Structural Glazing

STRUCTURAL GLAZING IN BUILDING–MODERN GLASS
Modern glass is not only a transparent beauty, but also has unmatched utility. Today, glass is an important building element, which not only offers aesthetics but also proves economical in the long term on account of very low maintenance expense.
Amongst the types of glass made in India,
(a) Float glass
(b) Sheet glass
(c) Plate glass
Float glass is a 100% distortion free glass, manufactured indigenously by three companies in India. Float glass is a recent development that produces a fire polished, nearly optically flat surface that does not have to be ground or mechanically polished.
Sheet glass, including the machine drawn cylinder process simulating hand-blown glass, was known for its wavy irregularities.
Plate glass, first introduced into the U.K. in the mid seventeenth century, was cast and then polished flat.





Undergraduate student, Department of Civil Engineering, NSS College of Engineering, Palakkad-08,Kerala
2.0 PROPERTIES OF GLASS

2.1 Transmission Properties
Transmission through glass can be modified by improving or modifying the
Chemical make-up of the glass to increase the absorption factor. Heat absorbing glasses are designed to absorb as much of the solar infrared radiation as possible while maintaining high transmission in the visible region. Some are also designed to absorb
visible radiation to reduce glare, which further reduces heat transmission. However, due to unavoidable impurities in the soda-lime-silica mix, typical window glass does absorb some radiation that might otherwise pass through.

2.2 Material Composition
The main components of clear glass are silica sand (73%), Calcium oxide (9%), Soda (13%) and Magnesium (4%). The amount and type of additional impurities within the glass affect the wavelength of the absorbed light and the amount of impurities determines how much it will be affected. Manufacturers often purposely add their own impurities such as ion oxides to manipulate absorption effects.

2.3 Thermal Conductivity
The thermal conductivity (K) of a soda lime glass between 5 and 7 Btu/hr Sq.ft F/in. is higher than that of insulating materials, but much lower than that of most metals. Glass of normal thickness offers negligible resistance to heat transfer between the inside and outside of a building and the thermal resistance is predominantly due to the surface films.

2.4 Strength of Glass
Glass is a brittle material and does not deform plastically before failure. It fails in tension regardless of the nature of loading. The potential tensile strength of glass is about 690107N/mm2, but failure occurs at average stresses far below this value because of the stress-raising effect of surface imperfections both inherent in the glass and mechanically created. Glass is most vulnerable at its edges, with surface imperfections from cutting and handling adding to the risk of failure. Because the effect of stress raisers is indeterminate, the allowable tensile strength of glass is determined statically and a sizable safety factor included. By using the value thus established breakage can be reduced to an insignificant level but not eliminated. Glass can be greatly strengthened by development of a “stressed skin” sandwich, where both surfaces are in compression & the middle is in tension. This can be accomplished by heating the glass to near its melting point and rapidly cooling both surfaces. The contraction of the middle (of the thickness) of the sheet develops the desired stress on final cooling.

2.5 Thermal Expansion
The co-efficient of expansion for glass is 4.5x10-6. Thermal expansion and contraction of glass is of importance in the design of glazing details, but it is even more important with respect to the development of stresses within the glass from temperature differential in a pane.

3.0 TYPES OF GLASSES

3.1 Safety glass
Safety Glass is defined as glass, which has passed specific impact tests, and must either not break or break in a manner that will not cause injury. There are 3 main types.
i. Toughened Glass
Toughened glass is obtained when a glass is heated to approximately 7000C in a furnace then chilled rapidly by cold air blown onto both its surfaces. This glass is 4 to 5 times stronger than float glass and, if broken disintegrates into small fragments with dulled edges, which are unlikely to cause serious injury, and therefore it is not termed as security glass. This is a safety glass, as it has the highest impact resistance when hit on the surface, although it is vulnerable to immediate cracking when hit on the edge. It also has a very high thermal resistivity, almost 5 times that of a normal annealed glass.
ii. Laminated glass
Laminated glass is obtained when two or more thin sheets of glass separated by layers of a plastic or resin material. The principal benefit of laminated glasses is their performance under impact, the glass may fracture, but any broken fragments will remain firmly bonded to the interlayer.
There are 2 main types.

