| The welds in the steel structure beam-column joints will more or less have defects.Under the action of external load,these defects will further expand and eventually lead to the fracture of the joint weld area.Under the action of earthquake and other loads,the defects in the joint weld have adverse effects on the bearing capacity of the structure.Therefore,sufficient attention should be paid to the quality of welds in the design of steel structural components and seismic design.In this dissertation,based on the consideration of initial crack(a0 and cumulative damage,a member model of spatial steel frame with accuracy and efficiency is proposed to study the influence of initial crack depth on structural fragility.The main contents and research results are as follows:(1)In this dissertation,the full-scale beam-column welding specimen model with initial crack(a0 is used as the research object.The extended finite element method(XFEM is used to simulate the whole process of the joint from crack initiation to lower flange fracture failure.Four loading modes are used to analyze the influence of different loading amplitudes on the performance of cracked joints.The research shows that the loading span has little effect on the performance of the joint.XFEM can better reflect the characteristics of component degradation.(2)The influence of the initial crack(a0 position on the performance of the joint is studied.The initial crack(a0 is set at the left,middle and right in the weld area of the lower flange of the beam.The failure mode,degradation characteristics,damage curve and fracture performance of the joint are analyzed.The research shows that the position of initial crack(a0in the lower flange weld area of beam has no significantly affect the performance of the joint.So,the initial crack(a0 can be set at the middle in the weld area to represent the initial crack in the weld area of the lower flange of the beam.(3)A simplified calculation model of joint connector member is proposed.The numerical relationship between yield point and limit point parameters is established by finite element simulation of joints with different initial crack(a0 depths from 0.0 mm to 8.0 mm in the weld area of lower flange of beam.The connector parameters are set according to the bending moment and rotation angle of the characteristic points of the joint,and the simplified model is established for different initial crack(a0 depths.(4)Based on the simplified jiont model,the member model of spatial steel frame is established,and the static elastic-plastic analysis is carried out.Different performance limits and failure states are defined according to the failure state of the jiont.The results show that the two parameters model considering damage is more reasonable than the single parameter model.And the two parameters model does not depend on the loading system.(5)In order to study the influence of initial crack(a0 on the seismic demand of frame structure,the spatial steel frame structure with the jiont containing initial crack(a0 is selected as the research object.The seismic demand model of this structure is established.Considering the initial crack in the joint as the structural uncertainty factor,a completely independent simplified method is adopted to analyze the correlation of initial crack(a0 at different positions in the frame structure.The initial crack depth of each jiont in the frame structure is different.The initial crack depths of the same numbered nodes in the sample are also different.The probability relationship between structural response and seismic strength parameters of steel frame structure with initial crack is established.(6)In order to study the influence of initial crack(a0 in the joint on the seismic fragility analysis of the structure,a 7-story,3-span steel structure is taken an example.The uncertainty factor of the joint with initial crack(a0 is considered as the nondeterminacy of the structure.The seismic fragility curve is obtained.The research results show that the model of steel frame member with initial defects can be used to analyze the seismic fragility of steel frame structure.Through experimental verification,numerical calculation and theoretical analysis,the influence of initial crack(a0 on seismic vulnerability of spatial steel frame structure is analyzed in this dissertation.The theoretical system of seismic vulnerability research has been improved and promoted,which provides strong technical support for the study of complex mechanical behavior of jionts with initial crack and analysis methods for actual engineering problems. |
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