Structural glass - Actions over the elements
On the second post of our structural glass series we summarized the design particularities of glass structures. Once this little brief is done, it allows us to dive deeper into the analysis of loads acting on glass structures and its particularities regarding glass behaviour against those loads.
Through this post we will try to summarize the different load types acting during the lifetime of a structure composed totally or partially by glass. The aim of this post is to analyse the characteristics of loads regarding glass structures, which particularities will make them quite different of the ones we are used to handle for calculation of steel or concrete structures.
Importance of glass cracks and flaws had been commented on previous posts. This importance turns essential the careful analysis of every action that could cause these cracks (no matter its size) on glass. This is one of the reasons why the entire stress history of the glass panes from its manufacture should be taken into account. This time history will be the most precise way to study glass behaviour and assess its strength. This states a very different approach than the one based on limit values usually taken in common construction materials. Nevertheless, actual standards and regulations only assume this las approach, giving limit strength values regardless the stress history on the glass units. If intrinsic strength of glass is neglected and only residual stresses produced by thermal treatments are considered, this approach through limit strength values is precise enough, while it simplifies enormously the calculation process.
We will summarize the phenomenon associated which each load, assessing the ways to analyse and evaluate its behaviour and how it affects to the structure. If at any time some detail is referred in terms of regulations, we will include the reference to the corresponding document, but each regulation has its particularities and its analysis is beyond the scope of this post.
Wind loads
The study of wind load action over glass elements is conducted in the same way than usual method on structure calculation. Wind action over the structure is usually characterized through two parameters, mean wind velocity and turbulence intensity. Calculation using those parameters fits most cases. On the contrary, when we are studying structures of great slenderness this assumption may not be accurate enough. If natural frequency of the structure is below 1Hz, resonance phenomenon may occur and a detailed study of interaction between wind flux and structure movements must be taken to ensure proper behaviour of the element.
Nowadays verification methods included on standards commonly rely on formulation obtained through probabilistic methods applied to the study of the phenomenon. This approach is not valid in every case, as an example, vortex and turbulences generated on element edges do not follow a normal distribution, therefore probabilistic methods should not be applied. For these cases structural behaviour of elements under wind action break away from standards. Therefore, wind tunnel tests or CFD (computational fluid dynamic) models that numerically model the interaction between fluid and structure may be applied. CFD technics are still developing, as it is a relatively new discipline and its use is progressively rising, as it requires a considerable amount of computational capacity for the resolution of complex models.
Wind load and temperature correlation
On glass structures correlation between wind load and temperature is especially important due to the common use of laminated glass. PVB, as the majority of the compounds used for the fabrication of laminated glass, is a viscoelastic material. This kind of behaviour turns many of its rheological properties time, temperature or load-dependent. Polymers used for the attaching of the glass panes experiment creep when submitted to long duration loads and its properties usually decrease when temperature rises. Therefore, all these conditions will have to be considered during verification of structural laminated glass elements. For this reason and as a simplification, shear transfer between glass panes is usually neglected when considering long lasting loads.
Nevertheless, when short duration loads are considered, shear transfer between laminated glass panes depends directly from the viscoelastic properties of the polymer interlayer bonding them. Considering the max wind load for a return period (T) acting simultaneously with the highest temperature for that same period will lead to a very unlikely situation and an over dimensioning of all the elements. Is for that reason that we use graphs as the following ones, introducing a correlation between the maximum wind load (and its duration) and the most likely temperature associated.
These graphs have been obtained based on Continental Europe data records and should be used in those countries. Different studies state that a G (shear module) value of 0,4 MPa should be taken for the maximum wind load analysis.
Seismic load and movements
Nowadays glass behaviour on extreme situations such as hurricanes and earthquakes is still being studied and, regardless the consequences of such situations on glass structures, there is not many bibliography regarding this. Generally, annealed and laminar thermally toughened glass has been stated to have a better behaviour through these events. It is also remarkable the fact that glass attached to the structure through adhesive bonding behaves substantially better than other fixation methods.
Impact loads
Every glazing unit placed anywhere where impact may occur should be designed to resist dynamic human impacts (e.g. glazing balustrades, glass doors or wall elements). There are many approaches to the analysing process of dynamic impacts on glass. As a first approach, a static load may be applied on the glass, it's magnitude will vary depending on the considered standard, but it will be about 1.5 kN at railing height (between 1 and 1.2 m). For non-standard glass elements (e.g. point supported balustrades) and load-bearing partitions (e.g. railings and balustrades, glass in curtain walls, etc.) a dynamic test should be held. For vertical glazing the tests are known as the soft body impact test (Defined on EN 13049). Hard body impact test may be also mandatory for overhead glazings. Prior to the test, a dynamic analysis through a finite element model (FEM) of the studied glazing could be made to analyse its behaviour and dimensioning to ensure its proper design.
Blast loads
Glass fragments are the primary source of injury in urban explosive events. Therefore, primary purpose of glazing protection is to minimize the number of injuries caused by fragments propelled from glazed openings when glass is subjected to blast. Secondary aims of glazing protection are to minimize damage to equipment and to allow re-occupation of the building within the shortest period of time.
