A consideration on intermediate diaphragm spacing in steel box girder bridges with a doubly symmetric section

Diaphragms in box girder bridges are implemented primarily to prevent premature excessive distortional deformation under torsional loading condition. Distortional warping and transverse bending stresses, which are the major stress components resulting from distortion, should be appropriately limited to a specific level for efficient use of the cross-section by installing adequate intermediate diaphragms. The objectives of the present study are to develop a thin-walled box beam finite element and to propose tentative design charts for adequate spacing of intermediate diaphragms. The developed beam element possesses nine degrees of freedom per node and the validity was intensively verified from a series of comparative studies using a conventional shell element. Also, performed herein are extensive parametric studies for continuous box girder bridges of doubly symmetric steel box section. The design parameters taken into account were the desired ratio of the distortional warping normal stress to the bending normal stress, the number of spans, the span length, the aspect ratio of the box section, and the spacing of the intermediate diaphragms. The results were summarized into tentative design charts indicating efficient spacing of intermediate diaphragms for the various stress ratios.