Glass Specifications

Glass in Architectural and Building Design

The majority of glass used today in architectural applications is float glass. Float glass is manufactured in a continuous process by melting glass batch (i.e., soda, lime, silica sand, and other materials) and, when melted, floating the glass on a bath of molten tin. The glass is slowly and carefully cooled in an annealing layer to produce glass with exceptionally parallel surfaces, high optical quality, and fire-finished surface brilliance. Float glass may be used as produced in a wide variety of applications, or it may serve as the base material for other products, such as fully tempered glass, heat-treated glass, laminated glass, insulated glass, mirrored and decorative glass. Non-architectural end-users include the automotive, aircraft, and transportation industries.

The Design Professional’s Responsibility

In the design and use of architectural glass, the responsible design professional must carefully consider the performance characteristics of glass as they relate to construction requirements. Safety glazing laws and local municipal building codes may set minimum requirements that do not relate to the specifier’s initial design criteria (i.e., wind loads, thermal stresses, solar or optical properties, and aesthetic considerations). In addition, other issues may affect the design criteria, such as break patterns, fall-out characteristics, acoustical insulation, and security demands.

Annealed Glass

Annealed glass has a surface strength that provides the wind-load performance and thermal-stress resistance needed in most architectural applications. In areas of high wind loads or in conditions where higher than normal thermal stresses occur, annealed glass may not be suitable. Annealed glass has poor resistance to hard, blunt objects or projectiles (such as storm-blown roof gravel) and, when broken, may fracture into large, sharp pieces. However, experience has shown that in-service annealed glass performs well when subjected to small, softer, low-velocity objects carried by low-level wind loads.

glass Glass Specifications

Heat-Strengthened Glass

Heat-strengthened glass is produced by heat-treating annealed glass under regulated thermal conditions. In this process, annealed glass that has been cut to size is heated in a furnace controlled between 1100-1500 degrees Fahrenheit (593-815 degrees Celsius) and air-cooled. This sudden cooling causes a compression envelope around the glass surface and edges and a balanced tension stress within the glass itself. This equilibrium of stresses increases the strength of the glass to approximately two times that of the original annealed product (when tested under uniform pressure such as wind loads). In addition, when broken, glass with a low-to-moderate degree of heat-strengthening generally exhibits few cracks and tends to break into large pieces that initially may remain in the glazed opening. (Note: glass should be removed and replaced as soon as possible after breakage.) A significant advantage of heat-strengthened glass is its ability to withstand high thermal stresses resulting from partial shading and heat build-up from solar loading. With its edge compression levels in excess of 5500 pounds per square inch (38 MPa) and surface compression levels in the 3500 to 7500 psi range, heat-strengthened glass performs well in demanding architectural applications. Heat-strengthening should be considered in all spandrel glazing when large lites are used, when heat-absorbing and coated glass is used, and when a likelihood of external shading and reflectance exists. The increased toughness of heat-strengthened glass also reduces the likelihood of glass breakage during shipment, handling, installation, and in-service use.

Fully Tempered Glass

Fully tempered glass is produced by heat-treating annealed glass under regulated thermal conditions. In this process, annealed glass that has been cut to size is heat-treated and then cooled quickly with air, creating an edge compression greater than 9700 psi (67 MPa) and a surface compression greater than 10,000 psi (69 MPa). Fully tempered glass may show more visual distortion of reflected images than heatstrengthened glass. Its key performance characteristics are increased strength and the ability to meet the requirements of safety glazing standards.

Under uniform static loads, fully tempered glass is about four times stronger than annealed glass of the same thickness, and twice as strong as heat-strengthened glass of the same thickness. It also has significant resistance to breakage from blunt projectiles. The increased strength of fully tempered glass (due to its compression stresses) makes fully tempered glass an option for many architectural applications.

