Fire engineering approach to major smoke control and fire pressurisation installations.

Most of the many standards written for the above, contain a lot of options, some of which are necessary, some of which are onerous embellishments. These latter serve not only to complicate the design, making it less robust and less user-friendly, but also make it more costly to install and to maintain.

By agreeing the scope of the objectives of the proposed fire protection systems, some simplification of the systems can often be justified on ‘first principles’ logic, and agreed with the Regulators. Sometimes the fire engineering approach also throws up the need for enhancements in published standards (For e.g. Atriums scope BS 5588/7 and expansion joints in fire rated ducts).

British standards are largely written by the industry that they serve, which may explain their often over-complicated solutions.

Areas that are prone to a fire engineered approach include:

• Fire resisting ducting specifications: the need for ‘burn out’ standard of ducts as per BS 476 Part 24 is not overriding in our experience, and other forms of ducting are often specified instead, including enhanced DW 144 systems. Type A and B systems to Part 24 are not necessarily interchangeable.
  
• Fire rated dampers and smoke dampers: identifying the function of these, method of control can simplify their complexity, control methods and numbers.
  
• Use of smoke screens and curtains: can be simplified and made less onerous
  
• Pressurisation systems can be integrated with smoke extraction systems from the accommodation to ease the duties of them, as can the use lobbies. Simple gravity pressure controls, and the use of lift shafts as pressure conduits reduce installation complexity and long term maintenance.

Ducting in Fire Engineering: the designation of compartments and areas within buildings, for security, noise and occupancy reasons, only increases. Buildings are also highly insulated thermally. Ducting is needed to service this compartmentalised layout but not compromise it in the event of fire. To provide passive protection and the facility to extract smoke from just one location, fire rated ducting is an essential tool of the designer. Lesser specification smoke extract ducts are similarly important. Most large projects have either smoke extract or fire rated ducting on them.

Case studies

• Millennium Stadium, Cardiff: use of fire engineered approach to duct performance, fan performance and kiosk fire resistance, reduced complexity of project. Saving of 15% on £ multimillion smoke control package, built and installed by general site duct contractor. Designed, Engineered and approval obtained by Airpocket Designs Ltd over a period of 2 years. Other consultants involved.
  
• Wales millennium Centre: simplification of smoke damper and temperature rated ducting design, afforded. In-duct fire temperature thermocouple monitors used, partly at the request of the brigade. Extensive Regulator negotiations, final agreement obtained by Airpocket Designs Ltd. Substantial cost savings on budgets.
  
• Odyssee Centre, Belfast: design rationalisation of 12 fire pressurisation systems. Use of gravity systems as opposed to variable speed initially proposed by client. Simplified controls, commoned-up fan procurement and duct arrangements. 50% saving on client’s budget. Agreement obtained with NI fire Service and Belfast City Building Control, by Airpocket Designs Ltd

Standards interpreted/implemented include

1. Morgan H.P. Ghosh B.K. et al. Design methodologies for smoke and heat exhaust ventilation. Building Research Establishment Report 368
2. B.S. 5588: 1991: Fire precautions in the design, construction and use of buildings: part 10: code of practice for shopping centers.
3. B.S. 5588: 1991: Fire precautions in the design, construction and use of buildings: part 5: code practice for firefighting stairs and lifts.
4. B.S. 5588: 1978: Fire precautions in the design, construction and use of buildings: part 4: code practice for smoke control in protected escape routes using pressurisation.
5. B.S. 5588: 1998: Fire precautions in the design, construction and use of buildings: part 4: code practice for smoke control using pressure differentials,
6. DW/144:1998: H.V.C.A.: Specification for sheet metal ductwork; low, medium and high pressure systems.
7. Fan and Ductwork Guide: Fan Manufacturers Association: 1993.
8. B.S. 848: Part 1: 1980: Fans for general purposes: Methods of testing performance.
9. B.S. 7346: 1990: Parts 1 & 2: components for heat and smoke control systems, natural ventilators/powered ventilators.
10. BS 476:1987: Fire tests on building materials and structures: part 24: Method of determining fire resistance of ventilating ducts
11. BSI: DD 240 Fire safety engineering in buildings: Part 1: 1997: guide to the application of fire safety engineering principles
12. Butcher E.G., Parnell A.C., (1979) Smoke Control in Fire Safety Design: E. & F.N. Spon, London

