in the grinding and polishing process.

In general, this cannot be used as safety glass, as the shards combine with the sharp mesh to present a serious injury hazard.

However, with a thicker-gauge mesh it can comply. Similarly, Georgian wired glass can provide up to 60 minutes (integrity only) and satisfy the BS 6206 Class C safety classification, which means it is the lowest grade of safety glazing. For 'critical' glazing in doors and side panels (as defined in Approved Document Part N) the width of each pane of Class C glazing cannot exceed 900mm.

LAMINATED GLASS Laminated glass is made by pressure-bonding sheets of glass to an interlayer, most commonly polyvinyl butyral (PVB), such that the composite panel has the combined attributes of each individual panel, although laminates made from two toughened glass panels will afford no structural integrity if both panels are broken.

However, it can be worked after manufacture as the strength of the bond between the layers is unaffected by cutting and chasing.

PVB comes in thickness multiples of 0.38mm. Three-ply panels with a 0.38 or 0.76mm interlayer are classified as safety glass. With a 1.53mm interlayer (or five-ply with two layers of 1.14mm) it is classed as manual-attack-resistant glass to BS 5544.

Bullet and blast-resistant glass are specified in BS 5051, the former weighing as much as 120kg/m 2 for a 50mm thick pane. A variety of laminates is now readily available providing intumescent, coloured or solar-control interlayers.

FIRE-RESISTANT GLASS In the same way that sound resistance relates to a composite (ie. a glazed screen) rather than an element (ie. an individual glass pane), so with fire resistance: failure in one part of the element compromises the whole. The fire-resistant classification of a glazed screen, say, is a mixture of integrity and insulation - the former ensures that the fire does not penetrate the element, the latter reduces the transmission of radiant and conductive heat.

Some fire-resistant glass is classified in accordance with BS EN 13501-2 in terms of integrity (E), insulation (I) and, where applicable, radiation (W) and will be contained on fire test certificates that are applicable only to that particular assembly or pane that has been tested. It is important to note that fire protection is related to the extant conditions, materials and detailing and claims that a certain glass is fire resistant should be treated with caution until read in conjunction with the whole assembly.

Austin Williams is the author of NBS Shortcuts. For more information visit www. buildingregs. com You can see shortcuts online at www. ajplus. co. uk

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