Hysteretic Behavior Of New Energy-dissipated Beam-to-column Connections In Steel Frame

Lots of brittle fractures were observed in the welded beam-to-columnconnections of steel structures during the1994Northridge and1995KobeEarthquakes. These observations provoked reconsideration of the ductility behaviorof steel frames. Meanwhile, with the development of performance-based seismicdesign conception, the new beam-to-column connections with self-centering and (or)easy to repair characteristics have been attached much importance to up-to-now.Shape memory alloys (SMA) are a class of intelligent materials with unique shapememory effect and superelasticity, also having large deformation capacity andsuperior fatigue properties. These unique behaviors make SMA suitable forstructural energy dissipation and vibration control. In view of this, newenergy-dissipation beam-to-column connections are proposed. The SMA threadedrods are used in these new connections to improve the performances of theconventional angle connections and to enhance the deformation ductility andseismic performance of steel frames. In this study, pseudo-static tests and theoreticalanalysis have been conducted on the proposed new connections. The main contentsand results are as follows:Due to few studies on the large-scale shape memory alloy materials, themechanical properties of big diameter NiTi bars were first investigated by tests andnumerical simulation. The SMA material tests under various loading conditions oftension, cyclic tension-compression and cyclic tension had been conducted. The testresults show that the size effect of SMA materials can not be ignored, and thelarge-scale NiTi bars can have good shape memory effect and superelasticity. TheSMA bars exhibit more stable post-buckling behaviors than the steel bars. Pre-cyclicloading is need for the origin SMA specimens to acquire stable cyclic behaviors.The martensite SMA specimens have better fatigue properties than the austeniteSMA ones. The fatigue properties of SMA are found to be related with thedeformation modes and the local plastic deformation will reduce the fatigue lifeseriously. Numerical study was then conducted to simulate the mechanical behaviorsof the specimens by using ANSYS program. A combined material model to simulatethe post-strengthening behavior of martensite SMA is proposed. The effectiveness ofthe original superelastic model in ANSYS is investigated for different loading cases.A uniaxial constitutive model considering the asymmetric tension-compressionbehavior and the different elastic modulus between austenite phase and martensitephase is proposed and incorporated into ANSYS software to enhance its simulation ability on the whole structures.Based on the conception of effectively controlling the damages limited to theconnecting components, new beam-to-column connections using SMA threaded rodshad been designed and fabricated. The pseudo-static tests were carried out on thesenew connections, and also done on the connections using steel threaded rods at thesame time as for comparison. Two cases are considered for the new connections:1)recovering the damaged connections by the shape memory effect of SMA; and2)reducing the residual deformation of connections by the superelastic behavior ofSMA. The test results suggest that the pure angle connections possess nonlinearpseudo-elastic behavior under small deformation due to the pre-stressing effectinduced during fastening the angles. Due to the good fatigue properties and theunique post-strengthening effect of SMA threaded rods, the SMA connectionsexhibit better and more stable hysteretic behavior and energy dissipation capacitythan the ones using steel rods. The fractures of steel threaded rods make thecorresponding connections brittle. The martensite SMA exhibit excellent repairingcapacity for connections under different damaged states due to its good shapememory effect, and the corresponding connections can experience two completeloading schedules due to its superior fatigue properties. The residual deformation ofconnections can be significantly reduced by the superelastic behavior of austeniteSMA, the extent of which depends on the contribution ratio between SMA and otherplastic components (such as angles) and the superelasticity degradation itself. Thebehavior of connections under small deformation can be significantly improved bypre-tensioning the austenite SMA rods.The refined finite element models considering the material nonlinearity,geometric nonlinearity and contact nonlinearity had been built for each typeconnection by using the ANSYS program. The effectiveness of the finite elementmodels is verified through the test results. The accuracy and features of differentsimulation approaches for pre-tensioning the threaded rods is analyzed. Thepre-stressing stiffening effect induced by the initial imperfection of angles and theeffect of the initial geometric imperfection of rods on the connection behaviors arealso investigated by the finite element method. The validity of improving theconnection performances by restraining buckling of the threaded rods is alsodemonstrated. The feasibility of the combined material model in simulatingmartensite SMA rods and the corresponding connections is analyzed.Based on the experimental and numerical results of the proposed connections,simplified hysteretic models considering the pseudo-elastic behavior, pinching, poststrengthening (or deterioration) and the sub-loop effect are proposed and incorporated into ANSYS program by creating a series of new user-defined springelements, which could consider seven general degree-of-freedom behaviors. Theaccuracy and effectiveness of the proposed hysteretic models and the user-definedelements are verified by comparison with the test results.Finally, on the basis of the experimental and numerical results of the newconnections, the prediction models of the initial elastic stiffness and ultimatestrength of the connection in view of its major bearing and dissipating components(angles and rods) had been proposed. These models can be used for performanceevaluation of the connection in the preliminary design stage. According to thecharacteristics of the new connections, several design advices are presented. Basedon the experimental results, the major design parameters of the threaded rods arediscussed, and the recommended design values are put forward.