Full-Range Behavior Of Uniplanar RHS Tubular Joint Based On Component Method

Tubular joint is the most common joint seen in the tubular structures,who’s mechanical behavior has significant impact on structures,including internal force distribution,stability capacity,seismic performance,progressive collapse resistance and so on.Current design codes or existed researches are focused on axial resistance of tubular joint,while other mechanical properties are less mentioned,including initial stiffness under axial force,initial stiffness and resistance under bending moment and full-range behavior.Therefore,this thesis presents component-based researches on uniplanar RHS(Rectangular Hollow Section)joint through experimental investigation,finite element analysis and theoretical derivation.Joint mechanical behavior under different loads were investigated,and joint component method model was then proposed,as well as the design method.Besides,joint axial full-range behavior was studied on the foundation of the proposed component method model.At first,this thesis reviewed the current design method and researches on RHS joint,and failure-mode-based theoretical model and resistance formulae were described.Then,the semi-rigidity and component method of open section joint,as well as the state-of-the-art research and extension to tubular joint of component method,were introduced in detail,so that the features and insufficiency of the method were concluded.Because of the complex joint load condition in practical structures,ten sets of uniplanar X joint under different brace loads were experimentally investigated,and the specimen mechanical properties,including failure mode,strain distribution,peak load,initial stiffness,load-displacement curves,were comprehensively analyzed.Besides,the mechanical behaviors of joint and plates were also horizontally compared,in order to study the performance similarity of the same joints in different loading conditions.Numerical simulation is the complement to the experimental investigation,by which the mechanical behavior of joint plate can be fully studied.Finite element package ABAQUS was utilized for the numerical simulation of tested joints.Shell element models with weld modeling,practical boundary conditions and simplified steel material property were created,to which the geometrical imperfection was considered through buckling mode and introduced.The comparison between experimental and numerical results shows good agreement in failure mode,load-displacement curve.Considering the subsequent demand of parametric modeling and calculation,the simplified modeling method considering the weld size and the corresponding equivalent brace width calculation method were proposed.And the method was proved reliable by load-displacement curves comparison of kinds of joints with different brace loads and geometrical configuration.For the purpose of studying general rule of joint properties,the database covering common RHS joint is demanded.Based on the modeling method stated above,python script was written for parametric batch modeling,analysis and post-processing of X joint under various load conditions and K joint under axial load,covering common geometrical and material parameters in practice.The mechanical properties and loaddisplacement curves show that: the initial stiffiness of joint under different brace loads is only relative to geometrical configuration,but has nothing to do with the direction of axial force and steel grade;Compression failure of chord side wall is the main reason of joint peak load,and the volume of peak load is related to geometrical parameters and steel strength;Similar capacity of side wall was observed in the joints under different brace loads;Brace end rotation restraint has some effects on joint buckling mode and post-buckling behavior,but the difference is rather minor,which can be ignored;codified resistance formula of K joint chord face is based on the axial deformation limit,showing some degree of discreteness in the resistance of joints with different geometrical parameters,which should be further studied.According to the experimental and numerical phenomena,component method model of X joint was built to calculate the initial stiffness and resistance.At first,based on the component method concept and rational assumption,X joint zone was divided into chord face and side wall,and the components were classified by brace loads as,specifically,chord side wall in tension/compression/bending-tension/bendingcompression,chord face transverse part in bending/torsion and longitudinal part in bending/torsion.Then,the simplified theoretical models of component resistance and initial stiffness were proposed and derived,and formulae for component mechanical properties were obtained based on the results of parameter analysis above.Finally,according to the brace load condition and relevant load transferring path,the brace,chord face and side wall components were assembled to be the mechanical spring.The springs were assembled to represent the mechanical model of whole joint,and the component-based design method was suggested for the calculation of joint resistance and initial stiffness.The results of proposed method were compared with that from experiments and numerical analysis,demonstrating the reliability of mechanical model.On the foundation of X joint component method model,full-range behavior model under axial force were built,extending the model application scope to the joint fullrange properties.Tri-linear and three-stage models were proposed,respectively,to illustrate the full-range axial behavior of chord face and side wall by their mechanical characteristics.And key parameters in the component full-range model were determined through theoretical derivation combined with empirical regression.After comparing the resistance corresponding stage in chord face full-range behavior of X/Y/T joint and K/N joint,some issues in resistance definition were discussed.Based on the concept of plastic condition load in full-range behavior,component method formulae of K joint chord face resistance were proposed by theoretical derivation.On the basis of component method model stated above and displacement superposition principle,the series connected chord face and side wall springs were merged together,and thus the joint axial full-range load-displacement curves can be predicted.The model was proved to be accurate and reliable by the comparison between predicted and experimental/numerical curves.At last,the conclusions of this thesis were summarized,and potential topics of further research were presented.