Required Safe Escape Time and Available Safe Egress Time - Case Study Example

Table of Contents Table of Contents 1 Part A: 2 Required Safe Escape Time (RSET) 2 MEZZANINE area 2 Uncertainty 8 Part B 9 Available Safe Egress Time (ASET) 9 Parameters used in ASET 10 Estimation of ASET 11 Fire simulation dynamics 12 References 14 Part A: Required Safe Escape Time (RSET) This is the time available after fire ignition to the time at which the occupants in a building are able to move to a safe place. The time for escape depends on factors such as the warnings, the detection, and the occupant’s alertness and behaviour in response to fire. BS 7974 provides a systematic method for estimating the time for evacuation of the occupants in case of fire occurrence. RSET has three main phases. RSET can be expressed in form of detection and alarm time, pre-movement time, and movement time. Therefore, RSET = td + tpm + tm as shown in the figure below. A picture showing components of egress time MEZZANINE area The Required Safe Escape Time (RSET) for MEZZANINE area is determined following the guidance of BS7974. The plans and information available are listed below. The area in question is a storage warehouse for building materials (which may include combustible materials) with dimensions 40m by 32m by 12m high as shown in the figure below 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 as shown below on with red colour, located 8m above the ground floor level and occupied by up to 15 members of staff and there will be no disabled access. A single open stair will also be located towards one end of the mezzanine as shown. Detection time, td – This is the time taken for fire ignited to be detected by an automatic system or by an occupant. Detection time is dependent on the fire scenario and the device used in the detection. It is assumed detection occurs when occupants become aware of smoke by either visual awareness or when the facility fire alarm system is activated through smoke detectors, sprinklers, or manual pull stations and the building alarm sounds. According to Biddle (2014), a modelled scenario, a smoke detection activates in 7.88 seconds (Biddle, 2014). For egress analysis reasons, the time taken for detection and notification is 8 seconds. Pre-Movement Time, tpm – This is the time between the moment an alarm is sounded and the moment the occupants start to escape. It is affected by the perception, interpretation and action of the occupants. It can be divided into reaction and action time. In the reaction time, the person decides that something has gone wrong, confirms it and decides on what to do. In action time, someone decides to alert other people or put an alarm (Bris, Soares and Martorell 2009, 1167). However considering the evacuation and escape time for a group of occupants, there are two phases that include: The time between the raising of alarm and the movement of the first occupant The subsequent distribution for the group. BS 7974-6 covers the behaviour of the individuals as they respond to a fire alarm and the effects of heat, smoke and gases. Before exiting, most people have tendency to take preservative action like collecting valuable or important items. When estimating pre-movement time, it is important to take into account the occupant’s cognitive function ability. Some people may not take the seriously the audible fire warning, but they wait for further information for example clarification by the management or notification from a neighbour before beginning to evacuate. Thus, when designing the time for escape, it is essential to recognize the behaviour of the occupants in a fire incident such that pre-movement is taken into consideration. Other considerations include the physical conditions and distribution of the occupants, the age, gender, and other conditions, are very important parameters used to estimate movement time (Kong et al., 2014). Since the occupants are well trained, they are considered to be awake and familiar with the building. In this case, the pre-movement and pre-movement distribution time is very short. Based on the studies done in the past, the first occupants spent few seconds to move at the sound of an alarm in a well managed situation such as level M1. The first percentile pre-movement time takes half a minute, and the 99th percentile pre-movement takes one minute. The total time taken for pre-movement is 90 seconds (British Standards Institution, 2004). Movement Time, tm – This is the time taken by the occupants to move to a safe place and ends with complete evacuation. The movement is affected by the distance to the exit and the speed of the occupant. It is determined by the physical dimension of the building the unhampered walking speed and distribution of the occupants. Due to the low density in the current case, it is assumed that there are no impediments of the occupants as they walk to the exit. Travel time is also affected by the movement of the occupants through an exit. It is influence by flow capacity assuming that all the occupants arrived at the exit route at the same time (Kong et al., 2014). Total movement involves walking, queue and flowing. Walking time is the average time taken by the occupants to move from the present locations in the building to the escape exit route. It depends on the walking speed and the distance to the exit route. The walking speed depends on the sex and age of the occupants. Based on the past studies of the office buildings, the average unimpeded walking speed on horizontal distance is 1.25 m/s (Proulx, 2002, 345-360). Since there population density in the building, the walking speeds of the occupants are independent from each other. It is also assumed that the occupants begin to evacuate at the same time, anad ther was no impedement on walking by the occupants. Density - This is the ratio of the number of staff occupying the area and the total area occupied. This ratio measures the level of crowdedness at an exit route. The density of the people in an area is obtained by dividing the number of occupants by the area as in the equation below. The distance in the 1st step is given by √(282+ 152) = 32m Distance for the 2nd step = 40m -15m -10m = 15 m Third step, √(122 + 82) = 15 m Time = The walking Speed x