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Advance Façades & Glazing Failures
By: Dasun Siriwardena
‘A façade is a building’s primary exterior face. It generally includes the main entry to the building and has the most elaborate architectural features. As the most public face of a building, a façade is particularly important to your business. Studies have shown that thoughtful design improvements often lead to greater sales for a business by attracting more customers’.
Looking at contemporary building envelopes (façades), it is obvious that various types of façade systems and materials have been innovatively and architecturally incorporated into buildings. As a majority, façade types mentioned in Figure 1 have been in practice in the sector.
In this article, I will be highlighting the various types of glazed façade screen structures and the safety precautions to be considered in the design and engineering stage by architects, façade consultants and façade design engineers, especially considering façade safety and security aspects.
Framed Façade Systems
Framed systems are designed to support each glass on two or four sides. There are plenty of different systems innovated as ‘Framed’ by respecting above mentioned support standards.
Stick Façade Systems
Stick-built glass façades are a method of curtainwall construction where much of the fabrication and assembly takes place in the field. The mullions of extruded aluminium may be prefabricated, but are delivered as unassembled “sticks” to the construction site. Mullions will be installed onto the building’s face to create a frame for the glass, which is installed subsequently.
Venner systems can be designed using various types of aluminium or steel profiles. Such a system can provide continuous support for the simplest and most minimal off-the-shelf glazing system, thus combining relatively high transparency with excellent economy.
Panel / Cassette
Panel systems are generally designed by framed glass units. The frame panel can be point fixed by a structural supporting system while the glass remains continuously supported on two or four sides.
Frameless Façade Systems
Frameless glazed systems are considered as the most expensive glazed system out of all types. These glass panels require perforations to accommodate the specialized bolting hardware. Cast stainless steel spider fittings are most 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.
Point-fixed-clamped system are designed to fix the glass panels without any perforation for supporting. In 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.
Mullion systems include a steel or aluminium sections positioned at every vertical joint in the glazing grid. These steel and aluminium mullions can be designed in ‘open’ or ‘closed’ positions.
Truss Facade Systems
Truss systems employ a planar truss design, often in a hierarchical system that combines element types and tension components. The truss systems stand as complex steel fabrication and are frequently manufactured to architecturally exposed structural steel (AESS) standards. Moreover, rods or cable elements may be incorporated into the truss design and lateral tensile systems are often used to stabilize the façade structure.
The mast truss utilises cable bracing as a strategy to reduce visual mass. This structural type is named after its nautical origins – i.e., a central compression element (or mast), which is stiffened by a cable bracing that incorporates spreaders to give shape to tensile elements. The brazing incorporated to the mast, adds to the stiffening, and reduces the length between supports to minimize its buckling force. This brazing can be mounted as bilateral, trilateral or quadrilateral symmetry about the centre mast.
This system design and engineering rely on the introduction of pre-stress forces into the tensile elements of the truss to provide stability. Depending upon the conditions of span and load, referring to its design calculation statics, the required pre-stress forces can be quite high, and must be resisted by the adjacent building structures. It is, therefore, important to identify these forces and incorporate those into the design static of the façade along with structural analysis.
Glass fin systems are quite simple in concept. In these systems, a glass fin is set perpendicular to the glass pane at each vertical line of the glass grid.
Strength of Annealed Glass
Strength of annealed glass is dependent on:
Surface condition and edge quality of the glass panel
Load duration on the glass panel
Environmental condition (Humidity)
Stress distribution on the surface
Size of the stressed area
Damage on glass surface (Flaws and cracks)
Why Tempered (Strengthened) Glass?
Increases apparent tensile strength due to compressive residual stresses on the surface of the glass.
Principally like “Pre-stressing” methods in structural engineering.
Improves the brakeage performance due to small, blunt pieces/splinters – hence it is called safety glass (A tempered glass)
Improves apparent tensile strength, but keeps high breakage performance in laminated glass after fracture. As in the case of laminated annealed glass (Heat strengthened glass)
Typical Failure Modes
Instability failure – Compression member or flexural member
Overstressing of the glass in tension – by excessive uniform load, blast, impact, thermal stress or uneven / inappropriate supports
Surface and edge effects
It is possible to determine the exact cause of the failure by considering the following:
Nickel Sulphide (NiS) Inclusions and Related Failures of Tempered Glass
Spontaneous breakage – sudden failure of thermally tempered glasses without external action
The phenomenon has been known since the 1960’s
For high-rise buildings, a big echo in media occurs generally as flying glass debris
The typical breakage pattern (Butterfly) is one indication, but not a sufficient indication of NiS
In recent times, the heat-soak-test is considered as the most efficient measure to bring panes with inclusions to failure in advance.
As a façade specialist, façade engineer or façade consultant, it is our responsibility to work and abide all the necessary design and engineering protocols to develop a perfect façade considering, sustainability, weather tightness, interaction with super structure, thermal gains and losses through the façade, occupant comfort and energy efficiency, shading, ventilation, natural lighting, fire behaviour of the building envelope, acoustic performance, safety and survivability, security, maintenances and durability. Moreover, façade failures are a vast area of study. One needs to have immense experience and knowledge to analyse the reasons for failures and provide exact solutions.
Head of Design and Engineering
Dasun Siriwardena is professional in dynamic façade designing and engineering, quantity surveying, structural simulation animator and project installation managing strategist with a 11+ year record of achievement and demonstrated success driving huge amount of unique façade growth in Srilanka, U.A.E, Iran, Oman & Ethiopia while providing leadership in highly competitive environments