Fire Safety Engineering - Assignment Example

Fire Safety Engineering Name Institution Warehouse RSET During the design of buildings, both residential and commercial, predictions of the movement of building occupants during egress is one of the most important factors of fire safety analysis. Generally, occupants are considered to be safe from fire if the required safe egress time (RSET) is less than the available safe egress time (ASET) where ASET is taken to mean the time it takes for fire conditions to become untenable to the occupants. RSET stands for the Required Safe Egress Time. Cooper (1985) defines it as the time taken from the sounding of the alarm for safe evacuation from the premises. This value is obtained through simulating evacuation movements of occupants where the modeler makes some assumptions regarding the reactions of people in a fire, the type of occupants, and their flow in case of a fire. This assignment asks us to determine the Required Safe Escape Time from the proposed Mezzanine section of the warehouse as per the guidelines in BS7974 (2004). The building is an industrial warehouse which has limited combustible materials with dimension of 52m by 60m by 14m high. Additionally, only trained staff can access the area. The proposed mezzanine will be located 9m off the floor with the occupancy rate estimated at 20 workers. 20m Figure 1: Ground floor plan showing location of exits. The RSET equations utilized are derived from PD 7974-6:2004- page-5. Some of the parameters considered when determining the required safe egress time are the detection time, and time until all workers are notified of the fire. In evacuation, there are two main types of behavior. The first is pre movement behavior which refers to the occupants’ reactions before they commence moving through the building exits. The second type of behavior is the travel behavior once the agents start moving through the escape routes. According to PD7974, the RSET calculation is given by: : : the required safe escape time. : Is the time from ignition to detection by an automatic system. : Is the time from detection to a general alarm. : Is the pre-movement time for the enclosure or building occupant. : Travel time of the enclosure occupants or building occupants. (PD 7974-6:2004 – page 7) The equations are then solved to give the Required Safe Escape Time. Each term in the RSET equation has its own parameters thus making it necessary to calculate all terms separately. All the equations used to solve for the RSET for the warehouse have been sourced from PD 7974-6:2004. is effectively zero as the warehouse has a level 1 alarm that activates on detecting a fire. In complying with fire safety standards, the building has an automatic fire detection system in place through the whole building which makes equal to zero. Since the building has an automatic system, the alarm to the occupants is immediate to all parts of the building. Since these are highly qualified workers, they are classified as level M1 which shows they are skilled with some training in fire prevention and maintenance drills. PD7974-6:2004 (p.6) provides a ranking for occupancy types. In this case, the warehouse occupancy type is industrial which means the occupants are awake and aware during the fire. Therefore, the is given by the following equations Tdet = Where RTI refers to the response time index Ujet is the velocity of ceiling jet in m/s Tjet is the temperature of the ceiling jet in 0C Tdet refers to the detector activation temperature in oC T0 is the ambient temperature of the containment Finding Tdet requires solving tow formulas as outlined in Alpert’s correlations for maximum ceiling jet velocities and temperatures. The first part is solving for Tjet Where: Q= heat release rate in KW R= horizontal distance of detector from the fire plume in m H is the height of the ceiling in meters Tdet= Detector activation temperature in 0C T0= ambient temperature in oC Referring to table 3 from BS7979 part 1 our fire growth rate is rated as ultrafast because of the building use. Therefore, the fire growth rate is 0.188 Assuming that the horizontal distance from the fire axis is about 7 meters, r= 7m H=14m Tdet= 55c To=20c Then r/H = 7/14 = 0.5 Rearrange 1712.232676205747 Q= 1712.23KW Tjet can now be calculated with the following equation Tjet=20+5.38(9500÷7) ⅔ (14)-1 Tjet=20+5.38 x (1357.14)⅔ (14)-1 =20+5.38x 122.579x0.0714 = 67.08650C **Now will calculate the ujet r/H and 7/14=0.5 therefore, ujet=(0.195 Q⅓ r-5/6 H1/2 ujet= =20.63 m/s Now will calculate tdet First thing RTI=50 Couse the building sprinkler (according BS7974 part 4) tdet tdet , tdet tdet= 11.008 ln (4.668) , tdet= 11.008 (1.54) tdet= 9.468s Total area and occupants in the warehouse: Is the time of total occupant population to flow though available exits. Where: number of workers in mezzanine according to ADB Approved Document B exit width is =7500mm=7.5m In order to find the total number of building occupants including those on the main floor of the warehouse, we need to use some equations: Approve Document B table C.1, one can deduct the space factor in order to estimated number of total occupants in the warehouse Where: Floor space factor 30 Area Therefore, total number of building occupants = In our case = = 52.99s According BS7974 part 6 According (AMc, 2008) Total travelling distance is 105m Average walking speed as per guidelines is 1.2m/s-1 Therefore: = 87.5s = 143.53s Finally: tRSET =∆tdet+∆ta+(∆tpre+∆ttrav) tRSET=9.468+0+143.53 =152.998s Part B The available safe egress