a) PVB laminated
Two or more sheets of glass are bonded together with one or more layers of polyvinyl butyral, a plastic interlayer in sheet form. The interlayer in laminated glass provides two additional benefits: sound transmittance is reduced, particularly at the higher frequencies and UV radiation is reduced by atleast 97%.
b) Resin laminated
Such glasses are manufactured by pouring liquid resin into a cavity between two sheets of glass, which are held together until the resin cures. They are principally used for decorative and acoustic purposes, where safety performance is normally of secondary importance.
iii. Wired glass
A steel wire mesh is embedded within the glass and is intended to hold it in place if cracked. The glass breaks in almost the same way as ordinary glass, into sharp dagger-like pieces, but mostly stays in place. The visual quality of the glass is impaired by the wire mesh but this has traditionally been accepted in low-cost fire resistant and roof glazing.
3.2 Security glass
There are similar to safety glass, which has passed specific impact tests and must either not break or break in a manner that will cause injury. Laminated glass is the only glass that qualifies as a security glass.
There are 3 main types
i. Anti-bandit glass
Designed to resist manual attack, such as might be made by a man armed with a hammer, crow bar or pickaxe, and to delay access to the protected space for a short period of time.
ii. Bullet resistant glass
A multi-layered variable thickness laminated glass can function as a bullet resistant glass. A laminated glass upto 80mm thickness can resist an AK-47 bullet. A glass laminated under variable thickness specification can function as a bulletproof glass.
iii. Explosion pressure resistant glass
Basically defined as glazing that affords a defined resistance against a specified explosive blast.

3.3 Solar control glass
This type of glass reduces the amount of heat and light from the sun transmitted through the glass. This is usually done by increasing the amount energy either absorbed within the glass reflected from its surface. There are 2 different ways of producing a solar control glass.
i. Body Tinted glass
This is a mild coloured glass, available in blue, bronze and grey colour tints. They are produced by the addition of metal oxides to the raw materials during float glass manufacture. It not only restricts transparency in glass but also most importantly restricts the sunlight glare, and thereby partially insulating heat transmitted via the sunlight.
ii. Coated Glass
Solar control is achieved by applying a very thin transparent reflective metallic coating to the surface of clear or body tinted glasses. There are two different coating techniques:
 Pyrolitic coating: This coating reduces the proportion of solar energy transmitted through the glass mainly by increasing the amount reflected.
 Sputtered coating: The coating enables the glass to reduce the proportion of solar energy transmitted by increasing the amount absorbed and reflected.
3.4 Low-E coatings
Low-emissivity coatings, called Low-E for short, act to reduce surface emissivity of glass. These glasses provide excellent energy conservation by reflecting majority of the heat and allowing more of visible light thus reducing the heat gain without having to sacrifice much light transmission resulting in dual benefit of less energy requirement to cool the interior of the building as well as less energy for lighting requirement which in turn will help to reduce air-conditioning and lighting cost.


3.5 Spectrally selective glazing
The aim of this glass is to screen out as much ultraviolet and short wave infrared radiation as possible whilst allowing through the maximum amount of visible light. Because this type of glazing can have a virtually clear appearance, they admit more day light and permit much brighter views to outside, whilst still providing order similar to that of dark, reflective tinted glasses
.
4.0 GLASS APPLICATIONS

All the above varieties of processed glasses have ensured safe and functional use of glass in many areas of human life. Amongst them, a few special examples are
4.1Glass skylite/Glass dome
From small to very large spans of ceilings can be covered with glass allowing transmission of natural light permanently. Here, although glass is above our head, there is no fear as these are designed safely. A laminated glass is an ideal concept for glass skylites. While a glass roof does transmit heat faster during the day it also has the benefit of losing heat faster during the night.
4.2 Glass flooring
Today it is possible to walk on glass and be absolutely safe and secure. Proper structural support and adequate cushioning between glasses as well as glass and structure are inevitable. Glasses can be surface treated to prevent skidding. If used carefully, a glass floor does not involve major maintenance.
4.3 Front façade
For every architect or designer, the front facade of any building is very critical. Glass has opened wide avenues, daring opportunities for providing a grand awesome look to the building. Clear view with minimum obstructions, while the inside is cut from outside with regard to noise and pollution, yet due to transparency two worlds are merged.
Toughened or ideally toughened laminated glasses are used here.
4.4 Glass wash basin units
A clean, vibrant, novel, beautiful appearance is derived from a glass washbasin unit.


4.5 Glass furniture
With the advent of skilled workmanship and unique designing techniques, glass can speak different language in versatile styles as an element of furniture. Fusing, blasting, blowing, etching, staining, glue chipping, sticking, colouring, sculpting, chipping are some of the known techniques for beautifying transparent glass into a piece of art. Coffee tables, dining tables, T.V.Stands, corner tables, glass chairs, partitions, lampshades etc. are few of the endless forms of glass interiors.


5.0 GLASS INSTALLATION

There are five main techniques for single glazing.

1. Into rebates without beads
The glass sits in a rebate, held in place by a suitable glazing material, such as putty or a plastic-based compound, which then provides weather proof seal.


2. Into rebates with beads
As before the glass sits in a rebate, but is held in place by the beading. The glazing compound provides the weather seal.

