C2I 2017: Travelling fire methodology behind the design of The Scalpel

The travelling fire methodology has been devised to more closely represent the behaviour of fire in open-plan spaces, and enable structural engineers to design accordingly

As the proportion of the world’s city dwellers continues to rise, so does the demand for workspace. Most people based in cities work in offices, and the most popular paradigm for office workspace is open-plan, particularly — and increasingly — in tall buildings. The new and growing mega-cities of the Middle East and Asia tend to be full of skyscrapers, their floors essentially constituting a tall stack of spaces that, apart from their supporting columns, are open throughout.

Over the past two decades it has become clear that there is a significant deficiency in the way such open-plan structures have been designed and built — namely, how they react to the most basic enemy of the built environment: fire.

All buildings should be designed to resist fire for long enough to enable firefighters to safely rescue the occupants. This design involves ensuring that the structure will remain intact and escape routes will not be blocked by falling debris. Features include checking that the structural elements themselves are robust enough and made from suitable materials, and employing additional ‘passive protection’ materials in key areas (such as fire-retardant panels in doors and ceilings).

As buildings get taller, potentially holding more people, they take longer to evacuate and therefore must remain structurally sound for longer. But events this century, starting with the fires caused in New York City’s World Trade Center on 11 September 2001, have revealed that the assumptions behind the methodology for structural fire resilience did not keep up with evolving building design.

The methodology was based on traditional workspaces where floors were divided into many small compartments — individual offices, store and utility rooms, meeting areas and so on — or on residential buildings or hotels, where floors were similarly divided. When such spaces catch fire, the inside of each room can be considered to be at a constant temperature; an entire floor is essentially a horizontal array of furnaces. So the job of the structural engineer is to ensure that the spread of the fire from one constant-temperature box to the next is slowed down.

The science is different in open-plan buildings, however, as shown by forensic examination of the remains of the World Trade Center and other fire-hit open-plan skyscrapers, such as Windsor Tower in Madrid that burned down in 2005 and the Plasko Building in Tehran that burned earlier this year.

In response to such findings, a collaborative effort led by civil engineering consultancy Arup — which began in 2007 — has produced a new methodology for designing open-plan buildings.

“We were working on a number of buildings with open-plan spaces and we identified that there was a major gap in knowledge about how fire behaved in these very large spaces,” explained Panos Kotsovinos, a fire engineering specialist at Arup. The firm contacted Dr Guillermo Rein, a mechanical engineer with a specialism in combustion science and expertise in the behaviour of fire in complex compartments, who was based at the University of Edinburgh in 2007.

Arup funded PhDs under Dr Rein’s supervision, as well as regular meetings between him and Arup engineers. From studies of the earlier fires it became clear that, rather than the temperature in the fire-affected compartment being constant, there were two distinct zones: a high-temperature zone corresponding to the flames themselves, and a lower-temperature zone with a temperature gradient falling away with distance from the hottest area, which moved around the floorplate of the open-plan area. Rein invented the term “travelling fires”.

This fire behaviour has a number of differences from fires in more compartmentalised spaces. For example, because the fire covers only a portion of the floor at one time, it can keep burning for a lot longer; in one example in Philadelphia in 1991, the fire lasted for 12 hours. This can have significant effects on the structural integrity of the building, which in some cases can be more severe than in fires that assume constant temperatures, and can trigger different structural mechanisms.

Rein and his collaborators, including the Arup engineers under Kotsovinos, devised a travelling fire methodology (TFM) to more closely represent the behaviour of fire in open-plan spaces and enable structural engineers to design accordingly. TFM features include: considering the two moving regions of temperature distribution; taking account of a spatially varying gas temperature distribution at the ceiling level; considering the effects of mixing air with smoke in the lower-temperature zone; and viewing the fire in an affected space as a family of fires, all behaving differently and with a range of effects on the structure, depending on the burning area of the floorplate. Considering this distribution of temperature “is one of the breakthroughs of this methodology”, Kotsovinos told The Engineer.

The primary concern of TFM is maximising the structural integrity of the burning building, he said, while stressing that this had a direct impact on ensuring that everyone inside got out safely. “It’s that which ensures that the emergency services can do their work, that emergency exits and evacuation routes remain clear and that evacuation proceeds smoothly, and this is particularly the case in taller buildings,” he explained.

The collaboration continued when Dr Rein transferred to Imperial College London in 2012, with Arup still sponsoring PhDs. “We’ve been the leader in structural fire engineering for 20 years,” Kotsovinos said. “Key to that is understanding how fire affects the structural elements of buildings and whether it can result in the failure of the structure. Prof Rein’s expertise in the behaviour of fire itself was the other component of the collaboration, which was managed in regular collaborative meetings where we discussed problems in real-life projects.”

Most recently, TFM has been applied to the design of a building in the City of London: 52 Lime Street, a 38-storey tower known as ‘The Scalpel’, which is scheduled for completion later this year and where each floor has a free open-plan space with an area of 1,500m2.

Meanwhile, experiments to confirm the TFM are still taking place, such as fires staged in an empty office building near Warsaw, Kotsovinos said.