...Fire engineering, of course! CM explores how a wildfire new science is being wielded in the battle between the steel and concrete lobbies

In February an epic fire raged through the Windsor Building, a prominent fixture of the Madrid skyline. It burned all night and all the next day, capturing headlines around the world.

When the fire finally went out the international media moved on, but elsewhere it ignited a flare up in the stand-off between the structural steel and concrete lobbies.

“Madrid fire highlights fundamental weakness of steel frames,” declared a press release issued by the British Association of Reinforcement.

The release proclaimed one-nil for concrete because, despite the unusual ferocity of the fire, the fact that it lasted 26 hours and engulfed all but the bottom five storeys, only the upper floors collapsed – floors which, uniquely in the building, had steel perimeter columns.

Soon after, Dr Pal Chana, technical director at the British Cement Association, called it “an example of the excellent performance of a concrete frame designed using traditional methods and subjected to an intense fire. It also highlights the risks when active fire protection measures fail or are not included in steel frame construction.”

But others drew a very different conclusion. Dr Barbara Lane, director of Arup Fire, wrote on Arup’s website that what Madrid actually showed was how her own, fast-spreading discipline of structural fire engineering (FE) can be used to protect buildings against fires like this. For instance, the fire spread upwards very quickly and, in this example of FE strutting its stuff, she hypothesises why:

“Although there is a requirement to fire stop the gap between the slab edge and the inside of the curtain wall, most [building] codes do not address the tie-back connection of the curtain wall to the structure. Therefore a light facade structural element can heat up quickly and ... [bulge] away from the slab edge, which can create internal flues... ”

Lane wrote that this kind of thermo-mechanical analysis of whole buildings on fire is essential, but is currently lacking in building codes.

These don’t look like opposite conclusions, but in a way they are. In the battle between the steel and concrete lobbies, they will be taken as the latest exchange of artillery fire. The stakes are high. Steel has snatched the multi-storey frame market from under concrete’s nose. In-situ concrete had around 50% market share in 1980, but that dwindled to just under 20% last year. Steel rose from 35% market share in 1980 to just under 70% last year. The data comes from research commissioned by steel producer Corus, but no-one is disputing the figures.

It’s important to point out that Lane positions herself outside the fray. FE works on any building no matter what it’s made of. But the fact is, FE plays to the advantage of the steel lobby, and they are making the most of it.

Common sense

But what is it? In short, fire engineers make new buildings safe by predicting how an occupied building handles a real fire. They measure a variety of factors – how temperature affects the load-bearing, how fast furniture burns, how fire spreads, what people do when they panic, how fire-fighters do their jobs. They run this data through their analytical sausage makers and the results inform the design of the building, sometimes radically.

For instance, you may find you can do without sprinklers on a part of the building if an FE analysis shows they don’t add extra protection. Lane said that at Plantation Place, a recent office development in London, structural FE showed that most of the secondary steel beams could be left unprotected with an intumescent coating. At the ExCeL exhibition centre, also in London, the received wisdom on compartmentation would never have allowed such vast open spaces, but an FE analysis said it was okay.

It sounds like common sense, so what’s so revolutionary about FE? Before FE, architects referred to tables for what to specify to keep the building standing long enough to get occupants out. The data backing up this approach comes from furnace-testing individual components. Analysing how a building performs in a fire when all the components are bolted together is relatively new.

FE makes steel attractive because it offers the potential to economise while meeting the obligation under the Building Regulations to provide safety to occupants. It also allows more flexibility in design, which architects like.

Many people can claim that they do structural design; very few can evaluate the performance of those designs in the event of a fire

Jose Torero, University of Edinburgh

In the early days, building control officers were wary about departing from the codes. Now, prominent examples, like the Greater London Authority, have paved the way.

FE took another big step earlier this year toward general acceptance with the publication of a new, FE-friendly British Standard. It’s actually a draft for development still, called DD 9999, but the task group that wrote it, led by Dr Brian Kirby of Corus Fire Engineering, predicts that it could replace the current Building Regulations guidance, Approved Document B for England and Wales. Observers call it a halfway house between FE and the more prescriptive method of specifying fire safety, an FE “lite” for those who still find FE a bit too “out there”. It presents a table of fire resistance periods in a prescriptive way, but the science behind it is fire engineering. If it becomes a British Standard FE might be more palatable for wary Building Control Officers.

Chana calls it a bit of a black art. He points out there is nothing inherently wrong with the science, but it is easily misunderstood, and subject to abuse.

“It’s like putting a 17-year-old behind the wheel of a Ferrari,” he says.