Horizontal distance = 1.25 x (32+15 + 15) = 77.5 sec The occupants queue as they wait for room to move forward at an exit. As a result, the speed of moving through an exit is less than the speed at which the occupants are arriving at the exit. Queuing affects the speed of movement as result in waiting in the queue, thus increasing the time for evacuation. Flowing occur if the occupants arrive at the exit together which also affected the movement time. However, there is no queuing if the density is low. Based on the guidance provided by Nelson and Mowrer the occupants move at their own speed on the stairs if the density is less than 0.54 persons /m2. Thus, the unimpeded travelling speed on the stairs is 0.85 m/s (Nelson and Mowrer, 2002, 368-370). Time used on stairs = 8m/0.85 m/s = 9.4 sec The queuing cannot occur because the population density is low. Total movement time = 77.5 + 9.4 = 87 sec In this case, the density of the occupants is low, and they are widely spread. In this case the time required for evacuation is mainly dominated pre-movement distributions and movement time. The total time needed for complete evacuation = detection time + pre-movement time + movement time = 8 seconds + 90 seconds + 87 seconds = 185 seconds. Thus RSET = 185 seconds Uncertainty The uncertainty factors affect the accuracy of egress time. But due to uncertainties, RSET is multiplied by a safety factor of 1.5. Therefore, RSET = 1.5 x 185 = 278 seconds The design evacuation time takes into consideration the uncertainties caused by the behavior of the people during an emergency. Safety factor takes care of uncertainties that occur as a result of impediment due to the behavior or the apparent conditions of the occupants such as sickness, age (Xie et al., 2012). The total time required for safe evacuation in Mezzanine area is 278 seconds. Various considerations should be made when designing fire safety. These can include technical consideration and human response to fire. The technical requirements include sufficient provision of the means to prevent fires, early detection and warning, means of escape, smoke control systems, fire extinguishers, control of the fire growth and others. The design of fire safety is focused at providing safe environment for the building occupants. Therefore, the provisions for fire safety include safe means of escape, and the effectiveness of the means of escape in a building depends on how the time is used by the occupants in case of an emergency. The response to the fire by the occupants is complex. The psychological response of the people depends on their perception about the existing situation, which in turn affect the overall escape time (Xie et al., 2012). Part B Available Safe Egress Time (ASET) ASET starts from the time the fire starts to ignite to the time when the conditions inside the building are untenable due to the heat, smoke and toxic gases. ASET is dependent on the growth of fire and the ensuing development of untenable conditions along the escape routes. In order to calculate ASET, there should detailed study of fire from ignition. The fatalities in building fires are caused by exposure to heat, toxic gases, reduction in the amount of oxygen and smoke. For example, it may not be easy to tolerate the temperature of over 700, depletion of oxygen below 15% and the heat radiation of more than 3kW/m2. In addition, the evacuation along the escape routes may become difficult if the visibility due to smoke increase above 3dB/m. It is important for the occupants to evacuate before the untenable condition is reached (British Standards Institution, 2004). The objective of evacuation is to safe life in case of an emergency such as building fires. All the building occupants are evacuated to a safe place such that the fatalities are reduced to a low value as much as possible. In order to achieve this, the RSET time for a building should be less than ASET. Margin of safety is the differences between ASET and RSET. Thus, if the value of ASET is greater than RSET, the evacuation route will be considered suitable. The delay before beginning evacuation would expose the occupants to vulnerability. The behaviour of the occupants during evacuation would affect their movement in case of fire, and the response is affected by the psychological and the physical state of the occupants at the time of fire recognition (Tubbs & Meacham, 2007). ASET provides the maximum time the occupants are exposed to a fire hazard before reaching intolerable conditions. The building occupants should be able to go out of the building before ASET is reached. There are guidelines, which provide principles for the applications of engineering and scientific principles related to fire safety. Fire Safety Engineering is an application based on the understanding impact of fire, its behaviour and response of the occupants in case of fire emergency, and also how the people and the property can be protected. Fire safety engineering standards like British Standard, BS 7974: 2001, is concerned with application of engineering fire safety approach in building design and is the related to published documents (PD). Fire safety levels is based on the complex relationship between different factors that include fire initiation, fire growth and spread, the response by the occupants in the building, the surrounding environment that include the floor, walls, tables, etc., and the response by the fire detectors and other fire safety measures (Tubbs & Meacham, 2007; British Standards Institution, 2004). Basically, ASET is sum of RSET and margin of safety. In other words, tASET = tRSET + tMargin Parameters used in ASET Life safety is determined by factors such as the amount of heat radiation, inhalation of poisonous gasses such as carbon monoxide and visibility. Untenable conditions occur when individuals are exposed to excess amount of these parameters. The effects of these conditions depend of the age and health status of an individual. ASET may be increased by protecting the escape routes through provision of ventilation and sprinklers. a) Heat radiation produced due to from the fire may produce excessive heat. Skin burns when it is exposed to heat. But a radiant heat that can be allowed for a short time is 2.5 kW/m2. High