Getting to the core of high-rise fires
01 May 2016
CHRISTOPHER MIERS* and MARTIN EDWARDS of Probyn Miers discuss the risks associated with the various types of external cladding in the wake of devastating fires that have hit the region.
The fire in the 12-tower Ajman One development in Ajman, UAE, on March 28, 2016 damaged both Towers Six and Eight, as the fire developed all around the external cladding of one 33-storey tower, then radiant heat and flaming panels borne on the wind ignited the cladding of the neighbouring building.
Coming close in the wake of the New Year’s Eve fire at the 302-m-high skyscraper The Address Downtown in Dubai, UAE, building owners, contractors, residents and insurers are asking questions about the combustibility of external cladding panels on other high-rise towers in the Middle East. Which types of panels are more at risk, and what are the risks?
It is widely suspected that the presence of aluminium-faced combustible core composite panels was responsible for the fire spreading alarmingly rapidly up the exterior of The Address, resulting in more than 40 storeys burning simultaneously. Similar fire spread appears to have occurred at Ajman One. To understand the risks, we need to look at some of the technical features of typical external cladding panels.
External fire spread
External composite cladding panels are made of thin outer metal skins of steel or aluminium with cores of insulating material, which historically have included combustible materials such as expanded polystyrene (EPS) or polyurethane (PUR). The recent Dubai fires involved polyethylene (LDPE) cores with aluminium facings.
Polymeric core materials such as EPS and PUR contribute to fire spread and, in well-developed fires, combustible cores burn with savage intensity. Composite panels can delaminate suddenly, exposing the combustible core which then intensifies and spreads the fire. Aluminium has a much lower melting temperature than steel and hence aluminium facings will fail earlier. Delaminated panels can fall off the building, raining down hot metal and burning foam insulation, with risk of injury and secondary fires. We can see this happening in various photographs of the recent fires in Dubai and Ajman.
UK Building Research Establishment’s (BRE) report entitled Fire Performance of External Thermal Insulation for Walls of Multi-Storey Buildings (2013) explains the mechanisms of fire spread:
• Initiation of the fire event: From a fire occurring inside the building or by an external fire in close proximity.
• Fire breakout: An internal fire may develop to flashover; if so, it is likely to break out through a window opening or doorway. Flames will typically extend 2 m above the top of the opening, regardless of the cladding type.
• Interaction with the external envelope: Once flames impinge on the external cladding, it may contribute to the external fire spread by surface propagation, or cavities. If flames become confined in cavities behind the cladding, they will become elongated as they seek oxygen. Flame extension may be five to 10 times the original flame lengths. Fire may spread rapidly and unseen, if cavity barriers are not installed.
• Fire re-entry: Window openings within the flame envelope provide a potential route for fire spread back into the building. Flames may break out again, threatening other openings further up the building.
• Fire service intervention: Where the external cladding system is contributing to fire propagation, fire may affect multiple storeys simultaneously, thus making fire-fighting more difficult.
Composite panels
Composite panels have had various alternative core materials, and in order of decreasing probability of fire propagation include:
• Polystyrene (EPS);
• Polyurethane (PUR);
• Polyisocyanurate (PIR);
• Phenolic; and
• Mineral fibre.
In the UK, influence from insurers and technical development within the composite panel industry has led to cores of polymer-cored external cladding panels changing from PUR to PIR to phenolic foam, progressively decreasing the fire hazard. In the Gulf, composite panels became widely used for the same reasons as in the UK. The manufacture of the composite panels in many countries included combustible thermoplastic cores.
Exterior cladding fires
In 2012, external cladding fires occurred at Al Baker Tower and Al Tayer Tower, both in Sharjah; and Saif Belhasa Building and Tamweel Tower, both in Dubai. Revisions were published to the UAE Civil Defence Fire Code in 2012, which significantly reduced the risk of similar fires in new buildings.
