Ultimate flexural strength of hybrid composite girders using high-performance steel of HSB600 at sagging bending

HSB600 is a high performance steel which is currently developed by a Korean steel company. HSB600 does not have an obvious yield point and yet undergoes large strains after proportional limit. Its 0.2% offset yield strength and tensile strength are over 450 MPa and 600 MPa respectively. Modern standards such as AASHTO LRFD and Eurocode 4 provide rules for determining the flexural strength of composite girders. These rules are based on the presence of a yield point in the stress-strain diagrams of steel materials. This paper presents experimental works conducted for the purpose of verifying the ultimate flexural strength of hybrid composite girders using the tension flange made of HSB600. Totally five composite girder specimens (one hybrid and a conventional composite girders (Series I), three hybrid composite girders (Series II)) have been tested. The results of the test are compared with the flexural strengths obtained by nonlinear moment-curvature analysis based on the stress-strain curves of each material and the code predictions. The test results show that, if the plastic moment is calculated with using 0.2% offset yield strength, the ultimate flexural strength is underestimated. Finally, based on the test results, some design recommendations regarding the flexural strength and the ductility requirement for hybrid composite girders with HSB600 are suggested.