Load Distribution Factors for Composite Multicell Box Girder Bridges

Composite steel-concrete multicell box girder bridges combine excellent torsional stiffness with elegance. While the current design practices in North America recommend few analytical methods for the design of composite multicell box girder bridges, practical requirements in the design process necessitate a need for a simpler design method. This paper presents an extensive parametric study—using the finite-element method—in which 120 bridges of various geometries were analyzed. The parameters considered are: number of cells, number of lanes, span length, and cross bracings. Results from testing a simply supported three-cell bridge model is used to substantiate the analytical modeling. Based on the parametric study, moment and shear distribution factors are deduced for such bridges subjected to AASHTO truck loadings as well as dead load. Saint-Venant torsional stiffness for composite cellular cross sections used in this study is also investigated. Recommendations to enhance the torsional stiffness are formulated. An illustrative design example is presented.