Structural glass - Connections
Once summarized fracture mechanics on the last post, we now will go through a short brief of main considerations regarding connections on glass structures. This is a crucial topic as, same as classic structures, joints and connections are usual points where structural failure starts when not designed or executed properly.
Traditionally, when assessing unions between glass and other elements, basic criteria is to avoid any contact between glass and more rigid materials as this can cause stress concentration points leading to glass breakage. Although this is still mandatory nowadays, progressive advances on the last years had made available a wide range of choices for glass connections design.
Architectural design progressively tends to more light and transparent structures. Due to that the use of structural glass elements has grown increasing the research and development of new connection solutions more efficient, light and increasing overall structural transparency. Advances on this matter tend to the use of smaller and stronger connection elements.
On each point where glass is at risk of contacting more rigid elements, intermediate materials should be placed, allowing the redistribution of concentrated stresses to avoid glass breakage. Those materials usually have a lower rigidity than glass, but with enough strength and durability to ensure proper transfer of loads acting on glass and the supporting structure. Usual materials are plastic, resins, neoprene, injection mortars, aluminium or gaskets.
Nowadays glued connections are more commonly used every time and many advances are taking place on chemical compounds used for them. This technology offers new solutions that will not be possible using mechanical fixings. Main problem regarding glued connections is usually its durability. Furthermore, in many cases an additional mechanical fixing is needed to ensure proper security levels.
Glass lineal supports
Lineal supports are commonly used when glass pane are installed on perimetral profiles, as in curtain walls, being supported on 2 or 4 sides. Generally this kind of support is used to transmit loads acting perpendicular to glass pane. Usually in curtain wall systems self-weight load of glass panes is transmitted to horizontal profiles through plastic setting blocks or neoprene layers. For transmitting loads acting perpendicular to glass pane glass is fixed to substructure profiles on its perimeter through linear pressure plates (metallic profiles that tighten glass against mullions and transoms). To avoid contact between metal elements and glass gaskets are disposed in between them.
On linearly supported systems over profiles is necessary for separation between profiles to be greater than glass pane dimensions. This way we leave a certain gap between glass edges and profiles, allowing for some possible mistakes on manufacturing process and mounting on the profiles grid and avoiding contacts between glass and profiles as long as this misalignments and errors keep under construction tolerances set prior to manufacture.
For calculation purposes, this kind of connections can be modelized as simple supports along glass perimeter as they allow for glass rotations and in-plane displacements.
As an alternative for the use of pressure plates, linear supports can be made through the use of adhesive connections. This kind of connections will be discussed on following sections.
A much more uncommon application for linear supports is the transmittance of in-plane loads. This kind of solutions require more complex calculations (as instability phenomena could take place) and careful design due to stress concentration points generated on glass edges.
Clamped connections are developed to minimize visual impact of linear supports, but its mechanical behaviour is similar. Glass clamping is made through punctual elements, generally made from aluminium, that press glass against supporting structure. This supporting structure could even be another glass elements, as reinforcement fins on glass structures.
There are two kind of clamped connections, frictionless clamps and friction clamps. Frictionless clamps act only against loads perpendicular to glass pane, not being able to transmit those acting parallel to that glass pane.
On friction clamps mechanical behaviour is consistently different. This kind of clamps are designed to address both perpendicular and in-plane loads. Typical example of this kind of connections takes place usually on glass fins, where two metal plates are joint through bolts, pressuring two glass panes in between them achieving this way certain continuity of glass element.
For friction clamps, as they act pressuring glass, special care has to be taken with PVB interlayers as they suffer high creeping, which could lead to a progressive drop on clamp pressure as PVB creeps out from the connection area. This drop on the clamp pressure could lead to a friction loss, causing the inability from the connection to transmit in-plane loads and therefore failure of the entire connection. To avoid this problem PVB is usually replaced by aluminium plates during glass manufacturing process on clamping areas.
When calculation models for glasses with this kind of connections are made it is very important to take into account that usually clamped connections do not allow for glass pane rotations. As a result of this attachment high stress concentrations could take place around clamping points, which should be taken into account when assessing strength of the whole system.
