Glass and the Terrorist Threat
The old Chinese curse is to live in uncertain times.
The threat of terror, real or imagined, has focused the minds of building owners, architects, engineers, technologists and planners to better design buildings that can withstand a whole new array of risks. It’s led to design teams taking a multi-disciplinary approach to assessing hazards – from power failure to cyber attack, from civil disorder to fire and explosive detonation – and arriving at risk assessments that, hopefully, illuminate how that that building should be designed and built.
Designing in safety is nothing new, and starts with actively assessing the possible risks against that building’s occupants, structure, resources and continuity of operations. There are a number of assessment methodologies to understand the potential threats, identify the assets to be protected, and how best to mitigate against those risks. That assessment then guides the design team in determining acceptable risks and the cost-effectiveness of the measures proposed.
The trouble, however, is that there is as yet no accepted methodology for assessing terrorist threats, or how to guard against them. Partly, that’s been about the fluid nature of the threat. However, it’s also that the technologies involved in containing those risks have changed considerably over the past few years. In our industry, what we are still seeing are designers and architects who are capable of assessing risk, but are unaware of the huge advances that we, and others, have made in strengthening the fabric of the building against attack.
At Wrightstyle, we’ve been supplying glass and glazing systems internationally for some years now, mostly to contain fire and provide safe evacuation routes. Requirements to guard against fire are, of course, integral elements in building regulations everywhere. But what is not included in building regulations anywhere, as far as I know, are guidelines on building in safety against terrorist attack? or a comprehensive understanding among architects of the options to make buildings safer.
In fighting the terrorist threat it is of course, the first objective of governments worldwide to remove explosives from their arsenal. However, it would be wrong to assume that high explosive alone is the main cause of death and injury. In urban areas, between 80-85% of all secondary blast injuries are caused by flying glass.
To understand how glass and glazing technology has changed, you have to go back to the attack on the Federal Building in Oklahoma City fourteen years ago. It was that atrocity that really focused minds both in governments and in the glass industry. Amid that carnage, 200 victims suffered from glass injuries. The images from Oklahoma City were compelling reminders that glass can be both a friend and an enemy.
When a bomb detonates, it produces gases at very high temperatures. This in turn leads to a rapid expansion of air and the creation of a shock wave travelling at supersonic speeds. The shock wave lasts only a few milliseconds and is then followed by an equally sudden but longer-lasting drop in pressure. It’s the enormous impact of the shock wave and the subsequent suction that shatters the glass and distorts the framing.
Until quite recently, the physical properties of glass made it impossible to guard against a severe detonation. However, the lessons learned from Oklahoma coincided with developments in steel technologies and glazing materials able to overcome glass’s inherent fragility. A few short years of research and development has enabled us now to provide architects with glazing systems that allow for aesthetically-pleasing structures with large spans of glass â€“ but which will remain intact in the event of an explosion.
The importance of glass in a terrorist incident cannot be overestimated. One of the foremost experts on blast injury is Eric Lavonas MD of the Department of Emergency Medicine, Carolinas Medical Centre. He wrote: “Secondary blast injury is responsible for the majority of casualties in many explosions. For example, the glass facade of the Alfred P Murrah Federal Building in Oklahoma City shattered into thousands of heavy glass chunks that were propelled through occupied areas of the building with devastating results”.
The day after the Oklahoma bombing, the US President instructed the Department of Justice to see what conclusions could be drawn in terms of protecting federal buildings. One of the DOJ’s key findings was to provide for [the] application of shatter-resistant material to protect personnel and citizens from the hazards of flying glass.
This echoes findings of the Applied Research Association Inc. Historically, the major contributor to injuries due to terrorist explosion in urban environments is the glass fragment hazard generated by breakage of windows. At Oklahoma, glass fragments were found six miles from the detonation. In New York, 15,500 windows were damaged within a mile of Ground Zero – nearly 9,000 within half that distance.
However, in the wake of Oklahoma, researchers from the Glass Research and Testing Laboratory at Texas Tech University reached a significant conclusion. They found that damage to property and person could have been reduced if laminated glass, at the very least, had been used in the buildings that surrounded the Federal building.
It’s a lesson being learned across the globe as architects struggle to balance form and function with the new requirement of additional security. For example, after Oklahoma, the US State Department started to make windows smaller and less numerous in several embassy projects.
However, and quite simply, none of us want to live and work in windowless environments and architects don’t want to design buildings where form and function are severely imbalanced. So it was in the US, and the State Department experiment was dropped on aesthetic grounds.
Partly this was in response to a report written for the US National Academy of Engineering. It said that “a more proactive approach is to develop glazing materials that meet aesthetic and functional design objectives but do not contribute to the explosion-induced projectile hazard.”
Specialist glazing companies have now carried out much research and development coupled with high pressure blast-resistant testing. The new systems that the industry is now bringing to market offer enormous advantages over older systems, and some peace of mind to the occupants of buildings where such systems are fitted.
The only caveat I would make is that anybody specifying a glazing system to mitigate against blast must ensure that both the glass and framing system have been tested together. The two components form part of one assembly so it doesn’t matter how strong the glass is if it’s held in an inappropriate frame, or vice versa.
We are seeing ingrained attitudes towards blast resistance changing, with more designers specifying rigorous test accreditation. However, even now, too many architects are, for example, using aluminium framing systems in large spans of curtain walling and covering the interior surface with a plastic coasting to hold the broken glass in place. This does have the advantage of being cheap, but it could also be deadly. Aluminium curtain walling is fine, but not for blast resistance – simply, it’s not strong enough and its structural integrity will be compromised.
Our own steel system has been tested independently, with a charge of 500 kg of TNT-equivalent explosive being detonated adjacent to the glazing system. That’s the size of a lorry bomb. We immediately followed that with a simulated car bomb attack on the same assembly (100 kg of TNT). The lorry bomb was detonated 75 metres from the test rig and the car bomb was detonated at a distance of 20 metres, producing a higher loading on the facade. Both tests were equally successful.
We’ve all had to respond to painful lessons. None of us in the industry really wanted to develop such safety characteristics to cope with a mad and bad world. We’d much rather that the baddies were caught or went away. However, the harsh reality is that we had to respond and we have now pushed the design envelope of what can be achieved safely and that’s good news for architects who can now safely incorporate large spans of glass into their designs, even for the most sensitive of buildings.
We may not have yet taken explosives from the hands of terrorists. But for the occupants of those buildings that incorporate the latest blast-resistant steel glazing systems, we have taken away an equally potent weapon: the glass itself.
A video of the blast tests can be viewed below; www.wrightstyle.co.uk