钢桥面板U肋嵌补段对接焊缝在车轮荷载反复作用下容易产生疲劳裂纹；这是钢箱梁典型的疲劳细节之一，直接影响桥梁结构的安全运营和耐久性能。为探明正交异性钢桥面板顶板U肋对接焊缝的疲劳开裂特性，采用有限元模拟建立钢桥面板局部模型，研究了嵌补段U肋对接焊缝的疲劳受力特征；然后设计4个足尺单U肋试件，并结合疲劳试验开展实际结构的疲劳性能分析。在此基础上，通过结构应力法提出适用于预测U肋对接焊缝疲劳寿命的修正主S-N曲线，并基于扩展有限元法（XFEM）探究该细节的疲劳裂纹扩展行为。研究结果表明：轮载作用下U肋对接焊缝应力纵向影响范围为2个横隔板间距，横向影响范围为1.5个U肋间距。圆弧过渡区域所受疲劳应力幅最大，应力集中显著，是潜在的疲劳易损点。试验观测到的疲劳裂纹均起裂于圆弧过渡处，并向纵肋底缘和腹板继续扩展。基于名义应力法评估得到对接焊缝平均疲劳强度为68 MPa，接近于欧洲规范规定的71 MPa等级。与基于等效结构应力法提出的主S-N曲线相比，该研究提出的修正主S-N曲线对预测疲劳寿命较为安全保守。采用扩展有限元法可有效模拟U肋对接焊缝的扩展行为。疲劳试验和扩展有限元均表明疲劳裂纹扩展方向取决于初始缺陷的位置。当初始缺陷出现在底板时，疲劳裂纹易于沿底板方向扩展；反之当初始缺陷位于腹板时，疲劳裂纹则易于沿腹板方向扩展。

As one of the typical fatigue details of the steel box girders, the U-rib butt weld in the embedded sections of the steel bridge decks is prone to fatigue cracking under the repeated action of the wheel load, and it directly affects the safe operation and durability of the bridge structures. In order to explore the fatigue cracking characteristics of U-rib butt weld on the roof of the orthotropic steel bridge decks, this paper first established a local model of the steel bridge deck using finite element simulation, and studied the fatigue stress characteristics of the U-rib butt welds in the embedded sections. Then it designed four full-scale single U-rib specimens and carried out a fatigue performance analysis of the actual structure combing with fatigue test. On this basis, the modified main S-N curve suitable for fatigue life prediction of U-rib butt welds was proposed by the structural stress method, and the fatigue crack propagation behavior of weld detail was explored based on the extended finite element method (XFEM). The results show that the longitudinal influence range of U-rib butt weld stress under the wheel load is two cross-spacer spacing, and the lateral influence range is 1.5 U-rib spacing. The arc transition area is subjected to the greatest fatigue stress amplitude and high stress concentration, which becomes a potential fatigue vulnerable point. The fatigue cracks observed in the experiment all start at the arc transition and continue to extend to the bottom edge of the longitudinal rib and the web. Based on the evaluation of the nominal stress method, the average fatigue strength of the welds is 68 MPa, which is close to the 71 MPa specified in the European Code. Compared with the main S-N curve proposed based on the equivalent structural stress method, the modified main S-N curve proposed in this research is safer and more conservative for fatigue life prediction. The extended finite element method can effectively simulate the expansion behavior of U-rib butt welds. Both fatigue tests and XFEM results indicate that the fatigue crack propagation direction depends on the location of the initial defect. When the initial defect appears on the bottom plate, fatigue cracks tend to extend along the bottom plate; but when the initial defect is located on the web plate, fatigue cracks tend to extend along the web.