Structural glass - General concepts
On this first post about the structural glass series we will resume the manufacturing process of structural glass and the main treatments and processes it could be submitted to before reaching the final customer.
First of all we have to define what a structural glass is. A structural glass is no more than any glass that, regardless of its origins, is submitted to any stress distribution. For that reason, almost all glass used on architectural applications should be considered as structural glass, as it will be submitted to many kind of loads such as wind, thermal or any other loads that could appear during its life cycle.
Historically speaking, glass has only been used on building construction. More specifically, glass has been used on the manufacturing of windows that permit the house insulation from the outside, while allowing the sunlight to get inside the room. For many years glass has been calculated by basic rules of thumb based on experience. With the passing of years, the use of bigger glass units of more complex geometries, and the application of higher quality standards, glass design and calculation has become nowadays a complex discipline itself.
To start characterizing the material, we will resume the processes of production and the main properties both physical and chemical. Also, we will explain the main treatments and processes glass can be submitted to nowadays.
Production and Properties
There are many different manufacturing processes that can be taken on glass fabrication but main steps are common to all of them. The manufacturing of glass units starts with the melting process of glass components at a temperature between 1600 and 1800 ºC. After that, conforming of glass takes place at a temperature between 800 and 1600 ºC. Finally, temperatures keeps lowing down to the cooling phase that takes place between 100 and 800 ºC.
From all available processes, the float glass is the most widely spread for the manufacturing of flat glass (About 90% of plane glass manufactured globally is obtained with this method). This is too the most used process on the production of glass for façades, inner glazings or automotive applications.
On a very schematic way, float glass manufacturing process can be resumed as:
1. Melting of glass components is produced on an oven.
2. Melted glass is poured over a melted tin pool
3. Due to differential densities melted glass floats over tin, forming a layer of uniform thickness
4. Glass is progressively rolled out the tin pool to the processing zone, where a set of rollers control its thickness, finishing after that with the annealing process
On the following link there is a Pilkington video illustrating the float glass process. Pilkington is one of the main glass manufacturers in the world and the first company to commercially use this process to obtain glass.
Regarding its composition, the SLSG glass (Soda lime silicate glass) is the most used on the construction industry. It is composed mainly by silica, sand and soda. For some special applications, such as glass with special thermal resistance or fire protection different compositions are used, as the BSG (Borosilicate glass).
On the contrary than in most solid compounds, electrons on glass are confined on fixed energy levels, this prevents the molecules to alternate between different energy levels, dissipating radiation. This way it is possible to energy to pass through glass without opposition. On real glass units, energy transference is not perfect due to imperfections on glass composition caused by inclusions of little amounts of iron oxides as Fe2+, which are the ones causing the characteristic green colour on SLSG glasses. Extra-clear glasses, also known as low iron, are named that way precisely for eliminating this coloration through the reduction of those iron oxides present on the glass composition.
One of the most valuable glass properties is its excellent resistance to chemical aggressive substances. This is one of the main reasons causing its wide spreading through chemical industry and the fact of being one of the most durable materials used on construction industry.
Regarding its structural behaviour, the following physical properties should be highlighted:
Density: 2500 kg/m3
Elasticity modulus: 70000 MPa
Glass density and rigidity properties are close to the aluminium ones, widely used material on construction, especially on secondary structures.
About glass resistance, taking into account the fact that it is a material with a fragile breakage mechanism, breaking of glass will be enormously linked to the presence of cracks on the element. This will cause that, regardless its theoretical tensile resistance could reach 32 GPa, this value should be neglected for its structural uses, as the breakage of glass will be produced at a much lower stress value. As an example, on the last draft of the Eurocode for glass on construction, for its bending calculation, a characteristic value of 45 MPa is being used for tensile resistance on annealed glass.
Once glass is obtained from its raw materials, it is common to take some additional processes to achieve the desired shape, capabilities and appearance on the glass units. We will explain on the following paragraphs the ones affecting the structural behaviour of glass, taking into account that so many more processes can be taken before glass unit reach the final customer.
In this post we will talk about thermal treatments on glass, its main types and how each one affects the final product.
The tempering process is the most crucial treatment for structural applications due to the fact that it is the one that most affects the glass resistance. The tempering process seeks the optimization of the remnant stress field, generated by the differential cooling of the glass parts during the manufacturing process, aiming to compensate the stresses the glass unit will be submitted to along its life time. After tempering process the outer glass surfaces remain in compression, while the inner parts closer to the mid plane remain in tension. This stress distribution not only compensates tractions on the outer faces when glass is under bending efforts, it also keeps compressed cracked zones (generated on glass surfaces during manufacturing process), improving significantly its structural behaviour.
There are two main types of thermal treatments, fully tempered glass and heat strengthened glass, differentiating the process of both treatments in the cooling speed. Despite the similarity of both treatments, effects produced on glass are substantially different.
Process consists on the heating of glass to a temperature between 620 and 675 ºC, slightly above the glass transition temperature, and a fast cooling through cold air streams.
Residual compressive stresses reached through this procedure vary between 80 MPa and 170 MPa for SLSG glass.
As a result of this treatment glass resistance is maximized and therefore the intern energy absorbed by glass before breakage rises too. Such high absorbed energy amount provokes tempered glass characteristic breaking pattern in very small fragments, as it is necessary a high amount of crack to liberate all the accumulated energy.
One of the biggest problems derived from this tempering process is the possibility of spontaneous breakage on glass units. This is a known phenomenon, provoked by nickel sulfide inclusions during manufacturing process. This inclusions, when heated (at not necessarily very high temperatures), start vibrating and creating punctual stress point that may lead to glass breakage. One of the ways to avoid this problems before glass reaches building site, is to take over each glazing unit the process known as HST (Heat Soak Test). This procedure lowers the number of spontaneous breakage of mounted glasses by forcing the breakage of the defective units prior to its installation, during test.
Heat strengthened glass
This process is very similar to full tempering, but cooling is made on a slower procedure. This leads to lower residual stresses.
For a heat strengthened SLGS glass, residual compressive stresses vary between 40 and 80 MPa.
The need of a controlled cooling process makes unable (generally) to heatstrength glasses above 12 mm thickness.
Breakage, on the contrary than fully tempered glass, is produced in much bigger fragments. This kind of breakage will make heat strengthened glass much safer for many applications (as will be commented on next posts).
We hope to see you in the next post, in which we will be talking about the design, verification and details of structural glass elements.