Extended travel distance in a fire – How far is too far?

The guidance of Approved Document B (AD B) provides general recommendations for all types of buildings and you are deemed to have met the requirements for Fire Safety of Schedule 1 to the Building Regulations if you have followed these recommendations. Consequently the conditions experienced in a fire situation in a building which has been designed to AD B are considered to be acceptable.

It is possible to design buildings in many different ways therefore other approaches can be used but the requirements for Fire Safety outlined in sections B1 to B5 of Schedule 1 to the Building Regulations must be met.

A comparative design approach can be taken – this entails comparing the level of safety achieved in the proposed design with that of the code compliant equivalent. If the level of safety in the proposed design is at least as good as the code compliant alternative, the proposed design will also have met the requirements of the Building Regulations for Fire Safety.

This is where fire engineering is of benefit – fire engineering uses a time based approach – all fires start small and grow, the speed of growth and the eventual size of the fire are affected by the amount, distribution and type of combustible items in the space, the amount of oxygen present, the size of the space, whether the walls offer any fire resistance and whether there is any form of fire suppression present. Balanced against this we have the time which it will take to get all the occupants of the building to a place of safety before conditions within the building become too difficult to escape.

The escape time is made up of different time steps – time from ignition to detection, time from detection to alarm, time between hearing an alarm and deciding that there is a need to leave the area (pre-movement) and the time taken to travel to a place of safety away from danger. All these time steps can be shortened by the introduction of different things – the time from ignition to alarm can be reduced by having an automatic fire detection and alarm system; the pre-movement time can be significantly reduced with good staff training including regular fire drills; the travel distance can be reduced by making the distances shorter from a place of danger to a place of safety.

Approved Document B restricts travel distance because this is an easily measurable parameter however travel distance makes up a very small part of the overall escape time. At an average walk speed of 1.2m/s it takes only 7.5 seconds to travel a further 9m however providing an automatic fire detection and alarm system can reduce the time from ignition to alarm by minutes. A lack of staff training and frequent false alarms can lead to people not leaving the building when a real fire occurs however regular staff training, maintenance of the fire alarm and reduction of false alarms leads to prompt evacuation of a building.

Fire engineering uses a combination of factors to take a holistic view of the building taking into account all the passive and active measures provided to deliver a building which is safe in the event of fire but allows greater freedom to designers to achieve their goals.

What design freedoms would you like to achieve – get in touch with Innovation Fire Engineering and we’ll help you to make them a reality in the most cost effective way.