temperatures may result in incapacitation of death. As the fire grows, the smoke and temperature increases. The amount of temperature that can be allowed for survival is 1200C for a short time. Less temperature is required if sprinklers are present (British Standards Institution, 2004). b) The presence of smoke can reduce the distance through which the occupants can be able to see. Poor visibility may result in the occupants not finding their way to the escape route. Thus, the visibility along the escape routes should be adequate. Visibility distance of about 12m is recommended, except in situation where the routes are very clear. c) Toxic gases such as carbon monoxide can cause incapacitation or death to an individual depending on their concentration and the length of time someone is exposed to. The concentration of carbon monoxide, for example, should be less than 650ppm (British Standards Institution, 2004). Estimation of ASET Lower layer height criteria can be used to estimate ASET on fire originating from the building. This criterion requires limited data input. This method utilizes the equation for the conservation of mass and energy. The building has openings to the surrounding to minimize build up of pressure because of expansion of gases. The assumptions which are made when using this technique include the following. There is formation of two layers with the upper layer consisting of hot smoke and the lower is free of smoke. The density of the upper layer is constant The heat lose at the boundaries very small and can be neglected The fire heat release rate is low with respect to the space enclosed. The following equations are used. Where LLH lower layer height in metres, H is the height of the room in metres, S is the area of the floor m2, α is growth rate factor for t-squared fires in kW/s2, ρg is the density in the upper layer in kg/m3, ρa is air density at a temperature T in Kelvin, g is the gravitational force and cp is specific heat in kJkg-1K-1 (Tosolini et al., 2012). Fire simulation dynamics Generally, the objective of life safety is to protect the occupants from being exposed to extreme conditions. This is made possible by providing adequate means of escape and protection in the space and along the routes of escape. The time available before untenable conditions is reached depends of factors such as the type of materials used to constructed the building or in the building, the type of fuel, the building geometry, and the amount of ventilation (Hurley, 2015). Estimation of ASET is carried out using computational zone models like FAST, especially in rooms that has less complex geometry. Computational fluid dynamics program is used in the analysis of the complex interaction of untenable conditions such as the interaction between space geometry and smoke movement. The main parameters which are analysed include the increase in temperature and smoke movement. Fire models also provide visualised information about the growth of fire and development of untenable conditions. This is important in analysis of smoke movement in order to achieve better smoke management (Hurley, 2015). In order to evaluate life safety in case of fire, it is important to find information about the fire size and the effluents produced. For modelling purposes, a fire scenario is predicted by extracting scenarios which are appropriate for a given design. Factors such as fire location, the rate of heat release, types of fuel and the presence furniture and equipment, and the production of smoke and toxic gases (Nelson & Mowrer, 2002). If a fire originate inside the building, it important to determine its visibility, in order to determine the time at which the occupants will become aware of the presence of fire through the presence of fire or smoke or through an alarm, as well as to determine how the will respond to it. For the purpose of calculating ASET in a building, it is important to know the fire size, the extent in which it can be contained, structural failures, heat and temperature change in the enclosure (Nelson & Mowrer, 2002; Hurley, 2015). The concentration of toxic gases and optical density of the smoke should also be determined. This information is needed when assessing the tenability of the building to the occupants, and to know how the occupants will escape out of the building and the contribution of the escape routes to the time of escape. The products of fire forms well defined layers of smoke and gases with different temperatures in an enclosure. The height of the hot layer and the heat radiant in the downward direction should be determined. Installation of fire protection equipment along the escape is necessary to extend the time for ASET. In some circumstances, fire brigade may intervene in securing the safety of the occupants (Hurley, 2015). References Bris R., Soares C. G., and Martorell S., (2009). Reliability, Risk, and Safety, Three Volume Set: Theory and Applications, Sebastian Martorell, CRC Press Biddle J. (2014). Fire and Life Safety Analysis for the AF Child Development, Center Standard Design British Standards Institution, (2004). The application of fire safety engineering principles to fire safety design of buildings, British Standards Institution, Hong Kong Kong, D., Lu, S., Kang, Q., Lo, S., & Xie, Q. (July 01, 2014). Fuzzy Risk Assessment for Life Safety Under Building Fires. Fire Technology, 50, 4, 977-991. Nelson, H.E. & Mowrer, F.W., (2002). SFPE handbook of fire protection engineering: 3rd ed. National Fire Protection Association Society of Fire Protection Engineers, Quincy, MA. Proulx, G., (2002). SFPE handbook of fire protection engineering: 3rd ed. National Fire Protection Association Society of Fire Protection Engineers, Quincy, MA. Tosolini, E., Grimaz, S., Pecile, L. C., & Salzano, E. (August 06, 2012). People evacuation: Simplified evaluation of available safe egress time (aset) in enclosures. Chemical Engineering Transactions, 26, 501-506. Tubbs, J. S., & Meacham, B. J. (2007). Egress design solutions: A guide to evacuation and crowd management planning. Hoboken, NJ: Wiley. Xie, Q., Lu, S., Kong, D., & Wang, J. (January 01, 2012). The effect of uncertain parameters on evacuation time in commercial buildings. Journal of Fire Sciences, 30, 1, 55-67. Read More