Bolted connections are not the most efficient way to support loads from a brittle material as glass, but due to aesthetic parameters is a widespread solution. There are many types of bolted connections but some recommendations and principles are common to all of them. On the following sections we will summarize main particularities of the most important types.
Main problem associated with this kind of connections is the appearance of stress concentration point around glass holes. For ductile materials such as steel, stress concentrations are redistributed through the materials by means of its yielding behaviour. In glass elements, due to their molecular structure, this yielding does not take place, therefore stress redistribution is not possible. This implies that it is not advisable to consider a uniform distribution of loads for different bolts in a glass connection. For this reason real challenge during design phases of this kind of connections would be to avoid this stress concentration points and contacts between glass and metallic elements.
For this reason real challenge during design phases of this kind of connections would be to avoid this stress concentration points and contacts between glass and metallic elements.
Contact between glass and metallic elements is avoided by disposing between them materials with the adequate mechanical properties, which means materials stiff and strong enough to resist forces acting on the connection but not too much in order to avoid stress concentration points on glass. It is also important for this materials to show a good behaviour against creep phenomena, fatigue or UV radiation. Some of the most used materials for this applications are EPDM, POM o Polyamides.
It is important to take into account that this materials help to redistribute compression stresses on glass holes due to contact between connection parts, but they do not help with tractions stresses provoked for the hole elongation.
- Through bolt connections
This connections are made drilling glass elements (prior to thermal treatments) and putting a bolt through them, allowing to transfer efforts between independent glass elements.
It was the first bolted connection used for glass structures and its use comes directly from the steel and timber industry. It is a fixation system in which in-plane acting loads are transferred through shear effort on the bolt. This kind of connections are very usual on glass structures due to dimension limitations on glass manufacturing processes.
- Point supports
Punctual supports are essentially bolted connections used on glass to glass or glass to substructure connections where there is no need of elements overlapping.
This kind of connections is not used to transmit greater in-plane loads that glass pane self-weight, as for this kind of loads through bolt connections are a much better option. Therefore, point supports are used mainly to transmit loads acting perpendicular to glass pane.
Originally, rigid systems where used in this kind of connections, leading to the same problems we stated on clamped ones, rising stress concentrations due to lack of rotation capacity. Modern systems on the contrary allow rotation of the glass when necessary, improving mechanical behaviour and allowing the use of thinner glass elements.
Progressive tend to reduce visual impact on structures has led to progressive increase on studies of glued connections, as they favour suppression of mechanical components, lightening connection elements and increasing transparency on glass structures.
Adhesive technology is usually unfamiliar for most engineers, even more fore civil engineers, but when speaking about glass structures adhesive bonds and connections are a widespread technology. Comparing with mechanical fixings, glued connections offer the opportunity to lower stress concentrations due to their flexibility, which will be a very helpful property due to glass brittle behaviour. Although nowadays adhesive technology is a field where many investigation is still in need, we have vast experience applying some of their particular solutions on both building and civil engineering fields.
The bond of elements forming glued connections takes place thanks to the chemical reactions between adhesives and element surfaces (glass, steel, aluminium, etc.). Therefore strong atomic bridges between the adhesive and the glass surface result in a strong adhesive joint. The glass surface consists of silicon atoms saturated with OH-groups and some metal ionic (i.e. Na) and it is therefore desirable to establish strong Si-O-Si bonds between the glass surface and the adhesive. This may be achieved by applying a silanized primer to the surface and an adhesive with a compatible molecular structure. The primer has the dual function of providing a reactive group for the glass surface in addition to a reactive group for the adhesive.
- Main adhesive types
Adhesive macromolecules will be composed by simple monomer units recurrently chained. Atoms of the molecule will be chemically bonded while each molecule is physically or chemically bonded to another one, forming a polymer. Regarding their thermomechanical properties, which will be defined by their molecular structure, we will find three different adhesive types.
Molecules on thermoplastics are weakly bonded, therefore it will be a material very sensible to temperature changes. This way thermoplastics will soften when heated, recovering their properties once the material cools. This process can be repeated softening and solidifying the material alternatively.
Elastomers are rubber like polymers, which macromolecules are cross-linked with a low density of cross-links. This composition allows elastomers to stretch many times their original length when stressed, returning to their original shape when stress disappears.
