基于能量法的组合梁桥拥堵车辆怠速振动所致疲劳损伤评估

针对拥堵车辆怠速振动造成的城市桥梁疲劳失效风险，利用ABAQUS建立车辆拥堵荷载作用下的车-桥耦合有限元模型，通过动力响应分析和应力循环计数，采用能量法以总应变能密度为损伤参量，计算4种拥堵工况下城市组合梁桥钢箱构造细节的疲劳累积损伤，并通过与规范标准疲劳车畅通运行状态时的损伤对比，综合评估车辆拥堵对城市组合箱梁桥疲劳性能的影响。结果表明：城市组合梁桥在车辆拥堵荷载作用下弹性应变能在总应变能密度值中占主导地位，其疲劳行为是典型的高周疲劳，疲劳损伤主要来自于怠速振动初始的3~5个高应力幅循环；钢箱底板-腹板焊缝构造细节的疲劳累积损伤为母材的2~5倍，焊缝是疲劳性能较薄弱的部位；当典型拥堵车流只包含小汽车及公交车时，拥堵状态下的疲劳损伤可偏于安全地采用1辆标准疲劳车怠速振动进行评估，但在某些未限制货车通行路段，车辆拥堵造成的疲劳损伤显著增大；由于发动机怠速振动动力效应致使等效应力幅增大65%，1辆标准疲劳车1次拥堵作用下的疲劳损伤是畅通状态下疲劳车单次过桥产生损伤的14.7倍，构造细节疲劳破坏风险增大；考虑车辆拥堵影响后疲劳累积总损伤增大，疲劳损伤受拥堵工况、交通量及拥堵时长影响，日均拥堵4 h情况下，考虑拥堵后的疲劳损伤可达不考虑时的3.5倍。该文可为类似桥梁疲劳评估提供借鉴参考，为保障既有城市钢桥的安全运营提供一定的基础理论支撑。

Evaluation of Fatigue Damage of Composite Bridges Caused by Congested Vehicle Idle Vibration Based on Energy Approach

Urban composite bridges often experience fatigue failure risk induced by congested vehicle idle vibration. To address this problem, ABAQUS was adopted to establish a vehicle-bridge coupled finite element model under congested vehicle loading, along with dynamic response analysis and stress cycle counting. Regarding the total strain energy density as a damage parameter, the cumulative fatigue damage of structural details in the steel box girder of an urban composite bridge under four congestion load cases was calculated relative to the damage caused by a standard fatigue vehicle in an unobstructed state. The influence of vehicle congestion on the fatigue performance of an urban composite box girder bridge was comprehensively evaluated. The results demonstrate that elastic strain energy substantially contributes to the total strain energy density of urban composite girder bridges under vehicle congestion loading; hence, the fatigue behavior is typical high cycle fatigue, whereas fatigue damage is mainly cumulated within 3-5 high stress range cycles at the onset of idle vibration. Accumulated fatigue damage to the structural detail at the bottom flange-to-web weldment is 2-5 times that to the base metal, the fatigue performance of the weldment is therefore more vulnerable. When typical congested traffic flow contains only cars and buses, fatigue damage can be safely evaluated by the idle vibration of a standard fatigue vehicle; however, in some unrestricted road sections, fatigue damage increases significantly due to truck congestion. Since the dynamic effect of engine idle vibration increases the equivalent stress amplitude by 65%, fatigue damage caused by a standard fatigue vehicle under single congestion is 14.7 times that caused by single crossing under unobstructed conditions, thus increasing the risk of fatigue damage of structural details. Cumulative fatigue damage increases with the influence of vehicle congestion, which is affected by congestion load case, traffic volume, and congestion duration. When daily congestion lasts for 4 h, fatigue damage owing to congestion can reach 3.5 times that without congestion. This study can provide a reference for the fatigue assessment of similar bridges and basic theoretical support for the safe operation of existing urban steel bridges.