Stability Evaluation of Steel Girder Members in Long-Span Cable-Stayed Bridges by Member-Based Stability Concept

The member-based design concept utilizing the buckling length of each structural member has been widely used to assess the buckling instability of steel structures. Since steel girder members in conventional cable-stayed bridges are generally exposed to large axial forces, the buckling instability of these members should be carefully investigated in the design stage. However, analytical approaches for obtaining the buckling lengths of steel members, such as the alignment chart, story-buckling and story-stiffness methods, may not be adopted to cable-stayed bridges because these approaches imply some theoretical assumptions that are adequate only for steel framed-structures. Furthermore, the boundary conditions of steel girder members supporting by cables are obscure to be prescribed in general terms. Numerical eigenvalue analysis may be one of the most excellent candidates for determining the buckling lengths of steel girder members in that this method can handle the interactions among members implicitly without any irrelevant assumptions for cable-stayed bridges. This paper discusses detailed procedures for obtaining buckling lengths of steel girder members in cable-stayed bridges by numerical eigenvalue analysis. In order to avoid the problem of generating excessively large buckling lengths in some girder members having small axial forces, a modified eigenvalue analysis is proposed by introducing the concept of a fictitious axial force. Practical application example for a real cable-stayed bridge is illustrated with some discussions on the effect of the proposed modification and stability evaluation by member-based stability concept.