Effects of Combustible Stacking in Large Compartments

Effects of Combustible Stacking in
Large Compartments
by
Filippo Gentili, Luisa Giuliani and Franco Bontempi
Reprinted from
Journal of
Structural Fire
Engineering
Volume 4 · Number 3 · September 2013
Multi-Science Publishing
ISSN 2040-2317

Volume 4 · Number 3 · 2013
187
Effects of Combustible Stacking in
Large Compartments
Filippo Gentili1, Luisa Giuliani2 and Franco Bontempi1
1School of Engineering, Sapienza Università di Roma, Via Eudossiana 18,
00186 Rome
2Civil Engineering Department, Technical University of Denmark,
2800 Lyngby, Denmark E-mail: filippo.gentili@uniroma1.it
ABSTRACT
This paper focuses on the modelling of fire in case of various distributions of
combustible materials in a large compartment.
Large compartments often represent a challenge for structural fire safety, because of
lack of prescriptive rules to follow and difficulties of taking into account the effect of non
uniform distribution of the combustible materials and fire propagation.
These aspects are discussed in this paper with reference to an industrial steel building,
taken as case study. Fires triggered by the burning of wooden pallets stored in the
premises have been investigated with respect to different stacking configurations of the
pallets with the avail of a CFD code. The results in term of temperatures of the hot gasses
and of the steel elements composing the structural system are compared with simplified
analytical model of localized and post-flashover fires, with the aim of highlighting
limitation and potentiality of different modelling approaches.
Keywords: Structural fire safety; CFD modelling; steel industrial hall; fire propagation;
distribution of combustible; ventilation conditions
1. INTRODUCTION
The design process for structural fire safety can be divided into three main steps [1]: first the fire action
has to be modelled, then the temperature of the elements has to be computed with a heat transfer model,
and finally the system response has to be evaluated with a structural model.
More or less sophisticated models are available at each design step. This paper focuses in particular
on the advanced design of the fire action and heat transfer by means of Computational Fluid Dynamics
(CFD) models, with the aim of highlighting limits and potentiality of this design approach.
1.1. Fire in Large Compartments
Advanced fire models obtained with the avail of CFD investigations become particularly important if
greater design flexibility is desired or untraditional architectural or structural solutions are employed.
In this respect, the presence of large compartments is often required in industrial halls and public
buildings and is nowadays also a desired characteristic of many offices and residential premises.
Nevertheless, prescriptive design and verification methods for structural fire safety can only be applied
to compartments not exceeding specific dimensions, while the design of larger compartments often
represents a challenge for architects and structural engineers.
In particular, even if large halls are typically neither heavily cluttered nor densely furnished, the
distribution of goods or furniture may be strongly inhomogeneous, leading to possible concentration of
the fuel load, whose effects need to be carefully investigated.
Corresponding author: Filippo Gentili, Sapienza University of Rome, School of Civil Engineering, Via Eudossiana 18, 00184 Rome, Italy;
Tel.: +39 06 4458 5224;

In order to highlight the effects of combustible stacking in large compartment, an industrial steel
structure is taken as case study and investigated with respect to a fire triggered by the burning of
wooden pallets stored in the premises. Problematic issues of the CFD modelling concerning the
presence of uncertainties, the objectivity of the solution, and the reduction of computational onus are
presented and discussed. The advantage of more realistic simulations that take into account the effects
of fire propagation and the distribution of the combustible are also stressed out.
The outcomes of the CFD investigations in terms of temperatures of the hot gasses and temperatures
of the steel elements are compared with the results obtained by using simpler analytical models such as
localised and parametric fires. In particular, it is shown that a high spatial variability of temperatures
characterizes some type of large compartment fires. In those cases, post-flashover fires, which assume
a uniform distribution of the temperature along the compartment, can be not very representative of the
real phenomenon and can possibly lead to an underestimation of element temperatures.
1.2. State of the Art and Current Design Practice
The use of CFD models is fire safety has been originally developed and mostly used for the prediction
of smoke movements [2], but recent advances is performance-based design have led to an increasing
interest and research on the use of CFD models for structural fire design [3], [4], [5]. Nevertheless, the
use of these advanced fire models, even if contemplated by several regulations [6] and guidelines [7]
and despite the increasing use of advanced numerical program in building design and the availability
of more and more powerful computers, remains quite limited in the current design practice, which is
mostly based on the use of post-flashover fire model such as parametric fires and, for a large part,
nominal fire curve such as the standard ISO834 fire curve [6].
One reason of that can be found in the complexity of advanced CFD investigations, which require a
careful calibration and validation of the models (as better discussed in the last part of this paper) and
trained engineers to carry out the analyses and correctly interpret the results [8]. Another reason lies in
the common belief that the assumption of a severe post-flashover such the one described by standard
fire curve would necessary determine the highest temperature in the elements and therefore to a
conservative structural design.
Even if true in most cases, this assumption is not valid in general and current design practice may
lead to an unsafe design of structural elements, as evidenced by several cases of structural collapses [9],
[10], [11] and recently pointed out by several independent studies [12], [13], [14]. In particular, proper
consideration of the fire duration and the effective fuel load density seems to be fundamental for the
assessment of realistic element temperatures. Both aspects present difficulties in case of a fire in a large
compartment, due to the unlikelihood of flashover and uniform fuel load density. The study of different
grade of combustible staking seems therefore of particular interest with respect to the structural design
of large compartments, such as exhibition halls [15] and hangars [16].
2. CASE STUDY
A steel industrial hall has been considered as case study. The structural system considered has been
taken from a report [17], where FEM investigations of the structural response of the hall are presented
with respect to a standard fire.
Advanced investigations of the structural response to fire [18] are not within the scope of this paper,
which focuses mainly on aspects related to the modelling of the fire for structural design.
2.1. Description of the Structure
The configuration of the structural system and the properties assumed for the compartment and the
combustible are described below and summarized in Tab. 1, Tab. 2 and Tab. 3 with respect to the
property of the compartment, of the combustible and of the structural elements respectfully.
2.1.1. Geometry of the Compartment
The premises, whose geometry is shown in Fig. 1, consist in a large hall, 40 m long and 30 m wide and
covering a floor area of 1200 m2.
188 Effects of Combustible Stacking in Large Compartments
Journal of Structural Fire Engineering