time is usually determined based on a set of general criteria such as temperature, toxicity of smoke, and visibility along the exit paths (Ng and Chow, 2006). BS7974 defines the available safe egress time as starting from the time of fire ignition to when tenable conditions in the building are attained (British Standards Association, 2004). That is, it the maximum amount of time that occupants have to escape without injury. Since the main aim of fire evacuation is to avoid injury, occupants should be able to exit the building before the ASET. After ASET has been achieved, the building becomes untenable. In such cases, the safety of occupants is subject to other factors such fire location, its growth, and behavior of occupants in response to the fire. The ASET can also be extended through the use of smoke control systems and sprinklers. ASET is estimated with the following equation: ASET = thaz – tdet - tala Where t haz is the time that the fire starts, tdet is the time taken until fire detection, and tala is the time it takes for the occupants to notice the alarm. ASET can be estimated through calculation of the time available between the start of the fire and the time tenability is reached. With this in mind, Hurley (2015) recommends this design as the maximum allowable time that occupants can be exposed to the hazardous conditions without incapacitation. Tenability limits One of the factors considered when determining the ASET is the tenability limits. These refer to the conditions at which occupants cannot escape without injuries. It is the building designer’s responsibility to ensure that occupants are not exposed to undue hazards such as smoke or heat when escaping from the building. The limits can be extended, sometimes indefinitely, through the use of fire resistant materials, sprinklers, and smoke control systems. Smoke During a fire, smoke is one of the main dangers to occupants other than the fire itself. The smoke may result in toxic gases and a loss of visibility. If the building has sufficient visibility, then the effect of toxic gases is greatly reduced. However, the designers should ensure that if occupants have to travel through the affected region, then they should not be subject to conditions that lead to loss of life. The Chartered Institution of Building Services Engineers (2003) gives some guidelines on toxicity levels where visibility in smoke should be enough to enable occupants to identify exits and the air layer in a compartment should have no smoke beyond a height of 2m form the ground floor. Calculation of ASET The Available Safe Egress Time is usually estimated using equations for the conservation of energy and mass. Such equations assume that the building has at least one opening to prevent the buildup of hot air. Tosolini et al 2012 give the following equations as estimators of ASET. Where, and (Tosolini et al., 2012) In the equation above, h is the total height of the room in meters, α represents a growth factor regarding t2 fires measured in kW/ss . LLH represents the height of the lower layer above ground level while S is the cumulative area of the floor in square meters. ρg gives the upper layer density 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 (He et al.,2002,p.193). A margin of safety is required for all buildings when calculating the ASET and RSET. A margin of safety is defined as the difference between ASET and RSET. The Margin of safety accounts for unforeseeable circumstances and scenarios. Modeling In recent times, fire modeling tools have become important in assessing multiple fire scenarios, including the estimation of full scale performance and in developing performance objectives. There are different models for different scenarios. The two main types of models used in fire modeling are zone models and field models. The computer aided modeling process begins with constructing a heat release profile to determine the history of heat release from the building materials or from other combustible materials. Field models, alternatively know as Computation Fluid Dynamics models are used to simulate more complex scenarios as they can handle millions of computational cells. According to Yeoh & Yuen, (2009, p.29) CFD models are extensively used to estimate complex solutions to the conservation of energy, momentum, and mass. Bibliography ADB, (2007) Approved Document B (Fire Safety) Volume 2- Buildings other than dwelling houses. UK. BRITISH STANDARDS INSTITUTE (2003) PD 7974-1 application of fire safety engineering principles to fire safety design of buildings. Part 1: Initiation and development of fire hin enclosure of origin (Sub-system 1). London: British Standards Institution BS 7974-6 (2004) Part 6: Human factors: Life safety strategies - Occupant evacuation, behaviour and condition (Sub-system 6). The application of fire safety engineering principles to fire safety design of buildings. UK. Cooper, L.Y. and Stroup, D.W., 1985. ASET-A computer program for calculating available safe egress time. Fire Safety Journal, 9(1), pp.29-45. DiNenno, P.J., 2008. SFPE handbook of fire protection engineering. SFPE. He, Y., Wang, J., Wu, Z., Hu, L., Xiong, Y. and Fan, W., 2002. Smoke venting and fire safety in an industrial warehouse. Fire Safety Journal, 37(2), pp.191-215. Ng, C.M. and Chow, W.K., 2006. A brief review on the time line concept in evacuation. International Journal on Architectural Science, 7(1), pp.1-13. . 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, 2601-506. 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