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Bounce back

Protecting the UK's built environment from terrorist attack is as important a battle for civil engineers as ever. Kevin Walsh witnesses blast tests for a new shatter-resistant film for glass on buildings.

Seven years to the day from the tragic events of 9/11, protecting high-profile buildings from terrorist attack is as serious a concern as ever for engineers.

The attack on the Pentagon in Washington DC and the destruction of the World Trade Center's Twin Towers in Manhattan left many wondering if more could have been done at the building design stage to reduce the catastrophic effects of the attack.

Debate continues to this day as to how risk assessments should be conducted and what is enough protection for new buildings considered to be at high risk of terrorist attack.

In the UK, upgrading protection for older buildings to meet the government's current terrorism threat warning level of "severe" – meaning that the UK is likely to be targeted by terrorists in the future – can be an expensive business.

One company working towards providing an economical and effective solution is 3M, which is designing a shatter-resistant window film that can be retrofitted to large commercial and public buildings.

The film can be applied to the internal facing of a window and secured using rubber or silicon. It is designed to deform under the pressure of the shock wave from a bomb-blast while staying in one piece.

So if a bomb is detonated outside a building the internal film layer will stretch under the pressure and hold the broken shards of glass together.

If the blast pushes the film to full stretch it snaps back to its original position, throwing the shards of broken glass outside.

Although the film does not confer any blast resistance properties on the window itself, it can be effective in limiting secondary damage to building occupants close to an explosion.

To test the effectiveness of the film and test its compliance to International Organisation for Standardization (ISO) and US General Services Administration (GSA) standards, 3M has employed the services of Carillion security consultancy TPS.

Speaking about the new film, TPS explosive effects engineer Benjamin Fry says: "It's all about stopping it failing immediately – making it fail safely, rather than letting it fail in a more hazardous way. You want it to respond to the blast and hopefully throw the fractured glass outwards away from the people you are trying to protect."

The film's ability to do this also depends on two factors outside its material properties – the system used to anchor the film to the window frame and the thickness of the film itself.

Seven different film prototypes were tested to determine their suitability, in addition to varying thickness and using combinations of silicon or rubber anchoring systems.

After testing, TPS analysed the results of the live tests using specialist software for measuring blast wave propagation.

The software is the result of a thesis undertaken at the Royal Military College of Science at Cranfield University in Shrivenham.

The program allowed the effect of the explosion on the film to be modelled to ensure the prototype's compliance with International standards and to define its effectiveness in proportion to the distance from a blast.

A report from the testing exercise was issued last month. "Most if not all of the tests were successful." says TPS principal engineer Ken Holt. In one, a 90kg explosive charge 33m from a window failed to make shards of penetrate the film.

More tests follow next year, before the product can see the light of day.


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