Principle of ASET-RSET and Guidance - Assignment Example

Name Course title Task Date Principle of ASET-RSET and guidance provided in BS7974 Part A: Required Safe Escape Time (RSET) from the MEZZANINE area RSET is the time taken by the occupants to move to a safe place after fire ignition, i.e. the target time for complete evacuation and includes the time the occupants remain safe within the time spent in the building. BS 7974: 2002 is a comprehensive document which provides a systematic technique for calculating the time of escape (British Standards Institution, 2004). Fire in a building may become lethal to the occupants, if the occupants are exposed to the products of fire for a long period. Evacuation time is used to determine the time required for occupants to exit to safety after recognizing fire and beginning to exit, and the success of fire safety strategies depends on timely evacuation of the occupants. Various factors are considered when dealing with the evacuation time. They include recognition time, pre-movement time, response time, walking time, flow time and travel time used in evacuation. Mezzanine is a compartment in a storage warehouse has the dimensions 40m by 32m by 12m high as shown in the figure below, The building will only be accessed by trained staff, while the floor of the mezzanine is located 8m above the ground floor level. It will be occupied by 15 members of staff and there will be no disabled access. A single open stair will be located towards one end of the mezzanine as shown below. For the purposes of estimation of the evacuation time for Mezzanine, it is assumed that: All the staff will begin to evacuate at the same time. There will be no interruptions on the flow of the occupants as a result of the decisions by an individual. The basic formula used to calculate the time of escape in a building is shown below. Where the evacuation time, is the time between the ignition and detection by automatic sensors. It is dependent on the detection systems. is the time duration between detection and the general alarm, is the pre-movement time which is the time from alarm to the time the occupants move out of the building. is the travel time for the occupants from the building to the a safe place (British Standards Institution, 2004). 1. Detection time, and alarm time is the time taken for the occupants to be notified through sound alarm, visual awareness or smoke smell. In previous studies using FDS model, smoke detection and notification time took 8 seconds. Therefore, in this calculation, = 8 seconds. This time is faster than the time taken for the occupants to receive notification through smell of smoke and visual awareness. 2. The pre-movement time This is the time taken by the occupants to recognize the alarm and the time they respond to the alarm and begin to move out. The pre-movement time can be divided into the time between alarm and movement of the first occupants and distribution of the occupants when exiting. The factors that affect the pre-movement time include recognition time, and response time, (Muckett and Furness, 2007, 83; British Standards Institution, 2004). a) Recognition time, - Recognition time in British Standards is the time taken by the occupants to respond to alarm. They respond after accepting that they need to respond. Recognition time varies with the characteristics of the occupants, the alarm and organization management. b) Response time, - This is the time taken by the occupants to receive and interpret the emergency information and prepare to exit. Respond time can be reduced by providing prompt, accurate and clear information to the people. Other factors which affect response time are physical and mental state of the occupants, level of training on response to warning, role and responsibilities and how much they feel they are in danger. The behaviour of the occupants during the fire situation such as the level of alertness affects time for recognition and response. Therefore, pre-movement time,, is calculated from the equation: . For the purpose of escape and evacuation, it can be divided into two phases. Time for pre-movement of the 1st occupants. Time for distribution of pre-movement for the occupant’s group. For the current case, since the occupants are awake, familiar with the building and well trained, the pre-movement time is short. The proposed time for the first percentile pre-movement time is 0.5min. The additional time for the last few people to move out is one minute (British Standards Institution, 2004). Thus, Pre-movement time = Δtpre (1st percentile) + Δtpre (99th percentile) = 30s +60s = 90sec 3. Travel time The travel time consist of walking time, queue time and flow time. The walking time depends on the speed of the occupants during egress, while flow time is the time taken by the occupants to move through stairs. Other factors such as the building layout and the familiarity of the building systems and its characteristics also affect travelling time. Queue time is the time taken to queue while waiting to evacuate. Travel time is calculated from: a) Walking time, is the average time taken by the occupants from their locations in the building to the escape route. This is affected by the distance to the exit and the walking speed of the occupants. This is determined by physical dimension of the building, and how the occupants are distributed. It represents the time required for the last occupant to exit and no allowance is made for impede walking since the density of the occupants is low. b) Queuing time,, is the time required from the alarm to the time the queue is formed at the exit. At the exit, the occupants queue as they wait for room to move forward. The speed of occupants when passing through the exit is lower than the rate at which they are arriving. The queue can be bulk or orderly queue that is based on first come first serve. Queuing at the entrance involves a number of steps that include approaching the queue, standing or waiting in the queue and moving through the exit. The effects of waiting in a queue would affect the movement of the occupants and will lead to lengthening of the time of evacuation. Waiting time in the queue has been studied extensively. Queue time is elaborated more in time to queue formation in BS 7974, which involves waiting and moving time. Waiting time can be improved through changing crowd density, exit selection and the number and width of the exits (Hurley, 2015; Billington et al, 2002, 60-80). c) Flow time,, is the time taken by the occupants to move through the exit, assuming that all the occupants are at the exit at the same time. The occupants from various areas of the building converge in an exit during an emergency. Thus, congestion occurs due to reduction in the flow rate, due to the large number of occupants evacuating at the same time. Density - This is the ration of the number of staff occupying the area and the total area occupied. It measures the level of crowdedness in 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.  