A Theoretical Study on Distortion Induced Fatigue of Slender Web Curved I-girders subjected to Pure Bending

The slender web of I-girder bridges subjected to pure bending undergoes large deformations at the compressive region. This can occur at load levels less than the theoretical bend buckling limit due to the presence of initial geometric imperfections. Secondary bending stresses amplify the web stress upon receiving additional loads, i.e., traffic load. While the total web stress does not affect the resistance significantly, the cyclic component of the stress can lead to fatigue cracks at the web-to-flange connection area. The phenomenon, which is referred to as web breathing, has been studied comprehensively for straight girder. However, there is a lack of knowledge on the breathing of curved web panels. The non-colinear internal forces of curved girders causes web lateral distortions and high membrane stresses that amplifies the web breathing compared to straight girders. Theoretical studies associated with the curved web can be divided into two categories. First, very limited research investigated the curvature effect at web panel boundaries that is critical for the fatigue limit state. The simplified methods, based on beam theory, are not capable of properly modeling the actual behavior of curved girders having slender webs. Second, the more accurate analytical models, based on plate theory, were conducted for establishing the ultimate web stress and lateral displacements corresponding to strength and stability limit state, respectively. This paper reviews the analytical approaches related to the breathing of straight girders and presents a theoretical method for defining the web stresses at the web panel boundaries under pure bending moment.

Learning Objectives:

• "web breathing" definition in slender I-girder bridges
• The cyclic out-of-plane web deformation due to buckling, results in stress concentrations at the intersections of welded elements such as stiffeners and flanges. This phenomenon is called "web breathing" and results in fatigue cracks at the web panel boundaries.

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