3. Into grooves or channels.
It is usually used for glazing into concrete, store or timber frames with a rebate and bead at the sill, but it can also be modified to suit frames that have beads at the head and sill and grooves at the jambs.



4. Using structural gaskets.
Gaskets are used to hold the glass in a groove or rebate and to provide a weather seal. They can include H-shaped, Y-shaped or single-sided gaskets.

5. Using non-structural gaskets
The glass is held in place by a combination of pressure beads and gaskets.


Check that the clearance is appropriate for the thickness and type of glass. If the glass is in direct contact with the framing materials, it could lead to breakage.

Frameless glass glazing
The use of glass without using wooden or aluminum frames has become a necessity in the present and future concept with the change in the methods of use of thicken float glass in spacious buildings like airports, seaports, multistory buildings, banks, showrooms and spacious offices etc. It is therefore important that the development of such a glass glazing should be accurate and well designed, in accordance with the requirement of the situation and the architectural layout. It is also an important factor that such glass glazing should be fully supported by the hardware used in it with a capacity to sustain wind pressure and structural strength safety based on the necessary material and technique.

6.0 MAINTENANCE OF GLASS

Site Clean up
It is essential that all external glass be thoroughly washed with clean water to eliminate all abrasive and chemical laden dust. Excess glazing compounds and sealants should be carefully removed from the glass and frame surrounds, taking care not to scratch the finished surfaces with tools or abrasives. A solvent such as white spirit or professional glass cleaner may be used to remove any glazing compound, finger marks or grease. Frequent washing is required whilst construction continues on site, since chemicals in dust and particularly in cement may be activated by rain and cause permanent corrosion of the glass surface.
Normal Cleaning
Use a mild liquid detergent solution, then rinse the glass well with clean water and dry off. Under no circumstances may abrasive cleaning products or contaminated cloths be used. If rainwater coming from cement mortar contaminates the surface of the glass, frequent cleaning is necessary to prevent permanent staining.


Regular maintenance
It is essential that all installations are inspected and maintained during the lifetime of the building at regular intervals as recommended by the sealant and framing manufacturers.

7.0 PROTECTION OF GLASS

Glass can be a dangerous material .When standard annealed glass breaks, it forms potentially lethal shards and splinters. Glass manufactures have developed a range of safety glasses adding strength and integrity to this beautiful building material and allowing glass to be used in areas where safety is critical in unprecedented situation. Safety glass is defined as glass, which must have passed an impact test (as per BS 6206: 1981). There are 3 levels of impact: C, B and A, ‘A’ being the highest. Each involves the glass being impacted by a leather bag containing 45kg of lead shot.

Class A - 1219 mm
Class B - 457 mm
Class C - 305 mm
All security glasses automatically qualify a class A safety glasses.
Glass in doors and side panels to doors must be at least:
# a class B safety glazing material if the smaller dimension of the glass is more
than 900 mm.
# a class C safety glazing material if the smaller dimension of the glass in less than 900 mm.
Non safety glass in small panes may be permitted under certain controlled circumstances.



7.1 Permanent screen protection
The use of annealed glass is permitted in a critical location if protected by a permanent, robust screen. The screen must prevent the passage of a 75mm diameter sphere and must not be climbable.
7.2 Bathing areas and areas of special risk
BS 6262 requires that any glazing, forming part of a bath or shower screen or adjacent to or surrounding a bathing area, swimming pool or other wet areas must be at least a class C material. Consideration should also be given to the breakage characteristics. The fragmentation of toughened glass into small dice-like particles would result in, should it enter the pool, being invisible practically impossible to remove, practically causing damage to pumps filters. This can be avoided by using a laminated safety glass an account of the glass fragments remaining adhered to the plastic interlayer. This requirement also applies to all glazing in areas of special risk such as gymnasia and other places of energetic activity. In such areas, the designer must consider whether a higher class is required, or if additional safeguards such as protective rails or screens, or manifestation are necessary.
7.3 Commercial frontage
Robust glass (non-safety thick annealed glass) when fully framed is considered suitable for use in large areas in non-domestic applications, for eg: forming fronts to shops, showrooms, offices, factories, and public buildings.
Glass thickness / size limits for annealed glass that may be used in these locations are shown in the Table 1. Wind loads & other load must be considered when selecting the glass thickness.
Table1 Glass thickness and pane size


Normal Glass Maximum pane size
thickness (mm) (four edge supported )(mm)
8 1100 x 1100
10 2250 x 2250
12 4500 x 4500
15 or thicker No limits


7.4 Glass in furniture
Standard advises minimum BS 6206 classification, glass thickness and support details to insure the reasonable safety of flat glass having a total area of at least 0.02m2.
1. Glass in tables or trolleys
Glass that is not supported over its entire area:
The minimum horizontal area of each support shall be 36mm2. Annealed glass shall be supported for not less than 50% of its total perimeter and the support should be in at least two non-adjacent regions and shall be not more than 100mm from the edge of glass. Minimum BS 6206 classification and nominal thickness for glass that is not supported over its entire area as shown in table 2.

