Glazed Facades

'Curtain wall' is a term generally applied to vertical, external elements of a building

Last Updated 13 March 2024

Overview

'Curtain wall' is a term generally applied to vertical, external elements of a building which are designed to protect the occupants and structure of that building from the effects of the external environment.

Many options are available in relation to look and function. It may be entirely glass or include other materials such as stone or aluminium panels. A curtain may contain features designed to enhance the appearance or manage the effects of the environment.

A curtain wall is not designed as a structural element and as a result removal or failure will not result in disproportionate damage to the structure.

glass facade

Key features

Glass curtain walling systems are generally highly sophisticated pieces of engineering. However, they can be classified into three broad types according to their method of fabrication and installation:

Stick systems comprise of individual vertical and horizontal members, normally referred to as mullions and transoms, that are connected to each other and the floor slabs. Members are generally manufactured from aluminium sections and can be provided in a multitude of sizes, colours and finishes.

The members are fabricated in the factory, delivered and installed on site piece by piece to create a grid. Once the members are erected the glazing and other panels are positioned in place. They are generally fixed in place with pressure plates. 

This system is usually used for low rise offices or ground floors in high rise buildings.

Unitised systems use the same components as a stick system to create each unit. These systems are assembled and glazed in the factory, transported to site and then installed. 

Factory preparation means the connection details can be more complicated and use materials which must be applied in a controlled environment. 

A major benefit is the speed of installation, which is likely to take a third of the time. As such, this type of system is normally used in high rise buildings.

Bolt fixing or "Planar" glazing is commonly used for areas of a building to create a special feature. Glazing panels are held in place by highly engineered bolt fixings capable of spanning large panes of glass.

The panels and fittings are prepared and taken to the site for assembly. Apart from annealed glass, the same glazing types can be used as in other curtain walling systems.

Systems may be supplied in different forms. If sourced as individual items, it is essential that the complete system is evaluated and its use coordinated by a suitably qualified engineer.

Further guidance on these systems can be found in the NPSA guidance document Introduction to Glass Curtain Wall Systems.

Vulnerability

A blast load may cause significant damage to a glazed curtain walling system. Depending on the size of the explosion and the design of the system, this may create serious hazards for people, both inside and outside the building.

View Video Transcript

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Glazed facades are widely used in modern buildings. Against a blast threat, they can be very vulnerable.

[Explosion 1]

[Explosion 2]

The glazing hazard can be reduced if CPNI mitigation measures are followed.

[Explosion 3]

For further guidance, please visit the CPNI website.

Blast Resistance

Blast resistance of curtain walling systems can be improved by focusing on the following elements:

  • the overall size of the window - this can impact on its blast resistance; bigger panes generally perform less well than smaller windows
  • the type and thickness of the glass used and whether or not it is a double or triple glazed - laminated glass is preferred for security applications, specifically on the inner sheet of glass
  • the type and thickness of any infill panels (referred to as spandrel panels) used in place of the glass - typically to cover services or areas where transparency is not required or desirable
  • the supporting structure for the glass/panels i.e. transoms and mullions - this includes the geometric shape, thickness, material, and also the mullion/transom connections and fixing of the framing system to the structure, connections of which need to be robust
  • the bite of the glass - the 'bearing area' of the glass onto the framing system
  • the fixing/anchorage of the glass to the frame (e.g. structural silicon or gasket) and whether the glass is secured on one side or both - this can impact on the systems' ability to retain the glass in position during rebound force

To improve blast resistance, all elements of the façade design need to be considered. Recommended mitigation measures for improving the blast performance of stick curtain walling systems can be found in NPSA's  Blast mitigation measures for stick curtain walling guidance note, with recommended mullion/transom connections found in the CSE Glazed Facades chapter. In addition, guidance on specification for blast enhanced fixings for bolt fixed glazing can be found in NPSA's Fixed-point glass: Specification for blast enhanced fixings.

Blast testing

As there are currently no national or international standards for assessing the performance of curtain walling against blast loading, NPSA has developed the NPSA Test Standard for the Explosion Resistance of Windows and Curtain Walling which has two parts:

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