The ‘Obora’ Building
The building where the x-ONE experiment was carried out is located in a rural area, Golaszew, within Warsaw West County in Poland. It is approximately 20 km west of Warsaw. It was originally constructed as a concrete farm building (a cow farmhouse) and we refer to it by its name in Polish, ‘Obora’. Photos of the building from the top and inside prior to the set-up of the experiment can be seen in Fig. 2. The building has one floor that is mostly open plan, and an attic. It has thick, external masonry walls. The beam and block concrete slab ceiling is supported by 18 reinforced concrete columns, beams, and external masonry walls. The compartment has 10 bays. The structure supporting the roof is made from timber.
The experiment was carried out in the open plan section of the building which has a floor area of 380 m2, though, air ventilated and smoke was allowed to move via the adjacent compartment at the East entrance. The floor of the tested compartment is 10.8 m wide, 35.5 m long and 3.19 m high. A sketch of the floor plan and sections of the building are shown in Fig. 3.
In order to obtain the precise dimensions and layout of the compartment, a three-dimensional (3D) laser scan was conducted prior to the experiment. Laser scanning is a technique that captures the shapes of spaces in which the laser scanner is placed by measuring the distances and angles between the points and the instrument. These points create a 3D point cloud that can be used to visualize and gather information from the space with an error of up to ± 10 mm.
Protection of the Structure
Upon initial inspections of the building, many concrete columns supporting the ceiling were found to be damaged with missing concrete cover and exposed steel rebar which, if exposed to fire, could lead to collapse. Therefore, the structure was protected. All the columns were protected with autoclaved aerated concrete (AAC) blocks with the dimensions of 48 cm × 24 cm × 12 cm. The ceiling slab and beam soffits were protected with 5 cm thick mineral wool (80 kg/cm3). Some of the applied protection is shown in Fig. 4. All the openings to the attic were closed to prevent the entry of smoke or flames to the attic in order to protect the timber structure supporting the roof.
Ventilation
There are 6 door and 31 window openings in the building. Opening dimensions are given in Table 1. All openings were left open in order to reduce the effects of walls on the fire dynamics and to represent an open-plan compartment.
Table 1 Opening dimensions along different elevations of the building used for the x-ONE experiment Only the door and the window openings to the adjacent structure along the North elevation of the building were closed to keep the fire compartment simple and with openings directly to outside only.
In total, 20% of the compartment wall surface area, \({A}_{w}\) = 56 m2, constituted openings with an opening factor of 0.0694 m1/2 (\({A}_{w}\sqrt{h}/{A}_{total}\), where \({A}_{total}\) is the total surface area) and a weighted average opening height, h, of 1.67 m as defined in Annex A of Eurocode 1 [2]. Opening factors are commonly used to quantify the relative amount of ventilation in a fire compartment.
Fuel Load
The fuel load was an arrangement of a continuous wood crib mixed with fibreboard (see Fig. 5). The final wood crib arrangement was based on a preliminary study on the burning of wood cribs (see Rackauskaite and Rein [27] for more detail). In total, the final fire load was approximately 370 MJ/m2 (19.4 kg/m2) and covered a floor area of 174 m2 (6 m × 29 m) as illustrated in Fig. 3. The fuel bed was placed 1 m from the West entrance. This fuel load was chosen to limit the risk of damage to the structure but to still be within the range of typical fuel load densities in office buildings (250–950 MJ/m2 [28]; average 420 MJ/m2 [29]).
In some previous large scale fire experiments carried out by Hidalgo et al. [23], and Rush et al. [24], a relatively slow flame spread has been observed in the range of ~ 13 to 54 mm/min for the first 237 min until acceleration along the remaining fuel [23], and at an estimated average of 55 mm/min for 7 h [24]. These spread rates are lower than could be expected in real large compartment fires where large amounts of combustible materials such as paper, plastic, and furniture are present [30]. In real accidental fires, flames have been observed to spread in the range of 150–1000 mm/min (2.5–16.7 mm/s) [31]. Therefore, to aim for faster flame spreads than in previous experiments, it was decided to introduce additional material within the wood crib. Wood fibreboard (4 mm thick with a density of 250 kg/m3) was selected for the experiment based on a set of small-scale exploratory experiments.
For the wood crib, softwood sticks with the dimensions of 3 cm × 3 cm × 100 cm, density of 426–466 kg/m3 (average 453 kg/m3), heat of combustion of 18.94 ± 0.14 MJ/kg, and moisture content of 9.3 ± 0.3% (on dry basis) were used. Wood fibreboard had a heat of combustion of 19.60 ± 0.18 MJ/kg. A sample of 50 wood sticks was collected on the day of the experiment and weighted to obtain a representative average wood density. A sub-sample of 15 wood sticks were then dried in the oven at 80°C to obtain their moisture content. Bomb calorimetry was used to measure the heat of combustion. These wood and fibreboard properties have been used to determine the fire load.
The wood crib was arranged to have 11 stick layers with 4 sticks/m in each layer with an additional 2 sticks/m as the bottom layer (i.e. 12 stick layers in total), and 2 layers of 4 mm thick wood fibreboard (cut into strips of 0.59 m × 0.79 m and placed between wood sticks) as illustrated in Fig. 5. This is based on the study reported in [27]. The wood crib was lifted from the ground by placing bricks every 1 m to reduce heat losses to the concrete floor and enable air supply and burning below the crib.
The fuel was ignited by placing a line of 12 pans (15 cm × 25 cm) filled with 0.5 l methanol each at one end of the wood crib (i.e. East Entrance). The objective was to ignite them simultaneously to ensure a uniform line of ignition. Fire across the wood crib was allowed to spread naturally. Illustration of the fuel load at ignition is shown in Fig. 5.
