Select Committee on Environment, Transport and Regional Affairs Minutes of Evidence


Memorandum by Stephen Ledbetter, Centre for Window and Cladding Technology (ROF 45)

FIRE AND FACADES

INTRODUCTION

  Modern buildings of medium to high rise invariably comprise a loadbearing structure and a non-loadbearing cladding system. Hence, the cladding system becomes a load on the structure and attempts are made to minimise its weight.

  The facade also fulfils many functions to moderate the internal climate of the building. The filtering of light, heat and sound leads to energy efficient buildings and greater comfort levels that lead to greater productivity in commercial premises.

  Last but not least, the facade provides the aesthetic of the building and contributes to the cityscape.

  Any changes to the facade to satisfy a single requirement such as fire performance will impinge on all other aspects of the wall's performance as well as its cost.

  Walls may be constructed as single layer curtain walling in which an aluminium (or other material) frame holds infill panels of glass and opaque materials. More commonly, a wall may be constructed as a layered construction. In these walls a metal frame contains infill panels and acts as the structural part of the wall. It also serves to seal the building against air leakage and frequently provides the thermal insulation of the wall. An outer layer then serves to shed water from the wall and provide the aesthetic of the facade.

FIRE PERFORMANCE

  Currently walls may be required to prevent the spread of fire into exit routes and from floor to floor but not suffer disproportionate damage in a fire.

  Fire resistant walls are constructed to protect stairways where, say, they are at a re-entrant corner of a building. Such constructions are expensive as a result of the material and skills used and are not economically viable for the prevention of fire spread from floor to floor in a building.

  Spread of fire from floor to floor may occur by the passage of hot gases between the floor edge and the inner face of the wall. Fire spread may also occur as a result of fire breaking out through the wall and burning back in again. Spread of fire behind the wall can be restricted by placing a fire stop with a fire rating equal to that of the floor slab and this is normal practice. Spread of fire by burning out through the wall will depend on the form of wall. For a layered wall with separate air barrier and rainscreen there is an internal cavity that can promote the spread of fire by acting as a flue. This happened at Knowsley Heights but may be prevented by the use of fire stops within the cavity. Guidance on this is given in publications by the BRE, and the Building Regulations. Standard and Guide to good practice for ventilated rainscreen walls by CWCT gives guidance on all aspects of layered walls. For a single-layer wall, spread will be from floor to floor and will depend on the nature of the fire and the materials used. In these respects the LPC report has looked at a restricted number of facade types. There is a European Standard under development for a method of testing fire resistant facades.

  The LPC report has identified the glass as being a weakness in the glass/aluminium wall. It should be understood that in many constructions we want the glass to break early so that it falls in its broken state as small pieces with rounded edges. If the aluminium frame or fixings failed first the large sheets of glass would fall from the building to the possible injury of those fighting the fire. I would argue that the mode of failure during the fire is as important as the period of fire resistance in many cases.

  I am a cladding engineer not a fire engineer but am sure that the use of sprinklers has a role to play in the performance of cladding in fires. As is so often the case in the construction industry, it seems that the costs of these solutions are seen in isolation. It would be possible to build fire-resisting walls on all buildings but the costs would be unreasonable and unnecessary. It would require elaborate testing for each project and yet for most walls (layered walls) we have no clearly defined tests and I believe that test specimens would have to be large (several storeys high).

  Before any radical changes are made to the way in which buildings are constructed inthe UK, it would be advisable to look at the very few incidents of spread of fire in medium and high rise buildings and to establish the contributory factors. A proper risk assessment should then be undertaken. I feel the work of the LPC has been driven by a few incidents that have hit the insurance industry and that we run the risk of using a test method because it exists not because it delivers real benefits to building owners or users.

Stephen Ledbetter

July 1999


 
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Prepared 17 August 1999