The structural system is composed by 5 main frames, connected by 9 transversal purlins sustaining
a steel-concrete deck. The main frames consist of 2 bays, spanning 20 m between 5 m height columns.
The beams have a pitched configuration, so that a maximum height of 5.5 m is reached in
correspondence of the mid-span of each bay, while the average height of the hall is 5.25 m.
The total area of the enclosure (floor, ceilings and walls) results therefore to be equal to 3135 m2.
2.1.2. Amount and Properties of the Combustible
The premises are devoted to storage of goods and are assumed to be empty at the time of fire. Only the
presence of 320 wooden pallets (Fig. 2) used to support and transport goods is considered in the
premises and the effects of different disposition and stacking of the pallets are investigated.
Filippo Gentili, Luisa Giuliani and Franco Bontempi 189
Table 2. Properties of the combustible
Combustible
Fuel Proper Fire growing rate α 0.156 kJ/s3
Calorific value of combustible H 17.5 MJ/kg
Weight of combustible G 4800 Kg
Fuel Load Total fuel load Q 84000 MJ
Fuel load density (floor) qf 70 MJ/m2
Fuel load density (enclosure) q 27 MJ/m2
Table 3. Properties of the structural system
Structural Properties
Steel Density ps 7850 kg/m3
Specific heat cps 450 J/(kg.K)
Resultant emissivity εr 0.5 –
Profiles Purlin section factor Ap/Vp 192 m–1
Rafter section factor Ab/Vb 134 m–1
Central column section factor Ac/Vc 162 m–1
Table 1. Properties of the compartment
Enclsure Properties
Size Width B 30 m
Length L 40 m
Height (average) H 5.25 m
Floor area Af 1200 m2
Enclosure area At 3135 m2
Openings Average opening height hw,av 2.8 m
Total opening area Aw 103.7 m2
Air Flow Factor AF 173.49 m5/2
Opening factor O 0.055 m0.5
Thermal Inertia Gypsum surface A1 1426 W.s0.5/(K.m2)
Gypsum thermal inertia b1 762 m2
Concrete surface A2 1’920 m2
Concrete thermal inertia b2 1200 W.s0.5/(K.m2)
Thermal Inertia b 1017 W.s0.5/(K.m2)

Each pallet has dimensions 1.2 m × 1.2 m × 0.15 m and a weight of 15 kg. The pallets are assumed
to be made of wood with a calorific value of 17.5 MJ/kg, so that the total amount of fuel load in the
premises results to be equal to 84000 MJ, as reported in Tab. 2.
Wooden pallets are typically stored in stacks of different height, so that each pallet stack can be
considered similar to a firewood crib [19] and a constant plateau of the heat release rate (HRR) can be
seen if the stack is higher than 0.5 m (SFPE).
y
x
20 m
20 m
40 m
5 m
Door
Height: 3.6 m
Width: 4.8 m
Window
Height: 1.2 m
Width: 3.6 m
5.5 m
7.5
m
z 30 m
Figure 1. Industrial hall considered as case study.
1.2
1.2
1.2
1.2
0.15
hs
Pallet height
Pallet area
Pallet weight
No. pallets in the hall
Moisture content
Stack height 3.0 1.5
10
3.86
32
178
1.2
8
3.28
40
189
0.6
4
2.10 MW
MW
80
242
m
---
---
20
6.81
16
157
No. of pallets in a stack
HHRmax of a stack
No. stacks in the hall
HRRmax, tot
0.15
1.44
15
320
4
m
m2
kg
---
%
Figure 2. Geometry (top) and property (bottom) of a single pallet (left) and of a pallet
stack (right).

The maximum heat release rate (HRRmax) of a pallet stack can be therefore calculated with the
expression proposed by Krasner [20] and reported in Eq. 1, where the HRR of a stack of pallets is given
per unit area of the floor occupied by the stack (indicated as HRRs,max). The HRRmax calculated for different
stack height is visible on the right of Fig. 2: for a stack of 3 m, the HRRmax results to be equal to 9.81 MW,
which is consistent with the value reported by La Malfa [21], who suggests a value of 6.81 MW/m2 of
occupied floor area for a 3 m high stack of pallets; the value is also quite close to the value of 7 MW
reported by Karlsson & Quintiere [22] for a 3m high pallet stack.
HRRs,max = 919.(1 + 2.14.hp).(1–0.03.M) (1)
where:
hp indicates the height of a stack of pallets
M represents the moisture content of the wood
2.1.3. Ventilation of the Compartment
Four doors and eight windows have been assumed to be placed with a symmetrical disposition on the
external perimeter of the hall: in particular, a 4.8 m wide and 3.6 m high door has been placed in the centre
of each external pitched bay, while a 3.6 m wide and 1.2 m high window has been placed at 3.6 m from
the ground in the centre of each bay of the secondary frames in the transversal direction.
In case all doors and windows are assumed to be open during the fire, as assumed in the following
investigations, the opening factor of the premises results to be O = 0.055 m1/2 and the limit heat release
rate due to maximum oxygen income (HRRlim) is equal to 306.3 MW, which ensure a well-ventilated
condition for the development of the fire.
2.1.4. Materials of the Enclosure and of the Structural System
The structural system is a framed system of steel beams and columns, with the profiles shown in Fig. 3.
In particular, the purlins are realized with hot rolled steel S235, while the rafters with hot rolled steel S355.
All beams are considered to be exposed to fire on three sides and insulated on the top flange by the
presence of the roof deck.
Purlin
Section: HEA 160
Material: S235
y
xz
Rafter
Section: IPE 500
Material: S355
Column
Section: IPE 450
Material: S355
Figure 3. Structural system and steel element profiles.