Instrumentation
Thermal
In total, 30 Type-K thermocouples (see Fig. 6) were uniformly distributed across the compartment to measure temperatures. The number of the thermocouples was carefully selected to be enough to capture the key fire dynamics phenomena of interest, i.e. spatial temperature distributions at the ceiling level along the expected fire path, and vertical temperature distributions at a few selected locations.
15 Type-K thermocouples were attached to 9 columns (TC) and 9 to the soffits of beams (T) at the mid-span. Thermocouples were attached to 9 columns in total. On all selected columns, thermocouples were attached at the height of 2.4 m with 6 columns having an additional thermocouple at the height of 1 m to capture vertical temperature variations. Columns of interest were selected on each side of the compartment. Beam thermocouples were attached to the soffits of the beams at mid-span at the height of approximately 2.9 m. The thermocouples were evenly spaced to capture spatial temperature distributions at the ceiling level along the length of the compartment. In addition, 6 thermocouples (TP) were installed above the blocks of mineral wool, in contact with the original ceiling, to measure the surface temperature of the ceiling and monitor the fire protection.
The thermocouple measurements were corrected for radiation errors using a simple methodology referred to in this paper as the \(\beta\)-method. It is assumed that each thermocouple is in thermal equilibrium (quasi steady-state), equating the convective and radiative components as follows:
$$h\left( {T_{g} - T_{TC} } \right) - \dot{q}_{rf}^{\prime \prime } + \varepsilon \sigma T_{TC}^{4} = 0$$
(1)
where \(h\) is the convection coefficient, \({T}_{g}\) is the gas temperature around the thermocouple, \(T_{TC}\) is the recorded temperature at the thermocouple, \(\dot{q}_{rf}^{\prime \prime }\) is the radiative incident heat flux on the thermocouple, \(\varepsilon\) is the thermocouple surface emissivity (0.9 [32]), and \(\sigma\) is the Stefan-Boltzmann constant.
The radiative heat flux received to the thermocouple due to the fire, \(\dot{q}_{rf}^{\prime \prime }\), is unknown but can be assumed to be proportional to \(T_{TC}^{4}\). Hence, \(\dot{q}_{rf}^{\prime \prime } = \beta \varepsilon \sigma T_{TC}^{4}\), where \(\beta\) is a dimensionless constant (defined as the correction coefficient) greater than 1, as the radiative heat flux due to the fire will always result in \({T}_{TC}\) greater than \({T}_{g}\). Thus, \({T}_{g}\) can be calculated as follows:
$${T}_{g}={T}_{TC}+\frac{\varepsilon \sigma }{h}\left(1-\beta \right)\cdot {T}_{TC}^{4}$$
(2)
\(h\), is determined by evaluating the Nusselt number for flow around a sphere as described by Welch et al. [33] assuming a local velocity of u=12 m/s. Due to variation in gas density with temperature, \(h\) is varies throughout the experiment.
The thermocouple corrections are based on parametric analysis varying \(\beta\). As shown in fire experiments reported by Drysdale [34], the maximum average flame temperature measured by corrected thermocouples in turbulent flames is 900°C. Therefore, by setting 900°C as the maximum value of \({T}_{TC}\), we can find a range of values for \(\beta\) through Eq. (2). The values of \(\beta\) were calibrated to correct the peak thermocouple temperatures measured in the follow up experiment x-TWO.1 (1100°C) [35] to produce a local gas temperature within the range of 900–1000°C. A corrected temperature range was, thus, selected for the \(\beta\) values \(1.17\le \beta \le 1.25\), which also captures the error associated with thermocouple measurements.
Visual
A series of visual cameras were used to capture the flame size and spread rate. Nine raspberry-pi cameras were attached to the internal wall near the South entrance of the compartment to capture the fire development. They were pre-programmed to take time-lapse photos every 10 s and send them to the computer on-site via a wi-fi connection. The cameras were protected with plasterboard and fixed to a wall. However, only one of the raspberry-pi cameras survived despite the protection. One GoPro was located outside the building close to the West entrance to record video footage and track flame spread from the East end of the compartment where the fire was ignited. Additional footage included photos and videos recorded with 7 handheld and mobile cameras.
All of this footage was used in the image processing analysis described in “Appendix 1” to measure the locations of the leading (i.e. flame front) and trailing (i.e. burn-out) edges of the fire. In the analysis, the tomographic reconstruction approach by Graham [37] was adapted in MATLAB. The steps in the image processing analysis included calibrating each camera to correct for image distortion; detecting the location of each camera with respect to the ‘Obora’ building; and flame detection in images, all of which were then used to determine the location of flames in the building. An algorithm was developed to determine geometric relationship between building voxels and projected flame pixel rays. The error associated with image processing and flame detection is estimated to be ± 0.25 m and ± 0.5 m for the leading and trailing edges, respectively (see “Appendix 1”).
3D Scan
A 3D laser scan prior to the experiment was conducted to obtain the dimensions of the compartment. After the experiment, a further 3D laser scan was conducted to capture any deformations that may have taken place in the compartment due to fire. The deflections after the fire experiment have been quantified by comparing the measurements from the two sets of scans.
Weather
A weather station was placed outside of the building by the East entrance at approximately 1 to 1.5 m above ground to track weather conditions. The weather station recordings measured the outside wind during the experiment to be, predominantly, from Southwest-West with wind speeds in the range of 0.3–2.0 (± 1.0) m/s (light breeze). Air temperature and relative humidity were measured to be in the range of 17.6–19.4 (± 1.0)°C and 75–80% (± 5%), respectively, throughout the duration of the experiment.