To study fire resistance and fire-resistant design method in steel-concrete composite bridge girders (steel structural bridge girders) encountering hydrocarbon fire exposure, three large-scaled steel-concrete composite bridge girders, including simply supported girders with box section, continuous girders with box section and double I section, were designed and manufactured. The fire tests in mid-span of simply supported bridge girders and localized fire tests in single span of continuous bridge girders under hydrocarbon fire (use of fuel for rapid temperature rise in the early stage and use of natural gas for high temperature maintenance in the later stage) were carried out. The cross-sectional temperature field, variation path of deflection in fire exposed span and non-fire exposed span, crack development mode, steel plate buckling characteristics, and fire mode were obtained. The fire resistance of composite bridge girders under hydrocarbon fire exposure was analyzed and influence mechanism of girder section and structural system on fire behaviour incomposite bridge girders was deeply revealed. The experimental results show that concrete has obvious heat sink effect. The temperature rise rate of steel girders under fire exposure is much faster than that of concrete slab. After heating is stopped, the temperature of steel girders decreases rapidly, while the temperature in concrete slab continues to rise.The temperature in upper layer of concrete slab still presents an increasing trend through 48 min after heating is stopped. The deflection of simply supported system steel-concrete composite bridge girders increases rapidly from initial stage of hydrocarbon fire exposure, and finally fails by excessive deflection. However, the deflection of continuous system steel-concrete composite bridge girders increases slowly in initial stage of fire exposure.Finally, plastic hinges appear in negative moment area at pier top and positive moment area at mid-span. The girders become a mechanism system and fail by rapid increase of mid-span deflection. The non-fire exposed span of continuous system steel-concrete composite bridge girders is arched first due to deformation coordination, and then deflects downward due to decline of stiffness in fire-exposed span. The fire resistance of girder with closed box section is significantly better than that of girder with double ribbed I-steel section, and its fire resistance are 48 and 42 min respectively under similar load levels. The continuous system steel-concrete composite continuous bridge girders have severe internal force redistribution and deformation coordination from beginning of fire exposure due to existence of redundant constraints, and its fire resistance can be increased by 100% as compared with that of simply supported bridge girders. The plastic hinge of continuous system steel-concrete composite bridge girders at high temperature is different from that at room temperature, and it is a kind of changeable plastic hinge with gradually decreasing stiffness.The research results of this paper can provide a guidance for fire-resistant test method of steel bridge girders and also can lay a theoretical foundation for its fire resistance design method. © 2022 Xi'an Highway University. All rights reserved.
