Parametric study of design variations on the vibration modes of box girder bridges

Abstract

Under a moving load, a bridge undergoes deflections and stresses which significantly exceed those caused by the same load when applied statically. These dynamic deflections can cause discomfort to pedestrians using the bridge, since human response is sensitive to accelerations and to the rate of change of acceleration. Another factor affecting human response is the type of vibration in the dominant mode. People tend to react more adversely to torsional modes of vibration than to flexural modes. This paper summarizes the results of an extensive analytical investigation, which was conducted to identify key parameters affecting the changes of dominant mode types and to study the influence of diaphragms and cross bracings on dynamic responses of a twin box girder bridge. The finite element method was used to carry out these analyses. The analytical results indicate that the provision of diaphragms within the boxes at each support, or of a cross-bracing system in-between boxes, can effectively reduce peak accelerations when the dominant mode of vibration is torsional. As a result, the human discomfort and perception to vibration can be improved. Key words: vibration, human response, torsion, flexure, box girder bridge, diaphragm, cross bracing, finite element analysis.