考虑平均应力效应的在役公路钢箱梁桥多轴疲劳评估方法

Fatigue cracking is concerned by field of bridge engineering recently. Fatigue assessment of in-service large bridge has also been one of critical issues by utilizing monitoring data. However, the mean stress effect and multi-axial stress feature have not been taken account among existing fatigue assessment methods of steel box-girder of in-service highway bridges, which is hard to consider the mean stress effect led by temperature and massive bridge structure. Thus, the fatigue assessment results are possibly inconsistent with actual situation.. Based on the long-term monitoring data of steel box-girder of in-service highway bridges, multi-axial fatigue assessment method is to be investigated in this project integrating the mean stress effect. The main research contents are presented as follows : 1) welding residual stress test, hot-solid coupling simulation and multi-scale structure stress analysis are conducted to establish the computation method of weld true stress spectrum; 2) the multi-axial feature of weld true mean stress and true stress amplitude is investigated on the basis of actual monitoring data to establish the multi-axial fatigue strength model for steel box-girder of highway bridges integrating the mean stress effect; 3) computation method of multi-axial fatigue reliability degree is presented for steel box-girder of in-service highway bridges integrating the mean stress effect, on the basis of which the influence of temperature and massive bridge structure on fatigue reliability is investigated to establish the multi-axial fatigue reliability assessment method for steel box-girder of in-service highway bridges integrating the mean stress effect.

公路钢箱梁桥疲劳开裂是桥梁工程界关注的热点问题。利用长期监测数据开展疲劳评估是大跨桥梁服役性能评估的重要研究方向。然而，已有的在役公路钢箱梁桥疲劳评估方法尚未考虑平均应力和多轴应力特征的影响，难以准确把握因温度和整体桥型差异导致的疲劳平均应力效应，可能造成疲劳评估结果与实际偏差较大。.本项目基于公路钢箱梁桥的长期监测数据研究考虑平均应力效应的多轴疲劳评估方法。包括：1）基于残余应力测试、焊接热-固仿真模拟和多尺度结构应力分析研究焊缝残余应力与结构应力的耦合机理，建立钢箱梁桥焊缝真实应力谱的计算方法；2）通过研究基于监测数据的焊缝真实平均应力和应力幅的多轴特征，建立考虑平均应力效应的公路钢箱梁桥多轴疲劳强度模型；3）研究考虑平均应力效应的钢箱梁桥多轴疲劳可靠度计算方法，考察温度和整体桥型对疲劳可靠度的影响规律，建立考虑平均应力效应的在役公路钢箱梁桥多轴疲劳可靠度评估方法。

通过建立焊接残余应力与车载应力的耦合应力分析模型，构建了焊接残余应力和车辆荷载耦合作用下应力比、等效应力幅等疲劳参数计算方法，最终形成了焊接残余应力与车载应力的耦合应力精细化计算方法。以江阴长江大桥为例，开展了车辆荷载和残余应力场对疲劳寿命的定性分析。分析表明：焊缝位置残余拉应力在叠加了以拉应力为主的循环车载应力后，纵、横向应力松弛大小均超过车载应力峰值，出现明显的应力松弛现象，而叠加以压应力为主的循环车载应力后，应力松弛效应并不明显；与仅考虑车载应力作用下焊缝位置应力状态相比，考虑焊接残余应力和车载应力耦合作用之后，压应力循环工况焊缝位置疲劳应力状态发生了本质变化，即由不需要进行疲劳验算的压应力状态变为拉应力状态；拉应力循环工况的疲劳状态虽未改变，但无论是受压工况还是受拉工况焊缝位置的疲劳寿命均会产生不同程度的降低。.采用Soderberg公式对焊缝处应力幅和平均应力进行等寿命的转换，建立考虑焊接残余应力影响的钢材疲劳抗力的全空间S-N曲线模型，并以世界最大跨度六线钢桁拱桥——南京大胜关大桥为例详细介绍该模型的疲劳评估步骤。首先，采用有限元数值分析方法模拟得到大胜关大桥顶板-纵肋局部焊接细节的残余应力分布，并通过试验验证其准确性。进而将局部焊接细节模型嵌入到钢桥面板整体多尺度有限元模型中，考虑焊接残余应力与列车荷载耦合效应，计算得到钢桥面板焊接细节在客/货车作用下的典型结构应力谱与真实应力谱。最终，采用雨流计数法对真实应力时程进行分析，并根据该桥2015年实际通车量，通过本文建立的考虑焊接残余应力影响的全空间S-N曲线模型，评估了该桥钢桥面焊接细节的疲劳寿命。分析结果表明：顶板上、下表面垂直于焊缝方向的纵向残余应力分别约为0.52fy（屈服强度）和1.04fy; 顶板上、下表面平行于焊缝方向的σZ分别约为0.47fy和0.96fy；在残余应力与车辆荷载耦合作用下的真实应力极值依然低于屈服应力，不能忽略平均应力对焊接细节疲劳寿命的影响；与欧三规范相比，项目提出的评估方法得到的疲劳寿命更为准确，为大胜关大桥能否通行货运列车提供了计算依据。

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