HC火灾下考虑支座高温性能退化影响的PC薄腹梁垮塌机理及抗火设计方法研究

Prestressed concrete thin-web girders are widely used in bridge construction and it has no definite collapse mechanism under fire condition. Due to its thin web and large span and existed prestressing strands and influence of high-temperature performance deterioration in bearing, the fire endurance of prestressed concrete thin-web girder cannot be obtained by failure criterion of RC beam in building. However, very little work has been done on study on fire resistance performance of prestressed concrete thin-web girder. To this end, this project will use a combined research method, including actual bridge sampling, model experiments, finite element theory and incremental iteration method of limit equilibrium state based on time-dependent boundary. The influence law on damage degree and effective prestress and deflection and the rate of deflection and bearing capacity, resulting from prestress degree and section type and load level and fire parameters based on time-dependent boundary conditions, of prestressed concrete thin-web girder under hydrocarbon fire condition is to be analyzed. Collapse mechanism of prestressed concrete thin-web girder based on high-temperature deterioration of bearing during the entire process of hydrocarbon (HC) fire is to be explored deeply. The limit state, including boundary, spalling, temperature, prestress degradation, deflection, the rate of deflection and bearing capacity, will be considered, and the reasonable failure criterion of prestressed concrete thin-web girder based on high-temperature performance in bearing under hydrocarbon fire is to be proposed. The calculation method of fire endurance and the fire-resistant design method on prestressed concrete thin-web girder under hydrocarbon fire condition will be also proposed. The data provision will be prepared for the guideline of fire resistance design of bridge structure and specification.

预应力混凝土（PC）薄腹梁在桥梁建设中应用广泛，然其火灾下的垮塌机理不明确，由于预应力混凝土薄腹梁腹壁薄、跨径大加之预应力筋的存在，同时受支座高温性能退化的影响，以建筑用RC梁的破坏准则无法获取预应力混凝土薄腹梁的合理耐火极限，目前国内外对此研究空白。为此，本项目拟采用实桥采样、模型试验、有限元理论和边界时变的极限平衡状态增量迭代法相结合的研究方法，考虑边界条件时变效应分析预应力度、截面形式、荷载水平及火灾参数对碳氢（HC）火灾下预应力混凝土薄腹梁的损伤度、有效预应力、变形、变形率及承载能力的影响规律，深入探索预应力混凝土薄腹梁在HC火灾下考虑支座高温性能退化影响的垮塌机理，综合考虑损伤极限状态（边界、爆裂、温度、预应力衰变、变形、变形率和承载能力），建立HC火灾下考虑支座高温性能退化影响的预应力混凝土薄腹梁的破坏准则，提出耐火极限计算方法和抗火设计方法，为桥梁抗火设计及规范编制提供依据。

油罐车碰撞或倾覆引发可燃物燃烧引起的碳氢（HC）火灾，可能导致局部构件破坏甚至桥梁整体垮塌。项目选取应用广泛的预应力混凝土（PC）薄腹梁为研究背景，针对半开放环境下（桥下火灾场景）预应力混凝土薄腹梁（箱形截面梁和T形截面梁）的抗火性能，通过试验研究、数值模拟、理论分析，开展了预应力混凝土梁火灾响应及承载能力衰退研究，揭示了高强爆燃火灾作用下预应力混凝土薄腹梁的损伤机理，并提出了一种预应力混凝土桥梁实用抗火设计方法。主要研究内容如下：.（1）采用场模型模拟了半开放环境下桥梁火灾温度场，对桥下不同净空、不同车辆类型和不同受火位置的桥梁火灾温度场进行分析，得到随火源中心水平距离、高度和时间变化的温度非定场，提出了空间位置任意点温度随时间变化的函数关系，建立了桥梁半开放环境火灾温度场模型。.（2）设计制作了5榀大比例PC简支缩尺模型试验梁（3榀箱形和2榀T形截面梁），开展了燃油加速升温碳氢火灾条件下PC梁局部受火试验，获得了梁截面温度场、挠度变化、有效预应力衰变、裂缝开展、高温爆裂和耐火极限的相关试验数据，分析了PC梁火灾损伤演化过程和破坏模式，研究了截面类型、荷载水平、预应力比和横隔板对PC薄腹梁抗火性能的影响规律。.（3）提出了火灾下PC桥梁火灾响应CFD-FEM耦合分析方法，该分析方法能够考虑简支、连续以及各类复杂截面的PC桥梁的真实边界条件（支座接触和梁端约束）和实际火灾温度场的不均匀特性。.（4）采用CFD-FEM耦合分析方法对PC薄腹梁在不同荷载、不同桥下净空和受火位置的典型车辆火灾场共同作用下的挠度发展、破坏模式和预应力衰变进行分析，获悉了火灾下PC薄腹梁的承载能力退化规律，揭示了火灾下PC薄腹梁的破坏机理。.（6）构建了桥梁结构实用抗火设计方法，考虑经济性能与结构性能的平衡建立了PC桥梁两类抗火设计目标，提出了“中度火灾可修复、严重火灾不垮塌”的PC桥梁抗火设计原则，获得了火灾下PC桥梁抗弯和抗剪承载能力简化计算方法。

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