Fire Engineering Report: ASET and RSET Analysis of a Warehouse

This report presents a fire engineering analysis focusing on a warehouse mezzanine floor. It evaluates the Required Safe Egress Time (RSET) and Available Safe Egress Time (ASET) to determine safe evacuation procedures in case of a fire. The study employs Computational Fluid Dynamics (CFD) using the Fire Dynamic Simulator (FDS) model to establish ASET values. Assumptions are made regarding the building's characteristics, including a low percentage of flammable materials and a class B1 building complexity. The analysis considers tenability criteria such as smoke-free layer height and maximum upper layer temperature. The report calculates both ASET and RSET values, determining a safety margin to ensure occupant safety during fire emergencies. The report also provides a detailed explanation of the formulas and parameters used in the analysis, referencing relevant codes and guidelines like CIBSE and BS standards.

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NAME
UNIT
ASSIGNMENT
ASSESSMENT NUMBER

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INTRODUCTION
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
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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].
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
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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;
ASET (t)=Cef 3 t g
2
5 H
4
5 ¿
Where;
Cef3 = 0.937,
H = the minimum smoke-free layer height
tg = 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:
z=0.20 Qp
2 /5
Where Qp = the convective heat fire output. The values of Qp is given by Qp=Q /1.5
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Q is given by Q=H c × R where Hc 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 Qp 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;
ASET=0.937 × 75
2
5 × 2
4
5 × ¿
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
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trained in safety policies to help mobilize and evacuate the occupants and attempt to
fight the fire if it can be handled.
RSET ANALYSIS
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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;
RSET ( tevac )=t pre+ttrav (mvmt ) +ttrav (flow ) [12]
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;
SM =ASET −RSET →677−140
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
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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.
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
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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:
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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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