Oxford Method Research Programme

External Cladding

The pressed steel wall panels that form the external cladding of the building consist of Stelvetite Plastic coated galvanised steel panels filled with a rigid polyurethane foam, 38 to 40kg/rn3 nominal density. The inner lining comprises two layers of 9.5mm-thick Gyproc wallboard that are bonded together and to the back of the wall panels. The plasterboard is also retained by steel angle trims or edging around the perimeter of the panels and around the perimeter of openings in the panels. Joints between panels are fitted with weatherproof gaskets and pvc clip-on cover strips. The panels are fastened via steel fixing cleats to the steel frame of the building. Aluminium framed window units are fitted within the external cladding. The glass in the window units is not specified but we have assumed that it is not a fire rated glass.

Plasterboard is a material which has been subjected to many fire resistance tests as part of partition, floor and ceiling systems. In a typical steel stud partition system the plasterboard would be screwed to steel studs, with the studs positioned at 600mm nominal centres. If subjected to a standard fire exposure, 9.5mm-thick type 1 plasterboard would develop large cracks and fall away from the studs after approximately 20 minutes. Where a second layer of plasterboard is fitted under the first, this would also start to fall away after about 35 minutes. However in this case the plasterboard is only bonded to the polyurethane foam and retained with perimeter angles and trims. This fixing arrangement will not provide such good support to the plasterboard. The steel angles and trims only support the plasterboard around the perimeter of the plasterboard sheets and the adhesive only remains effective until the paper faces of the plasterboard degrade. As such we estimate that the two layers of plasterboard would only remain in position for about 25 minutes when subjected to a standard fire exposure from inside the building.

Of more concern is the presence of the rigid polyurethane foam within the panels. When exposed to fire the foam will readily combust, producing much flaming. If exposed to fire from the outside, the weatherproof gaskets and pvc cover strips will rapidly degrade, allowing the fire to penetrate to the foam. Also, with only thin gauge steel sheet between the fire and the foam, the heat transfer through the steel will cause rapid deterioration of the foam. We estimate that within 10 minutes of a standard fire exposure the flames from the polyurethane foam would appear on the non-fire side of the panels from any gaps or openings. Even for fire exposure from the inside of the building the foam would be susceptible to the fire penetrating any small gaps or openings around the edges of the plasterboard. Once the first layer of plasterboard fell away, probably after about 15 minutes, the cracking of the inner layer of plasterboard would allow the fire to penetrate to the foam, with the result of much flaming.

Once exposed to a fully developed fire, the glass in the windows (assuming it to be non-fire rated glass) will shatter, allowing the fire plume from a fire inside the building to expose the outside face of the building above the window to flame and hot gases. It will also allow a fire outside the building to pass to the inside. Aluminium, when directly exposed to a standard fire, will soften and melt after about 20 minutes. The aluminium window frames will therefore melt after about a 20 minute fire exposure.

Due to corrosion of the external steel panels in some locations, the external facade of some buildings are being fitted with a skin of brickwork, 105mm thick. This brick skin will provide protection to the panels behind the brick skin from an external fire exposure of at least 120 minutes. However the comments above on an internal fire exposure and on the windows still apply.