Chana believes unqualified quacks are putting occupants at risk by practising a bogus strain of FE that amounts to mere skimping on fire safety, especially in low-tech industrial buildings. As it happens, leaders in FE agree with him wholeheartedly. Professor Jose Torero of Edinburgh University (see interview, below) admits the industry should be careful because the tools are new and only the initiated understand them. You can get a masters degree in fire engineering now, and there is a professional body, the Institute of Fire Engineering.

There is disagreement on other points, though. Chana says FE relies on a series of optimistic assumptions and that when one of them fails the whole rickety edifice topples. Concrete has inherent passive fire resistance qualities, so why risk something new and fancy? “Passive must always be better. It doesn’t rely on anything working for you.”

Nonsense, says Lane. She insists good fire engineers will assume everything will go pear shaped. The sprinkler system will fail utterly, every piece of the structure will burst into flame simultaneously and all the occupants, down to a man, will run around in tight circles with eyes shut and ears plugged.

Tried but not tested

The main objection – that steel structures simply can’t withstand fire – was blown away in 1996 when full-scale tests at Cardington showed that they performed better than had been assumed. (See what Prof Torero has to say about that.) The steel lobby is still riding high on those results, and they taunt the concrete lobby for not submitting whole structures to fire tests, for relying on old data, and generally for barricading themselves behind the status quo.

“Fire safety is the worst thing for them to throw at us,” chortles one steel lobbyist. Lane stays neutral, insisting she couldn’t care less what a building is made of, but even she wants more research on concrete frames because it will allow her to flex her FE muscles. After all, FE is about prodding at the boundaries with science.

“Without validation, you just have to be more conservative,” she says.

That’s partly why everybody is anxious to get to Madrid. Chana is going and so will Lane and Torero. The reason is that usually a fire spreads up and down slowly so that the first floors to ignite burn out before the whole event is finished. But at the Windsor building it was closer to total flashover, where numerous floors are blazing simultaneously. In the absence of Cardington-style tests on concrete frames, engineers hope the charred skeleton of the building will yield valuable information about how such structures behave.

Fire engineers will be looking to bring concrete deeper into the fire engineering fold, and the concrete lobby, on the back foot since Cardington, will be looking for vindication.

What happened in Madrid?

Completed in 1978, the Windsor Building totalled 32 storeys, 29 above ground and three below. Tenants included accountants Deloitte and Spanish legal firm Garrigues, but the building was being refurbished and so was empty when the fire broke out at around 11pm on Valentine’s Day this year.

A concrete core and concrete frame supported the first 20 floors. Above that was a central support system of concrete columns, supporting concrete floors with steel perimeter columns.

Two concrete ‘technical floors’ gave the building more strength, one just above the ground level and the other at the 20th floor.

The tower was built using normal strength concrete and before modern fire proofing standards, without any sprinkler system. It was undergoing a complete refurbishment, including, ironically, the installation of active fire prevention and resistance measures.

The fire started on the 21st floor and quickly spread both above and below. Firefighters could only mount a containment operation. The fire eventually finished 26 hours later, leaving a complete burn-out above the fifth floor. The steel-glass façade was completely destroyed, exposing the concrete perimeter columns. The steel columns above the 20th floor suffered complete collapse, partially coming to rest on the upper technical floor.

Crucially, the building remained standing.

Source: Dr Pal Chana, British Cement Association

“A building must be modelled to explain its performance in a fire”

In what sense, if any, has fire engineering (FE) made UK building regs awkward or outdated?

UK Building regulations have opened the door for FE, thus are not being outdated. What is happening is that construction technology is evolving very fast and requiring the use of FE more. The alternative route of using the classic regulatory approach is not being used as much as before.

Which country do you admire for its approach to building codes and why?

Many countries such as Australia, New Zealand, Sweden or the UK have been visionary enough to reform their regulatory practices to allow for engineering based solutions. I truly admire that. What is frustrating in almost all countries is that that regulatory vision has never been accompanied by the subsequent investment required to educate enough people to make FE a truly viable approach. In that sense I admire the Swedish model where education in this field is supported and encouraged. I believe that Sweden is the only place in the world where enough fire engineers are trained to satisfy local demand.

How deeply has FE penetrated the UK over the past five years as a discipline trusted by clients, designers and building control officers?

FE has penetrated quite deeply and very fast. Many buildings now make use of some form or other of FE. Fire Brigades and Building Control are being asked to face the challenge of evaluating engineering solutions on a routine basis. There is also a greater awareness within Universities of the need to produce qualified professionals. Government, through offices like the ODPM are beginning to realise that educational and research needs are not being met and are trying to generate the means to enhance the framework that supports ever-more prevalent FE.

