Structural Glazing
 

     Structural glazing systems incorporating spacers and adhesives are now a widely accepted glazing option and are used in a growing share of curtain wall, window and storefront installations.

 

     In fact, many people have come to regard structural glazing as the superior method of glazing. When properly installed, the structural sealant forms a continuous, waterproof seal against leakage and air infiltration. The sealant and spacer transmit wind load movement to the structure and withstand flexure, tension, compression and differential thermal shear stresses. Silicone resists UV, ozone and other environmental exposures and does not take on a compression set or lose resiliency. While structural sealants generally carry a 20-year of exposure still demonstrate elasticity, adhesion and strength.

 

     The major issues that govern the design of structural glazing systems are sealant performance air and water infiltration, structural design requirements, and the use of right type of glass. These requirements directly affect aluminium profile sizes, sealant widths and join sizes.

 

Sealant Performance: There are two major glazing systems named semi-unitized and unitized system, sealant is the heart of the glazing systems. Proper sealants has to be selected as per the requirements of the particular project, and required bite should be used which is determined by testing of aluminium profiles, glass, spacer tapes etc by the sealant company.

 

Air and Water Infiltration: Air and water infiltration requirements usually have few visual implications on structural glazing systems. However proper locations of gaskets, weather strips and sealant are critical. The system must be designed to resist water infiltration through the above gasketing and properly designed gutter and flashing.

 

Structural Requirement: The framework must be designed for wind pressure, corresponding beam displacement, spacing of verticals and deflections: variable movements due to expansion and contraction of the building.

 

The Energy Efficiency Factor: Energy efficiency of the glass of a building means using less energy for cooling and lighting. Relative heat gain and shading co-efficient factors are two of the most important factors to determine the energy saving performance of any glass.

 

The Concept of OTTV: The Concept of OTTV, (overall Thermal Transmission Value) which is a measure of the average heat gain into a building through its envelope, was introduced with the main objective of achieving adequately designed building envelopes to cut down external heat gains and hence reduce the cooling load of air-conditioning systems.

 

     The OTTV concept takes into consideration three basic elements of heat gain through the external walls and windows of a building. These are heat conduction through opaque wall, heat conduction through glass windows, and solar radiation through glass windows, which is measured in W/m2.

 

The Importance of Laminated Glass: Laminated glass is a safety glazing material made by bonding layers of glass, using heat and pressure, with one or more interlayer of polyvinyl butyral (PVB). It can be made with sheets of annealed, tempered, chemically strengthened or heat strengthened glass, depending on design needs and exhibits important structural performance and safety features along with other benefits.

 

Safety & Security: The principal feature of laminated safety glass is its performance under impact. Unlike ordinary glass which brakes into sharp pieces causing serious and sometimes fatal injuries, the PVB interlayer in the laminated glass absorbs the energy of the impact, resisting penetration, and even though it breaks, the glass fragments remain firmly bonded to the interlayer considerably minimizing the risk of injuries. In glass skylights, sloped glazing installations and curtain walls there is always the possibility of glass breakage, and without laminated glass that could mean a big safety problem. Laminated glass can be designed to withstand bullets & bomb blasts by using multiple or thick layers of glass and PVB.

 

Sound Reduction: Laminated glass has proved to be an excellent barrier to noise, having a higher sound reduction index than monolithic glass of equal thickness between the frequencies of 125 Hz and 4000 Hz. This sound dampening is due to the viscoelastic properties of the PVB interlayer.

 

Solar Energy Control: While natural light plays an important role in architectural design, too much sunlight can also mean too much heat. Tinted laminated glass can reduce heat gain from sunlight to lower air conditioning costs, and it can also control glare.

 

Low visible Distortion: Laminated building glass is usually glazed in an annealed form avoiding the distortion caused by roller waves in tempered and heat-strengthened glass. Sharp reflected images are possible with curtain walls construction of laminated annealed

 

Selecting Glass

 

Glass Selection Criterion: The correct choice of glass, by the design professional for a particular glazing application requires the consideration of a number of characteristics.

 

The Colour and Appearance: The use of reflective coatings on the second surface enhances the tint of the base glass. In general glass colours are subtle and need to be carefully evaluated.

 

Thickness: The thickness of glass to be used is usually decided by the strength and stiffness requirements. The colour and appearance of tined glass will change with the glass thickness because thicker glass absorbs more transmitted light and hence appears to be darker.

 

Visible Light Transmission: Interior daylight levels will be determined by this value. Residential applications generally require higher levels than in commercial buildings for obvious economic reasons.

 

Solar Transmission and Absorption: The Shading coefficient is the best measure of how much solar energy is admitted through a glazed opening.

 

Solar Energy Transmission: Expressed as a percentage of the solar energy, ultraviolet, visible and near infrared energy (300 to 3000 nanometers) that is directly transmitted through the glazing. The lower the percentage the more effective the glazing is in reducing solar heat gain.

 

Reflectance: The percentage of the solar energy reflected away from the exterior surface of the glazing. The higher the percentage, the greater the total reductions in solar heat gain.

 

U-Value: A measure of thermal conductivity or the heat gain o loss through glass due to the difference between the indoor and outdoor temperatures. The lower the U-value, lesser heat is transmitted through the glazing material.

 

Heat Gain: Heat added to the interior of the building by radiation, convection, or conduction.

 

Relative Heat Gain: The total heat gain through glazing for a specific set of conditions. The value takes into account indoor and outdoor air temperature differences and the effect of the solar radiation.