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Burning issues

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Cladding systems for high-rise buildings must be carefully detailed to prevent fire spread across the cavity barriers

The '80s was a time for refurbishing the high-rise housing that had been built in the late '50s and '60s. Part of this involved fitting types of external cladding systems, the most common form of which is external wall insulation, overlaid with render. If expanded polystyrene is the insulation layer this can greatly contribute to fire spread inside the cladding.

As a result of a fire in Knowsley, Manchester (see panel, below), the Building Regulations were changed so that product performance 'Class O' (limited combustibility) applied to the inside and outside of external cladding systems and recommended that the gap between the cladding and the wall of the building be firestopped. The requirements are not retrospective and many properties do not comply with the regulations. That is to say, many buildings now in use may not be constructed in accordance with the known causes and actions of fire spread - knowledge which has become available since their construction and refurbishment.

A government report that followed a fire in Irvine, Ayrshire, (see panel, below) 'does not suggest that the majority of external cladding systems in the UK pose a serious threat to life or property in the event of fire'.

However, the study, Potential Risk of Fire Spread in Buildings Via External Cladding Systems, does recommend a change to the method of testing for resistance to fire spread in cladding systems and for local authorities and registered social landlords to assess their multi-storey cladding systems.

It recommends that 'competent fire safety assessors be called in to evaluate what work may be necessary to ensure that no undue risk is posed by any of these systems'. Although aimed at local authorities, it would be advisable for designers, landlords and owners of other high-rise properties - even those being constructed today - to take similar steps to riskassess the following areas:

As more high-rise buildings are built with combustible exterior claddings, exterior fire spread problems become more important as rapid fire spread from floor to floor can happen very quickly.

In the case of very tall buildings, accessing the fire with fire-fighting equipment can prove difficult.

Fire spread can be particularly dangerous in buildings such as health-care units and detention centres as the occupants in these buildings may not be able to escape to a safe place without assistance.

Considerable property losses due to wall damage as a result of an exterior wall fire.

Fire spread on combustible cladding systems could also compromise sprinkler systems if the fire were to spread into several floors, setting off sprinkler heads at each level and creating a greater demand for water than can be supplied.

The regulations for England and Wales regarding the fire safety of external cladding systems are part of the Building Regulations 1991.

Schedule 1, Requirement B4 states: 'The external walls of the building shall resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building.'

Requirement B3 'Internal Fire Spread (Structure)' can also apply, particularly with regard to rain screen cladding. Guidance on meeting the requirements is given in Approved Document B 'Fire Safety' but this is recommended guidance only and other means can be used if they satisfy the building control officer that they meet the requirements of the Building Regulations.

There is some question about the adequacy of these regulations with regard to the fire safety of external cladding systems. This concerns the surprisingly fast spread of fire involving these systems and their effect on available escape time, fire-fighting access and building damage. There is also concern over fixtures, plastic cladding material and degradation.

Not all combustible exterior claddings can sustain vertical/horizontal flame spread. Combustible cladding materials, such as cellulose fibre cement sheets, do not significantly contribute to fire development.

As for composite materials, the propensity for vertical/horizontal flame spread is influenced by the outer protective layer applied to the core material and its integrity when exposed to fire. Provided that adequate outer-layer protection is applied to the core, combustibility of the core material presents less danger and can be used safely in a facade assembly under some circumstances.

The structural components and fixings that make up the cladding system can also be susceptible to the effects of a fire, suffering deformation or destruction. Early deformation in particular can create gaps, which allow the passage of hot gases into the cladding.Where plastic or similar components are used, these may melt and form burning droplets, spreading fire inside the cladding or falling on people below. In addition, cladding panels may become detached and fall from the building, endangering those below.

The use of some constructional features and facade geometry, such as vertical and horizontal projections, can reduce the effect of exterior fire spread while others may contribute to the intensity. For example, balconies, by disrupting vertical air movement, can serve as 'flame deflectors' and reduce the effect and intensity of upward spread. However, vertical channels, which may be formed by building recesses, exterior service runs or privacy screening, channel the fire upwards and can increase the intensity by as much as 50 per cent.