When an explosion occurs, high pressures expand rapidly into the surrounding medium forming a shock wave of compressed air travelling radially away from the bust point, gradually reducing its peak pressure but increasing duration of its effects. This shock wave will cause an almost instantaneous increase of pressures, with a gradually decay after peak, usually followed by a negative pressure effect or suction, with a much lower peak than the positive one and a larger duration. Intensity of this shock wave will decay with the distance to the blast point, while its duration increases. It is unusual for this shock wave to cause strong structural damage, but it often causes widespread damage to light cladding and glazing.
The shock wave phenomena act parallelly with the so called "blast wind", which is caused by the movement of air molecules causing dynamic pressure associated to the blast. However, on unconfined explosions effects of shock wave are vastly larger than the ones associated to blast wind. Ground surface and surroundings configuration could amplify the blast (often referred as reflected pressure), and therefore they will be very influential on the design blast load calculation.
Blast loading evaluation should start with a risk analysis of the probability of the blast explosion and the analysis of its consequences. In general, defining an explosion scenario is not easy, as it depends on the type of explosion, size of explosive artefact and its location. Some countries provide guidelines and professional advice on threat assessment.
On glass design regarding blast loads two different approaches exist. The first consist on assuming a "no break" scenario, where glass panes are designed to resist blast load. This usually results in too thick and expensive glass panes that should be mounted in stiff and heavy frames. This approach is a non-optimized way, and should only be taken when its mandatory. The second and more usual approach tries to reach a safe glass breakage. This could be achieved through the use of anti-shatter films, bomb blast net curtains or the combination of tempered laminated safety glasses with PVB interlayers of the proper thickness with robust frames. On this approach, blast energy will be absorbed through glass breakage and interlayer viscoelastic deformation. Actuation methods of each solution stated are recorded on this video.
For calculation of blast loads on glazing, unfortunately, there is no software available able to simulate the behaviour of broken laminated glass. Consequently, prototype testing is the usual approach. The aim is to assess the hazard consequences by measuring the distance that the glass fragments are projected into a test cubicle where studied glazing is installed when whether an explosive charge or a shock tube impacts the glazing unit.
Internal pressure loads on insulating glass units
Differential pressure between the enclosed chamber of an insulating glass unit and the environment may cause important loads and deformation on glass panes. These differential pressures may be caused by the difference on the altitude between glass fabrication place and site of installation, increments or decrements of temperature or variations on the atmospheric pressure. Those pressures may lead to high stresses when glass is too thick, too small (big rigidity) or too enclosed, avoiding deformation. Curved IGUs are much stiffer than plane ones, and therefore stresses caused by internal pressure increase.
Furthermore, not only stresses caused by internal pressures may be considered, as in too deformable glass panes (usually flat glazing units, due to their size or thickness) deformation may cause visual distortions. This is the reason why, usually, thicker glass panes are placed on the outer side of IGUs while narrower ones are placed on the inner side, so deformation takes place towards the inner side of glass, avoiding distortion of reflexions on building façades.
Thermal stress
Thermal stresses on glass are usually associated with a temperature gradient acting across the glass surface. The source of this heating energy may be either the sun or a heating device placed close to glass surface.
In case of sun as source of energy, as we explained in the first post of our structural glass series, a part of the incident energy on glazing units will be absorbed due to its imperfections, increasing its temperature. Therefore, on framed or under partial shading glass, only unshaded areas will suffer this rise of temperatures.
When the source of energy is a heating device, differential temperature gradient will be generated between the hotter regions, close to the energy generation point and the cooler ones, at a bigger distance from the generation point.
Hotter regions of glass will start expanding, while cooler regions close to them will keep their size, which will generate a tensile stress field acting over cooler regions. If differential temperature between hotter and cooler regions is high enough, the generated tensile stresses will cause breakage of the glass. Generally speaking, risk of thermal breakage is much higher for annealed glass than for heat strengthened glass or fully tempered glass.
The strength of glass against thermal stress failure is usually given as an allowable maximum temperature difference. If the calculated temperature difference is less than the allowable temperature difference the panel is thermally safe. There are several existing calculation methods. As an approximation, allowable temperature differences for different glass types and edge qualities will be around the following:
Surface damage
Usually, for non-structural glass elements or for structural elements with its surfaces protected, the amount of acceptable surface damage is controlled by optical acceptance levels.
Nevertheless, many structural glass elements may be exposed to accidental impact, vandalism, heavy wind-borne debris or other factors resulting in surface flaws that could be substantially deeper that the ones caused by production and handling.
At the instant of damaging the glass surface, the glass is subjected to an elastic stress intensity. If this stress intensity exceeds the fracture toughness, instantaneous failure will occur. Predicting the crack path or fracture pattern in glass is a complex issue involving dynamic fracture mechanics, this problem will be explained in future posts. If the instantaneous stress intensity is less than the fracture toughness, some local surface damage may still occur. This damage reduces the strength of the glass element significantly.
However, due to the current lack of information on how the qualitative assessment of the potential damage may be translated into quantitative design values usually requires for specific testing and a considerable amount of engineering judgement.
In the next post we will be talking about the particularities of structural calculations of glass elements.