Rise in structural steel temperatures during ISO 9705 room fires

Fire Safety Journal 46 (2011) 480–496

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Rise in structural steel temperatures during ISO 9705 room fires Khalid A.M. Moinuddin n, Jamal S. Al-Menhali, Kailas Prasannan, Ian R. Thomas Centre for Environmental Safety and Risk Engineering, Victoria University, P.O. Box 14428, Melbourne, Victoria 8001, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 September 2010 Received in revised form 29 July 2011 Accepted 8 August 2011 Available online 16 September 2011

An experimental programme was undertaken to study the temperature rise of protected and unprotected structural steel during a fire within a small enclosure (an ISO 9705 room). The fuel (wood crib) was placed at two locations (front and back) within the ISO room. Each location had two fire scenarios present: the first fire scenario was for recording the temperatures of protected steel members within the enclosure, and the second fire scenario was to measure the temperatures of the directly exposed members. Six steel columns and two steel beams were strategically placed, and their temperatures were measured. Other data recorded were gas temperatures and heat release rates (HRRs). Thermocouples were kept in identical locations during the tests with protected and unprotected steel members to facilitate direct comparison. Despite the natural variability in fire development in identical situations, data up to E20 min were found suitable for direct comparison between protected and unprotected steel members. Comparison of these results with Fire Dynamics Simulator (FDS) version 5.3.1 modelling (with prescribed HRRs) results is presented to show the usefulness of the data collected. & 2011 Published by Elsevier Ltd.

Keywords: Heat release rate Structural steel ISO 9705 room Steel temperature CFD modelling

1. Introduction Many high-rise office buildings are built using structural steel because it can economically support the loads imposed. However, the steel structures need to be fire protected to shield them from high temperatures if an uncontrolled fire occurs. The steel structure should be protected to withstand the building load for a sufficient duration to allow the fire brigade operation (which includes evacuation, fire fighting and search and rescue) to be conducted before the building collapses. This collapse takes place because the steel temperature will increase in the event of a fire, and its strength would be reduced to a level such that it would no longer support the building load. The steel temperature (either protected or unprotected steel) can be determined either experimentally or numerically. It is important to have reliable experimental data of both unprotected and protected structural steel in identical fire scenarios to validate numerical models. During the early 1990s, a series of full-scale office fire tests was conducted in Australia [1]. This was aimed at determining whether fire protection was required for the steel beams for a particular high-rise building. In that building, concrete was used to protect the steelwork around the inner core and the external columns. In a non-sprinklered situation, a gas temperature of 1254 1C was achieved, and the unprotected beams themselves

n

Corresponding author. E-mail address: [email protected] (K.A.M. Moinuddin).

0379-7112/$ - see front matter & 2011 Published by Elsevier Ltd. doi:10.1016/j.firesaf.2011.08.001

reached a temperature of 632 1C. However, no comparable test was conducted using protected steel beams. Instead, a comparable sprinklered fire situation was tested in which the steel beam temperature rose by a few degrees only. Over a period of 10 months (September 1995–June 1996), British Steel conducted a series of six fire tests [2] on an eightstorey composite steel frame structure in a large-scale test facility at Cardington. During the first two tests, structural steel members were heated by a gas-fired furnace. During the remaining four tests, structural floor systems were tested under natural fire situations with a fire-load density of 40–45 kg/m2 (wood crib). In these six tests, primarily bare steel structure temperatures were recorded in addition to strains, deflections and rotations. However, no comparable tests were conducted using protected steel beams. Earlier, British Steel conducted another series of natural fire tests in a large compartment (23 m long  6 m wide  3 m high) [3]. In this study, the temperature rise of both protected (using Vicuclad and ceramic fibre board) and unprotected steel members were recorded. Unfortunately, the protected and unprotected members were placed symmetrically with respect to the compartment and fuel location. As the protected and unprotected steel temperature was not recorded in identical situations, a direct comparison between results could not be made. Furthermore, during these tests [3], neither the mass loss rate nor the heat release rate (HRR) was recorded. The National Institute of Standards and Technology (NIST) conducted an experiment involving a 2 MW heptane spray fire in

K.A.M. Moinuddin et al. / Fire Safety Journal 46 (2011) 480–496

Nomenclature

Greek symbols

c d F Hc HRR h k Nu Pr q00c q00r Re T t x

e a r s

specific heat (kJ/kg/K) diameter (m) configuration factor for radiation heat of combustion of the fuel (MJ/kg) heat release rate (kW) convective heat-transfer coefficient (W/m2/K) thermal conductivity (W/m/k) Nusselt number Prandtl number convective heat flux (W/m2) radiative heat flux (W/m2) Reynolds number temperature (1C) time distance in x direction

a 7 m long  4 m wide  4 m high compartment, and unprotected steel components were placed within the fire compartment [4]. The recorded surface temperatures of uninsulated steel elements were compared to predictions made by a widely used computational fluid dynamics (CFD) fire model Fire Dynamics Simulator (FDS; developed at NIST, USA) [5] coupled with a finite-element model of the steel. However, no tests were conducted using protected steel components. Overall, to the best of our knowledge, no test data of comparable protected and unprotected steel temperature are available for ideal natural fire modelling with CFD methodology, though there have been some studies to measure the temperatures of protected and unprotected structural steel members that were heated uniformly in a furnace. The aims of the current study were the following:

 To undertake an experimental programme to measure pro  

tected and unprotected steel temperatures, as well as HRRs during a natural room fire. To compare the temperatures of protected and unprotected steel. To assess the ability of one of the fire-protecting materials to reduce the steel temperature. To compare selected experimental results with FDS modelling results (with prescribed HRRs) to show the usefulness of the data collected.

In the follow-up of this paper, a comprehensive comparison between the experimental data and the results of the numerical simulation with prescribed and non-prescribed HRRs will be presented.

481

emissivity absorpivity density (kg/m3) Stefan–Boltzman constant (Wm2 K4)

Subscripts f f–TC g i s TC w w–TC xi

fire fire to thermocouple gas wall/surface number, i¼1, 2, y, n solid thermocouple wall wall to thermocouple cell number of solid, xi ¼1, 2, y, n

2. Experimental technique 2.1. ISO 9705 room The experimental programme was conducted in the ISO room facility (see Fig. 1) at the Centre for Environmental Safety and Risk Engineering (CESARE) in Victoria University, Australia. The tests were conducted in a standard ISO 9705 room [6], except as noted below. The ISO room walls and ceiling were constructed using 1 mm thick steel sheeting with an internal lining of 39 mm thick plasterboard. The plasterboard, which would be in direct contact with the fire, was supported by the exterior steel sheeting. Firerated plasterboard that was 13 mm thick was also placed on the floor to protect the concrete floor from spalling. The room was ventilated solely by a doorway 2.0 m high by 0.8 m wide (as specified in ISO 9705) located at the centre of one of the 2.4 m wide walls. The doorway was fully open during all of the tests. The outgoing products of combustion were collected by an exhaust hood and directed to an oxygen calorimeter for the measurement of HRRs [7]. The size of the hood was 6 m  6 m  1 m, located 3 m above the floor. The ISO room and hood was placed within a large closed shed to remove the effects of the outside environment, such as wind. 2.2. Location of the structural steel members The structural steels chosen for these experiments are commonly used for high-rise building constructions according to the universal steel standard [8]. There were six universal columns (UCs) and two universal beams (UBs) used for these four tests. The size of the columns were 200 UC 46.2 kg/m  2200 mm high and

Fig. 1. ISO room. (a) Plan view. (b) Elevation view.

