Plastic Strength of Hybrid Steel Beams subjected to Bending and Axial Thrust

Abstract

In responce to the recent tendency of building extremely large vessels, partial applications of high strength steels to main hull constructions of ore-carriers have been attempted for their weight-saving purpose. In this paper, an analysis was made on the plastic moment of hybrid steel beams consisting of high strength steel flanges and/or plate connected by a mild steel web, to estimate the ultimate strength of such structures as deck plating or bottom shell plating together with the longitudinal beams or girders, which are subjected to bending caused by lateral loads as well as the axial thrust due to longitudinal bending of the ship. Interaction curves for axial thrust vs. plastic moment relations were first prepared for the design use.Supplemental tests were then made on both a mild steel and a heat-treated high strength steel, in order to examine the influence of the strain rate and the thermal effects of welding upon the yield strength of the steels.Having obtained the fundamental data on the mechanical properties of the steel in the inelastic range of the material, experimental tests were carried out on hybrid steel beam specimens of I-section and of stiffened plate-sections, which were subjected to axial thrust and lateral loads in a multi-loading type testing machine.It was confirmed from the test results that the interaction relation between axial thrust and plastic moment of hybrid steel beams, which has been obtained by the simple plastic analysis, is generally valid. Also found is the fact that, when the axial thrust is small the plastic moment of the hybrid steel beams could be remarkably increased by using a high strength steel flange in the stiffener, while in the case of large axial thrust acted in the beam, high strength steel plate should be arranged in the plating in order to sustain a large amount of the plastic moment.