Stick systems
Stick systems comprise individual vertical and horizontal spanning members (’stick’) known as mullions and transoms respectively. A typical system will be connected to individual floor slabs, with large glass panes providing a view to the outside and opaque spandrel panels installed to hide the structural frame, see Figure 1.
Mullions and transoms are generally manufactured from extruded aluminium sections, which can be provided in a variety of cross-sectional sizes, colours and finishes. These are connected using angles, cleats, a simple locating pin or bolted connections. A variety of sections and connections are available with different load capacities to create the required design. The dimensions of the sections are governed by the horizontal span between mullions, the height between floor slabs of the building, the environmental loads i.e. wind, and the weight of the glass. The glass is designed to be compatible with the system and is typically provided by a separate glass manufacturer.
Figure 1: Elements of a curtain wall
The individual elements of a stick system are commonly fabricated (cut to length and prepared for connection) within a factory and then shipped to site as a kit of parts which is then installed by a team of specialist contractors. Once the mullion/transom grid is erected, the glass panes and spandrel panels are positioned and typically held in place by pressure plates which are masked by cover caps. An alternative method uses toggles located in a channel between the inner and outer glass panes, to clamp the inner glass pane only. The two fixing details are shown in Figures 2 and 3.
Due to its versatility and cost, these systems are typically used in shopping centres and low-rise office buildings.
Figure 2: Components of a stick system with a pressure plate connection
Figure 3: Components of a stick system with a toggle connection
Unitised systems
A unitised system utilises the component parts of the stick system to create individual prefabricated units which are fully assembled in a factory environment, delivered to site and then fixed to the structure, see Figure 4.
Factory preparation of a unitised system means that more complex designs can be achieved, and materials which require tighter quality control measures can be used, to achieve a high-quality finish. The improvement in achievable tolerances and a reduction in site-sealed joints can also contribute to improved air and water tightness compared to stick systems.
Figure 4: Unitised curtain wall
With minimal on-site glazing and fabrication, a major benefit of using a unitized system is the speed of installation. When compared to stick systems, the factory assembled systems can be installed in one third of the time. Such systems are well suited to buildings requiring large areas of cladding and where there are high costs associated with access or site labour (e.g. high-rise buildings in London and New York).
Within the unitised family of curtain wall systems, some sub-categories exist which also benefit from the increased speed of installation and the re-distribution of labour costs from construction site to factory floor. Such systems include:
Semi-Unitised Systems
Semi-unitised curtain wall facade systems are a hybrid between stick curtain wall systems and unitised systems.
Like stick systems, semi-unitised systems comprise of individual vertical and horizontal spanning members (’stick’) known as mullions and transoms respectively. The mullions and transoms are assembled on site to create the frame of the curtain wall system.
Once the mullion/transom grid is erected, pre-assembled modules (or units) comprising of glazing with a perimeter carrier frame are secured into the openings of the frame, with hidden screwed connections completing the semi-unitised curtain wall system (Figure 5 and 6).
Figure 5: Semi-unitised curtain walling system
Figure 6: Detailed cross-section of a typical semi-unitised curtain walling system
Like other types of curtain wall systems, semi-unitised systems are designed to be connected to the edges of the floors of a framed structure and are required to support their own self-weight i.e. the system does not form part of the main structure of the building.
Semi-unitised curtain wall systems are typically utilised in medium-rise office and commercial buildings. Though less commonly used within the UK, this type of system combines higher quality control due to the preassembly of units off-site with the versatility of a stick curtain wall system on-site.
Bolt fixed glazing
Bolt fixed or ’Planar’ glazing is typically specified for glazed areas of a building which an architect or client has reserved to create a special feature e.g. entrance lobby, main atrium, scenic lift enclosure, shop front.
Rather than having infill panels supported by a frame on 4 sides i.e. aluminium mullions and transoms, the glass panels are supported by bolts, typically at the corners or along the edge of the glass. These bolt fixings are highly engineered components capable of carrying panes of glass with large spans between supports, see Figure 7.
The glass panels are delivered to site with pre-drilled holes along with the stainless-steel bolt fittings. The system is then assembled on site.
The different types of glazing specified for use in traditional curtain wall (toughened, insulated, laminated glass) can also be used in bolt fixed glazing if the manufacturer is sufficiently skilled to have developed and tested such technologies. Annealed glass is not used in bolt fixed glazing because the holes in the glass are too weak. The thickness of the glass will typically be thicker, due to the minimal number of support points.
Figure 7: Bolt fixing
Fixings which attach through the holes in bolted glazing are designed to allow relative movement between the glass and the building structure. Movement could be caused by settlement, live load or thermal expansion and contraction. Bolt fixed glazing will require a structural frame to support it, and this could be a steel trusses, glass fins or stainless steel tension systems. Since the performance of the glass is critical to the overall performance of such systems, it is important that risk assessments are undertaken to ensure the failure of one glass panel cannot lead to the progressive collapse of the entire structure.
Once installed, the bolt fixed systems are typically weather proofed by a silicone weather seal applied between the adjacent glass panels.
Bolt-fixed systems are offered to the market in different forms ranging from complete systems in which the glass and fittings are designed, manufactured, and supplied from a sole source, through to products which are designed by a structural engineer and sourced as individual items. When sourcing bolt fixed glazing as individual components, it is essential that the system is evaluated and coordinated by a suitably qualified contractor or engineer.