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the beams were 410 UB 59.7 kg/m  3200 mm long. Each beam was supported by two stands for each, as shown in Fig. 2. These stands were protected by 16 mm fire-rated plasterboard. The columns were placed 200 mm away from the two side walls. The two beams were positioned 800 mm apart (see Fig. 3). All beams and columns were positioned at least 200 mm away from the ISO room walls. The steel beam and column members were kept in the same locations to maintain similarity of all experiments.

2.3. Overview of the tests A total of four tests were conducted. During two tests, the fuel package was placed at the back of the ISO room. One of these two tests had protected structural steels, whereas the other had unprotected structural steels. These tests are correspondingly designated as tests Back-P and Back-UP, respectively. Similarly, two tests (one with protected steel members and the other with unprotected members) were conducted by placing a

Fig. 2. Beam supported by two stands.

fuel package at the front of the ISO room. These tests are named correspondingly as tests Front-P and Front-UP, respectively.

2.4. Method of steel protection There is a significant risk associated with structural steel when it is exposed to a fire. A protected steel structure attains a considerably lower temperature and a slower heating rate than an unprotected one, even though it takes a longer time for protected steel to cool down [9]. In Fig. 4, the top curve is the parametric fire temperature, followed closely by the temperature of the unprotected steel with a surface area to volume ratio of 200 m  1 (similar to the members tested in this study). The bottom

Fig. 4. Protected and unprotected temperatures exposed to a parametric fire (taken from [9]).

Fig. 3. Beam and column positions within enclosure.

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two curves show the theoretical temperatures of steel protected with 15 and 50 mm thick insulating material, respectively. A number of protecting materials have been widely used to protect steel elements and avoid a quick rise in the steel temperature. Some of these methods include the board system, the spray-on system, intumescent paint, concrete encasement, concrete filling, water filling, etc. In this study, during the tests Back-P and Front-P, all columns and beams were insulated with a board system (16 mm fire-rated plasterboard). The plasterboard used was considered to have uniform thermal performance across its surface and crosssection. A thickness of 16 mm was adopted for the plasterboard used to protect the columns and beams. The objective was to ensure a slow transfer of heat to the steel based on the principle of delaying heat transfer rather than providing protection to steel over any specific time period during the experiment. As shown in Fig. 5, the structural members, including their edge, were enclosed with gypsum plasterboard. The inner material of the board was designed to remain in place with no damage throughout heating. The gypsum board’s performance was enhanced by water of crystallisation, which was driven off during heating. This drying process gave a time delay at about 100 1C, but the strength of the board after exposure was very significantly reduced, as it turned into powdery form held together by the reinforcing fibreglass and other additives. Proprietary information on wallboard is available from the manufactures, and there is a range of diverse thicknesses to suit particular applications. An external corner bead was also used to help clamp the plasterboard to the beams and columns. A wire ligature was then wrapped around the beams and columns to help hold the plasterboard in place; the wire ligatures were spaced at approximately 300 mm. A basecote was then used to seal all joints to stop any hot gases affecting the steel temperature.

2.5. Fuel type and location Two timber cribs were used as the primary fuel in all the tests. The cribs were cut from 90 mm  45 mm non-treated pine timber.

16mm plasterboard Seals by Basecote

483

They were cut in half to form approximately 43 mm  45 mm sticks (E4 mm  45 mm slice was lost due to cutting by the blade). The crib size was 765 mm  765 mm  765 mm high. Timber cribs was used as the primary fuel as their burning rate is much slower than other type of fuels (such as liquid or plastic fuels), and this would generate a lot more data for analysis and future research. As shown in Fig. 6, the location of the cribs for the tests Back-P and Back-UP was 200 mm from the back wall. For the tests Front-P and Front-UP, the cribs were positioned 200 mm inwards from the open door. Two cribs were used for each test. Two litres of methylated spirits was used as the ignition fuel in all of the experiments. The ignition fuel was ignited in two separate trays, each containing one litre of methylated spirits and placed under each crib. The detailed weight of the cribs is shown in Table 1. The effective heat of combustion (Hc) of the timber used (Australian Pine) in the experiment was measured to be 14.5 MJ/kg using CESARE’s cone calorimeter. The total fire load and fire load per unit area were then calculated using the experimentally obtained value for Hc and are presented in Table 1.

2.6. Instrumentation 2.6.1. HRR measurement Section 2.1 provides brief information about the arrangement for measuring HRRs. As oxygen calorimetry [7] is a standard procedure, no further detail is needed.

Table 1 Weight of wood cribs. Test name

Crib 1 (kg)

Crib 2 (kg)

Total fire load (MJ)

Fire load per unit area (MJ/m2)

Back-P Back-P Front-P Front-UP

112.14 102.00 110.89 114.10

113.50 110.00 111.16 110.60

3272 3074 3220 3258

379 356 373 377

Table 2 Thermocouple location heights. Height from floor (mm)

Thermocouple number Back tree

Wire ligature

Corner bead Fig. 5. Structural steel member protection with plasterboard.

2200 2000 1750 1500 1250 1000 750 500

1 2 3 4 5 6 7

Middle tree

Front tree

8 9 10 11 12 13 14

15 16 17 18 19 20 21

Fig. 6. Fuel locations during the tests. (a) Tests Back-P and Back-UP. (b) Tests Front-P and Front-UP.

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2.6.2. Gas thermocouple trees To measure the gas temperature inside the enclosure, in all experiments, 1.5 mm diameter (dTC) type K Mineral Insulated Metal Sheath (MIMS) thermocouples were used; the end junctions were not exposed, which may lead to a slightly slower response. These were placed evenly in the room using three supporting stands. Each stand had seven thermocouples attached. Stand 1 (holding thermocouples 1–7) was positioned centrally 200 mm from the back wall, named hereafter as the Back tree. The Centre tree (holding thermocouples 8–14) was positioned in the centre of the room, and the Front tree (holding thermocouples 15–21) was positioned close to the open door, 200 mm inside the room and 800 mm from one side of the wall. The thermocouples were located at various heights as listed in Table 2. The precision in temperature measurement was 72 1C as per the data sheet provided by the manufacturer. However, the radiation correction was applied as per the section below. 2.6.3. Gas temperature correction Radiation correction to the gas temperature measurements was carried out as per the methods described in [10]. The relevant parameters used in this method are given in Table 3. Global parameters were mainly selected as per [10]. Ff–TC values were calculated as 0.1–0.8 for the three thermocouple trees. The flame temperature for the tests Front-P and Front-UP was taken as

900 1C [11] between 5 and 34 min with a quadratic (time2) rise from 0 1C at initial conditions and subsequent linear drop to 0 1C at 60 min. However, it was taken as 850 1C between 8 and 28 min, with a similar rise from 0 1C at initial conditions and subsequent drop to 0 1C at 40 min. 2.6.4. Steel thermocouples Type K MIMS thermocouple wires (3 mm) were used for the steel temperature measurement. This type of thermocouples was used due to their durability and extreme robustness, which is suitable for high-temperature environments and can easily be attached to the steel members. While the columns were vertically positioned, the thermocouples were placed 100 mm away from the edge (see Fig. 7). The placement of the thermocouples at the top of all the columns was to avoid any misleading temperature recordings. All thermocouples were spot welded to the steel sections. The thermocouple locations on three columns (A, B and C) are shown in Fig. 7. At similar locations, thermocouples were spot welded to the other three columns (D, E and F), which can be found in [12]. It can be observed in Fig. 7 that at a cross-section of the column, three thermocouples were placed as recommended by AS4100 [8]. In Fig. 8, the thermocouple locations on Beam A are shown. They were welded at four longitudinal locations at equal distances (named Open End, Loc 2, Loc 3 and Back End) and at each such location (around the cross-section), four thermocouples were placed

