Analysis Of The Hysteretic Behavior Of Gapped K Tubular Joint

In recent years,steel pipe structure has been widely used in the construction of large buildings by virtue of its excellent performance,convenient processing and beautiful appearance.The seismic performance of joints is closely related to the strength and stability of structures.When joints failed,they were generally presented by a variety of failure modes.In this paper,the influence of branch cross section on the damage accumulation mode of joints under strong earthquakes is explored,and the study of joint damage accumulation is supplemented by finite element simulation.Considering the application scenarios of gapped K tubular joint in arch,truss and other large-span spatial structures,two types of intersecting joints are selected to carry out a comparative study on seismic performance.The research contents of intersecting nodes mainly include:By making two kinds of K-section steel pipe intersecting joints,material property tests were carried out on the Q235 steel cut from the joint pipe fittings in the early stage,and low hysteresis pseudo-static loading tests were carried out to simulate the mechanical response behavior of the joints under strong earthquakes,record the whole test phenomenon and analyze its failure mode;The data obtained from the low hysteresis load tests of the joints were collated,and the influence of branch cross section on the seismic characteristics of the two joints was evaluated by seismic performance indexes;The implicit integral equation of mixed reinforcement was derived and the UMAT material subroutine for coupling damage was written in FORTRAN language according to the existing material damage theory.The applicability of UMAT material subroutine for coupling damage was verified by an aluminum ring calculation example.The full-scale finite element models of the two test nodes were established and the material subroutine was called to conduct low-hysteresis loading simulation on the ABAQUS platform.The stress development path and damage accumulation of the nodes were compared and analyzed,and the influence and rule of the branch cross section shape of the two nodes on their mechanical behaviors were concluded.By studying the results of quasi-static low hysteresis backloading test and finite element simulation,the following conclusions are drawn in this paper,in order to provide reference for the seismic engineering design and research of joints in steel pipe structures:(1)When axial reciprocating load is applied to the main pipe of the two kinds of joints,brittle failure occurs at the point of concentrated stress and cracks occur in the joints of the two kinds of branch pipe cross sections.Primary party branch party node branch pipe for loading under reciprocating load extrusion head wall,make the head wall generates a "bump" the turn of the deformation and fracture development envoys point rotation and lateral displacement amplitude increase the head wall partial plastic failure,finally the lateral branch from the connection weld root fracture,bearing capacity fell sharply and sabotage.In the test of the main square branched circular joint,the branched pipe has little influence on the plastic deformation of the main wall,and the lower part of the crack has wider characteristics and shorter length,while the upper part of the crack has longer length and smoother crack.Finally,the upper branch pipe of the circular node of the main square branch is pulled away from the main wall and destroyed.(2)From two kinds of joints of the hysteresis curves,skeleton curves,ductility seismic performance parameters such as the curve comparison found: after the yield of plastic working status of both are in spindle type,the hysteresis curve shape is full,the primary party branch round node yield strength and ultimate strength were greater than the main party branch node,higher bearing capacity of the main party round node energy dissipation capacity and the main party branch node close,the crack development stage primary party branch party stiffness degradation is more apparent.(3)Based on the theory of elastoplasticity and damage theory,a UMAT user material subroutine with mixed reinforcement and coupled damage parameters was written and applied to the simulation of joint hysteresis tests,and a good simulation effect was obtained.In general,the finite element simulation results are more accurate in accordance with the initiation time and development trend of joint cracks in the test,but there are also errors caused by the lack of considering the influence of weld on joint stress path.Combined with the simulation results,the process of joint damage accumulation was described from a mesoscopic perspective.The damage accumulation in the joint domain at the intersecting line was studied,and it was found that the damage accumulation speed of the inner wall of the joint was faster than that of the outer wall,which provided a reference and supplement for the study of joint failure mechanism and strengthening measures.