Thermosets permanently solidify when heated, this is an irreversible process. They consist of a three-dimensional polymer network with a high density of cross-links between them. This makes thermosets a very rigid compound due to the restriction of movements between macromolecules.
- Mechanical behaviour of glued connections
Under external forces glued connections show three different types of deformations, that take place superimposed, the ratio of each one will depend on molecular structure of the adhesive material. For low cross-linked polymers two last modes will be more important than the first one.
· Spontaneous elastic deformation due to changed valence bond angles of atoms in chemical bonding. This is a spontaneous and reversible deformation.
· Viscoelastic deformation due to stretched molecular chains. This is a time dependent and reversible deformation.
· Viscoplastic deformation due to molecular chain movements. This is a time dependent non reversible deformation.
When subjected to shear deformations, glued connections behaviour will be time-dependent. For short-term loads adhesive materials will behave as usual construction materials, where the shear deformation is proportional to their shear modulus. For long term loads adhesive materials will commonly show creep behaviour provoked on a first step by stretching of molecular chains (primary creeping), then by sliding of those molecular chains balancing lost and gained molecular connections (secondary creeping), and finally causing bond breakage when molecule connections lost exceed the gained ones (tertiary creeping).
Glued connections can be classified regarding its mechanical behaviour between flexible connections and rigid connections. On flexible connections usually structural silicone bonds together the different parts of the joint and its elastic and flexible behaviour will allow certain relative movements between those parts. On the contrary, on rigid connections mainly acrylic or epoxy adhesives or polyester resins closely bond connection parts, being more restrictive regarding movements on it, favouring that way the appearance of stress concentration points.
Flexible glued connections
Flexible connections are usually materialized through silicone bonding between glass and metal elements replacing mechanical connections. There are two types of structural silicone sealants available.
- One-component silicone
One component silicone cure with interaction with moisture in the air, therefore its curing starts as soon as it gets in contact with ambient. To guarantee correct curing a minimum of a 50% humidity is needed and some geometrical principles must be followed. Thickness should no be under 6 mm and width should be kept below 20 mm, this way we ensure humidity will be diffused through all silicone and curing takes place on the entire volume. Curing process could last up to 3 weeks, if seal is too thick or too large some parts could never cure completely.
- Two-component silicone
Two-component silicones cure thanks to a polymerization reaction triggered by the mixing of two different components. The first of it is a base compound (90% of the final mix volume) and the second one is a catalyst (10% of the final mix volume), no outside chemical components are needed for the polymerization to take place. Two-component silicone has relatively fast curing times (around 3 days) comparing with one-compound and it is not limited by seal geometry. Nevertheless this geometry should be kept above 6 mm thick and below 50 mm width due to mechanical limitations. Proper mixing is very important for the two-component silicones to achieve proper mechanical properties when cured, therefore its application should be strictly checked making difficult to correctly apply this solution on site.
Structural silicone joints are normally designed in terms of allowable stresses, which are in turn based on the ultimate strength and a safety factor of 6 for short-term loads and that of 60 for long-term loads. The allowable strain range under shear deformation due to differential thermal dilatation is 12.5% of the ultimate strains. At this relatively low level of strains, it is sensible to assume elastic behaviour.
Silicone has a very low modulus of elasticity allowing to reduce stress concentrations. This also allows silicone to absorb high amounts of energy, showing a very good behaviour in case of protective glazing when combined with laminated safety glass. On the other hand, due to this low modulus of elasticity, structural silicone sealants are not suitable to transfer high shear forces required for built-up sections of glass. Due to this lack of shear force transmission capacity joints where silicone is attached to three or more faces should be avoided.
It is very important when designing silicone connections to check chemical compatibility between silicone and the other compounds of the connections, as chemical interactions or migrations may take place varying their properties and possibly causing damage to the connection. Usually manufacturers held compatibility tests between their silicone compounds and usual materials they get in contact with (EPDM, neoprene, PVB, etc.).
When assessing structural integrity, the optimum result is to reach loss of cohesion inside silicone seal rather than adhesion failure. It is also very important, when designing a silicone connection to consult their adhesion compatibility to involved surfaces with the manufacturer.
