Design requirements for longitudinal stiffeners for horizontally curved box girders

Continuous curved box girders are frequently used in horizontally curved highway bridges today. The bottom flange of a box girder is subject to compression in the negative moment zones at interior piers. The steel box girders are built up with thin walled components for flanges and webs. Whenever thin-walled elements are subjected to compression, the possibility of local buckling must be carefully examined as it significantly reduces the load carrying capacity of a box girder. The buckling strength of the thin-walled plate element can be increased by the addition of longitudinal stiffeners.; Current AASHTO Specifications present design rules for the longitudinal stiffeners that yield an impractical and unreliable design. An extensive literature review revealed that a simple yet reliable design rule could be developed for the design of longitudinally stiffened flanges of steel box girders.; A straight stiffened plate model was selected first to have the simply supported boundary condition and to be uniformly compressed. The effect of the bending rigidity of the longitudinal stiffeners on the buckling strength of the straight models was characterized through a series of parametric studies and a regression equation for the minimum required moment of inertia of the longitudinal stiffeners was derived. The regression equation thus obtained contains all relevant variables that are reflected in a simplified theoretical equation.; The subtended angles in a practical curved box girder are expected to be very small in order to satisfy the requirement of limiting the magnitude of the distortional stress to be less than 10 percent of the vertical bending stress. It is, therefore, expected that the derived equation for the minimum rigidity of the longitudinal stiffeners for the straight girder can be applied to the horizontally curved girder. In order to validate this hypothesis, a series of parametric studies were conducted on three dimensional finite element models of the entire curved box girders. Results of these numerical studies confirmed that the minimum required stiffener rigidity is relatively unaffected by the curvature within the small subtended angle.; The inelastic buckling strength of the longitudinally stiffened flange and the minimum required stiffener rigidity in the inelastic transition zone were examined by incremental nonlinear analysis, taking into account the material and geometric nonlinearities and the initial out-of-flatnesses. The proposed equation for the minimum required stiffener rigidity in the elastic range was shown to be sufficiently valid in the inelastic transition zone. An incremental nonlinear analysis of a series of horizontally curved box girder models also showed that a parabolic transition curve could be used to predict the inelastic buckling strength of horizontally curved box girder flanges.