ASET RSET Calculation: Fire Detection And Alarm System In A Warehouse Mezzanine Floor Space

Write an individual report detailing an ASET RSET calculation for a specified area of the building and ventilation arrangement you were responsible for modelling in the group project based on the following scenario. 

Scenario The scenario for the ASET RSET calculation should be based on the following information: 

• The work area at the mezzanine level will be occupied by up to 50 members of staff and there will be no disabled access. 
• Two open stairs will be located at both ends of the mezzanine and these will be the only ways to access and leave the mezzanine 
• Recommend a fire detection and alarm system to the building and determine the detection time based on your recommendation. 

You are required to perform an ASET RSET analysis. You should identify the worst case scenario for evacuation. You should justify your choice of design parameters such as management level, building complexity, walking speed, flow speed, and etc. After the analysis is finished, you should discuss how smoke ventilation system affect the ASET/RSET analysis and how to protect fire fighters. 
To obtain the ASET you do not need to remodel the building including the mezzanine. Instead use the results from the CFD modelling you performed in the group project of your building and your individual ventilation arrangement to estimate the ASET value. 

With the structural developments in existence at the moment, buildings are growing more and more complex. With some growing taller, a lot of them are also growing in terms of area occupied laterally making them more and more complex. With this, it is becoming harder to be able to protect and safely account for people occupying these buildings were a disaster to strike. While relevant codes of construction have given enough advice on construction techniques, no one standard has provided for disaster management. While some practices during the operation of a building’s function may be controlled to reduce such risks, it is impossible to out-rightly come up with codes provide construction methods that totally protect against these natural disasters [1].

Among these disasters is fire which can be a result of natural causes or a result of human error. Fire in buildings is among the most dangerous disasters leading to the second largest percentage of deaths caused by elemental disaster. This is primarily because fires cause fatalities owing to the smoke, chemicals, fire damage and the heat itself [2]. In most cases, fatalities during fires occur mainly because the victims in these fires were trapped and could not get out in time. While obstruction is a major cause of trapping, smoke inhalation is also a huge factor leading to individuals being trapped because the smoke, apart from obscuring the paths to egress points, also has compounds which are poisonous to the human lung [3].

This therefore necessitates the evaluation of the Required Safe Egress Time (RSET) and the Available Safe Egress Time (ASET). This time is important as it helps in directing occupants in any evacuation procedures in the case of a fire hazard in office spaces, industrial buildings, residential structures or any generic enclosure where occupant’s evacuation in case of a fire should require prior planning [4]. The evaluation is usually carried out by taking and comparing some several predetermined or stochastic performance criteria, analysing the duration taken for every considered element to reach the established threshold limit and this is done for every one of those preselected criteria [5]. Computational fluid dynamics (CFD) testing was done using a Fire Dynamic Simulator (FDS) model which presents an ASET performance criteria [6]. A comparison between the values of a quick estimation of the ASET results and those of the computer generated FDS results in an analytical assessment of the conditions in enclosure shown is documented below.

The building being tested is a warehouse mezzanine floor space. This space is not considered to be a storage facility and in coming up with the values, an assumption will be made that the percentage of the room occupied with highly flammable material is 0 - 10%. With this assumption, we can assume that very little to no toxicity will be encountered courtesy of the stored material [7]. The building complexity will be taken as class B1 as warehouses are, in most cases, plain and open in order to maximize on storage space [8]. The alarm level will be A2. This is assumed because, owing to the fact that the building is a warehouse, protection of goods from fire hazards is of paramount importance [8].

Design Parameters

A management level of M1 will be considered owing to the assumption that all staff and occupants of a warehouse premises have some degree of training on fire emergencies owing to the high safety level of environment expected in a warehouse [8]. This is assumed because of the industry regulations involving occupant safety in industries, construction sites and warehouses. This indicated that the response and evacuation time for the staff will be relatively quick. Firefighting equipment is also present in warehouses as a regulatory measure and thus any obstruction due to fires and smoke will be cleared by the trained staff [8].

For the ASET analysis, a tenability criteria is derived from assuming the maximum level of exposure to the fire hazard that the warehouse personnel can tolerate from the fire without being incapacitated. From the CIBSE guide E, we see the growth rate of a fire in a warehouse characterized as ultrafast [9]. While this is a mezzanine floor with possibly little storage facilities and hence low storage capacity, and while the CIBSE value is still dependent on the load, the worst case scenario is that of a fire spreading from the storage area or starting at the mezzanine and spreading to the storage area. With that information, the time taken will be 75 seconds while the constant α is 0.1876 kW/s² [9]. The mezzanine floor’s dimensions are 100m in length on each side, making up an area of 10000m².

