LOAD DISTRIBUTION FACTORS FOR COMPOSITE MULTICELL BOX GIRDER BRIDGES

Abstract

The use of multicell box girders in bridge deck construction can lead to considerable economy. This type of construction leads to an efficient transverse load distribution due to excellent torsional stiffness of the section. Further utilities and services can be readily provided within the cells. In India, there are no design codes or guidelines for composite box girder bridge design. So, Indian designers are forced to take recourse to state-of-art research or codes of practice of European or North American Countries. Therefore the present study aims to undertake some aspects of the composite box girder bridges, specifically the composite multi-cell box girders. The current design practices in North America recommend few analytical methods of composite multicell box girder bridges. Box-girder analysis and design should take into consideration stresses due to longitudinal bending moment, shear force, torsion, distortion, shear lag and transverse bending. The instability of flange under compression and web under shear and plates and stiffened panels is also to be considered. Among the refined methods, the FEM is still the most general and comprehensive technique of analysis capturing all aspects affecting the structural response. But it is too involved and time consuming to be used for routine design purpose. Practical requirement in the design process necessitate a need for a simpler design method. This thesis work presents an extensive parametric study using the Finite element method in which nine, two-lane bridges of various geometries has been analysed. The parameters considered are number of cells, span to depth ratio, and cross bracings. Results from published literature are used to substantiate the analytical modeling. Based on the parametric study, moment and shear distribution factors are deduced for such bridges subjected to IRC loadings as well as dead load. Recommendations for optimum spacing of cross bracings has been suggested. An illustrative design example is presented.