For the purpose of thermal inertia calculations, the walls are assumed to be made of gypsum while
the floor and the ceiling are considered to be made of concrete, as shown in Tab. 1.
2.2. Simplified Fire Models
Analytical models for describing the temperature evolution of the hot gasses and of the elements in a
compartment can be synthetically distinguished in pre- and post-flashover models.
Pre-flashover models can be used for describing localized fires in case of either a low flame [23] or
a flame impinging the ceiling [24]. For the purpose of structural fire safety design however, generally
only post-flashover conditions are assumed. Simplified models describing post-flashover fires refer to
either nominal fires, such as the standard ISO834 curve, or to parametric fire curves, characterized by
a heating phase, a peak and a cooling phase.
Parametric fire curves were first introduced by the Swedish school [25] for describing post-flashover
compartment fires. The compartment was assumed to have a standard thermal inertia and an air inflow
capable of limiting the burning rate of the combustible. The method led to a graphical formulation of
temperature-time curves of the hot gasses and of the steel elements for different opening factors of the
compartment and fuel load densities of the combustible material. Further refinement of the model also
allowed considering different values of the thermal inertia and following studies [26], [12] based on the
same approach led to parametric curves described by analytical expressions. In particular, in the Danish
regulation [27] a unique expression describing both the heating and cooling phase is used (Eq. 2):
(2)
where:
Tg is the temperature of the hot gasses in °C
t is the time in min
b is the thermal inertia of the compartment in W.s0.5/(K.m2)
O is the opening factor of the compartment in m0.5
q is the fuel load density in MJ/m2 of enclosure surface of the compartment
The parametric curve indicated by the Eurocodes [28] instead uses two different expressions for
describing a natural fire: the heating phase is described with a monotonically increasing temperature,
while the cooling phase is represented by a linear decrement of the temperature, whose gradient varies
with the duration of the heating phase.
In this model, the assumption of ventilation controlled fires is removed and a different expression of
the fire curve can be used in case of well ventilated fires. A punctual comparison of design resulting
from the use of the Danish or of the Eurocodes parametric fires is presented in Giuliani et al. [13]. To
the purpose of this paper however, it seems relevant to point out that the use of the EN parametric fire
with linear cooling may lead to a strong reduction of the fire severity: this can be observed on the left
of Fig. 4, where the parametric fires calculated for the considered case study are compared with the
standard fire; on the right side of Fig. 4 the temperature curve of the rafters related to the Danish
parametric fire is instead reported, which results to be heated up to 400 °C during the fire.
It has to be pointed out however that the use of the EN parametric fire is recommended for fuel load
density calculated with respect to the enclosure are not trespassing the lower limit of 50 MJ/m2 and for
compartment not exceeding 500 m2 of floor area and 4 m of height. The same prescriptions on the
compartment size also apply to other parametric fire such as the Danish fire curve, limiting their
applicability to small compartments. This is due to the fact that parametric fire curves assume a
flashover-like fire with uniform temperature in the compartment, condition which hardly will occur in
large hall and atria. These limitations however are often disregarded in the practice, both because no
additional information can be found in the code for a simple modelling of fire in large compartments
and because other codes and literature references [29], [30] indicate that these limitations can be safely
T t
t
t t
g
=
+ +( ) 
−
( )
.log . .
. . / ma
20 345 8 1
1 0 04
10
Γ
xx
. max
/ .
/( )



=
( )
( )
=3 5
2
0 04
1160
with :
O
andΓ
b
t 77 8 10 3
. . − q
O

ignored at the expenses of a less economic but conservative design. In particular, thanks to the
symmetry of the structure considered as case study, exact results can be obtained by referring to 1/4 of
the compartment, as indicated by Hertz [12], in case a flashover is assumed in the compartment. By
using this method, the floor area of the compartment can be reduced to a size compliant with the
prescriptive limits. Always according to the same document, conservative results are obtained for
compartment higher than 4 m, provided that all the openings above 4 m will be ignored.
The case study presented here, refers to a compartment which is symmetric about the horizontal and
transversal centreline and could be therefore reduced to 1/4 of the original floor area, which becomes
equal to Af’ = 300 m2.The resulting opening factor of the reduced symmetric compartment will be
0.045. The parametric curves and the corresponding element temperatures shown in Fig. 4 have
therefore been calculated with reference to 1/4 of the compartment and to this reduced opening factor,
as according to the procedure described in Hertz [12].
2.3. Advanced Fire Models
The limitations of parametric fire curves can be overcome by using more advanced fire models. In
particular, the temperature evolution of hot gasses and elements can be obtained by CFD investigations
as a variable of time and space, and possible non uniform distributions of the temperatures in large
compartments can be highlighted.
Advanced fire models allow taking into account the ignition of secondary objects that represents a
critical event in the fire development. Radiation from the flame, which depends on the configuration
factor, assumes considerable importance for the ignition of nearby items. Many parameters such as fire
location, size and shape of flame, and material proprieties affect the phenomenon [31]. Among the
possible criteria for determining the ignition in a code, one based on flux-time product (FTP) provides
results in good agreement with experiments [32]. In this paper, for sake of simplicity, the ignition
temperature of pallets (275 °C) was considered as a criterion of ignition.
Non uniform distributions of temperatures in a compartment stem either from non-uniform
distribution in space of the combustible or from non-uniform burning time of the combustible:
i. in the first case, concentration of the fuel load in a relatively small area of the compartment could
lead to an underestimation of the flame height and of the temperatures above:
ii. in the second case, a slow propagation of the fire would determine a longer fire duration than in
case of a flashover-like burning of the combustible is assumed.
Both situations are likely to occur in large compartments (typically atria, auditoria, warehouses,
industrial halls, etc.), where the need of free stream of people or goods requires a low density of
furniture and encumbering materials, which can be either piled up, leading to fuel load concentration
(i) or placed far one from the other, leading to slow propagation of the fire (ii). Either way, the
ISO 834
DK parametric
EN parametric
800
600
400
200
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
Gas temperature
Rafter temperature
800
600
400
200
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
Figure 4. Comparison between nominal (ISO834) and parametric (DK and EN) gas
temperature (left) and temperature of the main beams according to the DK
parametric fire curve.

