Damage limitation

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.