Low-cycle Fatigue Life Of I-shaped Steel Braced Frame With Gusset Plate Connections

Braced frame with gusset plate connection has been widely used in the field of multi-story and high-rise steel structures because of its excellent seismic performance and convenient construction.The earthquake investigation shows that the failure of the braced frame system occurs not only in the brace which is the energy-consuming member,but also in the gusset plate connection.The too strong or too weak design of the gusset plate connection would have a negative effect on the overall fatigue performance of the braced frame.At present,there is a lack of low-cycle fatigue life prediction methods which can comprehensively consider the interaction and fatigue damage between the brace and connection.How to choose the geometric information and connection form of the brace and gusset plate,so as to match them synergistically and make the whole fatigue performance of the braced frame to achieve the optimal,needs further research.In order to solve the above problems,this paper conducts an in-depth study on the overall low-cycle fatigue performance of the braced frame through fatigue experiments of welded standard specimens(uniaxial and multi-axial)and the braced system with gusset plate connection,finite element simulation(using ABAQUS and Open SEES)of brace components and barced frames,and fatigue damage analysis.The fatigue parameters of commonly used steel were obtained,and the prediction abilities of different life prediction models were compared.A fatigue mechanics term of the fiber model is proposed,which can consider the local buckling of the bracing member.A probabilistic prediction model of the low-cycle fatigue life of the braced frame is established,which can consider the interaction of each member and can be used in the seismic design of the braced frame.The main research contents are as follows:To study the low-cycle fatigue life of the welded connection of the gusset plate,the low-cycle fatigue experiments of 15 welded steel bars under uniaxial loading and24 welded tubular specimens under multi-axial(tension-compression and torsion)loading were conducted.The fatigue performance of these specimens were studied,including cyclic hysteresis,capacity degradation,local plastic development,fracture location,and low-cycle fatigue life.The effect of the biaxial strain ratio on low-cycle fatigue performance was also analyzed.Moreover,the fatigue parameters of base metal and corresponding welding materials of common steel(Q235 B and Q355B)are given for fatigue life prediction.The low-cycle fatigue life prediction method of the brace and its prediction ability are systematically investigated.Based on the finite element analysis of the low-cycle fatigue experiment of 53 I-shaped brace components and 5 diagonal braced frames,six life prediction models(including equivalent strain model,maximum shear strain model,FS model,KBM model,LZH model,and ductile damage model)of three kinds of material life prediction methods(including equivalent strain method,critical plane method,and ductile damage method)are extended to predict the low-cycle fatigue life of brace.The prediction ability is compared systematically,and the sensitivity analysis of relevant parameters is made.In addition,based on the finite element analysis of the fiber model of 39 I-shaped brace components,5 diagonal braced frames,and 8 inverted-V braced frames,the fiber strain modified by the brace parameters(including slenderness ratio,the width-to-thickness ratio of the web,the width-to-thickness ratio of the flange,and the yield strength)was proposed as the damage term,which is called the representative strain.Thus,the prediction formula of the fatigue life based on the frame fiber model is established,which makes up for the failure of the fiber model to consider the local buckling of the braces.The prediction ability of the above 4 fatigue life prediction methods(including 7 fatigue models)is evaluated.The research results can provide a reference for the fatigue life prediction of the braced frame and the measurement and selection of critical parameters.To study the low-cycle fatigue performance of the braced system,the low-cycle fatigue experiments of 12 braced systems with the gusset plate connection were conducted.The fatigue failure mode and the whole fatigue performance of these systems were studied.The interaction mechanism between the bracing member and the gusset plate connection is obtained,and the influence of the design parameters of each member on the whole low-cycle fatigue performance of the braced system is analyzed.According to the Bayesian updating criterion,the calculation formula of the effective length factor and probabilistic prediction model of low-cycle fatigue life of the braced system are proposed.Based on the life matching relationship,the low-cycle fatigue design method of braced frame with the gusset plate connection is proposed.The finite element analysis and fatigue damage prediction of 192 braced frames with gusset plate connections(the slenderness ratio of bracing member is 50～100,the width-to-thickness ratio of flange is 6～11,the width-to-thickness ratio of web is 20～27,the clearance distance ratio is-6～2,brace angle is 35°～45°,the connection coefficient 1～1.6,and the interstory drift ratio is 1/100～1/50)were conducted.The influencing factors and the fatigue life prediction models of brace and gusset plate connections are studied and obtained respectively.Based on this,the fatigue life ratio of the gusset plate to the brace is proposed as the critical term to express the matching relationship of fatigue damage and failure sequence of each member in the braced frame.The prediction formulas of the life ratio under different reliability are established with the obtained influencing factors.The low-cycle fatigue design method of the braced frame with the gusset plate connection is proposed,and thus some relevant suggestions of optimization design and code improvement are given.