Study on Ultimate Limit State Design Method for Horizontally Curved Girder Bridges from Viewpoint of Fatigue Strength

本研究以曲线梁桥的疲劳性能为研究对象，从1988年到1989年的两年时间里，对曲线梁桥基于疲劳强度的极限状态设计方法进行了研究，但尚未得到广泛的研究。在曲线梁桥的疲劳强度评估中，由于桥梁轴线曲率的影响，给桥面横梁、横撑等部位的应力估算带来了许多难题。在这些问题中，曲线腹板平面外弯曲引起的疲劳强度，即所谓的“腹板呼吸”，通过制作5个独立的曲线工字梁试件，在1988年第一年进行了检验。从分析结果来看，…提出了曲腹板平面外弯曲应力的近似计算公式，并用试验结果验证了该公式的正确性。疲劳试验表明，翼缘和腹板的焊接部位在循环次数超过N=2×10~(-6)后仍未发现裂纹，应力范围几乎等于母材的许用单位应力。因此，在腹板设计为抗屈曲安全的情况下，疲劳校核是没有必要的。1989年，制造了一个由两根主梁和横向支撑连接的曲线型工字梁，研究了主梁和次梁交界处的肋板和节点板的静力性能以及应力集中情况。试验结果从不同角度进行了检验，并与有限元分析的结果进行了比较，有限元分析是本系列研究的一部分。通过静载试验，对模型梁进行了疲劳破坏试验。仔细观察了肋板和节点板疲劳裂纹的位置、加载循环次数以及疲劳裂纹的扩展情况。最后，对限幅梁桥的疲劳强度进行了讨论，并与中国船级社提出的S-N曲线进行了比较。这些试验结果为今后曲线梁桥抗疲劳安全设计提供了有价值的数据。较少

This study was focused on the fatigue behaviors of curved girder bridges and attempted to find a method for developing a limit state design method, based on fatigue strength, for curved girder bridges during two years, from 1988 until 1989, since this subject has not yet been researched extensively. In evaluating the fatigue strength of curved girder bridges, there are a lot of difficult problems, caused by the curvature of the bridge axis, to estimate the stress in floor beams, lateral bracings and so on. Thus, the induced stress is somewhat different from that of ordinary straight girder bridges and there are no distinctions between primary and secondary stress in the curved girder bridges, which yields complicated 3-dimensional state of stress.Among these problems, the fatigue strength due to out-of-plane bending of curved web plate,the so-called "web breathing", was examined by fabricating five isolated curved I-girder specimens in the first year, 1988. From the analytical results, … More an approximate formula to estimate the out-of-plane bending stress in curved web plate was proposed and its validity was checked by means of the test results. Moreover, it was shown from the fatigue tests that the cracks were not found at the welded portion of flange and web plates even af ter the number of cycles exceeded N=2xlO^6 with the stress range almost nearly equal to the allowable unit stress of their base metal. Therefore, it was concluded that the fatigue check is not necessary in case where the web plate is designed safely against buckling.In the second year, 1989, a curved multiple I-girder with two main girders connected by floor beams and lateral bracings were fabricated to investigate the static behaviors, as well as stress concentrations of the rib and gusset plates at the junctions between main girders and secondary members. The test results were examined from various points of view and compared with the results of the FEM analysis, which was derived as a part of this series of study. After these static loading tests, the model girder was tested up to the fatigue failure. The locations, the number of loading cycles and the propagation of fatigue cracks in the rib and gusset plates were carefully observed. Finally, the fatigue strength of curbed girder bridges was discussed and compared with the S-N curve proposed by JSSC. These experimental results offered valuable data for designing the curved girder bridges safely against fatigue in future. Less