Structural glass - Design concepts
After summarizing the main properties of glass regarding its structural behaviour on the first post of our structural glass series, we will now do a little brief of the design particularities of structural glass elements.
The structural glass elements design is done usually in three sequential steps taken on an iterative loop until the optimal design is achieved. First of all a predesign step is done considering a few simplifications to obtain an approximate dimensioning of the element. The second step consists on a detailed analysis through analytical methods that will define the element design. After them a test should be made on a glass element sample to fully verify the design before its implementation. The requirements of this tests vary depending on the glass elements and the standards applied.
As in any other structure, we need to know the requirements the structure has to stand to before starting the design process. The usual way to stablish this requirements is through the definition of the so called limit states. This includes the ultimate limit states (ULS) for the strength of the structure, the stability and the equilibrium checking. In addition to that, additional states should be considered, as safe breakage checking or the study of the post-breaking behaviour. On the contrary, service limit states (SLS) remain the same as traditional structures. We will summarize in this post the main considerations that should be taken into account during the definition of this design situations, including the affection on them of the different glass element configurations.
It is crucial when designing glass structures to have in mind that glass, on the contrary than other constructions materials, has a brittle behaviour. This means that, when breaking, glass is not able to redistribute stresses by the mean of local yielding. This implies that the whole design and detailing process has to be taken very carefully and understanding properly the behaviour of any material involved, as any contact between glass and any other more stiff material as steel or aluminium will certainly cause glass breakage. This is the reason why analytical models for this kind of structures are very complex (joint modelling, glazing supports, interaction between elements, etc.)
Risk analysis
It is crucial for this kind of elements to perform a proper risk analysis and to state a suitable strategy to manage them. We have to take into account that, generally, it is impossible to design a structure to stand any imaginable threat. We will then try to design the structure with enough capacity to develop an acceptable behaviour under most likely threats.
There are two main threats to a structural element: to undertake loads not considered during design, or being badly planned or built. This situations will imply a threat to the structural integrity of the element. To stand this, different strategies can be assumed:
The structure can be designed to withstand the threat
Measures to reduce the likelihood of the threat can be taken
The threat can be accepted as an unmitigated risk
If the structure is designed to withstand particular threats, it is mandatory to precisely define each threat considered. Based on that particular threats a set of accidental situations will be obtained. Those situations will be taken into account through the design process together with the common actions. This approach is generally unsuitable for most glass structures as it would entail a ‘no-break’ scenario that often results in very thick glass elements and visually obtrusive sub-frames and connections.
The most recommendable strategy will be the one based on taking measures to reduce the likelihood of the threats considered. This will simplify the complexity of the different elements, lowering the amount of material used on the structure. In Neikō we are strongly convinced that the most important aim on the construction industry nowadays should be to decrease the carbon footprint associated to the construction of a structure, and therefore this strategies are the ones contributing the most to achieve this goal. The most safe and productive way of adopting this strategies should be to integrate them from the very beginning of the design process of a structure.
For the last, there is the option to admit the possibility of a structural failure. This can be suitable if the probability of occurrence of the studied threat is low enough, if the consequences of the structural failure are harmless enough, or any combination of these variables regarded suitable. This risk analysis should be made systematically and under the requirements of the appropriate standards for each case.
Typical glass products
Usually glass in construction is not used as simple glass sheets as a result of many undesired behaviour which we will speak about later on. To achieve the desired behaviour of glass components many different compositions are available for glass products. We will here do a little brief of the main compositions used on the construction industry.
Monolithic glass
Monolithic glass is composed by a single glass sheet.
Laminated glass
Laminated glass is composed by 2 or more glass sheets, bonded together through one or more transparent layers to make a unitized element. The material composing the bonding layer is usually PVB (polyvinyl butyral), but there are so many different options as DuPont SentryGlass, or less common alternatives as etal-vinyl acetate (EVA) or synthetic resins. Despite it is commonly true, not every laminated glass is regarded as a security glass, it will only be classified as so the ones which, in case of breakage, the interlayer (element bonding glass layers) will allow to retain the glass fragments lowering the risks of producing wounds by cutting or penetration as EN ISO 12543 states. This will be summarized on the need of an, at least, 3B classification on the pendulum test defined on EN 12600.
Insulating glass units (IGU)
Those are glazing composed by two or more glass panes, monolithic or laminated, separated by spacers, leaving among them a sealed chamber filled with air or gas. Its main application is to improve the thermal behaviour of glass.
Post-breakage behaviour
During the risk analysis associated to every glass structure, the post-breakage behaviour of each one of the compositions explained previously will be crucial. As we stated many times, glass brittleness makes its breakage unpredictable and therefore substantially dangerous as it does not permit to react to its occurrence allowing to avoid the possible damage. Because of that it is necessary to ensure that when structural failure occurs on glass elements, this keeps integrity enough to guarantee no serious damage allowing to replace it safely
A proper post-breakage behaviour can be achieved both regarding the single element as for the whole structure. For the single element, it is achieved by ensuring that the element fails only partially or in a ductile way. For the whole structure a redundancy based design should be taken to allow load redistribution.
Redundancy and redistribution strategies aim to avoid the whole structure failure caused by the failure of one of its elements, but sometimes this is not enough, as the failure of a single glazing unit may lead to unacceptable consequences. On walkable glass or glass railings the failure of a single element could have terrible consequences if a certain residual integrity is not guaranteed after breakage.
Glass itself has no ductility, as a result of that it is necessary to achieve it through other means. Laminated glass with PVB interlayer usually ensures a good post-breakage behaviour, acting somewhat like reinforced concrete. Broken glass, unable to transmit tensile stresses holds the compression stresses, while PVB layer transmit tensile ones. Glass will then hold a good post-breakage behaviour as long as external forces are in equilibrium with this internal mechanism. It understandable then to reach the conclusion that a PVB laminated glass always ensures an acceptable security level, but it is not enough for every laminated glass compositions or every use of glazing units.
As an example, a laminated glass with two fully tempered glass layers bonded with a PVB interlayer is not capable to maintain the necessary integrity after breakage. This is due to the fact that fully tempered layer breaks in very tiny pieces, incapable of transmitting compressions once broken, which makes impossible to reach the "lock" effect, fundamental to ensure stability after breakage. This behaviour is usually known as wet towel effect. To avoid this behaviour it would be enough to heat strengthen one of the glass layers instead of tempering it, ensuring the rigidity of the compound once broken. We could also apply a stiffer bonding interlayer such as SentryGlass.
Regarding the whole structure, proper post-breakage behaviour is obtained by means of redundancy load path mechanisms. It should be then taken into account that a redistribution of the loads acting on the structure as a result of a failure of one of its components lead to a increase of the load acting over the rest of the elements. This additional load should be considered during design stages to avoid a cascade failure that will lead to the total or partial failure of a considerable section of the structure.
To illustrate the post-breakage behaviour of glass elements we provide you with the following video, where the behaviour of 3 different glass compositions against an impact load is compared.
In the next post we will be talking about the loads acting on structural glass elements and the calculation process considerations.