Where,  = population in a group (persons)  = Area occupied by this group (m2)  = Density (persons/m2) Thus, The travel speed for the occupants when they are exiting is determined by their density in the area. Experiment has shown that for a density of less than 0.55 person/m2, the movement of the occupants is not affected by the density. Thus, since the density is 0.022persons/m2, the speed is assumed to be approximately 1.2m/s. The travel distance is assumed to be the most remote part of the building. The time for evacuation is obtained by dividing the travel distance by walking speed. Distance from the furthest corner of Mazannine to the stair = 28m + 15m + 26m = 69 m. Travel time on horizontal distance = 69/1.2 = 58sec According to PD 7974-6:2004, an unimpeded speed on stairs of about 0.8 m/s for travel downwards Time taken on stairs = 8m/0.8m/s = 10s Time travel = 10s + 58s = 68s For safe evacuation and to allow for uncertainties, a safety factor of 1.5 is used. In this case, RSET times 1.5 is 249 seconds. The objective is to maintain tenable conditions within the evacuation route during evacuation. Factor of safety may not be taken into consideration when analysing conditions system failure scenario or near fire. The total time can be summarized as shown in the table below. Total evacuation time, Detection and alarm, (Sec) Pre-movement time, (Sec) Travel time, (sec) RSET (sec) 1.5 x RSET (sec) 8 90 68 166 249 Life safety in a building is determined from the difference between the required theoretical time for evacuation, Required Safe Egress Time (RSET), and the time available for evacuation of the occupants, Available Safe Egress Time (ASET). The total evacuation time, which is 249s in the current case, is the real time needed for all the occupants to be move to safety (Billington et al., 2002, 66). The safety of the people depends on the emergency procedures, which makes use of the designed fire safety measures and takes account of the occupants’ behavior in an emergency. The design takes into account the behavior of the people in case of fire for effective design of the emergency procedures that can overcome the occupants’ behavior. This is aimed at making sure that the building occupants are not exposed to smoke, heat, or fire and that the people are exposed to such incidents does not develop serious problems. Safety factor takes care of uncertainties which may occur for example poor response by the occupants due to their health status and age. Part B: Available Safe Egress Time (ASET) The definition of ASET as adopted by BS 7974 is the time from the fire ignition to the moment at which the tenable conditions in the building is reached due to heat, smoke, or toxic emissions. ASET was introduced in 1983 by Cooper. It provides the maximum exposure time to hazard from the fire which may be tolerated with no one being incapacitated. Therefore, all of the occupants in the building should be able to escape from the building before ASET is reached. ASET is calculated as follows: Where is the detection time, is the time for the onset of hazardous environmental conditions, and is the notification time. ASET can determine through the use of fire modelling technique. This begins by establishing heat release history of the combustible materials present in the building as well as the product of their combustion such as carbon monoxide and soot. This information is used as an input to a calculation tool like fire model to establish the time available after initial fire ignition, through which the occupants must evacuate before untenable condition is reached due to the presence of fire and smoke (Peacock et al., 2011). The pre-established tenability criteria used to determine the ASET include the following. The temperature that exceeds 600C Visibility of at least 10m The concentration of carbon monoxide must be below 1400ppm. ASET is the maximum time that an individual can tolerate exposure to a hazard without being incapacitated. This means that all the occupants should be able to evacuate before ASET is reached. This means that the time taken by an individual to reach a safe place should not exceed the time required for the conditions to be untenable. The margin of safety is the difference between ASET and RSET (Peacock et al., 2011, 253). The building is safe if ASET > RSET as it is shown in the figure below. It is essential determine the margin of safety to ensure that the occupants are evacuated into a safe place without causing serious injuries. ASET can be increased by reducing the amount of combustible materials, providing adequate separation between fuel packages, installation of fire suppression systems to minimise heat released or install smoke control systems. A safety index can be obtained from the relationship between REST and the margin of safety as follows. Safety index = Safety index is used to determine the safety of the occupants in a building. An increase in this value indicates an increased level of safety. It is necessary include a safety factor in the calculation to allow for uncertainties in the calculation (Jordaan, 2005, 514). Tenability limits designs The major factor in application of RSET/ASET criteria is to set up the performance design which will be used in defining the untenable conditions. In this context, tenable conditions are factors that support human life (NFPA, 2006). Life safety design is determined based on parameters such as visibility, thermal exposure and inhalation of toxic gases. The parameters are summarised in the table below. Criteria for the safe evacuation of the occupants Visibility Greater than 10m Toxicity Carbon monoxide with the concentration of less than 800 ppm in 30minutes Temperature Less than 600C Hot layer Greater than 2.1 m above the level of the floor Radiative heat flux Less than 2.5kw/m2 Visibility The visibility through the smoke should be adequate for the escape routes to be identified. A visibility of 10m is usually considered to be reasonable, but it can be less if other measures are provided to help in exiting. Temperature The occupants can be incapacitated or die if they are exposed to elevated temperatures. The increase in fire causes rise in temperature as well as production of hot smoke. The dilution of smoke may cause increases in the toxicity and optical density of smoke. The maximum temperature that can be allowed for the escape