Fire Safety Engineering Concerning Evacuation: Available Safe Escape Time - Term Paper Example

 Fire Safety Engineering Concerning Evacuation: Available Safe Escape Time Introduction The concept of performance based life safety design has been used to design safety systems for many buildings as regards fire safety management for many years. There are many different attributes of this concept that has been well researched on with many authorities providing a lot of content in literature about how it can be used to ensure effectiveness in fire safety management (Standards Policy and Strategy Committee 2003). According to Cote (2000), performance based life safety design is largely defined by the comparison between the time that is required for escape (Required Safe Escape Time (RSET) and time to loss of tenability referred to as Available Escape Time (ASET). By definition RSET refers to the time that is calculated between ignition of a fire and the time at which people that were previously at risk of the fire within the building are able to escape to safety (CIBSE Guide 2003). As is the case in most situations, there may be need to create a temporary space where people caught in the fire within a building are evacuated temporarily before they are eventually taken to the place of safety outside the building that is not under any immediate danger and at a safe distance from the danger of the fire (Standards Policy and Strategy Committee 2003). On the other hand, ASET is defined at the calculated time available between ignition of a fire and the time at which tenability criteria are exceeded in a given space within a building (NFPA 2010). These two terminologies refer to concepts that both include a number of stages that have to undergo through different processes requiring a range of data to be input in the consideration (CIBSE Guide 2003). There are problems that are presented for engineers dealing with fire safety for buildings that require serious consideration since there are cases where different stages require different amount of detail and hence focus when dealing with them. It is therefore incumbent on the design engineers to know how to allocate the resources and focus they have for the different stages involved in order to be able to realize desired outcomes of ensuring safety for people at risk of fire injury within a building (CIBSE Guide 2003). Design Approaches for Different Fire Conditions in Fire Safety Management System There are different approaches that can be used in designing a fire safety management system using the ASET and RSET concepts based on different conditions. For this exercise, the following are the specifications for the building condition in which the fire safety management system is to be designed: The area in question is a storage warehouse for building materials (which may include combustible materials) with dimensions 40m by 32m by 12m high (See Figure 1). Figure 1 – Ground floor plan of warehouse The building will only be accessed by trained staff and not members of the public The owner of the warehouse wants to add a mezzanine floor (see area in red on Figure 2; The arrow on stairs indicates travel upwards). The floor of the mezzanine will be located 8m above the ground floor level. The mezzanine will be occupied by up to 15 members of staff and there will be no disabled access. Figure 2 – Proposed mezzanine arrangement A single open stair will be located towards one end of the mezzanine (see Figure 2) and this will be the only way to access/exit the mezzanine. There are different principles that guide the design of fire safety management system for the defined building that have to be considered in order to ensure a good design is attained. In general, the main aim for this design is to ensure that there is a safe environment for building occupants whenever they would want to remain in the building and to provide a safe means through which they can escape in the event of a fire incidence. According to Babrauskas, Fleming & Russel (2010), traditionally, such designs have concentrated on physical provisions for means through which occupants can escape. This prescriptive methodological approach is advantageous because by it, it can easily achieve fairer levels of safety by applying simple guidelines that it uses which have been used and trusted before. Inasmuch as the traditional approach is not usually time based, the various elements that are used in the approach relating to exit and stair provisions usually have time-based calculations that have to be done well and to the required levels of accuracy and efficiency (CIBSE Guide 2003). For the RSET time line, the engineers should give most of their time and attention on the travel time component representing the actual physical movement of people as they move from the place of the fire through the escape routes towards a designated place of safety (Chenebert, Breckon & Gaszczak 2011). However, there is a lapse of time that has to be accounted for and considered where the occupants prepare before actually starting the movement through the escape routes. This is referred to as the pre-movement time which is occasioned by people’s behaviours before following escape routes such as panic attacks, confusion, and preliminary attempts to put out the fire where applicable (Chenebert, Breckon & Gaszczak 2011). This time is actually a considerable component of the total escape time that has to be considered by the engineers working on the escape time calculations. According to Babrauskas, Fleming & Russel (2010), these pre-movement time is dependent on different key features that include occupancy type, occupant characteristics, warnings, fire safety management strategy, and building complexity among others. In this regard, it is recommended that for a practical approach in designing the fire safety management system, the engineer should apply pre-movement time distributions based on monitored evacuations and fire incidents within