Do You Want to Know a Secret?

June 1st, 2009

With security such a sensitive issue, discovering what type of high resistance glazing is needed can be tricky. But however shy the client, it’s vital to find out.

Were Robert Hooke alive now, and looking down from the Monument he completed in 1677 to commemorate the Great Fire of London, he might well have been dismayed earlier this year by the events unfolding in the City streets below, during the G20 riots. Alternatively, he could have drawn some consolation from the fact that, even as the security windows installed in the Royal Bank of Scotland’s City HQ were attacked by angry protestors, the resulting damage from missile throwing was proof of his theories that presaged Newton’s first and second laws of motion. Such consolations were unfortunately not afforded executives at RBS, saddled with the repairs to their street level glazing; but the damage is also a textbook example of another kind to architects – to ensure that when it comes to high-performance glazing systems, they are sure that they are specified correctly.

According to Edward Wills, technical manager for the special processing division at glazing manufacturer Solaglas, the problem is that there is very little specific guidance out there to go on, just a plethora of general. In an industry that is usually highly governed by performance standards ‘blast proof or bullet proof glazing is strange, solely because from an architects’ point of view there’s little by way of directly applicable standards. Actually, I should say the word resistant rather than proof,’ he says, correcting himself mid-conversation, ‘If a bullet or bomb is big enough then a hole can be made in glass, and people need to be aware of that’. But by his own admission, reticence tends to characterise the conversations Wills has with architects and clients on the subject.

And that’s mainly due to the fact that, with security such a sensitive subject, negotiations are generally ‘cloak and dagger’ affairs. Clients do not really want to divulge their particular requirements, with manufacturers sometimes even left unaware of the location or nature of the building their products are being fitted into. This causes big problems when it comes to specification, says Wills. ‘The difficulty is that there are fundamental differences in the responses of bullet, intruder and blast resistant glass, so it’s vital the architect and manufacturer get a measure from the client of what they are supposed to be protecting against. Everything depends on the nature of the perceived threat,’ he says, emphasising that there is no ‘one glass that fits all’ scenario. He adds that a clear idea of use also leads to a better pricing regime, as over-specification can have exponential effects on the final tender price.

The general requirements for how glass or plastic achieve greater ballistic and blast resistance are based on the thickness of the material and the presence of polybutyl plastic interlayers that maintain the integrity of the glazing and prevent spalling after it is distressed, but how the glazing is precisely made up after that remains an industry secret. The difference in behaviour between the two types of applications however, is fundamental, and the design of homogenous cladding systems key. A technical advisor at cladding firm Schüco puts it succinctly: ‘You’ll want flexibility with blast resistant frames, and rigidity with bullet resistant systems. It is an oxymoron to assume the two are compatible.’ The glass frame, then, performs two different functions. For bullet resistance it holds the glass in place for localised but very high force ballistic attack, but for blast it allows the glass to ‘pillow’ quite radically across its surface – deformation can be in excess of 150mm post event. Specifiers should look to current guidance on both scenarios, comprising MoD guidelines, based on ‘weak’, ‘normal’, ‘enhanced’ and ‘special’ performance criteria: BS EN356 for resistance to manual attack, BS EN1063 for bullet attack and ISO 16933 for blast-resistant glass.

Architects also need to be clear about the limitations that might be imposed on their designs. Solaglass say a ballpark figure for glass thickness to resist a 9mm handgun bullet would be 22mm, whereas for an AK47, for instance, it would be around 50mm. For armour piercing ballistics, go up an order of magnitude again, to about 73mm. But as Wills points out, fitting glass of these thicknesses becomes a logistical as well as optical issue – 1sq m will weigh as much as 175kg, and optical performance is reduced. At this point, specifiers may want to go down the road of polycarbonate glazing products. ‘Thickness is significantly less, although the cost is much higher,’ he explains, ‘But this extra expense can be offset against the reduced manpower and logistical requirements for installation.’ Bear in mind that design and sectional thickness of the cladding system will be affected too. Ballistic rigidity will necessitate greater mullion section thickness. For blast resistance, expect the presence of more transoms and mullions than you might actually want.

But Schüco, like Solaglas, is very clear that correct specification is about a systems approach. ‘We use the term “harmonisation”,’ says a company glazing technical adviser. ‘Basically no one element of the glazing, frame or structure can be stronger or weaker than another, and unlike some firms that base their calculations on finite element analysis, we actively build test systems to destruction to ensure that’s the case. Under European standards, specifiers should make sure that the BR (bullet resistance) rating for a frame, for instance, matches the BR rating of the glass (both 1-7 depending on performance). For blast resistance, EXV ratings of frame and glass should likewise tally. According to Brian Waldron, chair of the committee on glass at the British Standards Institute, who saw the effects of mis-coordination of frame and glazing when he visited the City three days after the 1993 Bishopsgate bomb, this consideration is critical. ‘For explosions, you want the glass to absorb the blast, deform and dissipate remaining energy through the cladding to the structure,’ he explains. ‘I saw an example at Bishopsgate where, as a result of the glass being too rigid, the cladding and glass had literally torn itself away from the slabs in the explosion, taking concrete with it. It turned out to be the building itself that wasn’t strong enough to resist the transferred impact load.’

With all the emphasis on dynamic physics and ballistics, one can forget the whole reason why the guidelines are there – to reduce the loss of life in the event of attack. But on occasion design development may bring that possibility starkly into focus. In a situation where you are specifying to mitigate the effects of terrorism, it’s a given that at some point in a blast scenario, consultants have to prioritise spaces either side of the glass. What doesn’t rupture will by default contain and reflect explosive waves, and the effects of that, especially on internal spaces, can be severe. Every action has an equal and opposite reaction. And where there’s a potential human cost to design decisions, it becomes an onerous consideration of Newton’s third and final law.


It protects installations in Iraq and Afghanistan, as well as Air Force One and Marine Corp One; hell, it’s even protected Barack Obama, so the bods at Defenshield Inc in the USA are no strangers to high pressure as well as high profile assignments. There’s a lot of competition in the market for permanent bullet-resistant glazing systems and body armour, but Defenshield thinks it has positioned itself in a very specific niche. ‘We like to think of ourselves as occupying the space between tactical personal protection and bricks and mortar architecture by virtue of the mobility of the product,’ explains company president Collins White.

Laminations of glass within a urethane binder, specified to meet the performance criteria of National Institute of Justice level 4, means Defenshield the product can catch a 9mm, 50 calibre bullet fired at over 3,000ft/sec, so the product can deal with armour piercing bullets. At 76in tall and 36in wide, and weighing 500-750lbs, it’s a (just) portable bullet-resistant glazing system – although White admits that while it has blast resistance, it is likely to suffer ‘secondary fragmentation’ once it has reached its threshold.

‘I got the idea for marketing the product after seeing how heavily armed the guards were at New York’s Syracuse airport at the end of 2001,’ he recalls, ‘and felt this could be overcome by more architectural security approaches’.

White, who has been looking at potential markets overseas, is yet to make a UK breakthrough – where SPT markets the product. Will he be quoting for the new US Embassy in London? ‘Well, we’ve had our product installed in the US Consulate in Kabul,’ he remarks, ‘so it’s tried and tested. We’ll have to see.’

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