The increase in compression stresses and equilibrium center-tension stress in fully tempered glass may, on rare occasion, result in spontaneous breakage. All heat-treated glass will break when its compression layer is penetrated. Thermal or wind loads or building creep may produce surface or edge damage that does not completely penetrate the compression layer, but may result in spontaneous breakage. In addition to surface or edge damage, spontaneous breakage may result from deep scratches or gouges in the glass surface; severe weld splatter on the glass surface; glass to metal contact; and nickel sulfide inclusions.

Nickel sulfide inclusions signify the presence of certain types of rare and very small, undissolved nickel sulfide stones that are extremely difficult to detect. Glass manufacturers take extraordinary steps to minimize the potential for nickel sulfide inclusions. Considering that a large furnace may produce up to 600 tons of glass per day, total elimination of contaminants is impossible.

(NOTE: Identical sizes and thicknesses of annealed, heat-strengthened or fully tempered glass lites will have the same centerline deflection when exposed to the same uniform load. Centerline deflections may be reduced by increasing the glass thickness or reducing its size.)

Laminated Glass

There are several laminated glass manufacturing processes:

  1. permanently bonding two or more pieces of glass together with one or more interlayers of plasticized polyvinyl butyral (PVB) resin under heat and pressure;
  2. permanently bonding two or more pieces of glass and polycarbonate together with aliphatic urethane interlayers under heat and pressure; and
  3. permanently bonding two or more pieces of glass together using cured resin as the interlayer material.

The bonding of materials to glass provides a variety of performance benefits in architectural applications. The most important characteristic is the ability of the interlayer(s) to support and hold the glass when broken. This provides increased protection against glass fall-out and penetration through the opening. Most building codes, for example, require the use of laminated glass for overhead glazing. Other applications include safety glazing, acoustical insulation, resistance to smash-and-grab burglaries, windborne debris, bullets, and blast hazard mitigation.

Laminated glass is 75% to 100% as strong as annealed glass of the same thickness depending on exposed temperatures, aspect ratio, plate size, stiffness, and load duration. The edges of laminated glass are less resistant than annealed glass to handling and installation damage. Laminated glass, however, can be made with both heat-strengthened and fully tempered glass for additional benefits, such as greater wind load, impact, and thermal resistance. Note: when heat treated glass pieces or plies are laminated together, there may be a reduction in transmitted optical quality, especially if the plies are relatively thin.

Other Types of Glass

  • Borosilicate — silicate glass having at least 5% boron oxide; used mainly for fire-rated applications and offering more resistance to thermal shock and harsh chemicals.
  • Ceramic — solid material, partly crystalline and partly glassy, formed by the controlled crystallization of a glass.
  • Plate glass — glass made through the process of pulling molten glass through rollers, and then exposing it to systematic grinding and polishing. Plate glass has been totally replaced by float glass and is no longer manufactured in the United States.
  • Rolled glass – glass made through the process of pulling molten glass through a series of rollers to produce such products as patterned glass (where the glass has a decorative pattern imprinted on it) and wired glass (where a welded steel mesh is introduced into the molten glass).
  • Sheet glass — glass made through the process of pulling a ribbon of glass directly out of the molten glass pool. Sheet glass is no longer manufactured in the United States.

The Nature of Soda-Lime Float Glass

Glass is a brittle material. It acts elastically until it fractures at ultimate load. That ultimate load varies, depending upon the type and duration of the loads applied and the distribution, orientation, and severity of the inhomogeneties and micro-flaws existing in the surface of the glass. Because of its nature, glass cannot be engineered in the same way as other building envelope materials with a predictable specific strength. In those cases, factors can be (and are) assigned to minimize the likelihood that breakage will occur at the selected design load. Because the ultimate strength of glass varies, its strength is described statistically.

Architects and engineers, when specifying a design factor for glass in buildings, must choose the anticipated wind load, its duration, and the probability of glass breakage (defined as x per 1000 lites of glass at the initial occurrence of the design load). Glass manufacturers can provide the appropriate data for determining the performance of their products. However, the responsible design professional must review these performance criteria and determine if they are suitable for the intended application.

