油罐车火灾致简支钢-混组合箱梁抗弯承载力衰变机理

Degradation mechanism of simply supported steel-concrete composite box girder under tanker fire condition

针对油罐车火灾导致的简支钢-混组合箱梁的极限破坏问题,选取某简支钢-混组合箱梁为研究对象,给出桥梁火灾与建筑火灾的区别及热-力耦合的火灾全过程数值计算方法和强度分区的等效计算方法,分析碳氢(HC)火灾下简支钢-混组合箱梁的截面温度分布特征,研究各构件强度衰变过程,揭示不同延火时间下正截面抗弯承载力的衰变机理,建立简支体系钢-混组合箱梁抗弯承载力与HC火灾的时程衰变阶段关系,通过截面抗弯承载力与荷载效应的对比得到简支钢-混组合箱梁在HC火灾下的破坏时间。研究结果表明:所提方法能够预测HC火灾下钢-混组合箱梁的温度响应和结构响应;截面测点温度平均值受各构件厚度的影响;有横隔板的断面温度峰值低于无横隔板的断面;HC火灾下简支钢-混组合箱梁跨中截面抗弯承载力随延火时间呈四阶段下降;前8min内跨中截面抗弯承载力保持初始状态,8～16min跨中截面抗弯承载力衰变率增大,16～48min截面中性轴特征逐渐发生改变,48min后结构破坏;简支钢-混组合箱梁在HC火灾下的灭火时间应该控制在8min之内;拟合建立的HC火灾下简支钢-混组合箱梁正截面抗弯承载力时程衰变四阶段计算公式,进一步明确了油罐车火灾下其抗弯承载力的衰变机理,该公式简单实用,可为桥梁抗火设计及智能评估提供依据。

Aimed at ultimate failure of simply supported steel-concrete composite box girder caused by the oil tanker fire, the difference between bridge fire and building fire was given, the numerical calculation method of the whole fire process of thermo-mechanical coupled and the equivalent calculation method of strength division was provided, according to the fire resistance of simply supported steel-concrete composite box girders. The temperature distributed within the girder section under HC fire was analyzed, and the degradation process of the strength in each component was studied. Further, the time-dependent variation relationship between the cross-sectional flexural capacity and fire exposure time of a simply supported steel-concrete composite box girder was analyzed. A formula predicting the flexural capacity with time of HC fire was derived. The failure time of a composite box bridge girder under a HC fire condition was obtained by comparing the flexural capacity and load effect. The results show that the provided method can predict the temperature and structural responses of a composite box bridge girder under HC fire conditions. The average temperature can be highly influenced by the thickness of each component. The peak temperature in the girder section without a transverse diaphragm is higher than that in girder section with a transverse diaphragm. The flexural capacity of the mid-span section of simply supported steel and concrete composite box girder can be grouped into four stages with HC fire. The flexural capacity remains in the initial state for up to 8 min, the rate of degradation becomes larger between 8 to 16 min, the neutral axis feature of section changed from 16 to 48 min, then it steps into faster degradation and fails after 48 min. The extinguishing time is suggested to be limited to 8 min. The time-dependent decay formula proposed to solve the flexural capacity of the cross section of a simply supported steel-concrete composite box girder is simple, The decay mechanism of the flexural capacity of simply supported steel-concrete composite box girder is further clarified, and it can provide data to guide fire resistance design and intelligent assessment after fires.