Facade Types
Overlooking building envelopes (façades), there are various types of façade systems and materials being innovated architecturally for buildings. The majority of the façade types below are illuminating in the sector. As this article specifically focuses on façade safety and security, I will highlight the various types of glazed façade screen structures and the safety precautions that should be considered at the design and engineering stages by architects, façade consultants, and façade design engineers.
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Framed
Framed systems are designed to support each glass panel on two or four sides. Various similar systems have been innovated as ‘Framed’ while adhering to the aforementioned support standards.
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Stick
Stick-built glass façades are a method of curtain wall construction where much of the fabrication and assembly takes place on site. Mullions made of extruded aluminium may be prefabricated but are delivered as unassembled “sticks” to the construction site. These mullions are then installed onto the building’s façade to create a frame for the glass, which is subsequently installed.
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Veneer
Truss systems can be designed using various types of aluminium or steel profiles. Such systems provide continuous support to the simplest and most minimal off-the-shelf glazing systems, thereby combining relatively high transparency with excellent economy.
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Panel/Cassette
Panel systems are generally designed using framed glass units. The framed panel can be point-fixed to a structural supporting system while the glass remains continuously supported on two or four sides.
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Frameless
Frameless glazed systems are considered the most expensive of all glazed systems. These glass panels require perforations to accommodate specialised bolting hardware. Cast stainless steel spider fittings are commonly used to tie the glass panels to the supporting structure. The glass must be designed to accommodate bending loads and deflections resulting from the fixing method.
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Point-Fixed-Clamped
Point-fixed clamped systems are designed to fix glass panels without any perforation for support. In the case of a spider-type fitting, the spider is rotated 45 degrees from the bolted position so that its arms align with the glass seams.
Façade Structures
- Mullion: Mullion systems include a steel or aluminium section positioned at every vertical joint in the glazing grid. These steel and aluminium mullions can be designed in either open or closed positions.
- Truss: Truss systems employ a planar truss design, often in a hierarchical system that combines various element types and tension components. These truss systems are complex steel fabrications and are frequently manufactured to meet architecturally exposed structural steel (AESS) standards. Additionally, rods or cable elements may be incorporated into the truss design, and lateral tensile systems are often used to stabilise the façade structure.
- Mast Truss: The mast truss utilises cable bracing as a strategy to reduce visual mass. This structural type is named after its nautical origins, where a central compression element (or mast) is stiffened by cable bracing incorporating spreaders to shape the tensile elements. Stiffening the mast helps reduce its length between supports, minimising its buckling force. These bracings can be mounted in bilateral, trilateral, or quadrilateral symmetry around the central mast.
- Cable Truss: The design and engineering of this system rely on the introduction of pre-stress forces into the tensile elements of the truss to provide stability. Depending on the span and load conditions, as determined by design calculations and structural statics, the required pre-stress forces can be quite high and must be resisted by adjacent building structures. Therefore, it is crucial to identify these forces and incorporate them into the static design of the façade and structural analysis.
- Glass Fin: The glass fin system is quite simple in concept. It utilises a glass fin set perpendicular to the glass pane at each vertical line of the glass grid.
Strength Of Annealed Glass
The strength of annealed glass depends on several factors. The surface condition and edge quality of the glass panel play a crucial role in determining its overall strength. Additionally, the duration of the load applied to the glass panel influences its performance over time. Environmental conditions, such as humidity, can also impact the integrity of the glass. The distribution of stress on the surface affects how the glass withstands external forces. The size of the stressed area is another important consideration, as larger stressed areas may lead to greater vulnerability. Lastly, any damage to the glass surface, including flaws and cracks, can significantly reduce its strength.
Why Temper (Strengthen) Glass?
The process increases the apparent tensile strength of glass by introducing compressive residual stresses on its surface. This method operates on a principle similar to “pre-stressing” techniques used in structural engineering. Additionally, it enhances breakage performance by ensuring that, upon fracture, the glass shatters into small, blunt fragments, making it a form of safety glass known as tempered glass. Furthermore, it maintains the improved tensile strength while preserving breakage performance in laminated glass after fracture, similar to the behaviour of laminated annealed glass, a characteristic of heat-strengthened glass.
These allowable stress limits should be incorporated into the design calculation simulation of each façade at its design stage, following the protocol to maintain a façade safety factor of 2+.
Typical Failure Modes
- Instability failure – Compression member or flexural member.
- Overstressing of the glass in tension – due to excessive uniform load, blast, impact, thermal stress, or uneven/ inappropriate supports.
- Surface and edge effects.
- Solid inclusions.
NIS Inclusions And Related Failures Of Tempered Glass
Spontaneous breakage refers to the sudden failure of thermally tempered glass without any external action. This phenomenon has been known since the 1960s. In high-rise buildings, a major concern in the media is the occurrence of flying glass debris caused by such breakage.
A common reason for spontaneous glass breakage is the presence of small nickel sulfide (NIS) inclusions, which range from 50µm to 500µm in diameter and undergo a volume change. The typical breakage pattern, often resembling a butterfly shape, can be an indication of NIS inclusions, but it is not a definitive sign.
To mitigate this issue, the heat soak test has recently been recognised as the most effective measure to detect and eliminate panes with inclusions before installation
Advanced Failures Of Glass Components
Vertical & Overhead Glazing:
Conclusion
Façade specialists, engineers, and consultants have the responsibility to adhere to all necessary design and engineering protocols to develop a well-designed façade, considering sustainability, weather tightness, interaction with the superstructure, thermal gains and losses, occupant comfort and energy efficiency, shading, ventilation, natural lighting, fire behavior of the building envelope, acoustic performance, safety and survivability, security, maintenance, and durability.
Moreover, façade failures encompass a vast area of experience and knowledge, which can be analysed and demonstrated in various ways with precise solutions and studies.
