In building design, one of the most important consideration is the safety of the occupants. The occupants are always in danger of a fire outbreak regardless of the class of building and the layout plans. Factories, residential houses, offices, apartments among others are always in danger of fire and this has to be prioritized in the design plans. As such, there are calculations that consider some of the aspects of safety in case of an outbreak and these include the distance to be traveled to a safe location, the size of the exits, the smoke prevention mechanisms among others.
There are two types of fire prevention measures which may be used individually or as in a combination. These two types are passive measures and active measures. The former refers to measures that prevent the outbreak and spread of the fire and generally limit the phenomenon. Therefore, they include insulation walls, design layouts, and fire detection systems among others. On the other hand, active fire prevention measures are those that are called upon in the case of a fire outbreak. The purpose of these latter measures is to douse the fire whenever the passive measures fail. These systems include fire extinguishers, sprinklers, and hydrants among others. Therefore, the measures that are used to protect the building occupants have to be highly prioritized during the design stages.
Performance-based design has continued to pick up over the decades because of the increase in the number of buildings which has been caused by the increase in population. Nevertheless, the criteria have to be in accordance with the building regulations and with the fire prevention standards of the specific locality. As such, it has been centered on the life of the individuals and is based on the travel time requirements. The travel time required by the individuals to find the building exit before being overcome by the fire has continued to gain momentum in building and layout designs.However,it is more of a traditional approach as compared to fire engineering but provides innovative and cost-effective criteria for the design of a building.
Fire engineering has been developing over the past decades but it is not at the same level of performance-based design. However, there are new methodologies in the discipline which may be more innovative that PBD.CFD is a modeling methodology that has evolved over the past five years and has continually been used at the heart of complex fire systems (Babrauskas, 1996). However, it is based on two important design scenarios that are centered on PBD; the available safe egress time (ASET) and the required safe egress time (RSET).The former is modeled on the fact that in case of a fire breakout, it is only a matter of time before the enclosed conditions become intolerable. Nevertheless, it considers only a single compartment and the egress time required. Therefore, it is the window between detecting the fire and the hazard. On the other hand, the latter refers to the short time window between detection and escape. It is, therefore, the time frame that individuals in areas that are in danger of being overcome by fire have to escape the building. Based on these two definitions, the RSET must be smaller compared to the ASET (Poon, 2014).
Regardless of the model that is used in prescribing the safety of the occupants in case of a fire outbreak, there are a number of fundamental factors that need to be considered.10 factors have been described by the international standards organizational committee that try to determine the number of models to be used in the fire prevention modelling.To begin with, the design has to consider a location that poses a challenge to the fire.Secondly,the most probable type of fire has to be determined according to the building layout and the amenities present.Thirdly,the likely outcome of the fire has to be determined in terms of the hazard that it poses.Fourthly,the systems likely to have an impact on fire have to be determined. The systems may either aggravate the fire or reduce the fire e.g. gas mains and water supplies respectively. The next important thing is to identify the characteristics of the occupants in case of the fire outbreak. This is followed by designing an event tree that demonstrates the significant factors in the scenario. Using the tree developed and engineering judgment as well as calculations, the next important thing is to estimate the probability of occurrence of each of the identified state. The consequences of each occurrence identified has to be determined using engineering judgment and analysis. This it is followed on by ranking the risks identified. Finally, according to the risk analysis conducted in step 9, the fire that is likely to have the highest risk is used in the analysis (International Organization for standardization, 2006).
In this analysis, the building of consideration serves as a factory. It is a 120m by 120 m building whose height from floor to roof is 12m.Moreover, the roof is flat and in the form of a slab. At the center of the building is a working area that is 4m in height. It is a square working area whose breadth is 5m. From the working area, there are two sets of stairs which are on opposite sides of the walling and should lead directly to the exit doors.
In case of a fire outbreak, the people in the working area are at the highest risk of incapacitation. This is considering that the height of the platform makes them more susceptible to smoke inhalation. Smoke is one of the major causes of fire deaths because it clouds the body and prevents an individual from escaping the scene. What follows after incapacitation is the death by fire and therefore is a priority in this design. The two stairs that allow the individuals to exit the building should also be highly prioritized in the design in terms of sufficiency. A smaller set of stairs may reduce the rate at which the individuals exit the platform while an oversize set of stairs may not be economically viable.
Therefore, working from the platform poses the highest risk to an individual. The workers have to go down the flight of stairs to the ground floor and to the exit doors. Therefore, there are two sets of time that are used in the analysis: time for going down the flight of stairs and the time from the point where the stairs touch the ground and the exits. Based on the analysis proposed by the international standards organizational committee, such a scenario that has the highest risk is the one to be considered for a qualitative analysis.
ASET RSET MODELLING
It is a time-based approach that is used in demonstrating the time requirements for evacuation. However, it is dependent on several factors that are specific to the building and to the occupants.Therefore, the performance requirements form the backbone of this methodology and if the occupants can reach a safe location after the onset of the fire, then the requirements are dully met (Shenzen, et al., 2016).