the same facility or where it is not possible, to engineers should use behavioural models using differently designed behavioural scenarios that are based on the key features that are listed (Chenebert, Breckon & Gaszczak 2011). Be that as it may, there are different challenges that can be identified when trying to make these determinations especially as regards evacuation of travel time key of which the assumption by most calculations that there are no interactions between the occupants and the fire effluent (Cote 2000). For instance, if the occupants of the building are exposed to smoke which irritates them and affects their visibility, their movement speeds are most likely going to be reduced and this requires a recalculation that factors in the effects caused by such irritant smoke for occupants depending on the levels of concentration of the smoke within the building. According to Babrauskas, Fleming & Russel (2010), the ASET time usually ends when there is a prediction of occupant incapacitation from exposure to the fire effluent which is largely dependent on the time-concentration curves for the main toxic fire effluents. This requires the engineer to put into consideration of smoke and toxic product density yields into the calculation formulas as the case may be depending on the conditions of the fire (NFPA 2010). The formula and mathematical relationships that have been developed and the ones that are used among engineers today only consider smoke density and carbon monoxide and treats yields as constant factors in the computation for cases where the fire is well ventilated (Babrauskas, Fleming & Russel 2010). Babrauskas, Fleming & Russel (2010) provide that in order to predict ASET times, it is important to estimate the time-concentration or intensity curves for the major toxic products, heat, and smoke in a fire. This is covered in PD 7974 to PD 7974-3 in the Production Document Series. As for RSET calculations, Babrauskas, Fleming & Russel (2010) hold as has been explained above that escape time depends on detection, warning, and different parameters that affect occupant evacuation movement and behaviour. There are two broad categories of approach that are used in understanding occupants’ evacuation behaviour and these include the following: Pre-movement behaviours – this refers to the behaviours that first occur for occupants before they start moving into the escape routes which may include time periods when the occupants are inactive and those where occupants are involved in movements but not movements that take them to the escape routes (Babrauskas, Fleming & Russel 2010). Research shows that this phase more often than not usually ends up taking up the longest time of the total escape time (Babrauskas, Fleming & Russel 2010). Travel behaviour – this refers to phase where occupants are involved in actual movements into and through escape routes leading them towards safety. Tenability Limits and Criteria for the Design Tenability by definition refers to the “maximum exposure to hazards from a fire that can be tolerated without violating safety goals” (Standards Policy and Strategy Committee 2004, p. 8). As far as hazard assessment is concerned, tenability criteria for such a design have the following four main considerations as far as the means of escape and life escape is concerned: The psychological effects on the occupants’ escape behaviour when they see the fire effluents in the absence of direct exposure The psychological and physiological effects that the fire incident attributes of heat, toxics contained in it and smoke has on the occupants’ escape behaviour and mobility The place where the exposure to the fire incidence leads to incapacitation The situation where the exposure to the fire incidence leads to death In this regard, good design guidelines require that there be reasonable tenability limits for occupants to remain in a given place that is considered relatively safe or use a given route that has been sanctioned as safe (Standards Policy and Strategy Committee 2003). In the case where there is zero exposure to fire, the simple criteria that can be used for tenability criteria is based on a minimum clear layer height of 2.5 m above the floor and a maximum upper layer temperature of 2000C. In this condition, the assumption is that the occupants are both able and willing to escape in clear air as the downward heath radiation is quite manageable and tolerable (Standards Policy and Strategy Committee 2004). In the case where smoke is mixed with the air nearing the floor levels, it may be difficult for some building occupants may be unwilling to enter such escape routes or may be unable to find the escape routes altogether. Based on these considerations, engineers can design limits for optimal density of smoke following the guidelines in BS 7899-1 and BS 7899-2 documents. Bibliography Babrauskas, V., Fleming, J. & Russel, D. 2010. “RSET/ASET, A Flawed Concept for Fire Safety Assessment”. Fire and Materials, vol. 34, pp. 342 - 355 CIBSE Guide. 2003. Fire Engineering. London: Building Services Engineers. Standards Policy and Strategy Committee. 2004. The Application of Fire Safety Engineering Principles to Fire Safety Design of Buildings - Part 6: Human Factors: Life Safety Strategies - Occupant Evacuation, Behavior and Condiiton (Sub-System 6). London: British Standards (BS). Standards Policy and Strategy Committee. 2003. The Application of Fire Safety Engineering Principles to Fire Safety Design of Buildings - Part 7: Probabilistic Risk Assessment. London: British Standards (BS). Chenebert, A., Breckon, T. & Gaszczak, A. 2011. "A Non-temporal Texture Driven Approach to Real-time Fire Detection". Proc. International Conference on Image Processing, (IEEE), vol. 1, no. 4, pp. 1781–1784 Cote, A. 2000. Fire Protection Handbook (18th edn). New York: National Fire Protection Association. NFPA. 2010. National Fire Alarm and Signaling Code. New York: National Fire Alarm Association. Read More