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Super tall, super smart | Higher fire

The Address Dubai fire morning after

Blazes in high rise buildings such as the Sulafa Tower and The Torch complex in the United Arab Emirates (UAE) have drawn attention to fire risk in tall buildings. New Civil Engineer takes a look at high altitude fire.

America’s National Fire Protection Association (NFPA) reports that during 2007-2011, 15,400 fires took place in structures in the United States that were designated as high rise buildings.

A high rise building in the US is defined as one with at least seven storeys above grade – around 23m in height measured from the lowest level a fire department vehicle can access.

Averaged over the five year period, high rise fires accounted for 46 deaths, 530 injuries and $219M (£166M) in property damage.

 wtc cropped

wtc cropped

The 9/11 attacks triggered renewed efforts to toughen fire codes in the United States

The NFPA reports that fire risk is lower in high rise buildings than in low rise buildings with similar property uses – possibly because high rise buildings in the US are more likely to have fire-resistant construction materials and sprinkler systems than more diminutive structures.

Revisions to regulations after the terrorist attacks on New York’s World Trade Center in 2001 have played a role.

Four years after the event, the National Institute of Standards and Technology (a scientific research body, part of the US Department of Commerce) released damning findings from its investigation into the building and fire safety. More than half of its recommendations were partially or not enacted. This includes the key recommendation to “prevent progressive collapse” through adoption of a code, standards and guidelines.

But now, 15 years on, structural engineers around the world have taken notice, with many tall buildings designed with redundancy –  a structure’s ability to take loads if a section fails. Focus has gone on to how beams and columns connect, as well has how floor systems affect the resilience of a building; the vulnerability of the WTC lay partly within lightweight steel floor trusses and brittle cementitious fire protection to the structural steelwork.

US regulations do now demand measures such as: lift lobbies with a 45-minute fire rating; a seven-fold increase in fireproofing bond strength; and continuing education for architects and structural engineers in fire protection principles.

But while in the US and around the world fire prevention and mitigation measures are strongly incorporated, recent events in the United Arab Emirates (UAE) provide a stark contrast.

The blazes at the 302m tall Address Downtown hotel in December 2015, the 352m Torch complex in February 2016 and the 285m Sulafa Tower in July have drawn attention to the need for Dubai’s skyscraper forest to be more fire resistant.

The Address Dubai fire night

The Address Dubai fire night

An electrical short circuit on a spotlight from a projecting ledge between the 14th and 15th floors is believed to have been the cause of the Address Downtown hotel blaze in December 2015. Police said the fire had started on the outside of the building, so smoke did not trigger the internal alarms.

An electrical short circuit on a spotlight from a projecting ledge between the 14th and 15th floors is believed to have caused the Address Downtown hotel blaze in December 2015. Police said the fire had started on the outside of the building, so smoke did not trigger the internal alarms.

The problem with high rises in the UAE tends to be that aluminium panels with thermoplastic cores have been a popular choice for construction.

An electrical short circuit on a spotlight from a projecting ledge between the 14th and 15th floors is believed to have caused the Address Downtown hotel blaze in December 2015. Police said the fire had started on the outside of the building, so smoke did not trigger the internal alarms.

Cladding problem

The problem with high rises in the UAE tends to be that aluminium cladding panels with thermoplastic cores have been a popular choice for construction.

These burn aggressively.

“The UAE code was revised this year to specifically deal with the problem,” says CWB Fire Safety Ltd fire consultant Phil Barry. “New buildings should therefore be okay as long as the regulations are effectively enforced. The real problem is what to do with the existing estimated 70% of high rise buildings out there that have been built using materials that promote rapid fire spread.”

Taking standards seriously

But some engineers feel the new standards are not being taken seriously. “UAE banned panels to be used in construction after this incident [the Address Downtown fire], but as far as I am concerned, the ban is lifted,” says Bogazici University Department of Civil Engineering in Istanbul assistant professor Serdar Selamet. “I believe that the standards are strict enough but the fire reaction tests and certification procedures have loopholes.”

But some manufacturers in the Emirates are starting to make alternative building components containing fire retardant materials.