The Average Physical and Mechanical Properties of Soda Lime Float Glass

  1. Modulus of Elasticity (E) — 10.4 x 106 psi (71.7 x 106 kPa)
  2. Modulus of Rigidity (Shear) (G) — 4.3 x 106 psi (29.6 x 106 kPa)
  3. Poisson’s Ratio — 0.23
  4. Coefficient of Thermal Expansion — 4.6 x 10-6 strain per oF (8.3 x 10-6 strain per oC)
  5. Density — 156 lbs. per cubic foot (2500 kg/m3)
  6. Modulus of Rupture (Flexure)
    1. 6000 lbs/sq. in. (41.4 mpa): (mean) (design: 8 breaks in 1000)
    2. Annealed Glass — 6,000 psi (41 MPa) 2,800 psi (19 MPa)
    3. Heat-Strengthened Glass — 12,000 psi (83 MPa) 5,600 psi (39 MPa)
    4. Fully Tempered Glass — 24,000 psi (166 MPa) 11,200 psi (77 MPa)

      Note a – These are approximate values for short load durations (under 1 minute) for undamaged glass in four-sided support.

      Note b – Probability of breakage — note that these values are for the surface of the glass (not the edge) and do not take into consideration area effects.

  7. Hardness (Moh’s Scale) — 5 to 6
  8. Specific Heat Capacity — 0.84-0.88 J/Kg x K)
  9. Thermal Conductivity — 0.9-1.0W/mk (.52-.57 Btu/hrftF)
  10. Mean Refractive Index @ Sodium “D” Line 1.5
  11. Chemical Composition: Silicon dioxide (SiO2) 69-74% Calcium oxide (CaO) 5-12% Sodium oxide (Na2O) 12-16% Magnesium oxide (MgO) 0-6% Aluminum oxide (Al2O3) 0-3%
  12. Softening Point — 715-729oC (1319-1345oF)
    Annealing Point — 544-548oC (1011-1018oF)
    Strain Point — 504-511oC (939-952oF)
  13. Emissivity — 0.84
  14. Reflection — 4% from each surface (for 3mm glass)
  15. Visible Light Absorption – 1 to 2% (for 3mm glass)
  16. Far Infrared Transmission — 0
  17. Chemical Resistance — Excellent
  18. Electrical Resistivity — High

Design Loads and Glass Strength

Once the design load and its duration have been determined and a suitable probability of breakage has been selected, the appropriate glass thickness or glass type can be chosen.

Thermal and Solar Optical Properties of Float Glass

Specifiers choose glass as a building envelope material for a number of reasons. Some of the more important reasons include architectural expression, interior lighting, and a view to the outside world. All of the benefits of glass relate to its thermal and optical properties, absorptance, and reflectance. Glass exposed to the sun will transmit, absorb, or reflect all of the sun’s energy.

The Solar Spectrum

The sun transmits energy in the form of waves. Each wave is defined by its length and is measured in nanometers (nm). There are three specific ranges within the solar spectrum:

  1. Ultraviolet light in the 300 to 380 nm range.
  2. Visible light in the 380 to 780 nm range.
  3. Infrared light in the 780 to 2100 nm range.

The transmission and reflection at each wavelength are affected differently by the glass and glazing products present in the building envelope. Measurements to determine the optical properties of glass are made with spectrophotometers.

Visible Transmittance

Daylighting or visible transmittance (Tvis) depends upon the portion of the visible light spectrum transmitted through the glass. The appropriate amount of visible light transmittance reduces the need for artificial light and greatly improves worker productivity, occupant comfort, and interior appearance. Too much daylighting can cause overlit workspaces and glare. Determining the right amount of visible transmittance depends upon the building orientation, the size of the glazed product in the building envelope, and the color, reflectance, or tint of the glass itself.

