Research On Ultimate State Of H-section Steel Members Subjected To Combined Biaxial Bending And Axial Force Under Different Loading Paths

Slender H-section steel members with large width-thickness ratio plates have a larger radius of gyration,bending rigidity and yield bending moment compared the members with the same section area due to their wide flange and thin web,which can achieve good economic benefits and can be applied to lightweight low-rise steel frame systems.However,due to the large width-thickness ratio of the plates,the failure modes of slender H-section members are mainly dominated by the local buckling.Since the plastic deformation is limited,the ductility and energy dissipation capacity of the members is low,which hinders the formation of the plastic hinge of the section.The application of such members in the seismic design zones is restricted.To obtain a reasonable and accurate ‘low ductility – high bearing capacity’ seismic performance-based design method of lightweight steel frame system and ensure the safety of lightweight steel frame in seismic fortification areas,it is of important value to investigate the seismic performance of slender H-section steel members.In this paper,taking the bidirectional horizontal seismic performance of slender H-section steel members as the main research object,the ultimate state and hysteretic performance under different loading paths are studied.The seismic performance of steel members under biaxial bending and the effects of different loading paths on the hysteretic behavior of members were fully reviewed.It is pointed out that clear and unified criteria for the ultimate state of H-section with the significant difference in the characteristics of two principal axes under biaxial bending loading are still missing.In addition,it is found that the researches on different loading paths have been mainly focusing on reinforced concrete members,square steel tubes,and circular steel tubes.Based on the above finds the shortcomings of the existing research were pointed out.Based on the energy criterion of equilibrium stability,a criterion for determining the ultimate state of steel members under combined biaxial bending and axial force members under arbitrary loading paths was proposed from the perspective of elastic-plastic stability.The ultimate state of the plastic hinge sections is defined as the moment when the plastic hinge is transformed into a geometric variable mechanism to achieve its ultimate state.While the ultimate state of the slender sections dominated by the local buckling of the plate is the moment of initial plate buckling.In the following chapters,the rationality and applicability of the criterion were verified by three loading rules: simple monotonic loading path,complex hysteretic loading path,and horizontal seismic excitation.ABAQUS was used to establish a refined parametric analysis model for H-section members with different axial force ratios,web width-thickness ratios,and flange widththickness ratios under monotonic loading at different loading angles,which was verified based on the collected experimental data.Three groups of typical specimens were selected to analyze the relationship between the bidirectional bending moments and the ultimate bending capacity.Meanwhile,the influence of loading angles on the ultimate behavior and the development mechanism of local buckling of the plate were investigated to verify the rationality and accuracy of the ultimate state criterion.Based on a large number of parameter analysis results,the interaction curves of biaxial bending moments suitable for different cross-section classifications were proposed,and the reliability statistics and analysis of the interaction curves formula were carried out.Based on the above finite element models,the numerical analysis models of H-section steel members under six different axial force ratios and complex hysteretic loading paths were established to investigate the ultimate state analysis and hysteretic performance evaluation under complex hysteretic loading paths.The effects of loading paths were analyzed from five aspects including ultimate bearing capacity and interaction curves,hysteretic performance,ductility,energy dissipation performance,and damage characteristics.Finally,the interaction curves of bending bearing capacity were refined to consider the effect of cyclic loading conditions.Subsequently,based on the damage characteristic index,the finite element models of the beam-shell hybrid element were established.The nonlinear dynamic time-history response analysis of the typical members with three different cross-section classifications was carried out.It is verified that the modified interaction curves can well predict the ultimate bearing capacity of H-section steel members under bidirectional horizontal earthquake.More importantly,the rationality of the finite element models of the beam-shell hybrid element,which can be adopted in future analysis for steel members and frame systems,was verified from two aspects of bidirectional bending moments development and bidirectional column top displacement.Finally,the research contents of this paper were summarized and further research prospects were pointed out.