Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Evacuation of buildings can be analyzed in different ways. Approved Document B (ADB) which provides guidance on meeting the requirements of the England and Wales Building Regulations with regard to fire safety, specifies width of exit routes such that the time required for occupants to flow out of a prescriptively designed enclosure, containing a maximum design population is 150s (2.5 minutes). Sometimes, a more flexible approach, such as that given in BS 9999 is also used for the evacuation assessment in buildings. Techniques offered in these guidance documents are generally straightforward and 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 have recently undertaken an evacuation modeling exercise for a complex public assembly building in London. From the modeling exercise it is identified that there are some limitations of the model and it does not necessarily reflect the variable nature of human reaction in a fire. This bulletin will discuss the key considerations of human behaviour 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. 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. That is how an evacuation model might simulate a fire situation. But it does not consider what is termed “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 such that only the alarm zone activated will evacuate and people in the adjacent safe zones do not enter the fire area the actual human behavior 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 behavior, to a large extent, is due to the fact that information initially available to people regarding the possible existence of a fire and its size and location is often ambiguous or inadequate.
In order to ensure an effective evacuation it is paramount to rely on the trained members of staff to manage the evacuation by motivating and guiding the evacuation of members of the public. In some cases, voice alarm can be used in conjunction with a managed evacuation to ensure the robustness of an evacuation strategy of public buildings. Voice alarm systems are largely becoming a more acceptable mode of informing occupants of a fire occurrence 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.
Factor in Human Behaviour
Defining human behavior in evacuation models for public buildings is not as simple as just to adjust the occupant travel speeds or change the shape or size of each evacuee. From our experience, the followings are some examples of human behaviors that should be considered when simulating an evacuation model in public buildings.
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 required to travel for significant distances along external routes before reaching a point from which they can leave the premises. Many of the available escape routes discharge to the external space which can essentially be considered as external corridors and the management of people in these areas must also be considered in the event of a large scale evacuation. 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, the 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 Re-enter the Building to Collect their Belongings
Based on the interview with the senior management team of a complex public assembly building in London, we identified that one problem faced by staff was encountered where people wished to re-enter the building to recover items from the cloakroom in 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. This is particularly true when there are a group of people with conditions such as dyslexia, dyspraxia or autism visiting a public building such as a museum. These people may not be aware of their impairment. Some of them may present unpredictable behaviour (including violent behaviour), which may impede staff in an emergency. To overcome the limitation of evacuation models regarding disable evacuation, 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 model of public spaces presents a challenge to evacuation modelers. 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 made 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 modelers should try to understand human behavior in an emergency situation, particularly the factors that have been discussed in this bulletin 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.
This bulletin was written by Ken Seow MSc
Peter J Gyere
Lawrence Webster Forrest Limited
LWF are fire engineering and fire risk management consultants with over twenty five years experience in the development of fire engineered technology and the application of fire safety standards including fire engineered techniques.
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