The tenability criteria that were used in this analysis were derived from the assumptions that a minimum smoke-free layer height of 2m above the floor would be provided for and that a maximum upper layer temperature of 200C would be felt by the occupants. The warehouse’s occupants were assumed to be capable of rapid physical movement and willing and able to evacuate the floor in clear air under such a smoke layer. The downwards heat radiation value was also considered tolerable for the occupants [10]. A ventilation system is designed in the FDS model and will be used to help divert the smoke away from the ground and allow some clean air to come in under the smoke layer thus enabling the height of the smoke free layer to remain constant even as smoke increases. The ventilation system consists of 4 exhaust vents and 2 make-up vents.

To conduct the ASET, some values in the ASET equations and tenability criteria have to be assumed in order to develop a characteristic model. The assumption however should, however be made as relative to the model as possible. For this project, a visibility distance of 10m has been assigned as the space is large (100 meters on each side) and the floor has minimum complexity. This will affect the response time and evacuation time in the building. It is also assumed that, while not many details are known, the calculations based on worst case scenario assumption should work in this floor in order to prepare the best safe evacuation margin. The formula for ASET is given below;

Tenability Criteria

Where;

C_ef3 = 0.937,

H = the minimum smoke-free layer height

t_g = growth time

A = Area of the room

Z = height of the first indication of smoke that can be noticed above the fire.

The formula for z is as shown below:

Where Q_p = the convective heat fire output. The values of Q_p=Q/1.5 is given by

Q is given by  where H_c is the heat of combustion and R is given as the mass rate of burning [11]. These values will be obtained from the CIBSE and all are related to the warehouse building. In assuming that most of the components burning are made of wood, we can then further assume that the heat of combustion is 13.0 × 10³ KJ/Kg. the calculated heat release rate is therefore given by 13.0 × 10³ × 0.5 = 6500 W = 6.5MW. The value of Q_p is therefore given as 4.333. Naturally, we get this height as 3.6m. The rest of the values in the ASET equation can be drawn from the tables in the CIBSE and the BS and in in replacing them, we get;

This gives 677 seconds.

These results indicate that the occupants of the building require that much time to vacate the building before they are incapacitated. The results indicate slightly over 11 minutes. While the figure may not be entire accurate on the site itself, it is acceptable to say that the amount of time arrived at is sufficient for every individual presently at the mezzanine floor would be able to get out within that timeline. This is a safe time and with that, it is impossible to foresee any congestion at the doors or stampede during exit.

The exposure time for the ASET fire analysis has been taken as short exposure. This helps us come up with the tenability criteria assumed in this work. The exposure is assumed to be short because the distance between the point of exit and the farthest point from the room, being 70.71m, can be covered in under a minute with an assumed walking speed of 1.2m/s. This means that the maximum time any individual can be within the smoke envelope after detection of the fire or an alarm is raised, even after a 5 minute response window in case some operation needed to be shut down progressively, is still under 6 minutes.

The exposure indicated above has leads on to the predetermination that, as the temperature in the hot layer above a room rises above 200°C the radiant heat limit of 2.5 KW/m² is reached. This hot layer should be above an individual’s head and therefore the height 2m is assumed for this. It is also assumed that, owing to the ventilation facilities provided, individuals should be exposed to minimal smoke as it gets discharged on rising up [12]. This would also be enough for any staff member trained in safety policies to help mobilize and evacuate the occupants and attempt to fight the fire if it can be handled.

Ventilation System

When conducting the RSET test, most values have been heavily borrowed from the BS code and the CIBSE. As seen in the hand calculation below, the formula for attaining the RSET value is given by;

As we can see in the calculation below, the time required to evacuate the occupants of the building fully is 140 sec. The RSET building fire calculations have also been identified as the worst case scenario therefore granting enough time for the last occupant to safely evacuate.

The safety margin is then provided by;

The safety margin is therefore 537 seconds.

This figure in the scenario simulation indicates that it is possible for every occupant at the level of the fire to exit the premises within the time required to do so and still have some time left [13]. A safe structure is one that has a higher ASET value compared to RSET value as the ASET represents the most amount of time present at the end of which fatalities are affected. With the project study below, we can also obtain the safety factor which is the ratio of the 2 results with the ASET value of the numerator [12].

On choosing Management level M1, alarm level A2 and building capacity B1, the properties of the pre-movement time, we get between 30 – 60 seconds. This is the time between when the first occupant clears the fire area and the rest of the 99% clear the fire zone. This figure is small compared to the rest because this is the type of building with least hindrances to egress. The building is also relatively plain and as stated earlier, very few obstacles are expected between the route of the fire and the exits including partitions [7]. The level of preparedness of the individual in the building is also expected to be high as warehouses and industries are locations where safety is highly prioritized. It is therefore expected that the staff there will evacuate the building in an orderly fashion.