should be less than 1200C. If the condition is humid as a result of fire fighting activities like sprinklers, the maximum temperature required for the survival is less than 600C (Hurley, 2015). Radiative heat flux Heat radiation emitted from hot smoke layer may cause excessive heat intensities. The lower layer of air at the height of 2.1m is likely to absorb heat radiation emitted from the layer under the ceiling. An excessive exposure to heat radiation can result in skin pain and severe burns. A radiant heat flux that exceeds 2.5kW.m-2 can cause a lot of pain. This term is the maximum value for exposure that is recommended but for a shortest time. Toxicity The smoke produced from a building under fire condition usually minimise visibility before producing toxic condition. The smoke consists of narcotic and irritating toxic product that incapacity, disorientation or death depending on the duration and their concentration. The concentration of carbon monoxide should be less than 700 ppm in one minute. Visibility Visibility may affect the ability of the occupants to access exit routes. Smoke can reduce the ability of the occupants to see properly and pass through physical barriers. The design should ensure that the visibility is adequate and the toxic level of smoke is low. To ensure that the visibility along the escape route is maintained, the layer of air should be maintained relative smoke free above the eye level. The effect of these criteria varies depending on the health condition or the age of individuals exposed to the fire condition. The occupants should not be affected by undue hazards due to heat or smoke while they are escaping. The escape routes can be protected by constructing with fire resistant materials or by installing smoke control systems (Lataille, 2003, 33-34). Scenarios The time taken for untenable conditions to be attained varies depending on factors such as the characteristics of fuel (type, amount, or arrangement), the amount of ventilation, materials used to construct the building (structural frame or interior finishes), compartment geometry (such as large area with high ceilings or small compartment with low ceiling), and the layout (open office). Thus, it is vital to consider different fire development scenarios when making an assessment (Tubbs & Meacham, 2007). A fire scenario is the development and the spread of fire and the products of combustion, including the occupant characteristics and the building design. A fire scenario for a building is developed by extracting scenarios that are suitable for the design from a range of possible fire scenarios. In a scenario, factors such as the quantity and type of fuel, their location, fire growth rate and heat release, as well as the products of combustion like carbon monoxide and smoke are considered. Fuels are all the components that are combustible in the building such as equipment and furniture, architectural components within the building and combustible building materials (Tubbs & Meacham, 2007). For fires to occur, fuel, oxygen and energy have got to be present. The ignition source provides the initial energy. This can be from a cigarette, arson, welding machine, or a faulty electrical wire. After the ignition, the fire may develop rapidly or may grow slowly. As the flame fire grows, the compartment may become so hot such that all the components in the building are ignited. The fire ignited is likely to be put off before spreading to other parts of the building if sprinklers are present. Fires usually reduce in size if the sprinklers are present or if the fuel is exhausted. Heat release rate curve is used to quantify the growth of fire over time (Ramachandran & Charters, 2011, 2). When a fire ignites in a compartment, hot gases and smoke produced spread throughout the compartment. The smoke and heat flows at a given rate. Since they are hot, the products of combustion including smoke rise due to buoyancy and fill the upper part of the compartment in layers. The upper layer consists of hot gases. Depending on the size of the compartment and rate of heat released, the smoke may affect the egress process by reducing the visibility and producing untenable conditions along the paths (Tubbs & Meacham, 2007; Hurley, 2015). Use of models Analytical models can be used to study the behaviour of fire and to develop fire safety measures. The models are based on mathematical relationships required for some fire conditions to occur. Computer based models can also be used to predict fire behaviour and it effects, taking into account their limitations, assumptions, parameters, proper application and accuracy of the data input and interpretation of the result (Hurley, 2015). The worst fire location and scenarios are envisioned by reviewing the operation and function of the building. Fire engineers usually consider the effect of stack and wind that affect the development and movement of fire and smoke. The parameters for calculating the spread of fire and smoke are based on the experimental equations provided in the design guidelines. Field simulation like fire dynamic simulator (FDS) can be used to determine the probable situations of the building in case of an emergency. The unsustainable situations can be found from thermal radiation, an enclosed temperature and the smoke layer. The time between the ignition and the beginning of the untenable situations will be considered to be the ASET for the people in the building (Peacock et al., 2011, 252; Hurley, 2015). References Billington, M. J., Ferguson, A., Copping, A. G., & Wiley InterScience. (2002). Means of escape from fire. Oxford: Blackwell Science. . British Standards Institution. (2004). The application of fire safety engineering principles to fire safety design of buildings - Part 6: Human factors: Life safety strategies - Occupant evacuation, behaviour and condition (Sub-system 6) PD 7974-6:2004. London: BSI. Hurley, M. J. (2015). SFPE handbook of fire protection engineering. . Jordaan I., (2005). Decisions Under Uncertainty: Probabilistic Analysis for Engineering Decisions, Cambridge University Press Lataille, J. I. (2003). Fire protection engineering in building design. Amsterdam: Butterworth-Heinemann, p 33. Muckett M., Furness A., (2007). Introduction to Fire Safety Management, Routledge Peacock, R. D., Kuligowski, E. D., Averill, J. D., & International Conference on Pedestrian and Evacuation Dynamics, (2011). Pedestrian and evacuation dynamics. New York: Springer. Ramachandran G. & Charters D., (2011). Quantitative Risk Assessment in Fire Safety, Routledge Tubbs, J. S., & Meacham, B. J. (2007). Egress design solutions: A guide to evacuation and crowd management planning. Hoboken, NJ: Wiley. Read More

