复杂环境下连续弯钢箱梁耐火性能提升方法

交通类火灾严重威胁钢结构桥梁的耐久性和安全性。为提升复杂环境(开放火灾和弯桥荷载)下连续弯钢箱梁的耐火性能,增强钢结构桥梁的安全服役寿命,选取大型立交桥枢纽工程中两跨连续弯钢箱梁为研究对象,通过建立耐火试验验证的钢箱梁与混凝土刚性基层协同工作的数值预测模型,深入揭示开放环境碳氢火灾下传热模式和结构特征耦合的箱梁力学行为演化规律。研究了局部环境火灾作用下结构的高温响应与失效模式,分析了复杂荷载状况、弯曲半径与支座布置方式对连续弯钢箱梁火灾响应行为的影响,提出了复杂环境下连续弯钢箱梁的耐火性能提升方法。研究结果表明:连续弯钢箱梁在火灾下的内外侧挠度差值不断增大,主梁内外侧支座反力的变化呈相反趋势,并且在受火初期支座反力变化程度剧烈;受火区域边缘靠近中支点的底板与腹板严重屈曲从而先形成塑性铰,然后在受火跨跨中形成塑性铰,随即整跨结构发生突然性垮塌;荷载水平的增大会显著缩短其耐火极限,受火前期及时撤离桥上的车辆荷载能够有效地延缓变形发展并且避免结构的突然性垮塌;曲率半径小于200 m会显著加剧连续弯钢箱梁高温下的弯扭耦合效应,增大主梁内外侧挠度差值与内外侧支座反力变化幅度,削弱火灾下结构的整体稳定性能;在钢结构桥梁抗火设计时中支点应设置抗扭支座,常温下支座的布置方式对火灾下连续弯钢箱梁的支座受力状况改善甚微,应在支座与梁端附近增设外部限位装置以防止结构变形过大。研究结论可为提升复杂环境下钢结构桥梁抵抗火灾的能力以及增强安全服役寿命提供设计依据。

Methods for Improving Fire Resistance of Continuous Curved Steel Box Bridge Girders Exposed to Complex Environments

Traffic fire poses a severe threat to the durability and safety in steel structure bridges. In order to improve fire performance of continuous curved steel box bridge girders exposed to complex environment (open fire scenario and curved bridge loads) and enhance the safe service life of steel structure bridges, a two-span continuous curved steel box bridge girder in large interchange hub engineering was selected as the research object. A validated numerical prediction model considering cooperative work between steel box bridge girder and concrete rigid base was established. The mechanical behavior evolution of box bridge girder is deeply revealed by considering coupling of heat transfer mode and structural characteristics under hydrocarbon fire in open environment. Structural behavior and failure mode of the structure were analyzed under localized environment fire exposure. The influence of complex load conditions, curvature radiuses and support layouts on fire response of continuous curved steel box bridge girders was investigated. The fire performance improvement methods were proposed for continuous curved steel box bridge girders exposed to complex environment. The research results indicate that the deflection difference between inner and outer sides of continuous curved steel box bridge girders exposed to fire increases continuously. The development trend in support reaction of the inner and outer sides of the main bridge girder is opposite during fire exposure, and the degree of change in support reaction is violent at the early stage of fire exposure. Buckling of the bottom flange and web near the central support at the edge of fire exposure exists and a plastic hinge is firstly formed. Subsequently, a plastic hinge at the midspan is formed, and then the whole span under fire exposure collapses suddenly. The fire resistance limit of that can be significantly shortened with the increase of load level, and timely evacuation of vehicle loads on the bridge girder during fire exposure can effectively delay the deflection development and avoid the sudden collapse of the structure. The coupling effect of bending and torsion in continuous curved steel box bridge girders at high temperature will be significantly intensified when the radius of curvature is less than 200 m. The deflection difference between inner and outer sides as well as variation range of inner and outer support reaction is increased, thus weakening the overall stability of the structure exposed to fire. The middle support should be placed as anti-torsion support in practical fire resistance design of steel structure bridges. Different support layout methods at ambient temperature barely improve stress state of supports in continuous curved steel box bridge girders exposed to fire. External limit devices located at supports and the end of girder are supposed to be added in case of excessive structural deformation. The research conclusions can provide design basis for improving the fire resistance of steel structure bridges in complex environment and enhancing the safe service life.