Time to make a quick getaway

The cost associated with providing a robust fire strategy for a building ranges between 1% and 10% of the total construction cost, depending on the complexity of the design and the size of the building.

Arguably, the single largest component of the strategy is that which deals with evacuation from the building – or Building Regulation B1 as it is known by those who are familiar with the dark art of fire safety engineering. This Regulation essentially requires that a building must be designed and built so that people can be warned about a fire and, once aware, can leave the building safely.

Approved Document B, issued in support of the Building Regulations, takes up 43 pages detailing relatively straightforward ways in which this goal can be achieved – and the various British Standard documents are no better in this regard. With the advances that have been made in fire safety engineering, it is now time to reconsider how we approach the issue of building evacuation and evacuation strategies, and to seriously question conventional wisdom on the width of escape routes, travel distances and the time allowed for people to leave a building.

The main weakness with the current simplistic codified approach to determining the width of escape routes is that it uses a hydraulic model to assess the adequacy or otherwise of the escape provisions. By convention, the width of exit routes from a building are determined by dividing the number of people to be evacuated by the number of routes available. This treats people as unthinking entities and assumes that they will distribute themselves equally between the available exits, and is based on a study of fluid flow through a system – hence the use of the term ‘hydraulic’.

Although this is an easy approach to understand, it is an oversimplification. A more rational approach is to recognise the fact that people exhibit characteristic behavioural traits, and will try to avoid adopting new behaviours even during an emergency. As an example, research supports the view that people will try to exit a building using the routes with which they are familiar, even in an emergency, and will often ignore dedicated fire exits that would give them a quicker route to safety.

There is some evidence to suggest that up to 70% of people will try to exit by the same route that they used to enter the building and, this being the case, the main access/egress routes should be made larger than suggested using the hydraulic model. In compensation, it would be reasonable to reduce the size of the other escape routes as they are likely to be under-utilised. Alternatively, integrating dedicated escape routes into the routine circulation routes within the building would promote more efficient use in an emergency.

Going the distance

Travel distances are another part of the evacuation philosophy that should be reconsidered. Travel distances within a building are based on the premise that any compartment should be fully evacuated in less than 2.5 minutes. This can impose severe limitations on the design and servicing of large or complex buildings. A more robust approach is to objectively evaluate how long is needed to enable escape from a space within a building, and then to compare this with the time it will take for conditions within that space to become untenable. In fire engineering parlance, the former is referred to as Required Safe Escape Time (RSET) and the latter as Available Safe Escape Time (ASET), and as long as ASET is greater than RSET, all will be well.

Once these two parameters have been quantified, either can be reduced or extended as appropriate, by the judicious use of fire systems. For example, in order to reduce the required escape time, it is necessary to look in more detail at the various components that contribute to the total evacuation time from a space within a building. Thus:

t_escape = t_recognition + t_response + t_travel + t_doors + t_stairs

where:

• t_escape is the total evacuation time
• t_recognition is the time taken for people to realise that an incident has occurred
• t_response is the time taken for an individual to begin to respond appropriately
• t_travel is the time taken to walk to the nearest available exit
• t_doors is the time it takes a person to pass through the exit door, including queuing
• t_stairs is the time it takes for a person to travel down a staircase to a final exit from the building.

If the pre-movement time – that is the recognition and response time periods – can be reduced, we can increase the travel time, thereby increasing the travel distance, without increasing the total evacuation time. The simplest means of reducing the pre-movement time is to use a voice alarm system in place of simple bells, sirens or sounders. Research from a number of large scale experiments has shown that pre-movement times in excess of 15 minutes may be observed with the use of alarm bells alone, compared to a pre-movement time of less than two minutes with a system of directive voice alarms.

As an example of how this approach can be used, consider the following example. A building designed in accordance with Approved Document B can have a maximum travel distance of 45 m from any point in the building to the nearest storey exit, ie doors to a staircase or a final exit from the building. That same building may, by analysis of the fire load and distribution, be calculated to have an ASET of, say eight minutes. Adopting a reasonable safety margin, we can decide that we want the space to be fully evacuated within four minutes. The provision of a voice alarm system can be expected to result in a pre-movement time of one minute, therefore leaving three minutes available for travel to an escape route.

Allowing a conservative travel speed of 0.6 m/s, this generally being suitable for ambulant disabled people, the travel distance could be as much as 100 m. No account has been taken of the need to queue at the doors from the space, but even allowing for this, travel distances of between 80 m and 90 m would not be unreasonable. It is interesting to note that the BSI document, DD 9999 Code of Practice for Fire Safety in the Design, Construction and Use of Buildings, recommends that in certain cases travel distances of 90 m are acceptable.

This is obviously a very simple example of a complex situation and a complete egress analysis will take account of factors such as the impact of merging flows in a staircase, the effect of occupant density on travel speed and movement up and down stairs, as well as a host of other variables.

There will also be an inherent time delay between the ignition of a fire and the time at which the fire detection system, if present, will raise the evacuation alarm. Conventional point-type smoke detectors rely on the convection currents generated by the fire to carry the smoke and other combustion products to the detector. Depending on the size of the fire, the height of the ceiling and the location of the nearest detector, this may take a number of minutes – and this delay can easily be further extended if the airflows generated by the HVAC system have not been taken into account when designing the fire detection system. In such cases, the use of an aspirating detection system will probably significantly reduce the delay to a more acceptable level.

Buying more time

The evacuation problem can also be approached from the ‘other end’, by buying more time for evacuees.

The time available for escape, ie the ASET, can be extended by the provision of other fire safety systems, such as automatic sprinklers, water mist systems, additional fire compartmentation and smoke management systems. As you increase the ASET, so you can justifiably increase the RSET within certain limits, but as these values increase, the effect of the fire on the structure of the building needs to be considered in more detail.

Fire engineers are regularly criticised for relying heavily on active fire safety systems in order to arrive at an acceptable fire safety strategy for a building, and for neglecting the role of passive fire protection. Although there may be some truth in this view, more often than not it is client-driven and we are responding to the needs of the architect. There is, and always will be, a need for fire resisting walls and floors, but the bigger and better buildings that dominate our cityscapes could not be realised without innovative building systems and fire safety systems are just one part of the overall solution.