Solar Heat Gain

The rate of heat exchange from solar energy is called solar heat gain. The amount of solar heat gain through a window or door depends upon the incident solar ratio and the optical properties of the glass. Two terms related to solar heat gain are solar heat gain coefficient (SHGC) and shading coefficient (SC).

  1. Solar Heat Gain Coefficient Solar heat gain coefficient is the fraction of solar heat admitted through a glazed product.
  2. Shading Coefficient Shading Coefficient (SC) is a term that has been widely used in the past; however, Solar Heat Gain Coefficient (SHGC) is more frequently used today. The shading coefficient of a product is the ratio of the solar heat gain of a specified product to the solar heat gain of a referenced standard (i.e., single-pane (1/8″) clear glass. SHGC is a more accurate method of stating the performance of a glazed product in a building envelope because it represents the amount of solar heat gain relative to that through an unglazed opening. For conversion purposes, the SHGC of a specific glazing is approximately 86% of its SC (under a specified solar incidence).

Solar Energy Absorptance

An understanding of the absorptance of solar energy is important in order to determine the appropriate glass type to specify to reduce the risk of solar-induced thermal stress breakage. Various glass tints absorb differing amounts of solar energy that in turn build up thermal stresses in the glass. Heat-strengthened and fully tempered glass provide increased performance and reduced risk of breakage due to thermal stress when compared to annealed glass.

Glazing Emissivity

The net amount of solar energy transferred into a building through vision glass areas depends upon the solar reflectance and solar absorptance and emittance of the glazing. Some solar energy will be reflected, some will be directly transmitted through the glass, and some will be absorbed and either conducted through the glass and re-radiated to the interior or re-radiated back out to the exterior. The solar reflectance of low emissivity coatings further reduces solar heat transfer to the interior. Solar reflectances range from minimal to significant, depending upon the type and nature of the low emissivity coating.

Low emissivity (or low-e) glass products are formulated to reflect long-wave length infrared energy (i.e., furnace heat, heat generated by people, artificial lights, absorbed shortwave length infrared energy emitted as long wave length energy, etc.) back to the interior. Low emissivity coatings on glass have a greater effect on unwanted winter heat loss from a building than uncoated glass. Uncoated glass has an emissivity or emittance of 0.84. Low-e glass products are currently available in emittance values as low as 0.03. Low emissivity coatings reduce the infrared heat transfer across the air space of double sealed insulating units. A low-e coating on either surface #2 or #3 will make the double glazing insulate as well as or better than uncoated triple glazing.

U-Factor

U-factor is the thermal transmittance of a material or assembled wall. It is often used to communicate the thermal effectiveness of glass windows, curtain walls, and doors. As such, the framing and support systems themselves affect the overall U-factor published for these products.

Coated Glass

The four basic properties of coated glass are solar reflectance, solar absorbance, visible light reflectance, and long-wave length heat reflectance (low-emissivity coatings). These properties may be present singularly or in combination. The differences between any two coatings are the amount and type of solar transmission (ultraviolet, visible or infrared) allowed through the glass. Solar reflective glass may have some type of metallic coating that reflects a greater portion of the entire solar spectrum. A reflective coating on vision glass areas reduces the solar heat gain within the building envelope and thereby offers economies in the sizing of HVAC equipment and reduces energy consumption. Low-emissivity coated glass has various layers of nearly invisible coatings that reflect a significant part of long-wave infrared energy. These glass coatings may be applied either during the float process (pyrolytic coatings) or to the finished glass surface (vacuum deposition coatings).

Conclusion

There are many different glass specification combinations available in today’s market. The glass specification utilised can help control the environment. The glazing systems design and finish can often play an important aesthetic role, effecting the structures impact on the surrounding architecture and environment.

Planning permissions and your environmental/ aesthetical desired effect all determine the above. However due to building regulations and environmental design control, a final combination of the above specifications may need to be fabricated within double glazed or even triple glazed hermetically sealed units.