碳氢火灾下钢-混组合梁破坏试验研究

为研究钢-混组合梁(钢结构桥梁)遭遇碳氢火灾时的耐火性能与抗火设计方法,设计制作了3榀大比例钢-混组合缩尺试验梁,包括简支体系箱形截面梁、连续体系箱形截面和双肋工字形钢截面梁。开展了碳氢火灾下(前期燃油急速升温和后期天然气维持高温)简支梁跨中受火和连续梁单跨局部受火试验,获悉了截面温度场、受火跨和非受火跨挠度变化路径、裂缝发展模式、钢板屈曲特征和破坏模式。分析得到了组合梁在碳氢火灾下的耐火极限,深入揭示了组合梁截面类型和结构体系对组合梁耐火性能的影响机理。试验结果表明:混凝土具有显著的热沉效应,火灾下钢梁的升温速率远快于混凝土板,停火后钢梁温度迅速降低而混凝土板温度持续升高,混凝土板上层的温度在停火48 min后仍然呈走高趋势;碳氢火灾下简支体系钢-混组合梁的挠度从初期就表现出快速增大的趋势,最终因挠度过大而失效;连续体系钢-混组合梁受火跨的挠度在初期增长较为缓慢,最终由于墩顶负弯矩区和跨中正弯矩区均出现塑性铰,梁转为机构体系,使得跨中挠度快速增大而破坏;连续体系钢-混组合梁非受火跨由于变形协调性先上拱,随后由于受火跨刚度衰退转向下挠;闭口截面箱梁仅外表面受火,其耐火性能显著优于双肋工字形钢截面梁,在相似荷载水平下其耐火极限分别为48 min和42 min;连续体系钢-混组合梁由于多余约束的存在,从受火开始就发生剧烈的内力重分布和变形协调,相较于简支梁,其耐火极限可提高100%;高温下连续体系钢-混组合梁出现的塑性铰与常温下的不同,是一种刚度逐渐降低的时变塑性铰。研究成果可为钢结构桥梁的耐火试验方法提供指导依据,也可为其抗火设计方法奠定理论基础。

Experimental Study on Failure of Steel-concrete Composite Bridge Girders Under Hydrocarbon Fire Exposure Conditions

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.