Cladding In an explosion, every fragment of glass in a building becomes a potentially lethal missile. Stephen Kennett looks at the best ways to protect ourselves

When the Federal Building in Oklahoma City was destroyed by a bomb in 1995, glass fragments were found six miles away. It’s a sobering thought and explains why fragments of broken window are a major cause of injury in terrorist explosions in urban areas. This was one of the issues Home Office minister Lord West highlighted in his review of safety in public spaces and the built environment in November. One of the security measures examined was the use of specific types of glass to minimise the risk of injuries. This could appear in new guidance on the subject.

So how can architects and engineers factor security into their buildings’ facades without designing modern fortresses? David King, technical director of architect HOK, says this is not easy and requires expert input. “There is no blanket solution and architects can’t tackle it on their own,” he says. Furthermore, new and existing buildings require different approaches. For any building at risk, the first step is to establish what the risk is and the level of mitigation needed.

Blast threats can come from many sources, but the most common are improvised bombs, says Darius Aibara of security consultant TPS. These can range from 1kg letter bombs to lorry bombs, such as the one used by the IRA in Bishopsgate in 1993, which was packed with more than a tonne of explosives.

These sort of devices can turn glass into a weapon. When a bomb goes off, it exerts an intense pressure on the surroundings for a few milliseconds. This is then reversed as the vacuum created by the rapidly expanding air sucks everything back towards the origin of the blast. This second force is less intense, but lasts for longer and pulls glass fragments out into the street.

The first line of defence is to prevent a device getting close to the building in the first place, says King, by creating an area around the building to which vehicles aren’t allowed access. The size of this “stand-off area” depends on the type of glazing used in the building. Creating a large enough area can often be impractical – for common 4mm annealed glass, the distance needed could be about 110m. “Plane float glass or annealed glass shatters into small fragments which travel at a high velocity of more than 100m/s,” says Aibara. “Heat-strengthened glasses are similar. They accept higher loads before they break but still form hazardous fragments.” With Georgian wire glass, there are not only glass splinters to contend with but wire fragments as well. Polycarbonate, often used in bullet-resistant glazing systems because of its strength, also shatters into hazardous splinters.

The second step is to try to prevent fragments of glass escaping, most commonly by using laminated glass. This is made by bonding two layers of glass to either side of a plastic sheet. In a bomb blast, the glass shatters but the fragments remain attached to the plastic layer which stretches to absorb the blast energy.

The stretching, however, introduces another problem: the glass tends to pull itself out of the window frame. With conventional rubber gasket seals, laminated glass quickly pulls itself out of a 15-25mm rebate. A way to prevent this is to put the pane in a much deeper rebate – anything from 35mm to 50mm – and use hard, neoprene gaskets to keep it in place. The downside is that this can make the overall frame membrane thicker and aesthetically not pleasing.

“It becomes a balancing act as you go up the protection categories,” says Lee Coates of Wrightstyle, a supplier of steel and glass cladding, which tackles the problem by using a steel framing system. “There’s obviously a cost to this, which is why people don’t tend to use steel unless it’s for a reason.”

Plane float glass or annealed glass shatters into small fragments that travel at a high velocity of more than 100m/s

Darius Aibara, TPS

Another option is to glue the glazing unit into the frame with structural silicone, which bonds it so tightly that the laminate will fail first. The disadvantage of this is that if the window becomes damaged for any reason, replacement will be difficult. “If you can only replace them from the outside and there are 40 floors, it can be expensive,” says Aibara.

A robust frame isn’t all. The fixing and the structure need to be strong enough to prevent the whole window being ripped out. It isn’t just the glazing that can cause danger. At one of the office buildings damaged at Bishopsgate, a problem with the stone cladding panels revealed itself. Building frames are designed to sway to absorb wind loadings. In a bomb blast they behave in the same way, only a lot more violently. The problem that arose was with the stone cladding panels, which are commonly fixed in place with glued threaded studs. “We found that a lot of those studs had failed and while the cladding looked fine, there was nothing holding it in place except the sealant holding each panel to the next,” says Aibara. “So, something to think about is whether to use energy absorbing fixings.”

When installing security features, new-build is challenging, but retrofitting can be even more so. For instance, creating a stand-off area around an inner-city building can be impractical. TPS creates computer models of existing buildings to evaluate how they will perform in different blast scenarios and what upgrades will have the biggest effect.

For lower risks, shatter films are available. Applied to the inside face of the glass, these work by holding the shattered glass fragments together. In a blast, the entire section will be blown out and wrap itself around the nearest object, but it won’t disintegrate into sharp fragments.

New techniques that bond the film to the frame are also being developed. The same rules apply here as to laminated glass – that is, the frame and fixings should be strong enough to withstand the force. Testing these options to guarantee their performance, however, can be tricky.

A more expensive option is to introduce a crumple zone – an inner barrier, stepped back from the perimeter facade by about 1m. This can be built from prefabricated metal panels, enhanced drywalls or, in some cases, glass. “In projects where we have done this we have also rearranged the internal layout to introduce a circulation space immediately in front of the facade. This is the area that the facade would crumple into and if anyone was sitting there, they would get quite a wallop,” says Aibara. The drawback of this method is that it steals valuable floor space.

As anti-terror measures move up the agenda, designers and specifiers will have to give them more consideration. The more glass there is in a building, the bigger the risk to mitigate against. However, as King says: “It’s also a case of not letting anti-terrorist measures dictate the appearance of the building.”