Table 3 Parameters for radiation correction. Wall

Front

Back

Side1

Side2

Ceiling

Floor

Fw–TC,i (Back tree) Fw–TC i (Centre tree) Fw–TC,i (Front tree)

0.1–0.11 0.23–0.29 0.72–0.79 0.9

0.91–0.98 0.29–0.36 0.12–0.14 0.9

0.32–0.41 0.49–0.64 0.27–0.33 0.9

0.32–0.41 0.49–0.64 0.4–0.52 0.9

0.43–0.76 0.65–0.97 0.4–0.75 0.9

0.26–0.4 0.37–0.62 0.25–0.39 0.6

ew,i Global parameters

eg

eTC

ef

ag

0.1

0.2

0.8

0.3

s (W/m2K4) 5.67  10–8

Fig. 7. Locations of thermocouples attached to columns.

kg 0.035

Nu 5.0

dTC (m) 0.0015

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485

Fig. 8. Locations of thermocouples attached to beams.

during the tests Back-P at E16 min and Back-UP at E17 min. In the decay stage of the fire, the rate of burning was limited by the available surface area of fuel. The decay stage of the fire began during the test Back-P at E23 min and Back-UP at E21 min. The HRR versus time profiles from these two tests are plotted in Fig. 9. The two profiles are very similar in shape. The numerical values are almost identical up to 7 min; beyond that, a difference of up to 15% is observed during the development stage, apart from the spike in the Back-UP profile. However, a larger difference (up to 23%) is observed during the decay stage. Based on these profiles, a good comparison of protected and unprotected steel temperatures can be made for the first 20 min.

3000

HRR (kW)

2500 2000

Back-P Back-UP

1500 1000 500 0 0

10

20

30

40 50 Time (min)

60

70

80

90

Fig. 9. HRR versus time profiles from tests Back-P and Back-UP.

as recommended by AS4100 [8]. The locations of thermocouples on Beam B (similar to the Beam A locations) are presented in [12].

3. Experimental results 3.1. Fire growth and development in tests Back-P and Back-UP The fire observations in tests Back-P and Back-UP were similar because of the same location of the fuel load in the enclosure during both experiments. After ignition, the liquid fuel began to burn and spread to the wood cribs. Initially, the convective plume of hot gas rose and impinged on the ceiling, which occurred at E2 min during both the tests. A ceiling jet was formed where the plume met the ceiling, it spread horizontally in each radial direction and formed a hot upper layer of smoke. This was formed at E7 min during both the tests. Radiant heat from the hot layer was received by the fuel, and the initial rate of burning was enhanced. However, limited ventilation of the room eventually resulted in a restricted rate of burning. As the fire continued, the upper layer of hot gas and smoke increased in volume, reducing the elevation of the interface between the upper and lower layers. Once this interface fell below the door soffit, the products of combustion flew out of the door opening, also pushing the flame outside the room. This phenomenon is an indication of a flashover occurring, which took place

3.2. Fire growth and development in tests Front-P and Front-UP The fire observations in tests Front-P and Front-UP were similar, with small differences from the previous tests Back-P and Back-UP because of the different locations of fuel loads in the enclosure room. The convective plume of hot gas reached the ceiling at E3.5 min and the ceiling jet was formed at E5 min during these tests. Before the start of the decay stage of the fire, the wood crib at the front collapsed in the test Front-P; this occurred at E18 min. Pictures before and after the wood-crib collapse are presented in Fig. 10. As a result of this collapse, the intensity of the burning initially reduced for 5 min, and then started increasing again. The reduction of the fire intensity may be attributed to the consumption of incoming oxygen by collapsed charred crib, as a result of which, the wood crib at the back was deprived of oxygen. The burning of the collapsed crib also resulted in reduced radiation feedback to the fuel. Once the collapsed crib was burned out, the crib at the back started burning and produced more radiation feedback. The final decay stage of the fire during the test Front-P began at E27 min. During the test Front-UP, a mini collapse of the wood crib at the front occurred at E19 min. However, this collapse did not affect the fire behaviour significantly. The HRR versus time profiles from these two tests are plotted in Fig. 11. The two profiles are very similar in shape in the development stage. The numerical values are almost identical up to 18 min, but a difference is observed during the decay stage. The shapes are also different. This is due to the collapse of the wood crib in the test Front-P. These profiles indicate a possibility of good comparison of protected and unprotected steel temperatures for the first 18 min.

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Fig. 10. Wood-crib collapse during test Front-P. (a) Before the collapse. (b) After the collapse.

Fig. 12. It can be seen that the HRRs from the case with the fuel at the back are marginally higher than with the fuel at the front. Although this finding is consistent with the finding of [13], a higher variation was expected. However, due to the limited testing in the current study, no conclusion can be drawn.

3000

HRR (kW)

2500 2000

Front-P Front-UP

1500

3.4. Gas temperatures

1000 500 0 0

10

20

30

50 40 Time (min)

60

70

80

90

Fig. 11. HRR versus time profiles from tests Front-P and Front-UP.

2500 Back-Avg Front-Avg

HRR (kW)

2000 1500 1000 500 0

0

2

4

6

8 10 Time (min)

12

14

16

18

Fig. 12. Comparison of HRR versus time profiles between cases with fuel at the front and at the back.

3.3. Comparison of HRRs between the cases with fuel at the front and at the back The HRR curves of Figs. 9 and 11 are averaged for the first 18 min (up to the point of wood-crib collapse), and are plotted in

The gas temperatures recorded by three thermocouples of each tree during all four tests are plotted in Fig. 13. The data recorded by other thermocouples are presented in [12]. These thermocouples (1, 3 and 5 of Back tree; 8, 9 and 11 of Centre tree; 15, 16 and 18 of Front tree) are chosen as they were located adjacent to the steel thermocouples. In the graphs, the time– temperature curves are shifted 300 units upwards from the previous curves, except for the first curve at 1250 mm height. In Fig. 13(a)–(c) time–temperature curves are compared at various locations during the tests Back-P and Back-UP. Similarly, comparison of gas temperatures are made in Fig. 13(d)–(f), which were recorded during the tests Front-P and Front-UP. As expected, the gas temperature curves generally followed the trend of the HRR curves. When the fuel packages were placed at the back of the enclosure, the maximum gas temperature was recorded by the thermocouple located at the top of the Back tree. In contrast, when the fuel packages were placed at the front, the thermocouple located at the top of the Front tree recorded the maximum gas temperature. It can be seen in Fig. 13(a) that, at all thermocouple locations, identical gas temperatures were measured up to E20 min. However, within this period, little difference is observed in Fig. 13(b)–(c), especially at the lower parts of the thermocouple trees. This is due to the variability of the fire dynamics during two almost identical fire scenarios. For the tests Front-P and Front-UP, a better similarity in gas temperatures is observed (Fig. 13(d)–(f)) until the collapse of the wood crib during the test Front-P (at E18 min). The gas temperature data confirms that a good comparison of protected and unprotected steel temperatures can be made for the first 20 and 18 min, respectively, for back and front fuel cases.

