Design and behavior of high performance steel I-girders with composite webs

This research is related to a project entitled "Innovative Bridge Designs Using Enhanced Perfonnance Steels" sponsored by the Federal Highway Administration. The objective of the project is to investigate the feasibility of using high perfonnance steel in highway bridges. This thesis investigates the design and behavior of high perfonnance steel I-section girders with composite webs. The composite web, composed of steel face plates and a core material, is expected to overcome web stability, deflection, and fatigue design limits which pose barriers to fully utilizing high perfonnance steel in conventional web I-girder highway bridges. The potential advantages for high perfonnance steel I-girders with composite webs are determined through a study which compares minimum weight composite web girder designs with minimum weight conventional web girder designs. Minimum weight designs were produced according to AASHTO LRFD bridge design specifications. The study indicates that composite web girder designs are lighter than conventional web girder designs, because a composite web girder can fully utilize the strength of high perfonnance steel up to a yield strength of 120 ksi (825 MPa). In addition, calculated deflections of composite web girder bridges are less than those for conventional web girder bridges. Design criteria are established for a composite web girder to suppress three modes of web instability: vertical buckling, bend bucking, and shear buckling. The criteria enable a composite web girder to be designed to suppress web buckling without using excessive steel in the web. In the thesis, the design criteria are used to design a laboratory test specimen, which was tested to verify the criteria and the overall composite web design concept. To build this specimen, composite web core materials and fabrication procedures are studied and selected. In conclusion, high perfonnance steel I-girders with composite webs have potential advantages when used in highway bridges. Composite web I-girders can reduce the amount of steel in the web by overcoming design limits associated with web stability, deflection, and fatigue design limits. However, to make this innovation feasible, economical core materials and fabrication procedures are needed.