14 Nov Maintaining the WOW factor
Posted at 10:41h in News
Mass transit design has become more complex over the years with hubs becoming recognisable, focal points to a city’s architecture. It is an opportunity to welcome people to the city and impress on visitors a strategic vision where the boundaries of design are stretched to combine architectural expression with functionality. With the unique occupancy characteristics which come with a mass transit development and the often underground nature of the buildings, how are engineers using evacuation modelling to ensure safe evacuation for occupants whilst maintaining the ‘wow factor’ of complex design?
Simple hand calculations have been the tool of choice for calculating evacuation time since its development in the 1960’s. However, with the increase in architectural complexity of buildings and computational power of modern computers, engineers are relying more on computer based evacuation models in developing an engineered solution to means of escape strategies, particularly in mass transportation hubs where there is a high number of occupants.
Evacuation modelling is often incorporated into design
As part of a ‘time-line’ method where the Required Safe Egress Time (RSET) is compared with the Available Safe Egress Time (ASET). The evacuation model provides the RSET whilst the ASET (i.e. time to untenable evacuation conditions) can be determined, for example, by the results of a Computational Fluid Dynamics (CFD) analysis. However, for mass transit systems there are often code specific requirements for clearing platforms or concourses such as NFPA 130 which assigns a maximum time to reach a place of safety.
For any evacuation model, it is integral to provide suitable input parameters which reflect the nature of the development in question and calls upon the engineer to assess the likely occupants on a case by case basis. For some buildings the regional location can have an implication on these parameters but the nature of transit systems is often that they introduce a wide variety of occupants from various regions, therefore allowing the parameters to be broader. Ideally, research into existing hubs can help develop these parameters, or it is left to a mixture of code guidance and research papers that are then discussed and agreed with the local approving authorities.
There are key difficulties associated with providing input parameters for mass transportation hubs:
Speed, size & flow
Research data for entity profiles is often regional when it comes to speed, size and flow rates. However mass transit systems can introduce occupant types that vary on a daily basis.
Often approval authorities enforce entity profile speeds, size and flow rates to meet local code guidance which is not always compatible with the calibrated data within the modelling software. Some engineers follow default options integrated within the software which has often been calibrated based on empirical data researched by the software company. However, this research is not always made public which can call into question their validity in forming a suitable engineered solution.
Pre-movement and alarm time is incorporated into a model to determine how long it takes for the alarm to be raised and how long it takes for an occupant to respond once the alarm sounds. Pre-movement phase can form a significant portion of the total escape time.
The pre-movement and time to alarm is an interesting parameter as some areas of stations are often not provided with detection, staff numbers can vary and occupant behaviour in the station will be affected by the nature of the emergency and the environment. For example where occupants can see fire or smoke, the pre-movement and travel behaviours will be different to those who cannot.
Karl Fridolfs 2010 Paper ‘Fire evacuation in underground transportation systems: a review of accidents and previous research’ stresses the complexities of analysing human behaviour as multiple factors are involved. He states that a major issue related to fire evacuation in underground transportation systems is that people are often reluctant to initiate an evacuation. Occupants are often focused on reaching their destination and do not want to be disrupted, there can be a lack of understanding and coherent information on what to do. Social influences can
also have a big effect on the decision making process. For this reason, incorporating human behaviours into a simulation is no easy task, especially as some codes only call for an evacuation time from the most remote part of a platform to a place of safety. This means some engineers do not include pre-movement time within their simulations.
BS 7974 – 6 The application of fire safety engineering principles to fire safety design of buildings -Human factors: Life safety strategies – Occupant evacuation, behaviour and condition calls for a pre-movement time based on 3 categories – the Alarm system, Building Type and Management. From this, a minimum, maximum and mean pre-movement time can be calculated and used in the model. Transportation buildings will incur longer pre-movement times to reflect the complexities of the building and occupants’ unfamiliarity with the building’s layout.
Whichever method is incorporated, these generalised calculations cannot really predict how occupants will react in a real life emergency situation.
Route choices have a significant impact on evacuation times and a number of theories have been developed that can be applied to the evacuation model.
Some modelling software packages have a “closest exit” default setting for exit simulated occupants’ choice. This is contradictory to published research results by J. Sime , which shows that occupants are more likely to use exits that they are more familiar with rather than assuming that occupants will use the exits closest to them. Assuming that occupants will use the closest exit can generate unrealistic, shorter evacuation times.
Exit choice is also affect by other social factors, such as ‘herd behaviour’ where people tend to follow others towards an exit; a behaviour rarely implemented in evacuation models.
Evacuation modelling for fire safety is a design aid only and it should not be viewed as a true reflection of human behaviour or what will occur in the event of an evacuation due to the number of assumptions that need to be incorporated. Based on this, sufficient factors of safety are vital in ensuring a safe design. Even though human behavioural analysis is difficult to apply in RSET calculations, this is not to say that it should not be used within the building design. It can be a key tool in assessing where additional fire safety measures can be applied to aid escape within mass transportation hubs particularly in relation to management. Passengers should understand that they need to evacuate in an emergency and not continue on their journey, staff should understand that no further occupants should be allowed into the building during an emergency, exits should be clear and signage concise and sufficient systems in place to minimise confusion and panic.