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

Collaborate To Innovate 2017
The built environment
Winner: Making the 38-story 52 Lime Street (Scalpel): Structural design of modern open plan buildings using the travelling fire methodology
Partners: Arup UK, Imperial College London

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.

travelling fire methodology

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.

Shortlisted – The built environment

Project name: Nuclear reactor seismic studies
Partners: EDF Energy; Atkins; University of Bristol; Plymouth University; Manchester Metropolitan University; City University London

All engineering projects involving nuclear reactor designs must undergo seismic studies to ensure the reactors will remain safe in the event of an earthquake. This
can be challenging, especially when the reactors in question are old.

One effective way of assessing the seismic safety of a structure is to put a scale model of it onto a specialised shaking table that simulates different strengths of earthquake. The University of Bristol has such a table and led a project to construct a scale model of an advanced gas-cooled reactor (AGR) as part of a wider project to extend the lives of these  reactors – which were built in the UK in the late 1960s – as far as the 2020s.

Believed to be one of the most complex shaking-table experiments ever attempted, the project brought the Bristol team, led by Adam Crewe, reader in earthquake engineering in the civil engineering department, together with EDF Energy, the owner of the AGR reactors, and civil engineering contractor Atkins.  The quarter-scale model measures 2.5 x 2.5 x 2m, weighs 10 tonnes, and contains 40,000 components and 3,200 sensors. It simulates the behaviour of eight layers of hollow octagonal graphite bricks that are linked together by ‘keys’ machined into the graphite and are now beginning to show cracks, often emanating from the edges of the keys, exacerbated by their ageing process within the high-radiation environment inside the reactor core. These cracks are expected to worsen over time.

Scale modelling of this type must include not only the geometry of the components but also their material properties. The team settled on an engineering plastic called Acetal to simulate graphite. Each brick in the core can rock with six degrees of freedom with respect to its neighbours, and every instrumented brick has 15 embedded Hall-effect magnetic sensors, supplemented by micro-MEMS-type accelerometers, and contains a bespoke miniaturised 32-channel data acquisition system. A machine-vision system corroborates the data from the instrumented bricks.

The data from the physical model is supplemented by a numerical model of the core developed by Atkins for EDF, which assumes each brick is a rigid mass linked to its neighbours by non-linear springs. These models were used to select the most appropriate configurations to test on the shaking table.

Project name: Sustainable design process for the Midland Mainline upgrade
Partners: Atkins, Carillion Power Lines, Network Rail, DARPA, Invincea Labs, PSI (Physical Sciences Inc)

The Midland Mainline, linking London and Sheffield, is being upgraded to increase its passenger capacity in a programme spread across the next five years. Elements of the project include electrification of the entire 200km line, and smaller capacity increase projects such as one between Kettering and Corby and another between Bedford and Kettering.

Carillion Power Lines appointed engineering contractor Atkins to provide ecological, environmental and sustainability services to support the programme’s design team. Believed to be a first for the UK rail industry, this collaboration integrated sustainability into the core design process, rather than treating it as a separate process.

Working through a series of face-to-face meetings and workshops, the project helped Carillion meet its sustainability key performance indicators and created a set of criteria to assess the impact on the environment of the materials and components used in the programme across their whole lifetime. These criteria include: embodied carbon; recycled content; distance of supplier from site; waste generation potential; maintenance requirements; durability; and potential for recycling or reuse after end of life.

Collaborate To Innovate (C2I) is an annual campaign run by The Engineer, including an awards competition and conference, established to uncover and celebrate innovative examples of engineering collaboration

The headline sponsors for C2I 2017 were Frazer-Nash Consultancy and Yamazaki Mazak

For information on sponsoring or supporting C2I2018 contact The Engineer’s commercial director Sonal Dalgliesh  sonal.dalgliesh@centaurmedia.com