火灾下混凝土空心板温度场及损伤规律研究

为分析火灾下混凝土空心板温度场分布变化，研究其结构受火损伤规律，从服役多年的空心板桥上截取长2.6 m的梁段，按ISO 834标准规定的升温曲线对其进行180 min的明火高温试验，通过预埋温度传感器实测获得空心板梁段温度场随受火作用时间的分布变化规律。在此基础上，结合统计得到的混凝土热工参数代表值、火灾试验升温加热过程和空心板实际受火热边界条件，对梁段的温度场进行有限元数值仿真分析，并将温度场理论计算结果与实测结果进行比较，分析温度场计算偏差的主要原因，探讨温度场有限元模型参数合理取值。采用300℃、500℃和800℃等温线法分别计算火灾下空心板的截面缩减系数。基于最小二乘法拟合得到空心板截面缩减系数与受火作用时间关系，对其火灾下截面损伤进行多项式量化。研究结果表明：空心板内部在试验前期升温速度较快，后期趋于平缓；同一时刻下，空心板内部温度沿梁高方向非线性递减，且梯度逐渐降低；空心板温度场有限元数值仿真结果与实测结果接近；所提出的热工参数代表值合理；空心板截面高度对其受火损伤范围影响不大；火灾下混凝土空心板截面缩减系数随受火时间呈二次抛物线递减，并随梁高递增。该研究成果可用于类似桥梁火灾下的温度场仿真分析和损伤状况评估。

Temperature Fields and Damage Pattern of Hollow-core Concrete Slab Exposed to Fire

An attempt was made to analyze the internal temperature field and determine the damage pattern in hollow-core concrete slabs (HCSs) exposed to fire. A 2.6 m long beam segment was removed from an HCS bridge that had been in service for many years, and a 180 min high temperature fire test of the HCS was carried out according to the International Organization for Standardization ISO 834 Standard Curve. The change pattern of the temperature fields of the beam segment with the fire exposure time was obtained by embedded temperature sensors. Based on this, combined with the representative values of the thermal parameters of concrete, the heating process, and the thermal boundary condition of the HCS, the temperature field of the HCS was analyzed by finite-element numerical simulation. The theoretical calculation results of the temperature field were compared with the measured results. The main reasons for the deviation between the calculation results and the experiment ones of temperature field were analyzed, and optimal values of the parameters of the finite-element model of the temperature field were determined. The section reduction coefficient of the hollow slab under fire was calculated using a 500℃ isotherm method and a 300℃ and 800℃ isotherm method. The relationship between the reduction coefficient of the HCS and the fire exposure time was fitted by the least-squares method, and the variation in damage to the cross section under fire was quantified by polynomials. The results show that the temperature inside the HCS increases relatively rapidly during the early stage of the test and gradually during the later stage. The temperature inside the HCS decreases nonlinearly in the beam height direction, and the temperature gradient gradually decreases. The finite-element numerical simulation results for the temperature field inside the HCS are similar to the measurement results. The representative values for the thermal parameters proposed in this study are reasonable. The height of a cross section of the HCS has an insignificant effect on the extent of damage at the cross section. The reduction coefficients of a cross section of the HCS exposed to fire decrease exponentially with the fire exposure time and increase with the beam height. The results of this study can be used in numerical simulations of the temperature fields in similar bridges exposed to fire as well as in the assessment of their damage.