assumption of a uniform temperature in the compartment may result in an underestimation of the
elements temperatures and possible structural failures.
Those aspects are better highlighted in the following, where the results of CFD investigations are
presented, which refer to fires triggered by different distribution of the combustible materials. The
investigations presented have been carried out with Fire Dynamic Simulator (FDS), which is a field
CFD code released by NIST [33].
2.3.1. Fire Scenarios
An overview of the fire scenarios considered for the investigations is reported in Fig. 5 with respect to
1/4 of the compartment.
In every scenario, the pallets are considered to be staked in the centre of the hall. The number of wooden
pallets piled up in each stack varies from a maximum of 20 in scenario C to a minimum of 4 in scenario D
and the number and height of the stacks in the hall varies accordingly (Fig. 2), as the total number of pallets
in the hall is constant: in particular, a maximum height of 3 m is reached by the stacks considered for
scenario C and a minimum height of 0.6 m is reached by the stacks considered in scenario D.
This dispositions lead to different extensions of the floor area involved in the fire, but also to
different values of the HRRmax during the fire, which, as explained above and summarized in Fig. 2,
varies with the stack height. In all scenarios, the mutual distance between the stacks is constant and
equal to 1.2 m and the fire is assumed to trigger always in the 4 central stacks. As a consequence, the
different values of HRRmax are expected to determine a different speed of the fire propagation.
In the following sections, the outcomes of the investigations in term of temperatures of the fire and
of temperature on the elements are presented, with respect to different locations within the compartment.
In particular, the temperatures of the hot gasses are referred to 8 thermocouples TC-1 to TC-8 placed
at 4.5 m from the ground, while the temperatures of the elements have been measured with the adiabatic
surface temperature method on the points AST-1 to AST-6. The position of the measurement device is
visible in Fig. 6 (right) and the coordinates of the points as reported as table in the same figure (left).
Scenario A
10 stacks
1.2 m high
Scenario D
20 stacks
0.6 m high
Scenario C
4 stacks
3.0 m high
Scenario B
8 stacks
1.5 m high
Figure 5. Fire scenarios represented on 1/4 of the model.

The outcomes of all considered scenarios are compared and three out of four scenarios are discussed
in detail in the following, as representative of the most significant fire phenomena.
Scenario A: uniform distribution of temperatures
This scenario considers that the 320 wooden pallets have been piled up in group of 8, forming 40 stacks
of height 1.2 m. The pallet stacks are placed at a mutual distance of 1.2 m in a regular pattern
(Fig. 5, top left), which covers a squared area of 162 m2 in the centre of the hall.
The outcomes of the investigation are presented in Fig. 7 in term of temperatures of the gas (left
column) and temperatures of the rafters (right column). The upper row refers to measurements taken
above the combustible, while the bottom row refers to measurements taken farer from the flame.
TC - 2
ID
TC – 1
AST – 1
AST – 3
AST – 5
AST – 2
AST – 4
AST – 6
TC – 3
TC – 5
TC – 7
TC – 2
TC – 4
TC – 6
TC – 8
X
5.0 6.0
6.0
6.0
6.6
6.6
6.6
6.0
0.0 4.8 16.8 0.0
7.5
15.0
4.6
4.6
4.6
16.8
16.8
4.8
4.8
7.5
15.0
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
10.0
15.0
20.0
5.0 12.0
12.0
12.0
12.0
10.0
15.0
20.0
Y Z X Y ZID
ID X Y Z X Y ZID
Thermocouple
Adiabatic surface temperature
TC - 4 TC - 6 TC - 8
TC - 7TC - 5TC - 3TC - 1
AST - 5
AST - 6
AST - 4AST - 3
AST - 1 AST - 2
Figure 6. Coordinates of thermocouples (TC) and devices for element temperatures
(AST) (left) and their graphical representation on 1/4 of the model (right).
1600
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
1000
750
500
250
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
1000
750
500
250
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
1600
TC – 6 AST – 4 Localised fire
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
TC – 5
TC – 1
Tg (DK parametric)
AST – 1
AST – 3
Ts (DK parametric)
Figure 7. Outcomes from scenario A in term of temperatures registered by the
thermocouples (left) and on the element surfaces (right) inside (top) and outside
(bottom) the area occupied by the combustible.

With respect to the latter, the temperatures of the gas in two different locations of the hall are
compared with the Danish parametric fire curve (Fig. 7, top left). Both the temperature registered by
the thermocouple most distant from the combustible area (TC-1) and by a closer thermocouple (TC-5)
show a good accordance with the temperatures provided by the parametric fire. The comparison
between the two fire models has to be intended just as confrontation of the shape and temperatures of
the fire, while a check on the starting time and initial growing rate of the two fires is hardly possible.
The reason is that the parametric fire is a post-flashover model, which assumes the simultaneous
burning of all combustible material present in the compartment. The fire scenario investigated instead
refers to a fire, which triggers more realistically in few stacks and then propagates to the adjacent ones.
Therefore also the initial phase of the fire is represented in the outcomes of the investigation.
The same accordance in term of shape and maximum temperatures stemming from simplified
models is observable with respect to the elements outside the combustible area (Fig. 7, bottom right),
where the temperatures of two rafters are compared with the steel heating calculated from the Danish
parametric fire for the rafter profile. Even if the position of the two rafter with respect to the fire is
different (farer for AST-1 and closer for AST-3), the temperatures of the two elements are very similar
and close to the steel heating curve.
The same uniformity of temperatures can be observed on all structural elements having the same
profiles, except those just above the area occupied by the combustible (AST-6) or spanning from that
area (AST-5). For those elements (Fig. 7, top right) and for the thermocouples above the combustible
area (TC6 and TC8 in Fig. 7, top left) the temperatures are much higher, since the flame of the fire is
impinging the ceiling. This is due to the relatively high pile of pallets, which gives a HRRmax of
4.72 MW per stack and leads to a potential height of the flame equal to 5.53 m from the floor, according
to the model of [23]. It seems therefore more reasonable to refer to a localized fire model for a
comparison of the temperature of those elements. The Eurocodes model for a localised high flame fire
[34], which is based on the above referenced model of Hasemi [24], would however lead to an
overestimation of about 25% of the element temperatures in this case, as visible in the figure.
Scenario C: non uniform distribution of combustible in space
In this scenario, a higher stacking grade of the combustible is considered and the pallets are assumed
to be piled up in group of 20, forming 16 stacks of height 3 m. The pallet stacks are placed at a mutual
distance of 1.2 m in a regular pattern (Fig. 5, bottom right), which a covers a squared area 72 m2 in the
centre of the hall.
Contrarily to the previous case, differences in the distribution of the gas and element temperatures
along the compartment can be observed in this case also outside the area occupied by the combustible.
In particular, two temperatures of the gas outside the combustible are reported and compared with
the Danish parametric fire (Fig. 8 bottom left). In spite of the temperature evolution of the parametric
curve is much faster than the observed temperatures, due to what explained above, a consistency
between the temperatures of the parametric fire and of the hot gasses can be observed only with
reference to a point very far from the flame (TC-1). The temperature increases by moving towards the
centre of the hall and even when the distance from the flame is still consistent (TC-5) a significant
difference (around 40%) is shown with respect to the other curve (TC-1). The same difference reflects
on the temperature of the elements measured at two different distances (AST-1 on the most external
rafter and AST-3 on the adjacent one) from the combustible (Fig. 8 bottom right). The temperatures
shown in Fig. 8 refer to the same measurement points as the previous case; however, since the stacking
grade of the combustible is much higher, the area coved by the combustible is just 70.6 m2 and the
devices distance from the fire are higher, so that a lower difference on element temperatures could at
first be expected. However, the HRR developed by the fire in this scenario (Fig. 11) is higher, since the
stacking grade of the combustible also affects the fire propagation, as better explained in the following
section.
The higher stacks also determine a higher flame length. Therefore, also in this scenario the flame is
impinging the ceiling and the temperatures above the combustible area registered by the thermocouples

