Abstract: Abstract: To study the fire resistance behavior of long-span double-layer suspension bridge under different vehicle fire scales, the extra-large-span double-deck suspension bridge with the main span of 2 180 m is taken as the research object. Firstly, the thermal analysis model of the suspension bridge is established by using the fire dynamics software FDS, and the influence of different fire source power, fire source location, environmental wind direction and other factors on the temperature distribution law of the double-deck suspension bridge is studied. Besides, the temperature-time relationship curve of the key components of the suspension bridge is obtained. Then, the thermal mechanical coupling numerical simulation analysis of double-layer suspension bridge is carried out by using the finite element software ABAQUS, and the thermal parameters of the steel and high-strength steel wire at high temperature are selected to study the time-varying characteristics of the high-temperature mechanical properties of the suspension bridge hangers, stiffening beams, and bridge decks. The structural stress, deformation, and component damage behavior of double-deck suspension bridges under the working conditions of environmental wind speed, truss height, fire source characteristics and location were compared to determine the key fire resistance parts and fire resistance requirements of the extra-large-span double-deck suspension bridge. Finally, based on the results of numerical simulation, a preliminary proposal for the fire protection design of the double-deck suspension bridge structure is put forward.The results show that when the vehicle fire position is located on the lower deck of the bridge, the lower deck structure is less affected by the fire due to the thermal insulation effect of the bridge deck pavement. The temperature of the upper structure is significantly higher than that of the lower part which is caused by the hot air flow, and the temperature of the upper longitudinal and transverse beams, bridge decks, hangers and other components near the fire source rises rapidly. Due to the thermal expansion effect of steel, the component is rapidly heated up and expanded, and the expansion is limited by the surrounding rod members, resulting in the gradual increase of compressive stress.The stress development and failure location of the bridge section under fire are closely related to the location of fire source, power and environmental wind direction. （1） Location of fire source: when the fire source is located in the middle lane of the transverse bridge direction, the high stress area is concentrated in the upper longitudinal and transverse beams and bridge decks of the non-motorized vehicle lane. When the fire source is located in the non-motorized vehicle lane of the transverse bridge, the strength failure of the upper deck near the fire source occurs. （2） Fire source power: With the increase of fire source power, the effect of the fire field on the high temperature of the bridge increases, the temperature of key components gradually increases, and the range of high temperature influence becomes larger. The fire source power is 30 MW, and no strength damage occurs within 6 000 s; the fire source power is 100 MW, and its fire resistance time is 653 s; the power of fire source is 200 MW, and its fire resistance time is 413 s. With the increase of the fire source power, the fire resistance time of the bridge structure is significantly reduced, and the maximum reduction can reach 93.11%.（3） Environmental wind direction: when the fire source is on the emergency lane of the transverse bridge, the upper deck connected with the trusses on both sides will suffer strength damage under the action of outward wind; under the action of inward wind, the upper deck near the non-motorized lane will suffer strength destruction. The fire resistance time of the bridge section under the inward wind is 528 s, increased by 3.0% compared with that under the condition of outward wind. For the long-span double-deck suspension bridges under vehicle fire, the fire resistance grades should be classified according to the risk of the fire-affected structure, and the fire protection design should be carried out according to the grades.

Key words: Key words：vehicle fire, thermo-mechanical coupled analysis;double deck suspension bridge, steel structure, fire resistance behavior, fire resistance time