Richard Rogers' design for Heathrow Terminal 5 has a 43 m glass facade to flood his elegant interior with natural light and lift weary travellers' spirits. But would it be safe in a bomb blast? Alex Smith talked to Pascall + Watson about how to specify for terrorproofing
For many UK air travellers, the worst part of their journey is not the flight, but the tortuous trudging between check-in desk and boarding gate. Travellers using London's Heathrow suffer more than most, as the four existing terminals are dark, low-ceilinged, overcrowded and difficult to navigate.

So when BAA commissioned its Terminal 5 at Heathrow, it was adamant that the design should be passenger-friendly. Architect Richard Rogers Partnership obliged with a light, airy and transparent building that will give passengers a sense of space and freedom of movement.

However, this space and light comes at a cost in a 21st-century airport terminal, as detail architect Pascall + Watson discovered. For example, how do you protect 43 m high glass facades in the event of a bomb attack? "We've had to struggle with dealing with the affects of bomb blast on the doors and windows," says Julian Carlson, associate architect at Pascall + Watson.

The design criteria were set by security consultant TPS Consult after an assessment of the risk was made. Once Pascall + Watson came up with the designs based on TPS's criteria, they then had to be independently tested. The expense of the tests meant that Pascall + Watson ensured the designs would work first time. "In these situations, there is a reluctance to test designs because of the expense," says Carlson. "We always design exactly to the criteria, so when we come to test it we know it will work."

There were two things the facade design had to do. The first was to stop glass panels falling away from the main structure as a unit; the second was to stop glass break into large shards that could injure people below.

For facades that require protection from bomb blasts, Carlson reckons that structurally bonded glass is the answer. There were two possible types: a fully unitised profile in which the glass is supported on four sides by steel transoms and mullions, and a system in which the glass is only supported at the top or bottom. The architect chose the stronger unitised system because of the large expanse of glass. In Pascall + Watson's designs, the glass is bonded to the steel using structural silicone bonding.

Carlson says that the transoms and mullions on the facades with the bomb-blast protection had to be very strong but also flexible enough to absorb the blast pressure. If the steel doesn't flex there is a danger of the glass falling out under the pressure of an explosion.

To prevent the window from breaking into large pieces and falling onto passengers, the architect specified laminated and toughened glass. The laminated plastic layers between the glass panes stretch under blast pressure, increasing the structural strength of the unit, holding it in place within the frame and stopping fragments from falling out. The laminated layers also provide the glass with the UV protection and thermal insulation required in the Building Regulations.

The fact that the glass is toughened, as compared with ordinary annealed glass, means it is more likely to shatter into smaller pieces than into lethal shards. Toughened glass is made by suddenly cooling the surface layer of ordinary heated glass, which is already shaped and customised to fit.

The level of blast resistance is not the same on all four main facades at Terminal 5. In an airport environment, the airside, where the planes are situated, is protected by the security checks carried out between the terminal entrance and the boarding area. The risk assessment therefore ascertained that this area would need less blast-resistant glazing, enabling the client to make cost savings. The south and north facades, on the other hand, both have partial protection because some of these areas are passenger side before the security checks and some are airside.

For economies of scale the same metal glazing profile has been used throughout the building, but the bomb-protected facades have stronger frame details as well as blast-resistant glass.

Pascall + Watson has now completed the designs for the glazing and is about to specify the system. The beauty of the blastproof facade design is that when the terminal opens in 2007, passengers will benefit from floods of natural light without Rogers' elegant architecture being spoilt by intrusive security features.

Security check: Rules and regulations for airport security and blast-proof glass

Need to know more about airport security rules? Never fear – help is at hand in the form of guidance produced by the Montreal-based International Civil Aviation Organisation’s Annex 17 document and the Paris-based European Civil Aviation Conference Document 30. In the UK, the National Aviation Security Programme offers restricted-access guidance for architects and designers. A government spokesperson said that the government assesses all airport developments to ensure the correct security arrangements are made. “We are well aware of the need to safeguard public safety within the UK’s airports and have a range of measures that we keenly enforce, which include assessing all airport developments – before, during and after construction – to ensure the highest standards are achieved.” When designing a blast-resistant facade in Europe, the rating of a glass panel and its fixings is governed by a number of European Standards including EN 13123-1 and EN 13124-1. The shock tube method in these standards creates large explosions by mixing compressed air in a steel pipe. The standards mimic the effect of 100-2500 kg of TNT at distance of 35 m and 50 m from the glass facades. Proposed range testing (as proposed in prEN 13123-2 and prEN 13124-2) will examine the effects of smaller handheld explosive devices at a lesser distance from the building. In addition to the European Standards, work is now commencing on International Standards (ISO). These are being led by IS TC160 and TC162 and will deal with both shock tube and range testing – including a 100 kg TNT equivalent range test that is not currently covered by European Standards.

An alternative to silicone bonding

Glass does not have to be silicone-bonded into a metal frame to be resistant. Glass that is bolted back to a building structure can also withstand blast. Although there is government data on the resistance of a wide range of glazing products, none covers point fixed glazing, which undergoes different stresses in a blast. Recently, two Pilkington Planar panels, which are point fixed, were tested under the Home Office-sponsored Comblast 2000 Blast Trials. The panels received a hazard classification of “low”.

What happens in a bomb blast?

When an explosion occurs, a rapidly expanding pressure wave is created in the air. When this wave comes into contact with a window, there is a powerful positive pressure on the glass known as a reflected overpressure. This is quickly followed by negative loading on the glass, when the vacuum created at the heart of the blast sucks in the surrounding atmosphere. The suction load on the window is not as great as the positive pressure, but it lasts longer and can be responsible for a window bulging or falling towards the source of the blast. Although the magnitude of the positive blast load is massive, its duration is very short (a few milliseconds) and a glass panel can be designed to be strong and flexible enough to prevent a deflection sufficient enough to cause the glass to fail.