The time taken between travelling out through the exit when looking at the movement is a factor of both the length of the room and the walking distance of the occupants. The walking speed is kept at 1.2m/s because, in the BS 7974, this is the speed an individual takes to cover an unobstructed distance when evacuating from a fire area [8]. The distance indicated there is the farthest point from which an individual can access the egress staircases. This is the mid-point of both walls adjacent to the walls with staircases. This distance can be calculated by the use of the Pythagoras theorem as it is the loci of this distance is the hypotenuse cutting between the midpoints of any two adjacent walls in that structure.

In calculating the travel flow, the constant 1.3 has been used as suggested by the BS 7974 in calculating the amount of time the 50 occupants of a building would take to vacate the facility through a 1.8m door [8]. The dimensions of the warehouse door have been assumed as they are not indicated in the diagram or assignment. However, a standard width for warehouse doors is taken as 1.8m. This is the widest single door from which a warehouse can use and therefore will be used in worst case scenario modelling. While there are 2 doors through which the occupants can exit the warehouse, calculations have been done for a scenario where one door is in use because the position of the fire is close to one of the exits making the vicinity around it untenable.

ASET Calculation Formula

Cumulatively, the RSET time has been found to be 140 seconds at worst. This is however specific for low to general exposure. This parameter has been adopted because individuals are not expected to be exposed to the smoke and poisonous gases for long. An indicator for this is the fact that the number of people is relatively little while the exits are two. With the explanation above highlighting the reason for the speed of the travelling movement, it is identifiable that occupants are not expected to be exposed to the fire for a long time.

The smoke ventilation system affects both ASET and RSET analyses as it helps buy time for the occupants of a fire hazard area. This is achieved by allowing the smoke to be discharged out of the room creating a smoke free envelope of clean air that would allow the occupants to move to safety. Such a system if especially effective if this warehouse contains materials that, when incinerated could release poisonous compounds into the atmosphere capable of causing fatalities [13]. This would be explained by the elementary physics principles that provide that hot air rises due to its reduced density. The density reduction occurs due to the expansion of the air’s volume while the weight remains constant. In this situation, the smoke being significantly hotter that the normal air would rise above the colder air [14]. Fluid displacement also provides for a situation where colder air contributes to this rise in hot air by displacing it owing to its much bulkier density. In the case of a fire, a ventilation system takes advantage of that phenomena allowing cool clean air in and hot smoky air out simultaneously.

This would be an advantage to fire-fighters from the fire brigade as it reduces the risk of fire damage to their protective head gear and minimizes their risk of explosion to smoke when fighting a fire. In a lot of firefighting instances, the obstruction caused by smoke leads to a lot of delay in identifying the surfaces with fire needed to be extinguished. Such a delay is leads to more property damage and risk of human life and should be avoided at all costs. Smoke in a fire hazard area, by obscuring visibility also makes it hard for firefighters to identify the people trapped in the fire either consciously or unconsciously. This is usually dangerous in situations where persons with disability or injured persons might still be trapped and in need of rescuing [14]. This is especially important as the building has no disabled access.

In regards to the building above, firefighters may be protected by providing them with the protective gear specifically for the heat and the smoke. This protective gear should be made out of an incombustible material e.g. asbestos fabric or asbestos-coated fabric. Headgear would also be important in order to protect them from accumulated smoke. The possibility of smoke accumulating despite the ventilation is especially heightened where little to no clean air gets in compared to the fire growth rate. As established earlier, the fire growth rate in a warehouse can be high leading to faster use of available, clean, oxygen-rich air by the fire and production of the poisonous smoke layer [14]. As such, it is also important to ensure the ventilation system can handle such a rapid rate of fire growth in the worst case scenario. A localized sprinkler system might be effected across the whole building as warehouses are also suffer most property damage compared to other buildings when exposed to a fire hazard [15]

Conclusion:

Based on the modelled room dimensions and requirements, the time available for the occupants’ evacuation is sufficient and a simulation would indicate no lives lost. The ventilation system in place allows for adequate smoke discharging but is concentrated on one side of the room making the lack of unevenness a possibility of smoke accumulation on one side of it. The projected time needed for the firefighters to take down the fire is unknown but as there is a ventilation system in place at the moment, clean air may be supplied in enough measure to allow for firefighting in the mezzanine floor before the building gets totally engulfed.

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