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

Temperature (°C)

Temperature (°C)

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0

10

20

30

40

50

60

70

80

90

1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

0

10

20

30

40 50 Time (min)

60

70

80

90

0

10

20

30

40 50 Time (min)

60

70

80

90

10

20

30

40 50 Time (min)

60

70

80

90

Temperature (°C)

Temperature (°C)

Time (min)

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

0

10

20

30

40

50

60

70

80

90

1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

Temperature (°C)

Temperature (°C)

Time (min) 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

0

10

20

30

40

50

60

70

80

90

Time (min)

487

1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

0

Fig. 13. Time–temperature curves at various locations during corresponding protected and unprotected fire tests. The curves shifted 300 units upwards from the previous curves, except for the first curve at 1250 mm height. (a–c) Comparison of gas temperatures during the tests when the fuel package was located at the back. (d–f) Comparison of gas temperatures during the tests when the fuel package was located at the front. (a) Back tree (Back-P and Back-UP), (b) Centre tree (Back-P and BackUP), (c) Front tree (Back-P and Back-UP), (d) Back tree (Front-P and Front-UP), (e) Centre tree (Front-P and Front-UP) and (f) Front tree (Front-P and Front-UP).

Fig. 13 shows that, despite marginal differences observed in the HRRs (Fig. 12), generally, higher temperatures were recorded when the fuel was located at the back of the enclosure. This is attributed to a large proportion of flame burning outside the enclosure during the cases with the fuel at the front. Therefore, the flame was not contributing to all of the heat inside the enclosure. 3.5. Steel temperatures in tests Back-P and Back-UP This section of the paper discusses the heat transfer from the gas to protected and unprotected steel members during the tests where the fuel package was placed at the back of the enclosure. 3.5.1. Steel beam temperature The temperatures recorded by four thermocouples located at a beam cross-section (as shown in Fig. 8) were averaged as per [8]. The averaged values at four longitudinal locations (Open End, Loc 2, Loc 3 and Back End in Fig. 8) for both beams are plotted in Fig. 14. Although data from the first 20 min is suitable for direct comparison, data is presented up to 40 min to show various physical

phenomena. As the beams were located symmetrically with respect to the room and fuel location, the time–temperature curves at corresponding locations are found to be generally identical. However, asymmetry is observed between 15 and 30 min at the location close to the back wall during the test Back-UP. No asymmetry is observed for protected steel beam temperatures. For the protected case, it has been observed that in the first 8 min of the test, temperatures of the steel remained unchanged, as the plasterboard was absorbing the heat during this period, before the heat reached the steel beam. After 8 min, the plasterboard transferred the heat by conduction directly to the steel member, resulting in increases of temperature. The temperature of the hot gases at this time was recorded at E900 1C (Fig. 13). Then, the steel temperature increased at a constant rate until 22 min, as shown in the graphs. At that stage, the steel temperature reached approximately 95 1C, and it remained constant until 25 min. This is due to the fact that the moisture inside the plasterboard reached the evaporation point (100 1C). After the moisture was evaporated, the temperature of the steel member (after 25 min) increased again. The temperature of hot gases reached a maximum of E1100 1C; however, the hot gas temperature decreased rapidly

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1100 1000 900 800 700 600 500 400 300 200 100 0

BeamA_Open End (Back-UP) BeamA_Open End (Back-P) BeamB_Open End (Back-UP) BeamB_Open End (Back-P)

Temperature (°C)

Unprotected

data for comparison

Protected

0

10

20 Time (min)

BeamA_Loc3 (Back-UP) BeamA_Loc3 (Back-P) BeamB_Loc3 (Back-UP) BeamB_Loc3 (Back-P)

30

40

Unprotected

Temperature (°C)

1000 900 800 700 600 500 400 300 200 100 0

Temperature (°C)

Temperature (°C)

488

data for comparison

Protected

0

10

20 Time (min)

30

40

1000 900 800 700 600 500 400 300 200 100 0

1100 1000 900 800 700 600 500 400 300 200 100 0

BeamA_Loc2 (Back-UP) BeamA_Loc2 (Back-P) BeamB_Loc2 (Back-UP) BeamB_Loc2 (Back-P)

Unprotected

data for comparison

Protected

0

10

20 Time (min)

BeamA_Back End (Back-UP) BeamA_Back End (Back-P) BeamB_Back End (Back-UP) BeamB_Back End (Back-P)

30

40

Unprotected

data for comparison

Protected

0

10

20 Time (min)

30

40

Fig. 14. Steel beam temperature during tests Back-P and Back-UP. (a) Beam location close to the opening, (b) Location 2, (c) Location 3 and (d) beam location close to the back wall.

Table 4 Maximum temperatures reached at various beam locations (back fuel cases). Beam location

Test Back-UP

Test Back-P

Beam A

Beam B

Beam A

Beam B

Front end Loc 2 Loc 3

795 1C at 24 min 900 1C at 23 min 995 1C at 23 min

780 1C at 23 min 885 1C at 23 min 985 1C at 23 min

240 1C at 79 min 260 1C at 82 min 290 1C at 75 min

240oC 1C at 78 min 260 1C at 81 min 285 1C at 71 min

Back end

1035 1C at 25 min

945 1C at 25 min

290 1C at 59 min

286 1C at 57 min

during the decay stage. Even though the room temperature was decreasing, the protected steel temperature reached its maximum temperature of 280 1C at E70 min. This is attributed to the continued transfer of heat provided by the plasterboard. After 70 min at Loc 3 and Back End, temperatures began to decrease at a slow rate. This slow rate of decrease was due to reverse heat transfer from the steel member to the plasterboard, which was released to the surrounding environment. At Loc 2 and Front End, temperatures continued to increase due to the conduction of heat in the same member from a high-temperature location to a lowtemperature location. Unlike in protected cases, for unprotected cases, the steel temperature started decreasing as soon as the room temperature started decreasing. At the location close to the back wall, the unprotected steel temperature reached its maximum temperature of 1040 1C at E24 min. At all other locations, the maximum temperatures recorded are shown in Table 4. In the table, arrows represent the general direction of heat flow. It can be observed that the 16 mm plasterboard was able to reduce the steel temperature by E700 1C.

3.5.2. Steel column temperature As per [8], the temperatures recorded by three thermocouples located at a column cross-section (as shown in Fig. 7) were averaged. The averaged values at three vertical locations (top, middle and bottom in Fig. 7) for all columns are plotted in Fig. 15. Columns A and F (data plotted in Fig. 15(a)), B and E (Fig. 15(b)) and C and D (Fig. 15(c)) are located symmetrically with respect to the room and fuel location. Therefore, the time–temperature curves at corresponding locations are found to be identical. However, asymmetry is observed between 20 and 33 min for columns C and D, which were located close to the opening during the test Back-UP. Minimal asymmetry is observed for protected steel column temperatures. As in the protected beam cases, the initial effect of heat absorption by the plasterboard and subsequent effect of its moisture evaporation were observed for all protected column cases. The maximum temperatures that were reached in the steel column members are shown in Table 5. It can be observed that the plasterboard could reduce the steel temperature by E 700 1C.