Generally the greater the width of the opening in the facade, the greater the risk of external ignition. So the avoidance of low and wide windows can reduce the risk, whereas tall windows project flames away from the walls, which produces relatively lower thermal exposure.

The fitting of sprinkler systems internally can reduce the risk of fire developing to the outside of the building and this is recognised in Approved Document B of the Building Regulations which states: 'If a building is fitted with a sprinkler system it is reasonable to assume that the intensity and extent of a fire will be reduced.' In cases where sprinklers are fitted the unprotected area may be doubled.

Kevin Thorpe is a senior fire consultant at Lawrence Webster Forrest, tel 020 8655 1605 or e-mail fire@lwf.co.uk

FIRE STATS

From 2003, cigarettes sold in New York state must be designed so that they will self-extinguish if the smoker does not take a puff within a certain time period.

In 1998 there were 90,000 arson fires in the UK costing, on average, £4million each day and resulting in 20 deaths and 80 injuries.The largest increase (17 per cent) was in malicious car fires.

It is estimated that 45 per cent of all fires attended by the fire service can be attributed to arson.

Less than 20 per cent of fires are reported to the fire service.The remaining 80 per cent are either put out by staff/public using portable fire fighting equipment or burn out.

Hospitals accounted for 52per cent of all false alarms.

CONTRIBUTORY FACTORS

Various mechanisms can contribute to fire spread to, and via, external cladding:

The cavity or air gap between the wall and the inner surface of the external cladding provides a 'chimney effect', which allows the spread of hot fire gases vertically and horizontally.

Combustible cladding, or cladding inner surface, allows the fire to spread through the inside or outside of the cladding by direct burning.But even where the cladding has a protective coating this can be destroyed by the intensity of a fire from a large opening and then ignite cladding materials within the opening.

External fire spread from storey to storey as a result of flames leap-frogging to other window openings.Flames through window openings may ignite the facade of adjacent properties.

Melting of metals and alloys.This happens as the intense heat from fire exceeds the melting points of alloys or metals and causes them to distort and fail.

Fire spread is possible through walls around service ducts, penetrations, gaps and openings within the duct.

Stairways are open shafts and act as chimneys for hot gases and flames to migrate.

HISTORY OF FIRE SPREAD

A fatal fire at a 13-storey block of flats, in Irvine, Ayrshire, in June 1999 spread from a fifth floor flat to the outside of the building and externally to the eight floors above. The upper floors were engulfed in flames within minutes. It was one of several fires in which the flames had spread externally up the building and into the upper floors with serious consequences.

Te Papa, Wellington, New Zealand (1997). The exterior cladding comprised a thin aluminium-faced panel with a polyethylene core, mounted over extruded foam polystyrene insulation board and building paper. During construction, there was a fire on the exterior facade as a result of burning building paper, which was ignited by a worker when heat-welding a roof membrane.

Apartment building, Munich (1996). The facade of the five-storey apartment building was made of a composite thermal insulation (about 100 mm thick) comprising polystyrene and foam plastics slabs and a reinforced covering layer. The fire originated in a rubbish container, ignited the cladding and extensively damaged the building facade.

Flames spread into rooms at upper floors through broken windows.

Knowsley Heights, Manchester (1991). A fire was started deliberately in the rubbish compound outside the 11-storey apartment block. The fire spread rapidly through a 90 mm gap between the building's rubberised, paint-covered concrete outer wall and a recently installed rain screen cladding (with limited combustibility). The fire spread all the way to the highest floor and seriously damaged the outer walls and windows of all the upper floors. This rapid fire spread was caused by the lack of fire barriers in the cavity gap, passing all 11 floors and providing a flue for hot gases to rise.

Basingstoke, Hampshire (1991). Spread two floors above the eighth.

Philadelphia, US (1991). Spread nine floors.

Winnipeg, Canada (1990). This fire involved an eight-storey unsprinklered apartment building with an open-air parking garage on the ground floor. The building's exterior walls were covered with a combustible Exterior Insulation and Finish System (EIFS). Fire initially started in the garage and quickly spread to the 25 cars that were parked there. Flames issuing from the garage exposed and ignited the EIFS on the exterior walls. The fire then spread to the top of the fourth storey.

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