# Study on Fire Smoke Distribution and Safety Evacuation of Subway Station Based on BIM
## 2 Calculation Method of Fire Evacuation in Subway Station  
There is a certain pattern in the development process of a subway station fire, as shown in Figure 1. According to the changing trend of temperature over time, the development process of the fire is mainly divided into ignition stage, golden stage and decline stage [17]. The trend and speed of fire spread are mainly reflected in the initial development stage, and the maximum scale of fire development is mainly predicted through the development stage. Therefore, the ignition and golden stages of fire are generally focused on. The space of subway stations is relatively closed and oxygen supply is insufficient, resulting in incomplete combustion and the release of smoke. Under high temperature conditions, some flame-retardant materials decompose to produce toxic gases such as CO, CO\({}_{2}\), SO\({}_{2}\), HCN and so on, and the concentration of O\({}_{2}\) decreases. When the concentration of CO is too high, it will affect the combination of hemoglobin and oxygen, leading to the reduction in oxygen content in the blood. When it reaches 2500 ppm, it will pose a threat to human health and is one of the important causes of casualties in fire. When the smoke is not discharged in time, heat accumulates in the building, the temperature rises and high temperature smoke will burn human skin and respiratory tract, making people gradually lose their ability to exercise. When the smoke temperature reaches 65 \({}^{\circ}\)C, the human body begins to have adverse reactions. In addition to toxic gases such as CO and CO\({}_{2}\), fire smoke also includes a large amount of dust and particulate matter. When dust and particulate matter float in the air, it has a strong light-blocking ability and reduces visibility to a large extent, leading to the inability of people to rapidly judge the location and evacuation conditions. In the case of insufficient visibility, if the people are not familiar with the environment of the subway station, it will be difficult to choose the appropriate evacuation path, which will make the flow of people in the station chaotic and increase the difficulty of rescue. In order to ensure that people can safely evacuate to a safe area, this study takes the height of human eyes as a reference, and selects the temperature, CO concentration and visibility at the height of 1.65 m above the ground as the judgment basis for whether the fire has reached a dangerous state. According to relevant data, the critical values of the related indicators are shown in Table 1: \begin{table}
\begin{tabular}{c c c} \hline \hline
**Project** & **Smoke Temperature** & **CO Concentration** & **Visibility** \\ \hline Critical value of danger & \(\leq\)65 \({}^{\circ}\)C & \(\leq\)2500 ppm & \(\geq\)10 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The critical value of related indicators. Figure 1: Fire development process. Subway stations are enclosed and deep underground. The structure of the station is complex, and various functional areas have a certain hindering effect on personnel evacuation [18]. In the case of sudden emergency, the safety evacuation of personnel depends on the available and required safe evacuation time. Available safe evacuation time (ASET) refers to the time from the occurrence of the accident to the threat to people's life safety, or the time when the accident further expands and intensifies the threat to people. The required safe evacuation time (RSET) is the time taken for all people to evacuate to a safe area. \"The Subway Design Code\" (GB50157-2013) stipulates that the stairs, escalators, entrances and exits of the common area of the station platform shall meet the requirements of evacuating all passengers and waiting people on the platform of an inbound train and reaching the safe area within 6 min in case of fire in the long term or in the peak hour of passenger flow control period.