K.A.M. Moinuddin et al. / Fire Safety Journal 46 (2011) 480–496

1000

900 Unprotected (A & F)

ColumnA_ Top (Back-UP) ColumnA_ Mid (Back-UP) ColumnA_ Bot (Back-UP) ColumnF_ Top (Back-UP) ColumnF_ Mid (Back-UP) ColumnF_ Bot (Back-UP) ColumnA_ Top (Back-P) ColumnA_ Mid (Back-P) ColumnA_ Bot (Back-P) ColumnF_ Top (Back-P) ColumnF_ Mid (Back-P) ColumnF_ Bot (Back-P)

500 400 300

Temperature (°C)

700 600

data for comparison

200 100

Unprotected (B & E)

900

800 Temperature (°C)

489

800 700 600 500 400 300

data for comparison

200

ColumnB_ Top (Back-UP) ColumnB_ Mid (Back-UP) ColumnB_ Bot (Back-UP) ColumnE_ Top (Back-UP) ColumnE_ Mid (Back-UP) ColumnE_ Bot (Back-UP) ColumnB_ Top (Back-P) ColumnB_ Mid (Back-P) ColumnB_ Bot (Back-P) ColumnE_ Top (Back-P) ColumnE_ Mid (Back-P) ColumnE_ Bot (Back-P)

100

Protected (A & F)

Protected (B & E)

0

0 5

10

20 15 25 Time (min)

Temperature (°C)

0

30

35

1100 1000 900 800 700 600 500 400 300 200 100 0

40

10

5

0

20 25 15 Time (min)

30

35

40

Unprotected (C & D)

data for comparison

ColumnC_ Top (Back-UP) ColumnC_ Mid (Back-UP) ColumnC_ Bot (Back-UP) ColumnD_ Top (Back-UP) ColumnD_ Mid (Back-UP) ColumnD_ Bot (Back-UP) ColumnC_ Top (Back-P) ColumnC_ Mid (Back-P) ColumnC_ Bot (Back-P) ColumnD_ Top (Back-P) ColumnD_ Mid (Back-P) ColumnD_ Bot (Back-P)

Protected (C & D)

0

5

10

15 20 25 Time (min)

30

35

40

Fig. 15. Steel column temperatures during tests Back-P and Back-UP (a) Column A and F, (b) Column B and E and (c) Column C and D.

Table 5 Maximum temperatures reached at various column locations (back fuel cases). Column

A B C D E F

Back-UP

Back-P

Top

Middle

833 1C at 24 min 965 1C at 25 min 1005 1C at 25 min 925 1C at 25 min 950 1C at 24 min 805 1C at 24 min

775 1C 926 1C 965 1C 875 1C 920 1C 765 1C

at at at at at at

Bottom 26 min 25 min 25 min 26 min 22 min 24 min

738 1C 847 1C 873 1C 800 1C 830 1C 735 1C

Column A and column F were exposed to lower gas temperatures than the rest of the columns, as these were located furthest from the two burning wood cribs. Hence, they attained lower temperatures than the other columns. The arrows shown in the first column of Table 5 indicate the general trend relating to column location and temperatures. 3.6. Steel temperatures in tests Front-P and Front-UP The heat transfer from the gas to the protected and unprotected steel members during the tests, where the fuel package was placed at the front of the enclosure, is discussed in this section of the paper. 3.6.1. Steel beam temperatures Steel beam temperatures (average recording of four thermocouples located at the beam cross-section) for both beams from the

at at at at at at

Top 25 min 24 min 24 min 26 min 23 min 24 min

247 1C 294 1C 292 1C 288 1C 283 1C 245 1C

Middle at at at at at at

57 min 54 min 55 min 55 min 56 min 57 min

209 1C 250 1C 249 1C 241 1C 240 1C 209 1C

at at at at at at

Bottom 77 min 73 min 73 min 76 min 76 min 77 min

175 1C 210 1C 211 1C 211 1C 209 1C 175 1C

at at at at at at

80 min 78 min 84 min 86 min 81 min 83 min

tests Front-P and Front-UP are presented in Fig. 16. Despite the symmetrical beam location, a significant asymmetry is observed at Loc 2 during the growth phase of the test Front-UP fire. A much smaller asymmetry is observed at the location close to the opening. Other steel beam temperatures at corresponding locations are found to be almost identical, especially at locations Loc 3 and Back End. Due to the variation in the HRRs (Fig. 11) and gas temperatures (Fig. 13(d)–(f)) during the decay stage due to the wood-crib collapse in the test Front-P, it is recommended that data beyond 18 min should be excluded from any theoretical analysis and validation. Similar to the tests with the fuel at the back, the unprotected steel temperatures started decreasing shortly after the room temperature started decreasing. At all locations, the maximum temperatures reached are shown in Table 6. During the test Front P, at Loc 2, Loc 3 and Back End, the steel temperature continued to increase beyond 90 min due to the conduction of heat in the same object from a high temperature (close to the opening) to a low temperature (towards the back wall).

490

K.A.M. Moinuddin et al. / Fire Safety Journal 46 (2011) 480–496

800

800 Unprotected

700

600 500 400

Temperature (°C)

Temperature (°C)

700

BeamA_Open End (Front-UP) BeamA_Open End (Front-P) BeamB_Open End (Front-UP) BeamB_Open End (Front-P)

data for comparison

300 200 Protected

100

600 500

300

data for comparison

200 Protected

100 0

10

BeamA_Loc2 (Front-UP) BeamA_Loc2 (Front-P) BeamB_Loc2 (Front-UP) BeamB_Loc2 (Front-P)

400

0 0

Unprotected

20 Time (min)

30

40

0

10

20 Time min

30

600

600

Unprotected

Unprotected

500

500

data for comparison

Temperature (°C)

Temperature (°C)

40

400 BeamA_Loc3 (Front-UP) BeamA_Loc3 (Front-P) BeamB_Loc3 (Front-UP) BeamB_Loc3 (Front-P)

300 200

Protected

100

data for comparison

400 BeamA_Back End (Front-UP) BeamA_Back End (Front-P) BeamB_Back End (Front-UP) BeamB_Back End (Front-P)

300 200

Protected

100 0

0 0

10

20 Time (min)

30

0

40

10

20 Time (min)

30

40

Fig. 16. Steel beam temperatures during tests Front-P and Front-UP. (a) Beam location close to the opening, (b) Location 2, (c) Location 3 and (d) beam location close to the back wall. Table 6 Maximum temperatures reached at various beam locations (front fuel cases). Beam locations

Test Front-UP Beam A

Front End Loc 2 Loc 3 Back End

776 1C 782 1C 595 1C 534 1C

at at at at

Test Front-P Beam B

25 min 28 min 31 min 32 min

Tables 4 and 6 show that temperature along a beam (whether protected or not) in a fire compartment is not constant. It is higher at a location close to the fire and lower where the location is far from the fire. This implies that there may be significant uncertainty if the steel temperature is calculated using a zone model as opposed to a CFD model. Comparing data between the tests Back-UP and Front-UP (Tables 4 and 6), it can be seen that much higher steel beam temperatures were recorded during the Back-UP test at all locations, except at the Front End. Even at the Front End location, temperature data during the Front-UP test were not higher than the Back-UP test. Similar trends can also be observed between the Back-P and Front-P tests. This implies that the fire-load burning at the end of an enclosure has a severe effect on steel beams compared to other fire locations. 3.6.2. Steel column temperature In Fig. 17, steel column temperatures (average recording of three thermocouples located at column cross-sections) for all columns

796 1C 781 1C 595 1C 534 1C

at at at at

Beam A 25 min 27 min 31 min 32 min

256 1C 222 1C 182 1C 151 1C

at at at at

Beam B 67 min 90 min 90 min 90 min

244 1C 222 1C 177 1C 151 1C

at at at at

74 min 90 min 90 min 90 min

from the tests Front-P and Front-UP are presented. Columns A and F (data plotted in Fig. 17(a)), B and E (Fig. 17(b)) and C and D (Fig. 17(c)) were located symmetrically with respect to the room and fuel location. Despite the symmetrical locations, a large asymmetry was observed at location Top in the front two sets of columns (A and F; B and E) during the growth phase of the fire during the test Front-UP. A much smaller asymmetry is observed at the location Middle in these columns during the same period. Other steel column temperatures at corresponding locations are found to be almost identical. Minimal asymmetry is observed for protected steel column temperatures. The maximum temperatures that were reached in the steel column members are shown in Table 7. As in the cases with the fuel at the back, the initial effects of heat absorption by the plasterboard and subsequent effect of its moisture evaporation were observed for all protected steel members during the tests, when the fuel was located close to the opening. It can be observed that the plasterboard could reduce the steel temperature by E500 1C while the fuel load was placed at the front of the enclosure.

