Optimum cost design of steel box-girder by varying plate thickness

Abstract

In recent years, steel box-girders have been used extensively and especially for long span due to its high torsional rigidity and rapid erection. In order to decrease the amount of costly steel material, the variations in thickness of web, top flange and bottom flange are always considered in cross sectional design. This design depends on the notable changing of the moment distribution along the length. Variation in plate thicknesses may represent a girder with the least weight; however, this may not be the most economical girder due to the increase in fabrication costs, which are associated with excessive variations in plate thicknesses. A parametric study is carried out to determine the optimum cost design of changing thickness of continuous beam over equal span. The objective function includes material and welding fabrication cost. The parameters varied are the length of span, the loading and the number of segments in each span. Both closed rectangular and open trapezoidal sections are considered. Finally, the relationship between cross-section dimensions of steel box girders is defined. The recommended design guidelines can be useful to the designer in the first stage of the designing procedure when he faces the problem of defining basic structure dimensions that would be close to the optimum ones.