On 21 February 2015, a fire started on the 51st floor of the 352-m Marina Torch tower. Burning material falling from above ignited a lower level, secondary cladding fire. There were two distinct columns of fire damage on different corners of the building: one high on the north corner, directly above the ignition point, and another at mid-height on the east corner, the result of burning debris landing on a broad ledge on the 30th floor.
In the fire of December 31, 2015 at the 63-storey The Address, up to 40 storeys of the building were alight. Hot metal and flaming core materials from disintegrated cladding panels fell and were carried by the wind to neighbouring streets and buildings. Fortunately, all occupants were evacuated, thanks to the swift action and determined efforts of the fire and rescue teams.
There has been speculation about possible late alarm (and sprinkler) activation in the Tamweel, Torch and The Address fires, but the likely reason is that smoke did not reach the smoke detectors, and heat did not reach the sprinkler heads, until the external fires had become sufficiently developed to break into the interior of the buildings.
The cladding on The Address is described by Alumco, the supplier/installer, as: “Aluminium plastic composite panels compounded with top and bottom layers of aluminium sheet, anti-toxic polyethylene core material.”
Polyethylene is a thermoplastic material, which (like EPS) melts and drips as it burns, spreading the fire downwards as well as upwards. The polyethylene core is typically only 2 or 3 mm thick. As the construction of The Address began in 2005 and finished in 2008, there was no obligation for the aluminium composite panels to comply with the 2011 Fire Code or the 2012 annexure regarding fire performance of exterior materials.
UAE fire safety regulation
Fire safety regulation in Dubai is described in the UAE Fire and Life Safety Code of Practice (2011), also known as the Civil Defence Fire Code, which is effective in all the UAE emirates. Its intention is to minimise the risk of fire and to ensure the safety of life and property, unlike UK Building Regulations, which are almost exclusively concerned only with life safety, rather than property protection. The UAE code is substantially based on NFPA standards (US), adapted for local purposes, and other national codes.
Following the major fires in Sharjah in early 2012, the Civil Defence Fire Code was revised, specifically regulating the external envelopes of new buildings. Annexure A.1.21. Revision 2 came into effect in September 2012 (for new approvals) and April 2013 (for installation of cladding). The Annexure is clearly intended to exclude the insulation and cladding materials most prone to external fire spread.
Annexure A.1.21. Revision 2 prescribes the required fire performance of exterior materials whilst keeping options open for procurement sources. It cites US, German, British and EU classifications and testing regimes, with some variation of options between them. Consequently, the required fire safety standards in Annexure A.1.21. Revision 2 are not entirely uniform. A table of equivalence of the various standards cited would be a helpful addition.
For an external cladding system, the UAE Code is much more demanding than the recommendations of the UK Approved Document B. The UK experience would suggest that the incidence of external envelope fires in the UAE, in buildings constructed from 2013 onwards, should be hugely reduced or eliminated by Annexure A.1.21. Revision 2, provided that the code is followed in both the design and construction.
Existing buildings
The remaining problem is the legacy of buildings with combustible cladding constructed before standards were changed.
The Annexure A.1.21 to the Civil Defence Fire Code states: “1.5. For the buildings that are existing and have cladding/curtainwall systems on the building envelope, it is highly recommended to the building owner to have the perimeter wall evaluated through civil defence approved house of expertise and resolve non-compliances through alternative solutions.”
The UAE’s The National newspaper has reported that a proposed update to the UAE Fire Code anticipated in 2016 will include a new section about liabilities.
Can anything be done about the legacy of buildings with combustible cored composite panels? It seems inevitable that there will be further fires involving aluminium-faced polyethylene core panels. It also seems inevitable that insurers will differentiate between buildings with and without combustible aluminium composite panels and will charge higher premiums for higher risks.
One or two more fires, or a fatal fire, could lead to insurance cover being refused if the risk is considered excessive. Insurance issues, bad publicity and loss of property value might then make retrofitting external cladding a viable option in commercial, as well as fire safety terms.
*Christopher Miers is CEO of Probyn Miers, International Construction ADR (Alternative Dispute Resolution) with offices in London and Dubai.