K.A.M. Moinuddin et al. / Fire Safety Journal 46 (2011) 480–496

800

700

Unprotected (A & F)

700

Unprotected (B & E)

600 Temperature (°C)

Temperature (°C)

491

600 ColumnA_ Top (Front-UP) ColumnA_ Mid (Front-UP) ColumnA_ Bot (Front-UP) ColumnD_ Top (Front-UP) ColumnD_ Mid (Front-UP) ColumnD_ Bot (Front-UP) ColumnA_ Top (Front-P) ColumnA_ Mid (Front-P) ColumnA_ Bot (Front-P) ColumnD_ Top (Front-P) ColumnD_ Mid (Front-P) ColumnD_ Bot (Front-P)

500 400 300 data for comparison

200

Protected (A & F)

100

500 ColumnB_ Top (Front-UP) ColumnB_ Mid (Front-UP) ColumnB_ Bot (Front-UP) ColumnE_ Top (Front-UP) ColumnE_ Mid (Front-UP) ColumnE_ Bot (Front-UP) ColumnB_ Top (Front-P) ColumnB_ Mid (Front-P) ColumnB_ Bot (Front-P) ColumnE_ Top (Front-P) ColumnE_ Mid (Front-P) ColumnE_ Bot (Front-P)

400 300 200 data for comparison

100

Protected (B & E)

0

0 0

5

10

15 20 25 Time (min)

30

35

5

0

40

600

10

20 25 15 Time (min)

30

35

40

Unprotected (C & D)

Temperature (°C)

500 ColumnC_ Top (Front-UP) ColumnC_ Mid (Front-UP) ColumnC_ Bot (Front-UP) ColumnF_ Top (Front-UP) ColumnF_ Mid (Front-UP) ColumnF_ Bot (Front-UP) ColumnC_ Top (Front-P) ColumnC_ Mid (Front-P) ColumnC_ Bot (Front-P) ColumnF_ Top (Front-P) ColumnF_ Mid (Front-P) ColumnF_ Bot (Front-P)

400 300 200 data for comparison

100

Protected (C & D)

0 0

5

10

15 20 25 Time (min)

30

35

40

Fig. 17. Steel column temperatures during tests Front-P and Front-UP (a) Column A and F, (b) Column B and E and (c) Column C and D.

Table 7 Maximum temperatures reached at various column locations (front fuel cases). Column

Front-UP

Front-P

Top A B C D E F

783 1C 680 1C 553 1C 555 1C 670 1C 800 1C

Middle at at at at at at

26 min 26 min 29 min 29 min 27 min 25 min

728 1C 620 1C 523 1C 523 1C 623 1C 745 1C

at at at at at at

Bottom 26 min 27 min 31 min 31 min 28 min 25 min

660 1C 580 1C 475 1C 480 1C 587 1C 680 1C

Tables 5 and 7 show that columns located close to the fire source experience higher temperatures than the ones located away from the fire. Early failure of a column close to the fire may accelerate collapse of a structure. During the World Trade Centre fire, Building 7 collapsed due to instabilities generated by a column failure on its 13th floor [14].

4. Numerical simulations Prediction of heat transfer from hot gases to protected and unprotected structural steel members is important in predicting the level of fire safety in buildings and in designing building fire safety systems. The FDS was chosen to use the experimental data for conducting a prediction exercise, as it is the most widely used CFD fire model by fire safety engineers. This model incorporates a simple pyrolysis/evaporation model, a Large Eddy Simulation (LES) turbulence model, a mixture-fraction combustion model,

at at at at at at

Top 26 min 27 min 31 min 31 min 28 min 25 min

274 1C 215 1C 163 1C 166 1C 204 1C 274 1C

at at at at at at

60 min 74 min 85 min 87 min 83 min 60 min

Middle

Bottom

231 1C 190 1C 149 1C 147 1C 182 at 231 1C

202 1C 168 1C 133 1C 133 1C 168 1C 213 1C

at 79 min at 89 min at 89 min at 88 min 90 min at 82 min

at at at at at at

83 min 90 min 88 min 88 min 90 min 86 min

a finite-volume radiative heat-transfer model and a simple convective heat-transfer model (which uses a combination of natural and forced convection correlations).

4.1. Heat-transfer model in FDS The details of how the temperature of solid objects is modelled in FDS is given in [15]. However, a brief description of modelling a non-combustible opaque solid’s (such as plasterboard and steel) temperature is given here, which is described in greater detail in Technical Guide [15]. A one-dimensional heat-transfer equation for the solid phase temperature Ts(x; t) is applied in direction x pointing into the solid (the point x ¼0 represents the surface):

rs cs

@Ts @ @Ts ks ¼ @x @x @t

ð1Þ

492

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where ks, rs and cs are the thermal conductivity, density and specific heat of the solid material, respectively. The boundary condition to calculate the surface temperature of non-combustible solid facing fire is set as ks

@Ts ð0,tÞ ¼ q00c þq00r @x

ð2Þ

where Ts is the temperature in the solid, ks is the thermal conductivity of the wall material, q00c is the convective heat flux and q00r is the radiative heat flux. On the back surface, two possible boundary conditions may be prescribed: (a) If the back surface is assumed to be open to either an ambient void or another part of the computational domain, the back side boundary condition is similar to that of the front side. (b) If the back side is assumed to be perfectly insulated, an adiabatic boundary condition is used: ks

@Ts ¼0 @x

ð3Þ

In an LES calculation, q00c is obtained from a combination of natural and forced convection correlations:   1=3 kg q00c ¼ hDT; h ¼ max C9DT9 , 0:037 Re4=5 Pr1=3 ð4Þ L where h is the convective heat-transfer coefficient (W/m2/K), DT is the difference between the wall and the gas temperature (taken at the centre of the grid cell abutting the wall), C is the coefficient for natural convection (1.52 for a horizontal surface and 1.31 for a vertical surface) [16], L is a characteristic length related to the size of the physical obstruction, kg is the thermal conductivity of the gas, the Reynolds number (Re) is based on the density and velocity of the gases in the middle of the first grid cell and the length scale L and the Prandtl number (Pr) is assumed to be 0.7. Since Re is proportional to the characteristic length, L, the convective heat-transfer coefficient (h) is weakly related to L. For this reason, L is taken as 1 m for all calculations. For opaque non-combustible solid obstructions, it is assumed that the thermal radiation from the surrounding gases is absorbed within an infinitely thin layer at its surface and the net radiative heat flux is given as X 4 q_ 00r ¼ ei Fi sTi 4  e|fflfflffl{zfflfflffl} ð5Þ s sTs i |fflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflffl} outgoing radiation

