LOAD DISTRIBUTION FACTORS FOR SKEW STEEL CONCRETE COMPOSITE BRIDGES

Abstract

It is more efficient to design bridges with skewed geometries in urban areas due to the lack of space required for more traditional right bridges. In addition, skewed bridges are common at highway interchanges, river crossings, and other extreme grade changes where skewed geometries are necessary due to limitations in space. The transfer of load from concrete slab to steel girder in a skew steel concrete composite bridge is a complex three dimensional phenomenon. Methods of analysis for this case include finite element analysis, grillage analysis and load distribution factor method. Even though Finite element analysis. is considered to be an accurate method, it requires much effort in data preparation, bridge modelling and analysis, and interpretation of results. And so it cannot be used as a practical tool for the routine design problem. The load distribution factor given by most codes of practice simplifies the analysis and design of bridges. These factors allow the design engineer to consider the transverse effect of wheel loads in determining the shear and moment in girders under the longitudinal as well transverse placement of live loads. As per load distribution method, maximum shear and moment in bridge are obtained firstly as if the wheel loads are applied directly to the bridge as a single. beam. Using the load distribution factors, design shear forces and moments in individual girders of the bridge are obtained. However, the use of inappropriate load distribution factors may lead to extremely conservative or unsafe design moments. The method given by AASHTO is too conservative for skew bridges, since the method does not consider the reduction in girder moments due to skew. Conversely, AASHTO LRFD equation produces accurate results, but it is considered to be cumbersome in practice. Several researchers have investigated the load distribution in skew bridges and presented empirical formulae for moment distribution factors for bridges subjected to AASHTO truck loading using different method of analysis. But only a few studies have been done on skew composite bridges subjected to IRC load and dead load. The main aim of this work is to conduct a parametric study to examine the key parameters that may influence the load distribution characteristics of a skew steel-concrete composite bridge. Three dimensional finite element modelling, using "ANSYS" software, has been used for these load distribution analyses. An extensive parametric study is done, in which 35 skew steel-concrete composite bridge models are analyzed to evaluate their load distribution factors for moment and shear under dead load and IRC live load conditions. Th'e key parameters ii considered in the study are the span length (20m, 30m and 40m), skew angle (0 - 60 deg), number of main girders (3, 4 and 5) and orientation of cross bracings. Based on the parametric study, moment and shear distribution factors are deduced for skew composite. bridges subjected to IRC loading as well as dead load. Results from publish