Advance Façades & Glazing Failures

By October 05 , 2017
Façade-design

‘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’.

Façade Types

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

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

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.

fig1

Veneer

Truss 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

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

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.

Façade-design-in-an-airport
Façade design in an airport

Mullion

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

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.

Mast Truss

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.

Cable Truss

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

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.

Resistance-to-wind
Resistance to wind thermal stresses and altitude are important factors to consider while designing and installing facades

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
  • Solid inclusions

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
  • A general reason for spontaneous glass breakage is small (50μm to 500μm diameter) that undergo volume change
  • 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.

Conclusion

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.