TC-6 (Fig. 8 top left) and TC8, as well as by the element devices AST-5 (Fig. 8 top right) and AST-6
are higher than in the previous scenario.
Scenario D: non uniform burning of combustible in time
This scenario considers a lower stacking grade of the combustible, where 80 stacks of 4 pallets each
and an height of 0.6 m are assumed to be placed at a mutual distance of 1.2 m, covering a squared area
of 229 m2 in the centre of the hall (Fig. 5, bottom left).
The development of the fire and the consequent temperatures in the compartment and on the
elements are very different in this scenario than in the previous ones. Due to the significantly lower
grade of staking, the fire propagation is very slow and one stack gets on fire when the fire on the
adjacent one is about to extinguish, as visible in Fig. 9. The fire therefore moves from one stack to
another, maintaining a low HRR and lasting much longer than a normal fire. This phenomenon has been
evidenced and investigated in recent studies [35], where it is referred to as travelling fire.
Results in term of temperatures of the gas and of the elements are reported in Fig. 10 with respect
to the first 30 min of fire, even if the duration of the fire is much longer in this case. With respect to the
previous cases, the temperatures obtained in this scenario are lower and spatially uniform in the area of
the compartment not occupied by the fire. Furthermore, they also show almost a constant trend with
respect to time, as it can be seen in the left part of Fig. 10, where the temperature registered by the
thermocouple TC-1 is reported. The same constant trend is shown by the temperature of the elements
outside the combustible area, as visible in the right part of Fig. 10 with respect to the temperature
registered for the device AST-1.
The temperatures of the thermocouples above the area occupied by the combustible show instead
peaks of temperatures of short duration and then they drop down to temperatures not very dissimilar,
even if slightly higher, to the temperatures outside the combustible area. Due to the significantly lower
HRRmax of a stack, the flame height in this scenario is lower than the ceiling and is not impinging the
1600
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
1000
750
500
250
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
1000
750
500
250
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
1600
TC – 6 AST – 4 Localised fire
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
TC – 5
TC – 1
Tg (DK parametric)
AST – 1
AST – 3
Ts (DK parametric)
Figure 8. Outcomes from scenario C in term of temperatures registered by the
thermocouple (left) and on the element surfaces (right) inside (top) and outside
(bottom) the area occupied by the combustible.

beams, which never get very hot as in the previous scenarios. The same peaks characterize the
temperatures of the elements inside the combustible area, which occur at different times, depending on
their distance from fire. The first peak is the one registered by the device AST-6 placed on the rafter
just above the centre of the fire, which is heated by the four central pallet stacks, assumed to get on fire
simultaneously, which then propagate the fire.
The above mentioned studies on travelling fire show that, despite the low HRR, this type of fire can
be detrimental for the structure, which is heated for a significantly longer time than in case of a
Time = 5 min Time = 14 min
Time = 30 minTime = 20 min
Figure 9. Graphical representation of the HRR at different time for the fire scenario D.
600
450
300
150
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
600
450
300
150
0
0 5 10 15 20 25 30
Time (min)
Temperature(°C)
TC – 1 TC – 8 AST – 1 AST – 6
Figure 10. Outcomes from scenario D in term of temperatures registered by the
thermocouples (left) and on the element surfaces (right) inside and outside the area
occupied by the combustible.

flashover-like fire. As well understandable, this situation can be critical especially in case of concrete
structural system [36].
This doesn’t seem the case in this particular fire scenario, where, even if the temperature of the
elements slowly increases with time, won’t get to very high values. This can be explained considering
that a low fuel load has been assumed in this study, in order to have the possibility of implementing
different stacking level of the combustible. When the combustible is concentrated in a small area, the
local fuel load density is high, which may lead to fire that are locally much more severe than what
expected from a compartment fire. However, when the combustible is relatively spread along the
compartment as in this case, the fuel load density is more uniform along the compartment and has a
low value.
Despite specific considerations on the temperature values however, it seems important to point out
that non uniform distribution in time and space of the temperatures, such as those generated by a
travelling fire, may have negative effect on the structural behaviour, e.g. in case cold elements hinder
the thermal expansion of hot ones [37].
2.4. Comparison
The results obtained from all the four fire scenarios considered (Fig. 5) are summarized in Fig. 11. The
left graph at the top of the figure shows the development in time of the HRR; the adjacent graph on
the right shows instead the temperature of the hot gases measured by the thermocouple TC-1, which is
the most distant from the combustible area, while the temperature measured by the thermocouple TC-7,
placed just above the combustible, are shown in the graph below (bottom right) of the figure; the graph
160
120
80
40
0
0 5 10 15 20 25 30
Time (min)
HRR(MW)
1600
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
TemperatureAST–4(°C)
1600
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
TemperatureTC–6(°C)
1000
750
500
250
0
0 5 10 15 20 25 30
Time (min)
Scenario A
Scenario B
Scenario C
Scenario D
Scenario A
Scenario B
Scenario C
Scenario D
Scenario A
Scenario B
Scenario C
Scenario D
Scenario A
Scenario B
Scenario C
Scenario D
Figure 11. Comparison of the outcomes for the different scenarios.