Fire Safety Approaches (comments by GC Miles, Fire Engineering Consultant)

• Prescriptive: these are experience based. They were originally based on the experience of Britain in the Second World War. Generally they consider burnout of the whole compartment, because in those days fire suppression and intervention techniques had not been developed much, past the fire hose stage stats not complete
  
• Probabilistic: this a statistical approaches to fire safety in Buildings. This is where the likelihood and severity of fire starts is related to the size and function of the building in question. It will be covered in the forthcoming BS 7974/7 and is an important part of fire engineering. The weakness is that the basis in the UK is the Home office statistics of fires, themselves sourced from operational brigades
  
• Fire Engineering: this is the application of science to fire safety judgements, BS 7974 Parts 1-6. It is termed ‘deterministic’ as there is a definite causal relationship between a given set of assumptions and the likely outcomes in the event of a fire. It also encompasses the life of a building and its conditions of use. This leads in to Risk Assessment. The causal relationships are equations relating fire parameters and typically allow predictions of the following relevant events

  Ignition time
  Detection time
  Alarm time
  Recognition time of danger
  Pre-movement time for escapees
  Evacuation time
  Approach time for Tenability limits for breathing
  Approach time for visibility limits for escape
  Time to flashover and ventilation control (see illustration below)
  Time for suppression devices to deploy
  Effects of wind direction and strength
  Time for damage to property to become significant
  Time for Brigade to intervene and control situation

• Burning: only gases burn, - an oft disputed fact. The prediction of volatile material emanation, from a fuel is the key indicator of most fire hazard parameters of the fuel. The question of compartment ventilation is also key, as fire is a gas-phase phenomenon

An event predictable by Fire Engineering: typical Ventilation Controlled Flame plume, volatiles starved of O2 in compartment

Risk assessment (RA): this should deal with confirming the assumptions that were made at the time of the design of the building. Are they still valid and are the control measures adopted in the fire protection design still valid? Many RA’s that we come across do not even acknowledge the presence of a strategy, that may or may not be valid. For example a key strategy assumption might be that occupants will escape under the influence of a fire alarm signal. If the occupants ignore the alarm, the whole strategy is floored and invalid, (Rhode Island Tragedy, February 2003). Most major disasters by definition display fire and occupant behaviour that is “outside the envelope”. Even worse, some disasters have occurred and no obvious envelope or assumptions were even documented. To guard against this the Workplace Regulations place the risk squarely on the shoulders of the building operator or owner.

To protect the owners the chart below relates these issues and obviously Airpocket Designs Ltd are involved at various points on the chart:

Audit Trail: before fire engineering arrived in the late 1980’s all buildings complied with the precise limits of Building Regulations. Apart from record drawings and good housekeeping the owner had a relatively straightforward set of responsibilities. All that changes with Fire Engineered buildings. In the event of a claim or complaint the burden of proof would be on the owner to demonstrate that the strategy was being fully implemented. The only conceivable defence would be that the original assumptions were not valid, for example the Twin Towers. Without this audit trail link to the strategy, negligence would be an obvious conclusion. Many owners are blissfully unaware of the risks they are taking both on behalf of their employees and themselves personally. The law also has been turned 180 degrees to shift all the responsibility onto the owner, via the Workplace Regulations. A realistic RA is required for all premises where more than 5 persons are employed. Terrorist induced arson is a threat that has highlighted the need for sound fire protection strategies. At the same time the facilities provided by the statutory authorities are in decline, with the Fire Brigade and police being effectively reduced in terms of per capita cover.

Fire Engineering and cost: it has been estimated that Fire Engineering could reduce Building costs across the EU industry on average by 3%. For building types particularly prone to Fire Engineering, ie complex ones, we estimate it could be 10%. The proviso is that they need to be kept under review by RA. This is a cost albeit a small one that can be seen as an ‘insurance policy’ on the continuing safety of the building. Fire Engineering gives good cost news and also the bad variety. Responsible designers weight both dispassionately.