In August, the Danube Group began producing Aluminium Composite Panels (ACP) that are fire-retardant. The company worked with the UAE Civil Defence and Alucopanel USA to develop a factory for the region designed to produce 4M.m2 in ACP material each year.

Future fire

Computer models have long been used to understand how fires spread inside and outside buildings.

“Computational modelling is used extensively on projects to predict internal fire development,” says CWB Fire Safety Ltd fire consultant Phil Barry. “In my opinion there is not much need for this to determine external fire spread. If external walls have linings that will promote rapid fire spread, the outcome is inevitable and this has been demonstrated in many fires around the world and particularly in Dubai.”

“The fire path, direction and speed depends on the burning rate (heat release rate) of the fire, the ventilation openings in the structure, and the smoke layer characteristics,” says Selamet.

 

 

 

dubai hotel fire dec 15

dubai hotel fire dec 15

Firefighters leave the 302m Address Downtown hotel in Dubai after it caught fire in December 2015.

When fires do break out in tall buildings, new technology in the form of drones can help firefighters. The Fireproof Aerial Robot System (Faros) has been developed by the Korea Advanced Institute of Science and Technology for just this purpose. Faros can fly and climb walls, and can endure temperatures up to 1,000ºC for more than a minute.

The drone is designed to carry out reconnaissance rather than actually fight the fire, but it might be possible to develop drones with firefighting capacity.

“Drones should be able to carry enough water or foam to suppress the floor fires where the fire fighters can’t reach,” says Selamet.

But he is cautious about relying on the ability to fight fires rather than prevent them. “I always believe designing buildings to be more fire resilient is a better solution than trying to suppress the fire in a fragile building system,” he adds. 

The Address Dubai fire morning after

The Address Dubai fire morning after

The morning after, at the Address hotel fire in Dubai.

CLT misconception?

Traditionally engineers have preferred to design tall buildings with steel and concrete frames. But other materials are now coming to the fore, notably timber.

As cross laminated timber (CLT), glue laminated timber and laminated veneer lumber (LVL) buildings gain height – albeit modestly compared to their steel cousins – should there be concern about the potential fire resistance of engineered timber structures?

“I have met several timber and fire experts in international conferences and they all advise that CLT is a very sustainable material and bound to be popular soon in high-rise construction, as long as timber buildings are constructed with other load bearing materials,” says Selamet. “Timber does not lose its strength as steel or concrete at high temperatures but it loses its cross sectional area with fire.”

The more flammable a material, the higher the fire load – and the faster a fire will spread. Timber is, of course, a flammable material, which surely means that engineered timber structures will have a high fire load?

Two hour fire design

“The misconception is partly true,” says Selamet. “But the slow burning rates of timber make the building still highly fire-rated. It is important to mention that no structure is currently designed for more than two hours of fire.”

The risk may come not so much from the material itself, as how it is manipulated during construction.

“I have not heard of CLT timber frame being any higher risk. I do have several examples where timber frame buildings have been poorly constructed, particularly with the poor fitting or omission of cavity barriers,” says Barry. A cavity barrier is a seal placed in cavities and concealed spaces in a building to restrict the spread of smoke and fire through a building.

“If timber frame is constructed correctly it is fine in my opinion. There is the problem of occupants who carry out alterations or contractors breaching compartment walls when fitting plumbing or IT equipment,” he adds.

 Final fire thoughts

Whether dealing with newer materials such as engineered timber, or constructing with more familiar techniques, the most important element is to modify design to prevent fires from starting or spreading.

But even building regulations cannot provide a definitive solution to fire risk. Barry says the main problem among high rise structures is that there will be no one-size-fits-all solution – each one will have to be assessed individually.

“External drenchers may work for some buildings, some coatings may provide sufficient protection,” says Barry.

An external drencher is a system that extinguishes, cools down or prevents radiant heat from spreading through a structure. A water source, pipes, control fittings and nozzles make up a drencher system. While sprinkler systems activate each nozzle one-by-one, a drencher system opens its nozzles simultaneously.

“We all know that firefighting operations above 30m are going to be problematical due to limits of fire service ladders. Hence the requirement in all the building codes that I am aware of worldwide for buildings above 15m to 18m to have external surfaces that will not promote fire spread,” he adds.

 

 

 

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