This study is aimed to check the effectiveness of U-shaped vertical stiffeners of the steel box girder support diaphragms. The results of the author's previous studies on the steel box girder support diaphragms indicate that such a diaphragm is collapsed by the local deformation with small waves at its bottom part near a shoe, and the torsional local buckling-like behaviour is observed at the bottom of the load bearing stiffeners. To avoid the torsional buckling, the stiffener should have more torsional rigidity such as U-shaped ribs. In this study, numerical analysis is made on the support diaphragms with the load bearing stiffener having U-shaped section to confirm the contribution of the torsional rigidity of the stiffener to the strength and behaviour of the diaphragms, and check the effectiveness of the U-shaped stiffeners. Dimensions and thickness of the U-ribs are adopted as parameters. Through the analysis, it is found that use of a U-rib type stiffener as a load bearing stiffener of a support diaphragm is effective to improve the strength of the diaphragm.
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