The UK Government is acutely aware of a sustained level of terrorist threat. While the security and intelligence services work to prevent live incidents, it is the work of responsible designers to incorporate risk-minimising strategies into the fabric of buildings considered to be at risk.
Foremost among these are transport infrastructure hubs and, as long ago as 1996, the Government considered the impact of design on safety with its Aviation Security in Airport Development (ASIAD) document.
The focus of development for the transport infrastructure in the UK at present, however, is the expansion of the railway network, with significant upgrade projects under consideration in the South East by Network Rail, the development of the new Crossrail line and the planning of the new High Speed rail link (HS2).
In view of this level of activity, the Security in Design of Stations (SIDOS) Guide was published in July 2012, bringing together the expertise of the Centre for the Protection of National Infrastructure, Department of Transport and the British Transport Police.
The objective of this Guide is the protection of the travelling public. It recognises the role that the design and planning of station infrastructure plays, including measures that will: Prevent, mitigate or deter attacks from terrorists using person-borne or vehicle-borne explosive devices.
However, the Guide also recognises that security is only one aspect of station design and that it is vital to ensure that making the station as safe as possible should not compromise the goal of creating places that are visually stimulating and which people enjoy visiting.
One key material used in many of our most iconic modern buildings is glass. Its inherent visual aesthetic is immediately appealing, while the design versatility introduced by its ability to be coloured and to capitalise on its reflective properties make it a popular choice.
However, its suitability for use in areas at risk of explosive attack has been questioned many times because of the way standard annealed glass behaves. The detonation of an improvised explosive device (IED) causes an instantaneous release of energy, creating a blast wave and heat. The effects of the blast wave can be felt as positive pressure, which carries destructive power out from the seat of the explosion, followed by negative pressure.
The blast wave itself can cause death and injury, but it is the secondary fragmentation from the structure of the building itself that should be the focus for building designers. Building collapse and glazing fragments are the main cause of trauma injuries in attacks involving explosive devices.
Most of the guidance produced on the use of glass in these high-risk buildings considers glazing used in windows, doors and shop fronts. Only very recently has there been a consideration of the requirements for glass façade systems.
When considering the suitability of such a system, the designer needs to consider three separate issues: The type of glass and its behaviour under blast conditions; the stability of the attachment of the glass to its fixings and the security of the mechanism used to fix the whole façade system to the building fabric.
The relevant standard here is ISO 16933:2007 Glass in Building Explosion-resistant security glazing. This is a procedure for determining the air-blast resistance of glazing, remembering that the majority of injuries are caused by glass fragments that are pulled away in the negative blast force that follows an explosion. The standard blasts in the test simulate both vehicle bombs and smaller satchel bombs.
Until recently there have been very few glazing systems able to meet the more stringent levels of this demanding standard, meaning that designers have been limited in their ability to specify glass systems in transport hubs. However, this situation is changing.
The UK is the location for one of the world’s leading facilities for testing the effects of blast and high explosives at Spadeadam in Cumbria. The idea of the test is to simulate a 100kg explosive essentially a car bomb at 15m range. The system is considered effective if no large glass fragments are released by the blast to cause injury.
If a decorative glass façade system is able to cope satisfactorily with the ISO 16933 tests then it should be considered appropriate for use both in overground and underground stations. This allows designers to access the myriad of colours and effects possible with fully glazed façade structures, either for new build or retrofit situations.
A quick review of the designs put forward to recent renovations and for the creative and forward-thinking stations of the Crossrail development will see that designers are not being slow to capitalise on this opportunity.
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