on the left at the bottom of the figure reports instead the temperature of the central rafter just above the
combustible, measured by the device AST-6.
By observing Fig. 11, it can be concluded that the primary effect of the grade of combustible
stacking is the propagation rate of the fire. The fire propagation is strictly related with the maximum
HRR achievable by the fire and therefore with the fire duration too. By moving from high to low grades
of combustible stacking, the peak of the HRR decreases and the fire becomes lower and longer, as
visible at the top of Fig. 11. If the HRR becomes sufficiently low for a given distance of the combustible
materials, a peak in the HRR curve cannot be evidenced anymore, as a different fire phenomenon
develops, where the fire moves throughout the compartment, as the combustible material of one area
burns out.
Among the three scenarios A, B, and C, the most severe fire in term of element temperatures seems
to be the one referring to the fire scenario C, which corresponds to a high staking grade of the
combustible. This result is reported in the figure for the temperatures of the elements above the flame,
but can be also evidenced in the temperatures of the elements outside the combustible area.
2.5. Problematic Aspects of CFD Models
The results presented above show that simplified models are not capable of providing with a
sufficient grade of accuracy the element temperatures in case of fire developing in large
compartments. A more realistic modelling of the phenomenon can be obtained by means of CFD
investigations, which can account for different distribution of the combustible and velocity of fire
propagation and are capable of describing possible inhomogeneous distribution of the temperatures
within a compartment.
2.5.1. Reduction of Computational Onus
As discussed in the previous sections, the above mentioned aspects are particularly relevant for the
design of large compartments. In case of large compartments however, CFD models requires generally
a particularly high computational onus. This is partially due to the greater extension in space and time
of the investigations: this problem can be avoided in case some symmetry lines are present in the
compartment. For example in the case study presented, the compartment was symmetric about the
horizontal and transversal central lines and a reduction of the computational onus has been obtained by
carrying out the investigations in 1/4 of the model (Fig. 12), where appropriate boundary conditions
Complete
model
Model
Complete
Partial
X
Dimensions
Y Z
40 30 6
20 15 6
Partial
model
Figure 12. Full model and reduction to 1/4 of the model.

have been considered for simulating the two symmetry planes. The validation of the reduced model
against the full model is shown in Fig. 13, where a complete accordance in terms of fire and element
temperatures can be observed.
Apart from the greater physical dimensions of the problem and possible longer durations of the fire,
more significant increment of the computational onus can be ascribed to the convergence problems,
which may arise in case of highly concentrated or highly spread combustible materials, both
representing situations likely to occur in large compartments: in the first case, difficulties stem from a
very high growing rate of the fire, while in the second case the slow propagation of the fire from one
object to another can be an issue. As a consequence, the grid size required for this kind of investigations
is generally smaller than for investigations of more conventional structures, as better explained in the
following.
2.5.2. Mesh Optimization
Even if FDS is one of the most acknowledged codes for fire modelling, the use of the program requires
a certain attention in case the HRR has to be predicted rather than specified, such as in case the
modelling of fire growth and spread is of interest. In these cases, the limits on the grid size are more
severe than the value recommended in the guidelines [38], [39], [40] and for an optimal representation
of the buoyant plume dynamic a careful mesh sensitivity study becomes fundamental. With respect to
the case study investigated, despite the property of the compartment did not vary, different physical
phenomena stemmed from the variation of the distribution of the combustible and different values of
the optimal grid size had to be identified for each case. In the following, the study of the mesh
sensitivity is reported for the case of scenario C. The most severe limitation from the Danish CFD guide
[7] recommends a mesh size around 0.7 m with respect to the HRR of this scenario. A starting size of
60 cm has been therefore used for the sensitivity study and then the mesh size has been decreased until
convergence in the temperature curves s is obtained. In Fig. 14 the results of the sensitivity study are
reported in term of HRR (top left), smoke height (top right), temperature of the gas registered by the
thermocouple TC-6 (bottom right), and temperature of the rafter registered by the device AST-4
(bottom left).
It can be seen that the mesh of 60 cm leads to inconsistent results for the HRR curve, for the smoke
height, and for the gas and element temperatures as well. A mesh of 30 cm would be sufficient for the
representation of the smoke movements; however, the description of the HRR curve would not
sufficiently accurate in this case and would lead to an underestimation of the gas and element
temperatures, which are of main interest in this study. Finer mesh sizes of 20 cm and of 15 cm are
therefore required for reliable results. Since the sensitivity study shows that the gas and element
temperatures don’t change whit a mesh finer that 20, this mesh size was therefore chosen for the
investigation of the scenario C discussed here.
1600
Partial model
Complete model1200
800
400
0
0 5 10 15 20 25 30
Time (min)
1600
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
Partial model
Complete model
Figure 13. Validation of the reduced model which refer to 1/4 of the compartment,
in case of scenario C.

3. CONCLUSIONS
Current design methods of structural fire safety refer to post-flashover conditions, where a uniform
distribution of the temperatures can be assumed within the compartment. These methods have been
extensively tested and safely used in small compartment, which are typically dense furnished and
present a reasonably uniform distribution of the combustibles along the floor area.
Large compartments may be instead less densely furnished and present therefore lower fuel load per
square meter of floor area, as shown by several statistic investigations conducted in the ‘70ies in
Sweden [41], [42] and more recently in Denmark [27]. Nevertheless, the assumption of a uniform
distribution of the combustible materials is generally unrealistic: especially in case of industrial halls,
furniture and other combustibles can occupy just a part of the premises or stored materials can be piled
up with different grade of stacking.
Since the flashover is unlikely to occur in large compartments, the Eurocodes [28] indicates a
relatively low limit on the compartment size for the applicability of parametric fire. If this doesn’t
represent a problem for traditional residential buildings, it hinders the use of simplified fire models for
a growing number of structures, given that nowadays longer span width and innovative solutions for
open spaces are made available by the constant advance in structural design. The need of a better
comprehension and modelling of the fire which develops in large compartment seems therefore a
critical aspect of the fire safety design.
In this paper a well-ventilated fire in a large compartment devoted to storage of wood pallets has
been investigated with respect to structural fire safety considerations. Four fire scenarios, each referring
to a different staking grade of the combustible, have been considered and the results in term of gas and
element temperatures have been discussed and confronted. These temperatures have also been
compared with those obtained by simplified fire models such as a localized fire and a parametric fire,
1600
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
160
120
80
40
0
0 5 10 15 20 25 30
Time (min)
HRR(MW)
5
4
3
2
1
0
0 5 10 15 20 25 30
Time (min)
Heightofsmoke(m)
1600
1200
800
400
0
0 5 10 15 20 25 30
Time (min)
Mesh 60 cm
Mesh 15 cm
Mesh 20 cm
Mesh 30 cm
Mesh 60 cm
Mesh 15 cm
Mesh 20 cm
Mesh 30 cm
Mesh 60 cm
Mesh 15 cm
Mesh 20 cm
Mesh 30 cm
Mesh 60 cm
Mesh 15 cm
Mesh 20 cm
Mesh 30 cm
Figure 14. Mesh sensitivity study for fire scenario C.

