Stress Intensity Factors for Structural Steel I-beams
Sanford, Robert J.
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The application of fracture mechanics to highway steel bridges has been hampered by a lack of stress intensity factor (SIF) solutions for cracks in I-beams. Previous work cannot provide satisfactory solutions. In this study, the finite element analysis method was used to develop accurate SIFs for two-tip and three-tip cracks in I-beams under tension or bending. Cracked I-beams were modeled with eight-node shell elements, with the web and flanges being fully joined along the junction lines. The region around the crack tips, singularity quarter-point elements were used. To ensure accurate and converging solutions, mesh patterns around the crack tips were studied. Also, different methods of extracting SIFs from FEA results were discussed based on benchmark problem studies. Governing parameters for cracked I-beams were determined. For two-tip web cracks, the SIFs are functions of applied stress, crack length, eccentricity, and flange-to-web cross-sectional area ratio. For three-tip cracks in web and flange, the SIFs are functions of applied stress, web and flange crack lengths, and flange-to-web cross-sectional area ratio. The flange-to-web area ratio describes the constraining effect of the flange on the web crack of a two-tip cracked I-beam; the interaction forces between web and flanges greatly affect SIFs for a three-tip cracked I-beam. The SIFs were calculated based on a total of 2,106 FEAs performed for a wide range of the parameters. The results were fitted with equations for ready use by practicing engineers. An example illustrates the calculation of SIFs for a three-tip crack in a composite steel-concrete beam of a steel bridge.