Table 2 Minimum BS 6206 classification and nominal thickness

Area of Nominal Thickness Minimum Nominal thickness
glass (m2) (mm) BS 6206 (mm)
annealed glass classification
Toughened glass Laminated glass


<= 0.25 >=10 Class C >=4.0 >=6.4

< 0.25 to >=10 Class C >=5.0 >=6.4
<=0.5

> 0.5 to >=12 Class C >=6.0 >=6.4
<=0.75

> 0.75 to >= 15 Class B >=8.0 >=8.4
< =1.5
>1.5 >=19 Class A >=10.0 >=10.8


Glass that is supported over its entire area.:
Glass which has an area not greater than 1.5m2 shall comply with the relevant nominal thickness given in table3 below.

Table 3 Nominal thickness of glass

Area of glass (m2) Nominal thickness (m)
Annealed
Toughened glass
Laminated glass
<=0.5 >=4.0 >=4.0 >=4.4
>=0.5 to <=1.0 >=5.0 >=4.0 >=4.4
>=1.0 to <=1.5 >=6.0 >=4.0 >=4.4
>1.5 N/A >=4.0 >=4.4

Contact of glass with other materials:
Hard materials such as other glass, metal or stone should not be allowed to come into direct contact with the edges or surface of the glass. Separation should be ensured by the use of suitable bushes and gaskets. Glass in furniture other than tables or trolleys. Horizontal glass supported over its entire area should comply with Table 3. Glass used to form the external surfaces (excluding horizontal glass supported over its entire area) and which is unbaked should comply with the requirements of table 4.
Table 4 Requirements of glass for external surfaces and unbaked conditions
Smaller dimension (width or height)
Less than 900mm

BS 6206 Class C

Minimum Thickness More than 900mm


BS 6206 Class B


Minimum Thickness
3 mm Fully framed 4 mm
4 mm Partially framed or unframed 6 mm
For sliding door and fixed glass retained in a rebate or groove, the edge cover provided by the rebate or groove shall be at least 4 mm.

7.5 Glass shelves
Glass shelves that are not fully enclosed in a cabinet shall be a class C safety glass to BS 6206 as a minimum. This requirement is most easily met by using toughened glass. Annealed glass is acceptable for use as shelves only when fully enclosed within a cabinet. The maximum evenly distributed safe load that a shelf can support is dependent on glass type, thickness, width and span of the glass between supports.

8.0 DEVELOPMENTS IN SAFETY GLAZING

8.1 Fire resistant glass products
Acceptable glazing products for fire – rated, hazardous locations include transparent glass ceramic and in tumescent glass. Glass ceramic are single glazed products that meet both impact resistance requirements and provide up to a 3-hour fire resistance rating.
In tumescent glass are double – glazed products with an inner layer of transparent sodium silicate that when exposed to high temperature, turns opaque and forms an insulating layer that can provide up to a two hour fire resistance rating. Unlike glass – ceramic, in tumescent glass becomes insulating when exposed to fire. It is also more expensive than glass ceramic.
8.2 Blast resistant glazing
When a building is subjected to bombing it is noted that 60% of the non-lethal injuries sustained in that event were due to shattered glass, shattered glass was found as far as 1 mile from the blast site.
The GSA standard defines performance condition based on the response of glazing to a specified blast event, and the level of protection provided by glazing system as shown in the table below.










Table 5 The level of protection provided by glazing system
Performance Criteria Protection Level Hazard Level Description of Window Glazing Response
1
Safe
None Glazing does not break.No visible damage to glazing or frame.
2 Very high None Glazing cracks but is retained by the frame. Dusting or verysmall fragments near sill or on floor acceptable.
3a High Very low Glazing cracks. Fragments enter space and land on the floor no further than 0.99m from the window.
3b High Low Glazing cracks. Fragments enter space and land on floor no further than 3m from the window.
4 Medium Medium Glazing cracks. Fragments enter space and land on the floor and impact a vertical witness panel at a distance of no more than 3m from the window at a height no greater than 0.6m above the floor.
5 Low High Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 3m from window at a height greater than 0.6m above floor.


9.0 CONCLUSION

1. Structural glass tends to influence modern architecture more and more. It defines not only modernity, but also value, richness and future technology.
2. There have been thousands of other developments in glass manufacture that make it one of the most useful and versatile materials available.