which assumes a ventilation controlled fire and a uniform distribution of the fuel load. The outcomes
of the investigations show that in case of highly concentrated fuel the beam temperatures of the beams
obtained with a parametric fire model are underestimated. The use of localized fire on the other side
would not be sufficient, as the temperatures of elements far from the combustible cannot be neglected
and show instead accordance with the temperatures obtained by using a parametric fire model. This is
not the case for the last scenario presented, where the element temperatures didn’t vary significantly in
space and were greatly lower than those obtained in the other fire scenarios or calculated from the
parametric fire.
It can be concluded that simplified analytical models may not accurately describe the element
temperatures, both in case the combustible is highly spread along the compartment and in case it is
instead highly concentrated in a small area of the floor.
With respect to the first case, a very low fuel load density deriving from spreading the combustible
over a large floor area, may result in very long fires, especially in case of a travelling fire, where the
propagation of the fire to the adjacent combustible materials occurs only after the burnout of the first
object. This phenomenon has been recognised to have been possibly responsible of major structural
failures both in steel [43] and concrete buildings [44], as the heating of the elements may be affected
by the fire duration more than by a higher gas temperature. Even if in the case study here presented
high element temperatures were not evidenced, when a spread distribution of the combustible was
considered, the occurrence of a travelling fire has been highlighted in this case.
With respect to the second case, a high staking grade of the combustible determines a non-uniform
distribution of the temperatures of both the gas and the elements, also outside the area occupied by
the combustible. Furthermore, due to the high local fuel load density, the temperatures of the
elements above the flame may be heated up to temperatures much higher than those predicted by a
flashover fire. This result may appear in contrast with recommendations for the structural fire design
of large compartments [12], which show that conservative results are expected if the limits on the
compartment size for applicability of parametric fire are exceeded. It has to be stressed out however,
that a value of 200 MJ/m2 of enclosing surface is recommended for domestic buildings, offices,
hospitals, schools and hotels, disrespectfully from the effective fuel load present [12] n the view of
the results presented above, in case a uniform distribution of the combustible is assumed in large
compartments, it seems essential to use a nominal high value of the fuel load, even when the amount
of combustible is known to be lower, in order to take into account the effect of a possible stacking
of the material.
ACKNOWLEDGMENT
The authors would like to thank Prof. Kristian Hertz for fundamental support to this study. The
contributions of Prof. Grunde Jomaas, Prof. Luca Grossi and Dr. Francesco Petrini are also gratefully
acknowledged.
REFERENCES
[1] Buchanan A.: “Structural Design for Fire Safety”, Wiley&Sons Ed., England, 2001.
[2] Hadjisophocleous G.V., McCartney C.J.: “Guidelines for the use of CFD simulations for fire and
smoke modelling”, NRCC-47740, National Research Council, Canada, 2005.
[3] Buchanan A.: “The challenges of predicting structural performances in fire”, proc. of the 9th Int.
Symposium on Fire Safety Science, Germany 2009.
[4] Byström A., Cheng X., Wickström U., Veljkovic M.: “Measurement and calculation of adiabatic
surface temperature in a full-scale compartment fire experiment”, Journal of Fire Sciences,
August 2012.
[5] Duthin D., Mc Grattan K., Khaskia A.: “Recent progress in fire-structure analysis”, EJSE special
issue on the 1st Int. Conference on Modern Design, Construction and Maintenance of Structures,
Hanoi, Vietnam, December 2007.
[6] EN1991-1-2:2002 - General actions - Actions on structures exposed to fire, 2002.