In Europe, the application of silicone structural glass sealant (SSGS) are regulated. A European Technical Approval (ETA) for any silicone used in structural applications is needed. The design method given in EOTA 1998 is limited to four-side supported glass (SSGS glued or mechanical fixed) panels with a linear SSGS joint over the entire glass edge. Furthermore, EOTA requires all SSGS connections to be made in the factory rather than on site because proper execution of an SSGS joint requires a controlled climate and clean surroundings. However, in all-glass structures some SSGS joints may have to be applied on-site. Therefore this will require special measures to ensure a proper environment and even more stringent quality assurance procedures. Even so, it should be noted that the structural quality of an SSGS joint cannot be tested non-destructively.
Rigid glued connections
As stated before, glass elements architecture tends everyday to lighter and more transparent structures. Glued connections favour suppressing mechanical components. Traditionally, silicone is used for glued connections, but its lack of strength and stiffness has led to an increasing investigation on rigid adhesive connections. Epoxies and acrylics had been used in other industries for a long time and are promising options for glass rigid connections, but its transfer to civil engineering and building industries is still in development as it carries hard challenges to adapt the technology to the particular needs of them.
One of those challenges is related to the distortions caused on glass flatness during their manufacturing process (e.g. heat treatments or preparation of laminar glass). Many rigid adhesives work as contact adhesives, which means that for properly working require a small adhesive thickness (below 1 mm) and, if manufacturing tolerances are above this level they will not work properly. For this applications, gap-filling adhesives are much better options.
Resistance to UV radiations is another challenging topic. For this applications, UV-curing acrylics are the most promising option, as once cured they are transparent and UV resistant.
Regarding its temperature related behaviour, common rigid adhesives tend to progressively lose stiffness, strength and adhesion capacity due to movements on their macromolecule chains if they are heated above their glass transition temperatures. Epoxies exhibit the best behaviour regarding temperatures, as their glass temperatures are higher than acrylic ones.
Another aspect to be considered is the lack of information about durability in this kind of connections. We know from the experience in other fields such as metallic connections that rigid adhesives are affected by the environment, mainly by water, but we do not know yet how aging affects rigid connections on glass structures.
Last consideration that has to be taken into account is the fact that rigid connections geometry has to be designed very carefully. Due to their limited capacity to redistribute stresses concentrations or to absorb deformations, sharp corners or geometrical singularities should be avoided generally by rounding their edges.
State of the art
Due to the fact that application of glass to complex systems as the ones stated before is a relatively new technology we had not yet explored all the possibilities of it as a construction material. Generally, we tend to adapt developed systems we know from other disciplines. With glass we use the same strategy we used when adapting steel for construction industry, where on its first steps steel structures adapted to the well know shapes of fabric structures (e.g. Ironbridge over Severn river).
Despite this, as the use of glass as a construction material becomes more popular, it is necessary to develop advances that improve its behaviour and increase the possibilities it offers. Regarding connections systems this development aims for strength improvements, safety redundancy, lower visual impact, easier assembling processes, etc.
On the following sections we will summarize some of the latest innovations although nowadays many improvements on glass connections systems and methodology are still under development phases.
Post-breakage behaviour improvement with fabric embeds
The use of embedded fabrics on PVB interlayers increases enormously post-breakage structural strength of glasses. This embedded layer are fixed to supporting structure so, when glass breakage is achieved, the fall of the glass pane from the structure is avoided as it will still hang from this fabric layer. This increase on security for laminated glass is especially interesting when designing walkable glass elements or overhead glazing.
Embedded supports on laminated glass
With the development of new interlayer as DuPont SentryGlass, a much more rigid and strong interlayer than PVB, possibilities regarding constructive solutions for glass raised drastically.
Thanks to that new kind of interlayer it is possible to embed support elements into the interlayer itself during glass manufacturing process. This allows to design connections which will not be possible to achieve with traditional blot connections, as it simplifies assemblage processes turning it close to prefabrication systems.
Due to the high strength of systems as SentryGlass structures as the following one can be made, where connections are barely visible and totally integrated on glass elements.
We hope you enjoyed this structural glass series, a discipline still in development with many potential and very interesting applications!