Exploration On Constitutive Model Of H-section Steel Biaxial Beam-columns Considering Degradation

The effects of earthquakes are multidimensional.Under multi-dimensional earthquake action,the structural lateral force members will be subjected to axial force,two-way horizontal shear force,two-way bending moment and torque,such a complex state of force will inevitably lead to complex structural response.Studying the hysteretic behavior of components under complex responses can more accurately reflect the true response of the structure under earthquake action.The biaxial bending constitutive model that takes into account the effect of local buckling is basic to conduct seismic analysis and study the seismic performance of thin frame structures.The work of the paper mainly includes the following aspects:First of all,this study reviews the commonly used constitutive models in current seismic research.A variety of different broken-line and curvilinear constitutive models are compared.The advantages and disadvantages,similarities and differences of commonly used constitutive models in different situations are pointed out.According to actual needs,the constitutive model form that can consider the hysteretic behaviors such as cumulative damage,stiffness degradation,and various performances after the limit is selected and improved.The finite element parameter analysis of H-shaped section steel members with different width-thickness ratios,axial compression ratios and loading angles under bidirectional bending is carried out and compared with the test results.The calculation results of the finite element model are analyzed.The ultimate load-bearing capacity and stiffness are discussed in terms of the failure modes and angles of biaxial compressionbending components.It is pointed out that under the action of biaxial bending,the two main axis often do not reach the moment of ultimate load-bearing capacity at the same time,and there is a complex nonlinear relationship.It is also found that when the loading angle changes in a small interval,the two main axis change greatly,but when the loading angle is large,the two main axis change not significantly.In order to study the constitutive model under biaxial compression bending,starting from uniaxial compression bending,based on the results of finite element parameter analysis,using numerical analysis methods,models under strong axis compression and weak axis compression are proposed respectively.The final uniaxial bending constitutive model can reflect the hysteretic properties of the H-shaped section under multiple failure modes.The model is divided into two degradation models according to whether there is increased reloading stiffness in the degradation stage.The first type,the general degradation model,was used for the bending around the weak axis with buckled-web and the bending around strong axis.The second type,increased reloading stiffness model,was used for the bending around the weak axis with unbuckled-web.After mastering the research method of the constitutive model under uniaxial compression and bending,a targeted study was made on the biaxial compression and bending constitutive model with a loading angle of 15°.The difference between the edge yield bending moment and the length of the hinge area under the action of biaxial compression and uniaxial compression is analyzed in detail.The decomposition method and unitization method of the bending moment-curvature relationship under the action of biaxial bending are pointed out.Through the specific observation and discussion of biaxial bending,a constitutive model that is suitable for reaching the limit moment when the two main shaft directions are different under the action of biaxial bending with a loading angle of 15° is finally proposed.The research methods of the constitutive model of biaxial compression bending are summarized,and ideas for the subsequent research on the constitutive model of biaxial compression bending from other angles are provided.The model proposed in this paper shows that the thin flexible H-shaped section still has a certain degree of ductility and energy dissipation capacity after buckling.This shows that it can be used in seismic design and provides a design basis for the performance-based design of light steel system buildings.Finally,the research content of this article are summarized and the direction of the research are pointed out.