Glass and the Terrorist Threat
Wrightstyle, based in the UK, supplies specialist glass and glazing systems worldwide. Denis Wright is the company’s chairman, and here explains how modern steel glazing systems have come of age.
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 – 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
façade 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 façade. 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 sound video of the blast tests can be viewed at;
http://www.wrightstyle.co.uk/wba/pages/blast-test.php
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