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行为为研究对象，从1988年到1989年的两年时间里，由于这一课题尚未得到广泛的研究，本研究试图寻找一种基于疲劳强度的曲线梁桥极限状态设计方法的发展方法。在曲线梁桥的疲劳强度评估中，由于桥轴线的曲率，在底板梁、侧支撑等处的应力估计存在很多难题。因此，弯梁桥的诱导应力与普通直梁桥的诱导应力有所不同，且不存在主应力和次应力的区别，从而产生复杂的三维应力状态。其中，在1988年的第一年，通过制作5个独立的弯曲工字梁试件，对弯曲腹板的面外弯曲（即所谓的“腹板呼吸”）引起的疲劳强度进行了研究。在分析结果的基础上，提出了估算弯腹板面外弯曲应力的近似公式，并通过试验结果验证了公式的有效性。此外，疲劳试验表明，即使在循环次数超过N=2xlO^6的情况下，法兰和腹板的焊接部分也没有出现裂纹，应力范围几乎等于母材的允许单位应力。因此，结论是，如果腹板被设计为安全的抗屈曲，则无需进行疲劳校核。第二年，1989年，制作了一个弯曲的多重工字梁，两个主梁由楼板和侧支撑连接，以研究静力行为，以及主梁和次要构件之间连接处的肋板和扣板的应力集中。测试结果从不同的角度进行了检查，并与有限元分析的结果进行了比较，这是作为这一系列研究的一部分导出的。静载试验结束后，对模型梁进行了疲劳破坏试验。仔细观察了肋板和扣板疲劳裂纹的位置、加载循环次数和扩展情况。最后，对约束梁桥的疲劳强度进行了讨论，并与JSSC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的抗疲劳安全设计提供了有价值的数据。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