In a paper on curtain wall design for buildings in warm climates presented at the Gulf Construction Conference Week* last year, Dr Leon Jacob of the Australia-based Jacob Associates emphasised that the ultimate selection of the glass type is critical in the performance of the curtain wall. Excerpts:

01 October 2004

Techniques used for the design of glass panels used in buildings have undergone dramatic development and transformation in recent years.

The early design charts were based on full-scale test results. They only linked the area of the panel to the applied pressure for the various glass thicknesses available.
Today, these design charts consider the aspect ratio (length/width), slenderness factor (width/thickness) to the laterally-applied pressure and have been developed from complex finite element analysis (FEA) based on the non-linear behaviour of thin plates.
The use of finite element techniques has improved our understanding of the interaction between the various parameters that influences thin glass panel behaviour. Figure 1 illustrates the stress distribution in a rectangular panel subjected to lateral pressures.
The first frame construction developed in the 19th century – in which the external wall did not have a load-bearing function – was the catalyst for the ultimate development of today’s curtain wall. Unitised curtain wall systems have focused the glass industries’ attention on the behaviour of the material and consequently enhanced our understanding of glass behaviour.
The more recent innovation (1980s) of low-e (emissivity) coatings has had a positive impact in the glazing market (especially in terms of solar control, a key issue in the Middle East), which is now extending into the curtain wall segment. These specialised coatings are available in different forms such as silver, tin oxide, silicon, titanium nitride and other oxides and their spectral properties range from high to low values in the solar spectrum. It is now possible to get a combination of solar control coatings with low-e characteristics.

Factors that influence performance
A major concern in the region's glazing industry is the amount of heat transferred to the interior. The U value is the measure of the transfer of thermal energy through the window when one side is hotter than the other and is dependant on the construction of the glazing. Table 1 provides a comparison in the spectral properties for various combinations and types of glasses.
The U value of an IG (insulated glazing) unit is influenced by the width of the air space, the gas within the air space, the emissivity of the glass surfaces and method used to measure the thermal characteristics of the IG unit.
The emissivity of the glass surface is a key factor in determining the U value of an IG unit. When a low-e coating is applied on surface two or three of the IG unit, its impact is found to be significant on the overall U value of the IG unit. Figure 2 illustrates this issue (calculations based on E N 673 specifications).
The shading coefficient of a glass type is the ratio of the total energy transmitted through a particular type of glass as compared to the transmission through a reference glass (generally 3 mm clear float glass).
Another term of reference for glass which is becoming popular is the solar heat gain factor, which is the fraction of the solar radiation that strikes the outside of the window that actually ends up inside the building.
Other factors that affect performance include the luminous efficacy, which is the ratio of the visible light transmission to the shading coefficient for a particular type of glass. Higher ratios of luminous efficacy indicate that the glazing is better at transmitting light than heat; and
The thermal emittance for clear glass is about 0.84; the first-generation low-e coatings had an emissivity of about 0.2. This reduction has resulted in significant improvements in the energy performance of windows generally.
Further development work resulted in the introduction of the dielectric-metal-dielectric coatings which have reduced the emittance from the glass surface to below 0.1 and as low as 0.05. At this level, the potential for further reductions in emissivity is becoming limited.

Determining spectral properties
The method used to determine the key spectral characteristics of a glass panel is an important factor in evaluating the energy-saving capability of a particular glass panel. The equipment used for measuring the transmission, absorption and reflectivity of a particular glass type has been tested and proven to be adequate.
However, equally important is the subsequent calculations used to determine the value, heating coefficient, solar heat gain factor and consequently the luminous efficacy. If the assumptions made in these calculations are inconsistent, then the results obtained could have a significant bearing on the performance of the curtain wall.
Historically, it has been generally considered that a low-e coating on glass will essentially only influence the U value of the product. In fact, a low-e coating on the appropriate surface of the glass will also improve the shading coefficient of a solar control glass panel.
In a warm climate, it is critical that the low-e coated glass be used in conjunction with a solar control glass, either body tinted or reflective. The ideal location for a low-e coating in an IG unit when used in a warm climate would be either surface two or four. A soft coating on surface four is not necessarily possible because the coating could be damaged.
The global architectural trend to allow more daylight in by using clear glass is not necessarily a good solution for curtain walls in warm climates. A theoretical assessment of the performance of clear glass in a warm climates will clearly show that the glass would be unsuitable from both the comfort and the utilisation aspects because once the short wave solar energy enters through the IG unit the heat is then trapped within the building in the form of long wave energy which then raises the temperature of the environment. In warm climates, IG units must have either a body tint or a reflective coating to minimise the short wave radiation entering the environment.
A hard coated low-e coating can be used on surface four of an IG unit. Another option would be using a hard coated low-e glass panel with a laminated solar control glass.

