车致疲劳损伤对钢-混凝土组合梁桥极限承载力可靠度的影响

钢-混凝土组合梁桥因其发挥了2种材料各自的优势，被广泛应用于中小跨径的桥梁结构中，而极限承载能力是评判其安全与否最直观的指标之一。为了对现役钢-混凝土组合梁桥的极限承载力进行更为准确的评估，提出一种确定钢主梁极限承载能力可靠度的新方法，该方法能考虑车辆荷载引起的疲劳累积损伤对钢主梁极限承载力的影响。首先建立了三维车桥耦合振动模型，并采用美国AASHTO桥梁设计规范中的Ⅰ形简支钢-混凝土组合梁桥、强度设计车辆荷载模型和疲劳设计车辆荷载模型作为算例进行分析。然后，基于建立的车桥耦合振动程序、S-N曲线和雨流计数法，获得不同桥面状态下强度设计车以不同车速过桥时产生的动力冲击系数和疲劳设计车以不同车速过桥时产生的疲劳损伤累积和最大应力，并根据卡方检验对在不同桥面状态和不同车速下获得的这3个参数的分布类型进行检验。最后，基于剩余强度理论，利用AASHTO规范中规定的桥梁承载力设计方程，建立能考虑桥梁全寿命周期内桥面处于不同状态时车辆过桥产生的累积疲劳损伤对钢主梁极限承载能力折减的极限状态方程，并以此对钢主梁极限承载力的可靠指标进行研究，获得其与疲劳设计车日均通行量的关系。研究结果表明：桥梁极限承载力可靠度会随着疲劳设计车日通行量的增大而降低；钢主梁疲劳累积损伤对其极限承载力折减具有重要影响。提出的方法为准确评估在役桥梁的极限承载能力提供了更为有效的途径。

Effect of Vehicle Load-induced Fatigue Damage on the Reliability of Ultimate Load-carrying Capacity of Steel-concrete Composite Girder Bridges

Steel-concrete composite girder bridges are widely used in small and medium span bridge structures owing to the respective advantages of both materials, i.e., steel and concrete. The ultimate bearing capacity is one of the most intuitive indexes to judge the safety of such bridge structures. To assess existing steel-concrete composite girder bridges more accurately, this study proposes a novel approach for determining the reliability index of the ultimate load-carrying capacity (ULCC) of steel-concrete composite girder bridges that can consider the effect of fatigue damage accumulation (FDA) induced by the dynamic vehicle loading on the ULCC of steel-concrete composite girder bridges. First, a three-dimensional bridge-vehicle coupled system (BVCS) was established. The bridge model, the fatigue truck model, and the design truck model adopted from the AASHTO bridge design code were used as examples. Then, based on the established BVCS, the S-N curve and rain-flow counting method under different road surface conditions, the impact factors induced by the strength design truck crossing the bridge with different vehicle speeds, and the fatigue damage and maximum stress induced by the fatigue design truck crossing the bridge at different vehicular speeds were obtained. The distribution patterns of the three parameters obtained under different road surface conditions and vehicle speeds were examined using Chi-square tests. Finally, based on the residual strength theory and the design function of the bridge bearing capacity specified in the AASHTO code, the limit state function (LSF) of the bridge steel girder under consideration was established. This LSF can consider the influence of the FDA caused by the fatigue design truck during the deterioration process of the road surface on the reduction of the ULCC of the bridge girder. Based on the established LSF, the reliability index of the ULCC of the bridge girder was analyzed, and its relationship with the number of daily passages of the fatigue design truck was obtained. Results show that the reliability of the ultimate bearing capacity of the bridge will decrease with the increase of the number of daily passage of the fatigue design truck, and the cumulative fatigue damage of the steel girder has an important impact on the reduction of the bridge ultimate bearing capacity. The proposed method provides a more effective and accurate assessment of the ULCC for in-service bridges.