The LWF Blog
Evacuation Modelling | The Human Behaviour issueAugust 8, 2013 11:36 am
Evacuation modelling techniques are designed, in the main, to be straightforward and simple to implement. They may be complicated by the size or complexity of the building, but they tend to assume that the people will react as they are supposed to. The humans in an evacuation modelling simulation will react immediately to the fire alarm and will make their way in a purposeful, but not panicked, way to the correct fire exit.
However, the issue faced by responsible persons in a fire situation is that not all people behave appropriately during fire alarm situations. The situation is complicated by various factors that often cause the building occupants to behave erratically.
Evacuation of buildings can be analysed in different ways. Approved Document B (ADB) which provides guidance on meeting the requirements of the England and Wales Building Regulations with regards to fire safety, specifies the width of exit routes, so that the maximum occupancy can exit the building safely within 150 seconds (2.5 minutes). Sometimes, a more flexible approach, such as that given in BS 9999, is also used for an evacuation assessment of buildings. Techniques offered in these guidance documents are generally straightforward, simple to use and suitable for most buildings.
However, in certain situations e.g. complex or innovative building designs, where the scope of traditional prescriptive design guidance cannot readily be applied it is necessary to consider a performance based design by developing a time based comparison of the time available for occupants to escape (Available Safe Escape Time – ASET) and the escape time (Required Safe Escape Time – RSET) to ensure life safety in the building. In this instance, evacuation models, based on engineering and computational tools, have been used to estimate the time taken to evacuate a building. While hand calculations as recommended in PD7974 remain a possibility to carry out such assessment, it can often be difficult where large numbers are expected or where more complex flow characteristics need to be considered. Also, evacuation models can offer more visual and easily understandable graphical outputs.
LWF has recently undertaken an evacuation modelling exercise for a complex public assembly building in London. From the modelling exercise, it was identified that there were some limitations of the model and it did not necessarily reflect the variable nature of human reaction in a fire. The key considerations of human behaviour need to be taken into account, so that a more comprehensive and predictive model can be developed for a building’s fire evacuation.
Understanding of Fire Precautions and Management Systems in place
The results of evacuation models could be misinterpreted if the fire precautions and management systems for a complex public building are not fully understood. The evacuation model may show that a fire alarm sounds in a public assembly building and everyone reacts promptly, only using designated stairways and exits, and making their way outside in a brisk but orderly manner.
However, the evacuation model does not take into account one factor of human behaviour – “exit choice behaviour” – the different exits that people will choose to leave by, often because they are also the entrances and routes by which they arrive. This is especially true where the occupants are unfamiliar with the layout in public buildings. In a real evacuation scenario this can lead to congestion in the main entrance/exits and queuing inside buildings at those discharge points. As a result, effective means of escape can be heavily dependent on robust management of the public by member of staff. All available alternative exits must be used to allow the quickest possible evacuation. If managed evacuation is not possible, the design should be examined and the exit choice ratio for evacuating occupants should be justified. A number of cases should be run to examine the effect of exit choice on evacuation time.
The simulation of the reaction of occupants in large public buildings which adopts a progressive evacuation should also be considered. Although it is possible to integrate fire alarm cause and effect functions into the evacuation models – so that only the alarm zone activated will evacuate and people in the adjacent safe zones do not enter the fire area – the actual human behaviour in a fire situation may not reflect this.
The psychology is that a fire alarm in itself is not necessarily regarded as an immediate call to action. This is particularly true where the alarm may have sounded, but no immediate threat is apparent or where occupants may have other activities such as making a purchase. However, even when the alarm is taken seriously, it is also a source of confusion, because the alarm is simply a loud noise. People often act inappropriately, but rarely panic or behave irrationally. Such behaviour, to a large extent, is due to the fact that information is not available to people regarding the possible existence of a fire and its size and location.
In order to ensure an effective evacuation it is paramount to rely on trained members of staff to manage the evacuation, by motivating and guiding the evacuation of members of the public. Voice alarm systems are largely becoming a more acceptable mode of informing occupants of a fire in modern buildings. Large premises, which are designed to cater mainly for the general public, will benefit greatly from a voice alarm system where voice messages can convey a greater amount of information to the occupants.
Final Exit of Evacuation Models
In most evacuation models, evacuation stops at the external exits from buildings. However, for some evacuation scenarios, people may still be required to travel for significant distances along external routes, before reaching a point from which they can leave the premises.
Congestion in the external routes should be assessed to determine whether there are extensive queuing times in some locations, particularly where merging flows occur at the external routes and external exits from buildings. In addition, scenarios such as people stopping at exits due to inclement weather should also be examined as this could reduce the flow of occupants in an evacuation.
People going back for their Belongings
Based on an interview with the senior management team of a complex public assembly building in London, we identified that one problem faced by staff was where people wished to re-enter the building to recover items from the cloakroom during an evacuation. Again, this behaviour can cause delay and congestion at exit points and can be difficult to manage for staff. The impact of such behaviour is very difficult to model.
Evacuation models do not take the behaviour of disabled evacuees into account. An extensive evacuation time may occur in the event of evacuating a large group of people with disabilities in buildings. The challenges may involve assisting people with a physical disability, or the potential issue could be one of understanding and reacting to the situation. This would be particularly true with conditions such as dyspraxia or autism. Staff may encounter unpredictable or fearful behaviour, including potentially violent behaviour. To overcome the limitation of evacuation models regarding the evacuation of people with disabilities, designers should show a level of design redundancy which provides a factor of safety in the models.
The lack of suitable experimental data to validate large evacuation models of public spaces presents a challenge to evacuation modellers. Most evacuation experiments are designed and conducted for practical purposes and not necessarily to support the development of evacuation models. Inevitably, certain assumptions are required to be made based upon the understanding of the fire precautions and management systems in place. Many of the assumptions are associated with the building’s reliance on a managed evacuation and the rapid response of staff. Therefore it is important to note that where effective evacuation management cannot be achieved this may lead to extended evacuation times beyond those determined by the evacuation models.
Evacuation models can only take us so far in designing safety. Apart from implementing effective evacuation management, evacuation modellers should try to understand human behaviour in an emergency situation, to influence the decision-making processes. By understanding those factors and processes, an evacuation modeler can then develop a more comprehensive and predictive evacuation model.
For more information on fire safety and evacuation modelling, please contact Peter Gyere, Marketing Director.
Tel: 020 8668 8663
Fax: 020 8668 8583