Sustainability
In 1987 the United Nations’ World Commission on Environment and Development released the Brundtland report which defined sustainability as: “meeting the needs of the present without compromising the ability of future generations to meet their own needs”. [1]
This definition has permitted a variety of approaches to the construction of curtain walls.
“Some are manifestos that call for rethinking architecture in its most basic formal terms. It has generally been postulated that the sustainability become the basis for architecture and therefore a search for relevant form and symbolic content.”. [2]
For instance, the US Green Building Council’s efforts to provide a national standard for what constitutes a ‘green building’ has seen it encourage the use of high-performance glazing products, photovoltaic technology and other construction devices for the building envelope.
It should be every designer’s intent to provide for the occupants well-being and comfort. More importantly, the economic return derived from the building design must be commensurate with the energy costs. The impact of minimising carbon dioxide emissions is a global problem and can be significantly improved by using better performing glass systems and designs.
A better approach would be a passive design which focuses on reduced energy demand by using selectively coated and specifically-designed glass products.
For over 40 years, reflective glasses have been essentially relied upon to provide a reduction in solar heat gains. However, the introduction of low-e coatings and more recently spectrally selective coatings has not only improved installation capacity but has significantly reduced the solar gain through the building envelope by increasing the visible light transmissions.
The advent of double-skin facades has been proven to significantly improve their energy conservation capacity and optimisation of daylighting within the confines of the curtain wall. The use of available daylight has a significant impact on the total energy consumption. It reduces the need for artificial lighting, which in turn reduces the electricity consumption.
The cavity within the double skin facades can be either vented internally or externally depending on the design criteria - summer cooling or winter heating.

Computer modelling
There are numerous commercially available computer software programs that can be effectively used to estimate the energy criteria for different combinations of glazing types. In a recent study using the DOE 2 software for a theoretical building in Darwin, which has a warm climate, the total energy use with and without day lighting was evaluated. Table 2 summarises the results obtained in the theoretical study.

Glass strength
Recent studies(3) have shown that using FEA software can eliminate the use of correction factors presently used for designing laminated glass. This software must be capable of handling both the panel non-linearity under a lateral pressure and the fact that the panel is a composite structure.
The effects of high temperatures on the capacity of the pvb interlayer should be modelled with the software for it to be a useful tool.
Laminated glass, like monolithic glass panels, experiences large centre deflections when subject to a lateral pressure. Tests have shown that the response of a laminated glass panel to be between the two extreme cases of monolithic and layered. Vallabhan[4] has also theoretically demonstrated that for certain aspect ratios the performance of laminated glass to be better than equivalent thickness monolithic glass. Jacob [5] also validated this finding.
The mechanical properties of the interlayer material have a direct influence on the behaviour of the laminated glass. New interlayer material now available can provide significantly improved strength and performance characteristics.
Practical tests on asymmetric laminated glass have shown that the load distribution between the different glass thicknesses in the composite is a function of the thickness squared and not the thickness cubed as would be expected.
Table 3 illustrates the stress build-up on the two different glass components of an asymmetric laminated glass panel. These results were derived from a stress analysis.
It is important to recognise that fractured laminated glass does not vacate the supporting frame.
Further, in full-scale tests, it was observed that on fracture generally one layer of the composite fractured. Only when the lateral load was maintained for some more time after the first fracture did the second layer of the laminated composite fracture. Even after both layers had fractured the laminated glass did not vacate the frame. This gives the product a unique safety characteristic.

Embodied energy
The embodied energy of any product is the energy required to manufacture products. So a designer must be aware of the embodied energy and select the building material correspondingly. For example, if one needs to reduce the embodied energy of the materials to be used in the next building, the information in Table 4 could be useful. It is possible that the building would have to be constructed with timber and concrete and insulated with fibreglass. However, a more careful look as to how these materials are selected and used would indicate a different scenario.
From Table 4 it would appear that glass had 11.8 times the embodied energy of concrete but when we look at the way the materials are used in a building (Table 5), it will be noticed that the concrete panel has 4.7 times the embodied energy of the glass panel. Glass can also repay its embodied energy.

Conclusions
Curtain wall design and construction now require a new approach to the concepts of design and material selection. Energy conservation, comfort and selection of material with minimal cost to produce are important issues.
Passive design, selective coatings and reduction in CO2 emissions will direct the curtain wall selection and design of the future.

References
[1] 1987 Brundtland Report, The United Nations World Commission on Environment and Development.
[2] Flynn D. Michael, Sustainability and Safety in High- Rise Curtain Wall Glass. Glass Processing Days, Conference Proceedings, Tampere, Finland 15-18 June 2003.
[3] Jacob, Understanding the Versatility of Laminated Safety Glass as a Glazing Product of the Future. Glass Processing Days, Conference Proceedings, Tampere, Finland 15-18 June 2003.
[4] Vallabhan, CVG., et al. Stresses in Laminated glass units and monolithic glass plates. Journal of Structural Engineering. ASCE Vol 113 No.1. January 1987.
[5] Jacob, L, New Limit state design model for laminated glass, GPD Conference July 1997.

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