[7] CFD best practice, Supplement to the Danish Building Regulations, Best Practice group,
November 2009 (in Danish).
[8] Torero J., Steinhaus T.: “Application of computer Modelling to Fire Safety Design”, proc. of the
53rd Jahresfachtagung de Vereinigung zur Foerderung des Deutschen Brandschutzes, Essen, June
2004.
[9] Meacham B., Engelhardt M., Kodur V.: “Collection of Data on Fire and Collapse, Faculty of
Architecture Building, Delft University of Technology”, Proc. of NSF Engineering Research and
Innovation Conference, Honolulu, Hawaii, 2009.
[10] National Institute for Land and Infrastructure Management (NILIM): “Report on the Windsor
Building Fire in Madrid, Spain”, Japan, Jul 2005 (in Japanese).
[11] Gann R.G.: “Final Report on the Collapse of World Trade Centre Building 7”, Federal Building
and Fire Safety Investigation of the World Trade Centre Disaster, NIST NCSTAR 1A, November
20, 2008.
[12] Hertz K., Parametric Fires for Structural Design, Journal of Fire Technology, Springer Science,
2001 (DOI: 10.1007/s10694-011-0246-5).
[13] Giuliani L., Budny I.: “Different design approaches to structural fire safety”, International Journal
of Lifecycle and Performance Engineering (IJLCPE), special issue on Fire Safety Design and
Robustness Considerations in Structural Engineering, accepted for publication on July 2012
(in press).
[14] Roben C.: “The effect of cooling and non-uniform fires on structural behaviour”, PhD
dissertation, University of Edinburgh, January 2010.
[15] Petrini F.: “Numerical analyses for Performance-Based Fire Engineering (PBFE)”, 4th
International Conference on Structural Engineering, Mechanics and Computation (SEMC’10),
Cape Town, South Africa, September 2010.
[16] Crosti C.: “Structural analysis of steel structures under fire loading”, Acta Polytechnica, Vol. 49,
No. 1:21–28, 2009.
[17] EUR 24222: “Fire safety of industrial halls – A valorization project”, Final report, Research fund
for coal and steel publications (RFCS), Luxembourg, 2007.
[18] Gentili F.: “Advanced numerical analyses for the assessment of steel structures under fire”,
International Journal of Lifecycle and Performance Engineering (IJLCPE), special issue on Fire
Safety Design and Robustness Considerations in Structural Engineering, accepted for publication
on August 2012 (in press).
[19] Drysdale D.: “An Introduction to Fire Dynamics, Wiley&Sons Ed., England 2008.
[20] Krasner L.M.: “Burning Characteristics of Wooden Pallets as a Test Fuel,” Serial 16437, Factory
Mutual Research Corp., Norwood, MA, 1968.
[21] La Malfa A., La Malfa S.: “Approccio Ingengeristico alla Sicurezza Antincendio”, Legislazione
Tecnica, 2009 (in Italian).
[22] Karlsson B., James G. Quintiere J.G.: “Enclosure Fire Dynamics”, Taylor & Francis Inc., 2009.
[23] Heskestad G.: “Fire Plumes”, SFPE Handbook of Fire Protection Engineering, pp. 2–9, National
Fire Protection Association, Quincy, MA, 1995.
[24] Hasemi Y., Tokunaga T.: “Flame Geometry Effects on Buoyant Plumes from Turbulent Diffusion
Flames”, Fire Science and Technology, 4, 15–26, 1984.
[25] Pettersson O., Magnusson S.E., Thor J.: “Fire Engineering design of steel structures”, Bulletin 52,
Sweden, 1976.
[26] Wickström U.: “Application of the standard fire curve for expressing natural fires for design
purposes”, Fire Safety: Science and Engineering, American Society for Testing and Materials
(ASTM), STP 882, Harmathy Ed., Philadelphia, 1985.

[27] EN 1991-1-2 DK NA: 2007 – Danish National Annex to Eurocode 1: Actions on Structure, Part
1-2: General Actions, Actions on structures exposed to fire, 2008.
[28] EN1991-1-2:2002 - General actions - Actions on structures exposed to fire, Annex A: Parametric
temperature-time curves, 2002.
[29] PD 6688-1-2:2007: Background paper to the UK National Annex to BS EN 1991-1-2: 2002-
Actions on structures. General actions. Actions on structures exposed to fire, 30 April 2007.
[30] Hertz K.D.: “Vejledning i dimensionering af bygningskonstruktioner for fuldt udviklet brand”,
(Guide for design of building structures for fully developed fires), version 2–3, National Agency
for Enterprise and Construction, Copenhagen, 2006 (in Danish).
[31] Jahn W., Rein G., Torero J.L.: “The Effect of Model Parameters on the Simulation of Fire
Dynamics”, Fire Safety Science 9: 1341-1352, doi:10.3801/IAFSS.FSS.9–1341.
[32] Baker G., Fleury R., Spearpoint M., Fleishmann C., Wade C.: “Ignition of Secondary Objects in
a Design Fire Simulation Tool”, Fire Safety Science 10: 1359-1372, doi:1038/IAFSS.FSS.
10–1359.
[33] McGrattan K., Klein B., Hostikka S., Floyd J.: “Fire Dynamics Simulator (Version 5), User’s
Guide”, March 2006.
[34] EN1991-1-2:2002 - General actions - Actions on structures exposed to fire, Annex C: Localised
Fire, 2002.
[35] Stern-Gottfried J., Law A., Rein G., Gillie M., Torero J.: “A Performance Based Methodology
Using Travelling Fires for Structural Analysis”, Conference of Fire Protection Engineering,
(SPFPE’10), Lund, June 2010.
[36] Law A., Stern-Gottfried J. Gillie M., Rein G.: “The influence of travelling fires on a concrete
frame”, Engineering Structures, Vol. 33, Issue 5, Pages 1635–1642, May 2011.
[37] Usmani A.S., Chung Y.C., Torero J.L.: “How Did the WTC Collapse: A New Theory”, Fire Safety
Journal, 38, 6, 501–591, 2003.
[38] NUREG 1824 - Verification and validation of selected fire models for nuclear power plant
applications, vol. 7: Fire Dynamic Simulator (FDS), Final report, US Nuclear Regulatory
Commission, 2007.
[39] Ascenzi G. Villi G., Vulpiani G.: “Ingegneria della sicurezza antincendio - Guida all’utilizzo di
FDS”, Flaccovio Ed., 2010.
[40] CFD Best Practice, Best Practice gruppen, Brandteknisk Selskab, 12. November 2009.
[41] Nilsson L.: “Brandbelastning i bostadslägenheter”, (Fire Load in Appartments), Statens Institut
för Byggnadsforskning, Rapport R34, 1970. Stockholm 1970 (in Swedish).
[42] Forsberg U., Thor, J.: Brandbelastningsstatistik för skolor och hotel (Fire Load Statistics for
Schools and Hotels), Stålbygnadsinstitutet, Rapport 44:1, Stockholm 1971(in Swedish).
[43] Gann R.G. Hamins A., McGrattan K.B., Mulholland G.W., Nelson H.E., Ohlemiller T.J., Pitts W.
M., Prasad K.R.: “Reconstruction of the Fires in the World Trade Center Towers”, NIST NCSTAR
1-5, September 2005’’.
[44] Fletcher, I. et al, “Model-Based Analysis of a Concrete Building Subjected to Fire,” Advanced
Research Workshop on Fire Computer Modelling, Santander, Spain, 2007.

Effects of Combustible Stacking in Large Compartments

Effects of Combustible Stacking in Large Compartments

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Effects of Combustible Stacking in Large Compartments

Similar to Effects of Combustible Stacking in Large Compartments (20)

More from Franco Bontempi

More from Franco Bontempi (20)

Recently uploaded

Recently uploaded (20)

Effects of Combustible Stacking in Large Compartments