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. In particular a perspective on glass and the terrorist threat.
Lee Coates is the company’s technical director, and here explains how modern steel glazing systems have come of age.
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.
Glass and the terrorist risk
In the glass 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 in every developed or developing country.
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 twenty years ago on the Federal Building in Oklahoma City. 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.
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.
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 that was learned across the globe as architects and designers struggled 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.
Specialist glazing companies have 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.
Specifying the correct glazing solutions
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 ineffective.
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 façade. Both tests were equally successful.
We’ve all had to respond to painful lessons. 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 sound video of the blast tests can be viewed on our Blast Resistant Curtain Wall product page here.