Here, ei is the emissivity of the ith flame or surrounding surface and es is the emissivity of the surface whose temperature (Ts) is being calculated. Fi and Ti are the corresponding view factor and temperature, respectively. Although obtaining q00r using Eq. (5) is, in general, very complicated, FDS has various algorithms for calculating it by solving the radiative-transport equation using a finite-volume method [15]. Eq. (1) is solved at each solid boundary cell for which thermal properties needs to be prescribed. The solid can consist of multiple layers of materials, e.g. a protected structural steel flange attached to the plasterboard. Each layer is partitioned into nonuniform cells, clustered near the front and back faces. The smallest cells are chosen based on the criteria: sffiffiffiffiffiffiffiffiffi ks dx o rs cs

ð6Þ

Interior cells increase in size by a stretch factor of 2.0 when moving inwards from the surfaces. By default, the stretch factor is 2.0, which can be changed by the user. The cell boundaries are

Table 8 Specified material properties and combustion parameters. Material

Properties

Value

Unit

Methylated spirit

Heat of combustion Soot yield

25 930 0.008

kJ/kg kg/kg

Steel

Emissivity Conductivity

Density

0.9 48 at T¼ 20 1C, 30 at T¼ 677 1C 0.45 at T¼ 20 1C, 0.60 at T¼ 377 1C, 0.85 at T¼ 677 1C 7850

Conductivity Emissivity Specific heat Density Thickness

0.158 0.6 1.09 800 0.039, 0.016, 0.013

kJ/kg/K kg/m3 m

Conductivity Specific heat Density Heat of combustion Soot yield CO yield

0.147 2.8 440 14,500 0.028 0.01

W/m/K kJ/kg/K kg/m3 kJ/kg kg/kg kg/kg

Specific heat

Gypsum board

Timber

W/m/K kJ/kg/K

kg/m3 W/m/K

incident radiation

Fig. 18. (a) Computational domain of an ISO room fire tests with strategically placed unprotected structural steel members and (b) simulation result with one temperature slice and fire represented by orange cells. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

K.A.M. Moinuddin et al. / Fire Safety Journal 46 (2011) 480–496

located at points xi. The temperature at the centre of the xith cell is denoted Ts,xi. The (temperature-dependent) thermal conductivity of the solid at the centre of the xith cell is denoted ks,xi. The temperatures are updated in time using an implicit

2200 mm EXP 1750mm EXP 1250mm EXP

400

800

1200 1600 Time (s)

1000 900 800 700 600 500 400 300 200 100 0

2200mm FDS 1750mm FDS 1250mm FDS

2000

2000 mm EXP 1750mm EXP 1250mm EXP

0

400

800

1200 1600 Time (s)

2000 mm EXP 1750mm EXP 1250mm EXP

0

400

800

1200 1600 Time (s)

2000mm FDS 1750mm FDS 1250mm FDS

2000

1600

1200

1400

800 600 400 1750 mm EXP-avg 1250 mm EXP-avg 2100 mm EXP-avg

200 0

1750mm FDS-avg 1250mm FDS-avg 2100mm FDS-avg

800

800

400

800

@xs,xi þ ð1=2Þ

ks,xið1=2Þ

n n Ts,xi Ts,xi1

@xs,xi þ ð1=2Þ

2200mm FDS 1750mm FDS 1250mm FDS

1200 1600 Time (s)

2000

2400

2000

2400

2000

2400

2000mm FDS 1750mm FDS 1250mm FDS

1200 1600 Time (s)

2000 mm EXP 1750mm EXP 1250mm EXP

0

1400

1000

400

1200 1100 1000 900 800 700 600 500 400 300 200 100 0

2400

n n Ts,xi þ 1 Ts,xi

2000 mm EXP 1750mm EXP 1250mm EXP

0

Temperature (°C)

1000 900 800 700 600 500 400 300 200 100 0

400

1200 1100 1000 900 800 700 600 500 400 300 200 100 0

2400

ks,xi þ ð1=2Þ

2200 mm EXP 1750mm EXP 1250mm EXP

0

2000mm FDS 1750mm FDS 1250mm FDS

2000

1200 1100 1000 900 800 700 600 500 400 300 200 100 0

2400

Temperature (°C)

Temperature (°C)

@Ts 1 ¼ 2ðrs cs Þxi dxxi @t

Temperature (°C)

Temperature (°C)

0

Temperature (°C)

Crank–Nicolson scheme:

Temperature (°C)

Temperature (°C)

1000 900 800 700 600 500 400 300 200 100 0

493

2000mm FDS 1750mm FDS 1250mm FDS

1200 1600 Time (s)

1200 1000 800 600 400 1750 mm EXP-avg 1250 mm EXP-avg 2100 mm EXP-avg

200

1750mm FDS-avg 1250mm FDS-avg 2100mm FDS-avg

0 0

400

800

1200 1600 Time (s)

2000

2400

0

400

800

1200 1600 Time (s)

2000

2400

Fig. 19. Comparison of gas temperatures from experimental and numerical studies. The curves shifted 300 units upwards from the previous curves, except for the first curve at 1250 m in (d) and (h). (a) Back tree (Front-UP), (b) Centre tree (Front-UP), (c) Front tree (Front-UP), (d) Average (Front-UP), (e) Back tree (Back-UP), (f) Centre tree (Back-UP), (g) Front tree (Back-UP) and (h) Average (Back-UP).

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K.A.M. Moinuddin et al. / Fire Safety Journal 46 (2011) 480–496

þ ks,xi þ ð1=2Þ

nþ1 nþ1 Ts,xi Ts,xi þ1

@xs,xi þ ð1=2Þ

ks,xið1=2Þ

nþ1 nþ1 Ts,xi Ts,xi1

!

the room. The walls of the ISO room were created as obstructions made of gypsum board (39 mm thick) and steel sheet (1 mm thick). The columns and beams were modelled according to standard dimensions as thin sheet obstructions, and six sided boxed plasterboard protections (16 mm) were applied for the protected case simulations. Inert stands/supports for the beams were also modelled. The floor was modelled as 13 mm thick firerated plasterboard with insulated backing. The set up of the thermocouples and other measuring devices was exactly the same as the set up in the experimental programme, with the only addition of temperature and velocity slices. When conducting CFD simulations of fire, the most important input is the HRR. FDS handles this in one of two ways by

ð7Þ

@xs,xi þ ð1=2Þ

for 1 rxirN. The width of each cell is dxxi. The distance from the centre of cell xi to the centre of cell xiþ1 is dxxi þ 1/2. However, the material properties ks, cs and rs are updated in an explicit manner, using the temperature information from time step n. The boundary condition is discretized as ks,1

n n Ts,1 Ts,0 þ 1Þ þ 1Þ ¼ q_ 00ðn þ q_ 00ðn c r @x1=2

ð8Þ

Finally, the surface temperature is calculated as Tw ¼Ts,1/2 ¼ (Ts,0 þTs,1)/2. 4.2. FDS input script

 predicting the mass loss rate of combustibles (i.e. reactant

An FDS input data file was created to resemble the experimental set up for the ISO room. The computational domain was extended beyond the enclosure to capture all of the combustion activity (Fig. 18). Two timber cribs (750 mm  750 mm  750 mm) were modelled as an obstruction made of 50 mm thick sticks (with 50 mm gap between them), placed appropriately in

 prescribing the HRR directly. This avoids the pyrolysis calcula-

production rate) through a simplified pyrolysis calculation, and

700

Top EXP Middle EXP Bottom EXP

However, the first method has more uncertainty than the second method [5]. A recent study shows that FDS could correctly

600

Top FDS Middle FDS Bottom FDS

Temperature (°C)

Temperature (°C)

600

tion, and the reactant loss rate is then converted from the prescribed HRR using appropriate heat of combustion values.

