عنوان مقاله [English]
The interaction of flexural moment and shear force in the eccentrically braced frames with intermediate link length is a major concern of the structural analysis and design. In eccentrically braced frames, if the link length is long, the flexural yielding dominates the inelastic response of the link. In case the link length is short, the shear yielding is dominant. For intermediate link length the inelastic response is controlled by the combination of flexural and shear yielding. Steel frames with intermediate beam length behave Like the links of eccentrically braced frames. Therefore, the interaction of flexural moment and shear force shall be considered in analysis of both intermediate link length and steel frames with intermediate beam length.
For design of steel I-sections with intermediate length, a yield surface considering moment-shear interaction is required for calculating the ultimate capacity. Nevertheless, for capturing the complete force-deformation curve by a nonlinear step-by-step analysis, a plastic flow rule is required in addition to the yield surface. In this paper, to investigate the plastic flow rule for the highly ductile steel I-sections, a study is carried out using nonlinear finite element analysis. The finite element analysis is verified using some available experimental results in the literature. Then, shear and moment deformations are calculated by using Timoshenko beam theory. Investigating the results of finite element analyses, a simple and practical non-associated plastic flow rule is proposed for the highly ductile steel I-sections. Finally, to show the applicability of the proposed plastic flow rule, an incremental (step-by-step) analysis is performed on a beam with intermediate length in which a mixed hinge is formed. This example is solved two times using both the associated and non-associated plastic flow rule. In this example, the load-deflection path resulting from the proposed flow rule is up to 7.5 percent less than the case without interaction. Results of this example indicate that the proposed non-associated flow rule leads to a softer force-deformation path and a larger ultimate deformation prior to the collapse. In general, the effect of flow rule on load-deflection path on structures is different case to case and cannot be ignored in some cases.