Movement Time, tm – This is the time taken by the occupants to move to a safe place and ends with complete evacuation. The movement is affected by the distance to the exit and the speed of the occupant. It is determined by the physical dimension of the building the unhampered walking speed and distribution of the occupants. Due to the low density in the current case, it is assumed that there are no impediments of the occupants as they walk to the exit. Travel time is also affected by the movement of the occupants through an exit.

It is influence by flow capacity assuming that all the occupants arrived at the exit route at the same time (Kong et al., 2014). Total movement involves walking, queue and flowing. Walking time is the average time taken by the occupants to move from the present locations in the building to the escape exit route. It depends on the walking speed and the distance to the exit route. The walking speed depends on the sex and age of the occupants. Based on the past studies of the office buildings, the average unimpeded walking speed on horizontal distance is 1.

25 m/s (Proulx, 2002, 345-360). Since there population density in the building, the walking speeds of the occupants are independent from each other. It is also assumed that the occupants begin to evacuate at the same time, anad ther was no impedement on walking by the occupants. Density - This is the ratio of the number of staff occupying the area and the total area occupied. This ratio measures the level of crowdedness at an exit route. The density of the people in an area is obtained by dividing the number of occupants by the area as in the equation below.

The distance in the 1st step is given by √(282+ 152) = 32m Distance for the 2nd step = 40m -15m -10m = 15 m Third step, √(122 + 82) = 15 m Time = The walking Speed x Horizontal distance = 1.25 x (32+15 + 15) = 77.5 sec The occupants queue as they wait for room to move forward at an exit. As a result, the speed of moving through an exit is less than the speed at which the occupants are arriving at the exit. Queuing affects the speed of movement as result in waiting in the queue, thus increasing the time for evacuation.

Flowing occur if the occupants arrive at the exit together which also affected the movement time. However, there is no queuing if the density is low. Based on the guidance provided by Nelson and Mowrer the occupants move at their own speed on the stairs if the density is less than 0.54 persons /m2. Thus, the unimpeded travelling speed on the stairs is 0.85 m/s (Nelson and Mowrer, 2002, 368-370). Time used on stairs = 8m/0.85 m/s = 9.4 sec The queuing cannot occur because the population density is low.

Total movement time = 77.5 + 9.4 = 87 sec In this case, the density of the occupants is low, and they are widely spread. In this case the time required for evacuation is mainly dominated pre-movement distributions and movement time. The total time needed for complete evacuation = detection time + pre-movement time + movement time = 8 seconds + 90 seconds + 87 seconds = 185 seconds. Thus RSET = 185 seconds Uncertainty The uncertainty factors affect the accuracy of egress time. But due to uncertainties, RSET is multiplied by a safety factor of 1.5. Therefore, RSET = 1.

5 x 185 = 278 seconds The design evacuation time takes into consideration the uncertainties caused by the behavior of the people during an emergency. Safety factor takes care of uncertainties that occur as a result of impediment due to the behavior or the apparent conditions of the occupants such as sickness, age (Xie et al., 2012). The total time required for safe evacuation in Mezzanine area is 278 seconds. Various considerations should be made when designing fire safety. These can include technical consideration and human response to fire.

The technical requirements include sufficient provision of the means to prevent fires, early detection and warning, means of escape, smoke control systems, fire extinguishers, control of the fire growth and others.

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