Insulating Glass Units (IGU’s) are designed to improve the energy efficiency of buildings. These elements consist of two or three panes of glass separated by a spacer, with the panes sealed together to
14 Picture from: C. Schittich, G. Staib, D. Balkow, M. Schuler, and W. Sobek, Glass Construction Manual.
Birkhäuser, 2006.
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create an airtight cavity. The space between the panes is typically filled with an inert gas, such as argon or krypton, to improve insulation by reducing heat transfer through the glass component.
At first glance, IGU’s appear to be an easy system to implement, given that they are one of the most commonly used solutions in terms of infill panels. They do, however, represent a significant advancement in engineering and architecture.
The way that buildings were illuminated was through natural light or oil lamps since it was the only option. Architecture accomplished this by incorporating large skylights and windows into the spaces, allowing in as much natural light as possible. When electricity was invented near the end of the 19th century, it was a way to change that, but it was too expensive and not a viable option at the time.
So, the solution remained using glass. However, even though glass can be a good insulator, in order to let the light in it had to be so thin that it would lose this capacity. So glass was necessary but was a problem too. As electricity got cheaper and heating or cooling systems were invented, glass started to be less used – its lightning capability was not necessary anymore. But still, people wanted to see the outside and engineers were trying to find a solution.
In 1934, Thermopane developed the first prototype of an insulating glass unit: by doubling the amount of glass, it was possible to keep the light in, while also benefiting from the insulation properties.
Furthermore, installing a cavity with dry air between them proved to be an effective insulation solution [16]. This was one of the factors that changed architecture (see Figure 2-15).
IGU’s can typically go up to three distinctive glass panes, and each one can be composed by laminated glass. It can also contain any type of glass that was previously mentioned.
Figure 2-15: Comparison of buildings before and after IGU’s. On the left, the former Shelton Hotel in New York, 1922-1924 (Arthur Harmon)15. On the right, the Seagram Building in New York, 1956-1957 (Mies var der Rohe)16.
15 Picture from: C. Gray, ‘Mr. Houdini, Your Box Is Ready’, The New Work Times, Mar. 26, 2009.
https://www.nytimes.com/2009/03/29/realestate/29scapes.html (accessed Dec. 11, 2022).
16 Picture from: M. Lamster, ‘A Personal Stamp on the Skyline’, The New York Times, Apr. 03, 2013.
https://www.nytimes.com/2013/04/07/arts/design/building-seagram-phyllis-lamberts-new-architecture-book.html (accessed Dec. 11, 2022).
15 MATERIAL PROPERTIES
The properties of glass are defined in the EN 16612 standard (described in Chapter 2.8), as presented in Table 2-1.
Table 2-1: Mechanical properties of glass.
Properties for glass
Density ρ 2 500 kg/m3
Young’s modulus E 70 GPa
Poisson number μ 0,22
The same standard suggests values of characteristic strengths for the different types of glass presented before – see Table 2-2.
Table 2-2: Characteristic strength of glass types.
Glass type Characteristic
strength [MPa]
Annealed 45
Heat strengthened 70
Thermally toughened 120
CONNECTIONS IN GLASS
Glass design connections are crucial because they contribute significantly to the overall strength and stability of the structure. The connections must be able to transfer loads from one element to the next and resist all structural actions. It also takes a part on the general appearance of the structure: it should be seamless and blend with the elements, creating a cohesive and visually appealing design.
The general approach for dealing with connections between glass and other materials is to avoid direct contact between the two, using softer materials – such as plastic or rubber. This way, the loads or movements are diverted from the glass elements. This also controls the effect of local imperfections, that have an impact on the behaviour of glass [4], [7]. As stated before, normally the stresses are concentrated in the points of connection with other elements, so connection design is also an important part of the process.
Also, during the past years the objective is to maximize transparency when using glass. This is also noticed in connections: from linearly supported glazing (mid 20th century) to the silicone sealants (1970s) and to the bolted supports (1980 and 1990s) – these glass supports are represented in Figure 2-16 [4].
16
Figure 2-16: Common glass supports types17.
The structure that is going to be developed in this work is going to use two methods of connection: the clamp and the bolted one. In Chapter 5.7 these types are going to be explored and explained.
STANDARDS
As the demand for structural elements in glass has risen, some design standards, draft standards, and technical guidelines have already been created. All have the same objective: determine the resistant capacity of a glass element based on its geometrical and environmental conditions, using simple calculations. However, further research and technical work still has to be done, as most of these standards are directed to the analysis of rectangular elements supported at all ends [4].
In Europe, some standards are being used as design guidelines; to avoid an exhaustive overview of all, this subchapter will present the two that are used for this work:
CEN/TS 19100:2021 "Design of glass structures". This technical specification document – the conversion of CEN/TS 19100 into a new Eurocode compatible with the revised 2nd generation Eurocode suite is an ongoing task, and the corresponding EN document is expected to be published by 2025 [17] – is divided into three parts: the first is the basics of design criteria, which presents the requirements for resistance, serviceability, and glass component failure consequences. The second and third part are oriented to the determination of the resistance when an element is under out-of-plane or in-plane loaded glass components, accordingly.
EN 16612:2019 “Glass in building – Determination of the lateral load resistance of glass panes by calculation”. This European standard gives the general guidance for lateral load resistance of linearly supported glazed elements used as infill panels. The standard presented before does not cover the action of cavity pressure variations on Insulating Glass Units.
17 Picture from: Matthias. Haldimann, Andreas. Luible, and Mauro. Overend, Structural Use of Glass. International Association for Bridge and Structural Engineering, 2008.
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DESIGN OF GLASS AND TIMBER STRUCTURES
In Portugal, wood was regularly used as a structural material century but has recently fallen into disuse. On the other hand, glass has been gaining structural capabilities. This chapter presents the main characteristics of both materials, as an introduction to the structure that will be designed under this thesis.
GLASS AS A STRUCTURAL MATERIAL