Damage And Degradation Behaviors Of Steel Frames Under Severe Earthquake

For members and connections in steel frames, the damage and degradationcharacteristics of strength and stiffness could not be efficiently simulated and predictedby traditional beam element model subjected to strong earthquakes. The deformation ofstructural system would be underestimated and the collapse performance would beoverrated, leading to potentially unsafe structures. In this thesis, the critical parameters,including damage and degradation mechanism, that influence damage evolution processof steel frames were firstly studied in depth. Then，a theoretical model was proposed tosimulate the damage and degradation behaviors of full frame structures more accurately.By developing the algorithm based on ABAQUS platform, elastic-plastic time historyanalysis with beam element model was implemented for the complete process ofdamage and degradation. An effective analysis approach of steel frames under strongearthquake was developed.The main work of this thesis consisted of six parts:(1) Damage and degradation properties of two series tests with ten damage indexmodels were compared. The accuracy of different damage index models for predictingstructural damage were investigated. The application scopes of different models werealso discussed.(Chapter2).(2)50specimens of Q235B and Q345B grade steel under10cyclic loadingpatterns were tested. A steel constitutive theoretical model, Steel Hysteretic ConstitutiveModel (SHCM), considering cyclic hardening characteristics was proposed. Based onthe finite element program ABAQUS, a corresponding calculative program wasdeveloped. Then the model was verified by available material and component tests,which provided the basic theoretical framework for equivalent constitutive modelconsidering damage and degradation behaviors.(Chapter3).(3) According to the referred component and connection tests results, using thesteel skeleton curve model considering cyclic hardening in Chapter3, the numericalanalysis method was proposed for accurately simulating the cumulative damage anddegradation phenomenon and efficiently predicting the complete process of damage.(Chapter4). (4) Based on the research results of Chapter4, a large number of parametricanalysis were carried out for components and connections. The influencing factors andimpact degrees of damage and degradation, the damage evolution process, as well as thedistribution law of the "tri-linear" degradation curve were analyzed in depth.(Chapter5).(5) An equivalent steel constitutive model considering damage and degradationbehaviors was established, and damage factors was defined to describe the seismicbehaviors of steel frame in details. Based on the ABAQUS software platform, auser-defined material program, Damage and Degradation Application Program (DDAP),was developed for analyzing the whole process of damage and degradation. The DDAPwas verified by available steel frame tests, including static cyclic tests and shaking tabletests. The model was also used for predicting the deformation and damage of a real steelframe damaged under Northridge earthquake.(Chapter6).(6) By comparing with shell element model, multi-scale model and the traditionalbeam element model, the proposed model considering damage and degradation wasproved to improve the precision of time history analysis and failure prediction of steelframe under strong earthquake significantly. The model was also a balanced solution inboth computational accuracy and efficiency, which provided technical assistance fordynamic analysis under severe earthquake. Based on this analysis method, the effectsand properties of cumulative damage and degradation on seismic evaluation indicatorsof multi-storey steel frames were obtained, and the predicting curve of the degradationindex was proposed for damage and failure control.(Chapter7).This dissertation was sponsored by the National Natural Science Foundation ofChina (NSFC) program (No.90815004and No.51038006).