How has FE as a discipline evolved over the past decade?

The discipline was generated on the basis of the large scientific accomplishments of the 1970s and 1980s. In the 1990s this knowledge was structured, an educational curriculum was published, codes allowed engineered solutions and the profession begun to achieve credibility as an engineering discipline. The traditional means of recognition (such as Chartered Engineer) evolved from there.

I have heard FE described as a ‘black art’, meaning a mix of witchcraft and solid science. Can it be trusted? Should the industry treat it carefully?

The industry should treat FE carefully. FE is based on science but it has not been digested to a point that we can have user friendly, robust and reliable tools that can be used by anyone. At this point it remains a specialist field, thus industry should seek for specialists with adequate credentials and a solid training. Many people without the proper qualifications exercise FE; they are the ones that introduce “witchcraft” as a substitute for proper knowledge. Industry should be very careful to make sure that the appropriate professionals are chosen when a job requires the use of FE.

What qualifications and experience should clients insist upon for FE consultants?

This is a very difficult question to answer because it depends on the job. I will say that Chartered Engineer with the Institution of Fire Engineers could be appropriate, nevertheless, this is not always the case. The reality is that certain jobs in FE will require much more than being a Chartered Engineer. I think the biggest discriminator is the tools that the engineer attempts to use. For example, if someone is to attempt to use Computational Fluid Dynamics (CFD) then this person should have a solid training in this area, the same is true for structural models (Finite Element Models). Fire scientists have created tools, but these tools are not ready for general use, and only highly trained users can get the correct answers. So this question can truly not be answered precisely. I think that given the complexity of the problem, and interim approach that could be taken is that, when a third party review is conducted, the credentials of the designers should be submitted and then scrutinised by the expert making the review.

Will you study the Windsor building? Can you explain
its significance?

I do hope I will have the opportunity to study that building. The importance of that fire relies on its magnitude. Generally fires propagate vertically (upwards and downwards) at a much slower pace, thus burnout is achieved in certain floors before the fire can propagate too far. So fires are rarely bigger than three or four flours. This was not the case for the Windsor Tower. It is important to understand well the mechanisms by which this propagation rates were achieved. Furthermore, the simultaneous intensity and extent of this fire is unique, there are few other examples where a structure has survived such an event. Again, the mechanisms by which the structure could handle this fire load need to be understood.

What were the most revealing findings of the Cardington tests in the mid 1990s?

The Cardington Tests, but especially their subsequent analysis, revealed that the geometry of the structure has a significant effect on the performance under fire. Issues such as thermal expansion appeared in some cases as dominant and in some cases alternate load carrying mechanisms appeared given particular design choices. Thus, some building designs, by their inherent shape, will have a particular performance under fire. Previously, the behaviour of a structure under fire was expected to be only a function of the deterioration with temperature of the material properties of its members. Furnace tests were considered as sufficient to predict performance. Now we understand that we need a lot more than just a furnace test. The evolution of a structure has to be modelled to understand its behaviour in a fire.

The steel lobby taunts the concrete lobby for not submitting concrete structures to Cardington-style tests. Is this valid?

One could argue this, but I do not believe that Cardington type tests are the only means to achieve understanding of structural behaviour under fire. It is necessary is for both industries to support fundamental research.

Furthermore, they should use their lobbying power to ensure that government keeps supporting this type of work. As I said above, Cardington has produced a large amount of new information, nevertheless after Cardington it has been decided that support for research in structures under fire can now cease and investment on fundamental work is not being made. Construction techniques continue to evolve together with innovative building design, nevertheless little to no investment is being made to understand the implication of these changes. Expensive studies like Cardington tend to generate a feeling of completion, which is far from being the case for steel structures. Cardington type tests could be an answer but it is not the only means. The only true answer is a continuous stream of research money that enables science to progress at the same pace as technology.

Furthermore, a continuous flux of research money will make viable educational programmes in Universities that will in turn produce the qualified professionals that we need desperately in the area of structural behaviour in fire.

I have a general comment; I think that where this industry is lagging is in investment towards the training and education of qualified professionals. Many people can claim that they do structural design; only very few can evaluate the performance of those designs in the event of a fire. Independent of the material used, concrete or steel, what we need is a critical mass of people that are competent professional that can use the science and tools available to properly assess structures under fire.

Source

Construction Manager

Postscript

To find out more see:

www.concretecentre.com
www.arup.com/fire/
www.ife.org.uk
www.steel-sci.org