2.2.1. Model Parameters

The fire source is located at the outermost side of the bridge mid-span (as shown in Figure 1), and its size is 12 m × 3 m × 3 m. Since the study of bridge fire mainly focuses on an extreme fire source scale [13,14], the fire power is set as the heat release rate of the tanker during fire. Inagason [15] suggested the heat release rate for a tank fire be set at 200 MW. According to the results of the French regulations [16], the growth stage of a 200 MW fire source is 600 s, and the stable stage is 4200 s, so the simulation time is 4800 s in this work. The type of simulated fire source is a t<sup>2</sup> -growth fire. The growth factor is calculated at the time specified by French regulations. It is appropriate to set the mesh size as 1/4–1/16 of the flame characteristic diameter (*D*\*) [17]. Employing the formula *D*<sup>∗</sup> = (*Q*/*ρ*0*CpT*0*g* 1/2) 2/5 [18], the flame characteristic diameter in this paper is calculated as 7.77 m, and thus, the appropriate mesh size is 0.49 m–1.94 m. Considering both simulation time and accuracy, 1 m is selected as the mesh size. The relevant parameters are shown in Table 2.

**Table 2.** Parameter settings of simulation.


The arrangement of thermocouples inside the utility tunnel is shown in Figure 2. The ceiling thermocouple is arranged 2 m apart, tiling the whole top deck of the bridge. spacing.

overall change of ceiling temperature. The height of the 2D slice is equal to the bridge deck

2.2.2. Operating Parameters In order to ensure that the maximum excess temperature of the hot smoke layer below the ceiling can be measured, the thermocouples are arranged 0.05 m below the ceiling [19].

In the paper, the definition of bridge deck spacing is the vertical distance from the ceiling to the bottom deck of the double-deck bridge, as shown in Figure 2. Effective

Figure 3 shows the influence range of high-temperature smoke on the ceiling at different bridge deck spacings. As can be seen from the figure, the range of the high-temperature area gradually decreases. With the increase in bridge deck spacing, the area where the flame plume hits the ceiling decreases. When the temperature exceeds 300 °C, the properties of steel begin to decrease, so the 300 °C isotherm is selected as the influence range of the fire temperature field. The spread distance of fire influence decreases gradually along the *x*-direction. This is because the spread distance of flame plume along the ceiling becomes smaller as the bridge deck spacing increases. However, the spread distance of fire influence first remains unchanged and then decreases along the *y*-direction. This is because the flame plume tilts along the *y*-direction due to ventilation velocity.

resource. In this study, six different bridge deck spacings, i.e., 9.8 m, 10.6 m, 11.4 m, 12.2 m, 13.0 m, and 13.8 m, are selected and investigated, while other parameters remain unchanged. In addition, a ventilation velocity (2 m/s) is set according to the perennial wind speed in Guangzhou. As shown in Figure 1, the ventilation direction is along the *y*-direc-

*3.1. Influence Range of High-Temperature Smoke on the Ceiling*

tion.

**3. Results and Discussion**

There are six thermocouples evenly arranged at the same intervals above the fire source on the inner surface of the truss (yellow points in Figure 2), and the thermocouples are arranged 0.05 m away from the surface of the truss. In addition, the 2D slice (blue graph in Figure 2) of the temperature is set at the bottom of the ceiling to observe the overall change of ceiling temperature. The height of the 2D slice is equal to the bridge deck spacing.
