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

本研究集中于曲线梁桥的疲劳性能，并试图找到一种方法，发展极限状态设计方法，基于疲劳强度，曲线梁桥在两年期间，从1988年到1989年，因为这个问题还没有得到广泛的研究。在评估曲线梁桥的疲劳强度时，由于桥轴线曲率的影响，对主梁、横撑等部位的应力估算存在许多困难，因此，曲线梁桥的诱导应力与普通直梁桥的诱导应力有所不同，没有主次应力之分，在这些问题中，1988年第一年，我们制作了五个单独的曲线工字形梁试件，研究了曲线腹板平面外弯曲（即所谓的腹板呼吸）引起的疲劳强度。从分析结果来看， 关于我们 提出了曲腹板面外弯曲应力的近似计算公式，并通过试验结果验证了该公式的正确性。疲劳试验结果表明，即使循环次数超过N= 2 × 10^6，应力范围几乎等于母材的许用单位应力，在翼缘和腹板的焊接部位也未发现裂纹。1989年，我们制作了一个由两个主梁通过楼板梁和横向支撑连接而成的曲线多工字梁，以研究其静力性能，以及主梁和次梁连接处的肋板和节点板的应力集中。试验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，这是作为本系列研究的一部分。在这些静态加载试验之后，对模型梁进行疲劳破坏试验。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。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