500 400 300 200

Top EXP Middle EXP Bottom EXP

500

Top FDS Middle FDS Bottom FDS

400 300 200 100

100 0

0 0

200

400

800

600

1000

0

1200

200

400

Temperature (°C)

600

800

1000

1200

Time (s)

Time (s) 500 450 400 350 300 250 200 150 100 50 0

Top EXP Middle EXP Bottom EXP

0

200

Top FDS Middle FDS Bottom FDS

400

600

800

1000

1200

Time (s) 800 Temperature (°C)

600

800

Open end FDS Loc2 FDS Loc3 FDS Back end FDS

Open end EXP Loc2 EXP Loc3 EXP Back end EXP

700 Temperature (°C)

Open end EXP Loc2 EXP Loc3 EXP Back end EXP

700 500 400 300 200 100

600

Open end FDS Loc2 FDS Loc3 FDS Back end FDS

500 400 300 200 100

0 0

200

400

600 Time (s)

800

1000

1200

0 0

200

400

600

800

1000

1200

Time (s)

Fig. 20. Unprotected steel temperature comparison between experimental and FDS values for Front-UP. (a) Column A, (b) Column B, (C) Column C, (d) Beam A and (e) Beam B.

K.A.M. Moinuddin et al. / Fire Safety Journal 46 (2011) 480–496

(Fig. 19(a) and (e)). In contrast, the model predicted much lower gas temperatures near the doorway (Fig. 19(c) and (g)). From Fig. 19(d) and (h), temperatures at the same height from the floor at three locations were averaged, and experimental and numerical results were compared. Good agreement was observed for the Front-UP case. However, lower gas temperature is observed for the Back-UP case, except during the early stages at 1250 mm above the floor. Similarly, gas temperature predictions were good for the Front-P case, but predictions were not as good for the Back-P case [12]. The difference between calculated and experimental gas temperature values for Back-UP and Back-P cases may be a direct result of a large amount of heat escaping through the doorway in the numerical model. However, this could not be validated, as no velocity data were collected at the doorway in the experiment due to the difficulty in measuring gas velocity at the fireenclosure opening. It is suggested that in future, for similar studies, laser-based techniques such as Particle Imaging Velocimetry (PIV) [18] be developed to measure gas velocities at fireenclosure openings.

determine the gas temperature for an ISO 9705 room pool fire test [17]. Therefore, for this study, the second method is adopted where the fire is represented by burners with prescribed HRRs. It was found that the difference in location of the burners on the modelled crib in the numerical simulation affected the flame height, and this interfered with gas thermocouples, directly causing large fluctuations in gas temperature recordings. After several runs with variation in the location of the burners, it was decided to model the burners at two levels (top-most and second-last levels), as shown in Fig. 18(a), as these produced comparable results. Material properties and combustion parameters for various materials used for the simulations are given in Table 8. All simulations used a grid size of 50 mm. To investigate the effect of a finer grid, a simulation was run with a grid size of 25 mm. However, this did not show any appreciable change in temperature results.

4.3. Simulation results: gas temperatures

200

0

100 90 80 70 60 50 40 30 20 10 0

Top EXP Middle EXP Bottom EXP

200

Open end FDS Loc2 FDS Loc3 FDS Back end FDS

400

600 800 Time (s)

1000

1200

400

600 800 Time (s) 80 70 60 50 40 30 20 10 0

1000

Top EXP Middle EXP Bottom EXP

0

200

90 80 70 60 50 40 30 20 10 0

Open end EXP Loc2 EXP Loc3 EXP Back end EXP

200

0

Top FDS Middle FDS Bottom FDS

Temperature (°C)

0

As gas temperatures in the test Front-UP (and also in Front-P presented in [12]) were reasonably predicted, steel temperatures from these two simulations are compared with the experimental

Temperature (°C)

Open end EXP Loc2 EXP Loc3 EXP Back end EXP

4.4. Simulation results: steel temperatures

Temperature (°C)

Temperature (°C)

Temperature (°C)

From Fig. 19, it can be observed that the gas temperatures calculated by FDS are generally in good agreement with the experimental results for the Front-UP case. Predicted gas temperatures at the rear of the ISO room are in good agreement with experimental measurements, regardless of the thermocouple location being near-field or far-field with respect to the fuel loads

90 80 70 60 50 40 30 20 10 0

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Open end FDS Loc2 FDS Loc3 FDS Back end FDS

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Fig. 21. Protected steel temperature comparison between experiment and FDS values for Front-P. (a) Beam A, (b) Beam B, (c) Column A, (d) Column B and (e) Column C.

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data in Figs. 20 and 21. It is expected that steel temperatures will be under-predicted by FDS for Back-UP and Back-P cases, as the gas temperatures were under-predicted. Although unprotected steel was reasonably predicted, as shown in Fig. 20, the predicted protected steel temperature was significantly lower than the experimental result (Fig. 21). Uncertainties of the material property values of plasterboard, one-dimensional heat-transfer calculation (as opposed to three-dimensional heat transfer), etc. can be responsible for such discrepancies.

plasterboard protected columns and beams than those without protection, when exposed to a room fire. However, all predicted steel surface temperatures were much lower than actual experimental temperature recordings, except that unprotected steel was reasonably predicted for the Front-UP case. In light of this study, the following future work is recommended:

 Estimate appropriate material properties of plasterboard and other types of fire-protection material.

 Develop laser-based PIV techniques to measure gas velocities at fire-enclosure openings.

5. Discussion and conclusion

 Equip CFD fire models with three-dimensional heat-transfer

This research programme was undertaken to record the HRRs, temperatures of hot gases and temperatures of protected and unprotected steel members in identical fire scenarios to facilitate comparison between protected and unprotected structural performance in fire and to facilitate validation of numerical modelling. Out of a total of four experiments, two experiments had the fuel load located at the back of the enclosure, and the other two experiments had the fuel load located at the front of the enclosure. The results from the tests when the fuel was located at the back of the enclosure, Back-P (steel members protected) and Back-UP (steel members unprotected), showed that both the HRRs and gas temperatures (for three different locations) were almost the same during the fire-growth phase (up to 20 min). Therefore, the protected and unprotected steel temperature data for the first 20 min can be used for testing any computational model in predicting heat transfer to structural steel members. Similarly, the results from the tests when the fuel was located at the front of the enclosure, Front-P (steel members protected) and Front-UP (steel members unprotected), the protected and unprotected steel temperatures data for the first 18 min can be used for such testing. The experiments show that columns located close to the fire source experience higher temperatures than the ones located away from the fire. Early failure of a column close to the fire may cause instability, leading to a structural collapse. It is also observed that beam temperature along a beam in a fire compartment is not constant, and that there may be a significant uncertainty if the steel temperature is calculated using a zone model as opposed to a CFD model. Furthermore, the tests show that fire-load burning at the end of an enclosure has more adverse effects on steel beams. Another obvious observation is that the experiments that had all steel members protected with plasterboard were able to reduce the steel member temperatures by several hundred degrees Celsius while exposed to elevated gas temperatures caused by natural fire situations that could be present within buildings. To demonstrate further usefulness of the collected data for numerical fire and heat-transfer modelling, the experiments were simulated using a CFD fire model FDS. The simulation was conducted with prescribed HRRs (obtained from experimental data). The FDS simulation predicted gas temperatures that were in good agreement with experimental measurements for thermocouples located at the rear of the ISO room, regardless of them being near-field or far-field with respect to the fuel loads. The model predicted lower steel surface temperatures for

 Enable CFD fire models to calculate the right amount of heat

calculations. and mass escaping through the fire-enclosure opening.

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