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Fire engineering - Controlling the big heat

Recent tunnel fires have highlighted the need for better fire engineering.Halcrow is on the case, discovers Mark Hansford.

These are tense times.

With terrorism seemingly an ever-present threat, security has never been a bigger issue for designers of buildings and infrastructure, and controlling fire in the event of a disaster a major factor in designs.

'We can contribute to a project where fundamental issues of safety have to be addressed from a strategic perspective, ' says Fathi Tarada, director of consultant Halcrow's newly formed transport and buildings fire safety engineering division.

'Providing early advice to clients can have a significant effect on costs and viability.'

Recent major fires have rammed home the importance of good fire engineering.

In February 2003 182 people died in an arson attack on a subway station in the South Korean city of Daegu. Just two days later 99 people died in a nightclub on Rhode Island, New York, when a pyrotechnic display set fire to the building's soundproofing.

Three years earlier, in November 2000, 155 people died in a funicular railway tunnel in Kaprun, Austria, when an electrical fault set fire to hydraulic oil. And a year before that, in March 1999, 39 people died in the Mont Blanc road tunnel between France and Italy when an HGV containing a cargo of margarine ignited. The fire burned for 53 hours and the tunnel was closed for nearly three years.

In all these tragedies, smoke was the big killer and the death toll could have been lower with better fire engineering, through improved understanding of either the structure's response to fire or of people's behaviour.

In Kaprun, for example, it was a design problem: The 45infinity incline of the tunnel created a chimney effect as hot air rushed upwards. Fire doors at the top of the tunnel did not close automatically, offering no hope of containing the blaze.

Meanwhile, most passengers instinctively tried to escape up the tunnel, and were suffocated by fumes. On Rhode Island only one of the club's four fire exits were used.

Arguably, this could have been predicted and managed.

There are also more mundane reasons for the increasing importance of fire safety engineering such as increases in building complexity, architectural innovation and, of course, economy - are there ways of satisfying fire codes more cheaply than in the past?

'If you used current fire codes on complex, multi-use buildings you couldn't even contemplate the project, ' explains Tarada.

'Plus, architects like large naturally ventilated spaces, and they are encouraged by building regulations, ' says Tarada.

'But if you consider the effect of smoke movement such spaces run counter to fire safety as there is no compartmentalisation at all, ' he adds.

Addressing these issues is far from simple. Halcrow uses three quantative methods to tackle structural factors - computational fluid dynamics (CFD), tunnel aerodynamic analysis and structural analysis for fires - and couples this with an understanding of how people behave.

Structural analysis for fires is a standalone tool that looks at the behaviour of composite steelframed buildings under fire conditions. The analysis is based on a finite element approach but looks at the performance of a building as a whole rather than individual elements.

The results are validated against Building Research Establishment Cardington fire data and show a lot of redundancy in structures. 'So you can eliminate a lot of passive fire protection, ' says Tarada.

CFD is a computational technology that studies the dynamics of fluid flow. Using CFD a computational model is built that represents a system - for example a building or a structure.

Then fluid flow physics - the Navier-Stokes equations for those who remember university hydraulics - is used to get a good idea of smoke movement.

It not only predicts fluid flow behaviour but also the transfer of heat, mass (such as condensation), phase change (such as freezing or boiling) and chemical reaction (such as combustion).

This makes it good for modelling smoke movement due to fires within buildings, and is being used by Halcrow on a new transport interchange planned for Liverpool South Parkway.

There, the building - a large, naturally ventilated space - is designed so that, in case of fire, cold air is drawn in at low level and then vented out at a high level, drawing the smoke directly upwards, preventing it from invading areas where it will do harm. By the time smoke hits the structure the air should have cooled it to the point where it does not endanger the structure itself.

'It is always a balance between fanning the flames and exhausting the smoke, ' explains Tarada.

'In Liverpool we will specify that furniture be non-flammable so the only thing that could burn is luggage, in which case the fire is fuel-limited rather than ventilation-limited.'

'In Abbey Stadium - a development in Redditch which combines sports facilities, restaurants and a hotel - it is the reverse, so we need to do something more active, ' he adds.

Abbey Stadium's retail areas are a high risk with a high concentration of combustibles. If a fire was to start in these areas and not be controlled, smoke would quickly get into the atrium and spread.

The solution proposed by Halcrow is sprinklers in the retail area to limit the fire size and fire-rated duct work above the glazing, separating the retail areas from the atria. This will vent smoke should the glazing crack.

The glazing then need not be fire rated, cutting costs.

Tunnel aerodynamic analysis is used in conjunction with CFD to assess the ventilation requirements of subways and tunnels. Ventilation design on the 2.1km long A303 Stonehenge tunnel is still flexible, as the Highways Agency has yet to decide whether to go with a traditional longitudinal ventilation system for handling extreme events, or whether to install sprinklers for the first time in a UK tunnel.

But Halcrow is working with the Agency on the pros and cons of introducing some sort of fire suppression system, with a decision expected by the end of the year (NCE 29 July).

There are four main types of suppression systems:

conventional sprinklers, deluge systems, water mist systems and compressed air foam.

Each has its own advantages and disadvantages, with most disadvantages focusing around the interaction between smoke and water and the effect this could have on visibility.

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