The two components of the model consider different factors in the modeling.To begin with, ASET considers the model of the fire, the design fire, the systems that may be used for fire suppression, the systems that may be used in the control of smoke and the model of the building (Tosolini, et al., 2012). On the other hand, RSET considers the time required for fire detection, the behavior of the occupants, the time required for movement and the wayfinding. All these form the core of the model but the eventuality is that ASET should be greater than RSET.From studies and analysis, the model has been known to meet the relevant performance requirements but the safety allowance must first be met. The safety allowance in some parts may be less than 76% while in other areas it may be greater than even 90% (Poon, 2014).
On the part of RSET, there are four values that are used in its analysis. All these values are time-dependent and include time for detection, time for the alarm, the pre-movement time and the time required for travel (Poon, 2014). The detection time refers to the time between ignition of a fire and the detection by either an automatic system or the occupants. On the other hand, the alarm time is the time between detection and the alarm. Thirdly, the pre-movement time is the time when occupants start to leave the building. This time requirement is made up of two components: the time when the occupants will know there are an alarm and the time that they will start to leave the building. Finally, the travel time is recognized as the timeframe required from movement from some specific part to the safe location. It is made up of the walking time and the flow time. Walking time refers to the speed of movement of the occupants while flow time refers to the flow of the occupants through doors and exits.
Because premovement times play a fundamental role in the safety of the occupants, there are a number of tables that have been designated for this use. These tables consider the level of management, the level of the alarm and the complexity of the building.
It is imperative to understand that there may be manual or automatic fire alarm systems to ensure that there is safety for the individuals present. However, ASET calculations depend on the design fire and the time available for escape.It is the nature of the fire that usually makes the place uninhabitable and therefore necessary to determine the materials present in the working area.
Some of the factors that are to be determined in the analysis majorly influence combustion and include oxygen concentration, the temperature of the fire and the elements that would aggravate combustion (International Organization for standardization, 2006). Because it is a factory used in the storage of wood and rubber tires, these are the most likely substances that will increase the chances of the fire spreading.
The design fire used in estimating ASET is quantitative and is measured using qualitative methods in order to enhance the safety of the occupants (British standards institution, 2004).Therefore, prior to implementing all the fire safety prior to the reaction parameters, it is important to use a design fire that will present the most possible fire case scenario and will, therefore, represent the challenge that is posed to all the fire protection features of the building. Nevertheless, the most important features of the design fire are the rate at which heat is released, the rate at which smoke is produced, the specific types of flames produced and the size of the fire. All these are variables that have to be taken into consideration in ASET calculation.
The inception stage of the fire is dependent on the amount of oxygen available and other factors that can aggravate the fire. It usually lasts for a few seconds to days and represents the initial stage of the expected fire. However, modeling around this stage is challenging because of the numerous variables that are involved. The second stage is the growth and represents the expected rate of growth of the fire. It is also dependent on a number of variables that may increase or reduce the fire. Therefore, it represents an increase in the surface area burning and the corresponding amounts of heat released. Nevertheless, the modeling of the stage is also very difficult as it is dependent on a number of variables that are not only changing in nature but also schotastic.Therefore, the growth rate that may is used by the fire department may be dependable on the judgment.Flashover is the third stage in design fire modeling and represents the movement of fire from the upper layer to all the combustible parts of the building. The layers in the upper region attain a temperature of about 500-600 degrees Celsius which results in the spread of the radiation downwards consequently leading to the combustion of all elements in the compartment. It is usually modeled in a linear scale from a small fire to a very big fire. Finally, there is the post-flashover that represents a phase where all the combustible elements in the compartment are under fire.
The aforementioned stages are used in modeling the fire and are used in the ASET analysis.
The fire detection system used should be strategically installed for proper fire detection. The radial distance of the system should ensure that it covers the whole building. Moreover, there may be a sufficient number of these elements to cover the whole building. Nevertheless, the system should be properly designed as per the building standards and the response time required.
The response time of workers depends on the health and age. Therefore, those working on the platform should be able to easily move and this requires an age bracket of about 20-40 years. As with the health, the workers should be healthy enough to be able to move without help.
Maneuverability will be achieved through proper building design. The stair system, as well as the exit doors, should have sufficient width to allow flow. However, the area between the working platform and the exit doors should be open to allow speedy movement of people.
The safety of the individuals that are usually in an enclosed area in case of a fire breakout depends on a range of factors which include the detection time, the time required for evacuation and other factors that may be dependent on the characteristics of the individuals. This time frame is calculated from the parameters that have been stated such as the alarm time, detection time, and premovement time among others.