Other factors which affect response time are physical and mental state of the occupants, level of training on response to warning, role and responsibilities and how much they feel they are in danger. The behaviour of the occupants during the fire situation such as the level of alertness affects time for recognition and response. Therefore, pre-movement time,, is calculated from the equation: . For the purpose of escape and evacuation, it can be divided into two phases. Time for pre-movement of the 1st occupants.

Time for distribution of pre-movement for the occupant’s group. For the current case, since the occupants are awake, familiar with the building and well trained, the pre-movement time is short. The proposed time for the first percentile pre-movement time is 0.5min. The additional time for the last few people to move out is one minute (British Standards Institution, 2004). Thus, Pre-movement time = Δtpre (1st percentile) + Δtpre (99th percentile) = 30s +60s = 90sec 3. Travel time The travel time consist of walking time, queue time and flow time.

The walking time depends on the speed of the occupants during egress, while flow time is the time taken by the occupants to move through stairs. Other factors such as the building layout and the familiarity of the building systems and its characteristics also affect travelling time. Queue time is the time taken to queue while waiting to evacuate. Travel time is calculated from: a) Walking time, is the average time taken by the occupants from their locations in the building to the escape route.

This is affected by the distance to the exit and the walking speed of the occupants. This is determined by physical dimension of the building, and how the occupants are distributed. It represents the time required for the last occupant to exit and no allowance is made for impede walking since the density of the occupants is low. b) Queuing time,, is the time required from the alarm to the time the queue is formed at the exit. At the exit, the occupants queue as they wait for room to move forward.

The speed of occupants when passing through the exit is lower than the rate at which they are arriving. The queue can be bulk or orderly queue that is based on first come first serve. Queuing at the entrance involves a number of steps that include approaching the queue, standing or waiting in the queue and moving through the exit. The effects of waiting in a queue would affect the movement of the occupants and will lead to lengthening of the time of evacuation. Waiting time in the queue has been studied extensively.

Queue time is elaborated more in time to queue formation in BS 7974, which involves waiting and moving time. Waiting time can be improved through changing crowd density, exit selection and the number and width of the exits (Hurley, 2015; Billington et al, 2002, 60-80). c) Flow time,, is the time taken by the occupants to move through the exit, assuming that all the occupants are at the exit at the same time. The occupants from various areas of the building converge in an exit during an emergency.

Thus, congestion occurs due to reduction in the flow rate, due to the large number of occupants evacuating at the same time. Density - This is the ration of the number of staff occupying the area and the total area occupied. It measures the level of crowdedness in 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.  Where,  = population in a group (persons)  = Area occupied by this group (m2)  = Density (persons/m2) Thus, The travel speed for the occupants when they are exiting is determined by their density in the area.

Experiment has shown that for a density of less than 0.55 person/m2, the movement of the occupants is not affected by the density. Thus, since the density is 0.022persons/m2, the speed is assumed to be approximately 1.2m/s. The travel distance is assumed to be the most remote part of the building.

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