All these factors are used in the following format:
RSET= time for detection+ time for the alarm+ (pre-movement time+ time for travel) (British standards institution, 2004)
Therefore, we have to consider a number of scenarios that will try to represent the case of a fire outbreak in the factory and the various time variables that are involved in the escape plan. Nevertheless, the time that is included in these escape plans has to consider the movement from the working area to the ground floor and to the exit. Furthermore, it may be assumed that there are two exit doors that are used by people using a single flight of stairs. The two opposite stairs, therefore, have four exit doors.
The number of workers on the platform is 10
The height of the stairs is taken as 4×1.25 which means that the stair distance covered is 5m
The floor area that is covered by the platform is 35 square meters while the total floor area of the factory is 14400
Therefore, the space available for maneuverability is 14375
The distance from the bottom of the stair to the nearest exit is 100meters.We assume that the stair takes up 15m of the floor area.
10 people working on a floor area of 10 square meters means that the density is 1p/square meter.
The management level is M1 which means that they are highly skilled
However, the total distance to be traveled by 5 workers from the platform to the nearest exit is 115meters
The walking speed (Working from a safe point of view) is 1.4meters
Therefore, the travel time is 115/1.4 which in turn means that each person will take 83 seconds to reach the nearest exit.
The time for the alarm and the time for detection depend primarily on the type of equipment used and the health of the workers as well as the situation of the workers
In this situation, the assumption is that the fire growth rate is at a maximum which means that the growth rate is 0.1876kW/s2 (Candy & Chow , 2006).However, this is based on the fact that the detector has to be placed at a position where it is optimal and maximum velocity of the gas which means that the detection time can be calculated from (Tosolini, et al., 2012)
On the other hand, the time for the development of the fire can be estimated as 5s
In turn, the heat release rate can be estimated at 0.94 kW.
The formula gives an estimate of the start of the working time of the detector. The detector will first detect the fire before sounding the alarm. This time is given by the formula
(Tosolini, et al., 2012)
From the above formula, RTI describes the index that describes the response time of the detector while r refers to the position of the detector under the roof in the radical position. Q is the rate of heat release by the fire. H refers to the ceiling of the height while To and T are the ambient temperatures and the new temperature respectively.
We may take a response time of the detector as 0.98 while the ambient temperature of the room at 25 degrees Celsius.
In this case, we may assume that the formula gives the 100 seconds from the detection to the sound of the alarm.
Another assumption is that the time required for the workers to start moving after hearing the alarm is 45 seconds
Therefore we obtain the REST value as 45+100+83 which means that the safe time for individuals to vacate the building is 227 seconds. This gives an estimate of the time but may not be sufficient for all people since the assumption is that all the workers are healthy and in good shape.
The safety margin is obtained by ensuring that there is sufficient ASET.Therefore, the ASET, in this case, should be greater than 227 seconds. It will ensure that the responsiveness of the workers is covered which in turn will ensure everyone gets to safety before the fire and smoke render the conditions inside the building intolerable.
Additional fire protection
Considering the active and passive fire protection systems, the best line of approach is to ensure that there is efficiency in the passive system. This may involve installation of heat insulation walling and even fire resistant ones. Moreover, considering that smoke is a danger when it comes to those working on the platform, a smoke removal system should be installed. This may involve the use of fans and proper ventilation systems.
Based on the calculations above, it is evident that there are numerous assumptions that are used in estimating the time required to estimate the safe movement of individuals from a building under fire. These assumptions have limited the use of the formula by itself with other models such as TET used alongside the formula. Nevertheless, the formula may prove to be effective provided all the parameters are properly utilized.The lack of information of the same because of the inability to determine all the optimum conditions and variables used in the formula is another shortcoming because of the unavailability of fire design criteria.
The materials present in the factory usually aggravate the fire more than others and this is something of concern. Materials such as explosives and fuel tanks may not be accounted for in such a design and it is therefore important to understand the surroundings when estimating the ESET and RSET values that are to be used.
Babrauskas, V., 1996. “Fire Modeling Tools for FSE: Are They Good Enough?”. Journal of Fire Engineering.
British standards institution, 2004. BD 7974:6. s.l.:s.n.
Candy, M. N. & Chow , W. K., 2006. A BRIEF REVIEW ON THE TIMELINE CONCEPT IN EVACUATION. International Journal on Architectural Science, Volume 7.
International Organization for standardization, 2006. ISO/TS16733 Fire safety engineering – Selection of design fire scenarios and design fires, s.l.: s.n.
Poon, S. L., 2014. A Dynamic Approach to ASET/RSET Assessment in Performance-based design. process Engineering.
Shenzen, Y., Xuefeng, H. & Mengjing, L., 2016. Accurate Assessment of RSET for Building Fire Based on Engineering Calculation and Numerical Simulation. Nanjing Jiangsu, s.n.
Tosolini, E., Grimaz, S., Salzano, E. & Pecile, L. C., 2012. People Evacuation: Simplified Evaluation of Available Safe Egress Time (ASET) in Enclosures. CHEMICALENGINEERINGTRANSACTIONS.