Study On Fracture Behavior Of Welded Joint In Steel Frames Under Earthquake Load And Assessment

Because of its light weight and good earthquake resistant performance, the steelstructure is widely used in the industrial fields and high rise buildings. But in recentgreat earthquakes, it is brittle fractures not the ductile fractures as expected whendesigned that happened in the welded beam-to-column connections in many steelmoment frames. It can be seen that a potential risk factor has not been considered. Inintensive earthquake,the steel structure will undertake a severe impact which the steelis very sensitive to. Under the strenuous impact, the structure will be damaged easily.It will lead to the falling of the strengths and the bearing capacity of the structure. So, the main purpose of this paper is to aim the fracture behavior under prestrainand dynamic load of the connections to perform extensive research. The effects of prestrain on the impact toughness of base metal and weld metal ofstructural steels Q345 and Q235B were investigated experimentally by use of theCharpy notch impact test at different testing temperatures. The results of the testsshow that: the impact toughness of both base metal and weld metal decreases withdecreasing temperature and the ductile-brittle transition temperature of the two kindsof material increases a little under prestrain. The ranges of ductile-brittle transitiontemperature of the base metal and weld metal of Q345 steel under 5% and 10%prestrain are lower than 0℃ and the room temperature(20℃) respectively, at whichit can be used safely. The ductile-brittle transition temperature of Q235B steel underprestrain is higher than 20℃ except that of the weld metal under 5% prestrain. Theconclude can be drawn that the prestraining effect should be considered whenestablish an objective toughness standard and carry out more effective safety analysis. The mechanical property tests of the base metal and weld metal on structuralsteels(Q345 and Q235B steel)are carried out under no prestrain and 10% prestrain.The results show that: both the yield strength and tensile strength increase underprestrain. The formula derivation result of the relationship between prestrain and theyield strength is consistent with the test. So, we deduce the mechanical property under5% prestrain. The value of CTOD of the base metal and weld metal of the two steel under 5%prestrain and 10% prestrain have been gotten. And the effect of prestrain on theCTOD has been investigated. The dynamic and static toughness of welds and base metal of Q345 and Q235Bsteel have been compared and analysed. From the result of CTOD under different IIloading rate, it can be concluded that the dynamic loading is beneficial to the Q345steel but disadvantage to the Q235B steel. At last, this paper provides two state-of-the-art Risk Assessment Procedures(RAPs). They are designed to enable an engineer to assess qualitatively the risk ofbrittle fracture. The procedures can also be used to determine measures to be used in arepair situation to reduce future risks to an acceptable level. The material of Q345 andQ235B under different prestrain has been assessed by using the two Risk AssessmentProcedures. It can be shown that the effect of fabrication and testing is great. Butwhen the material has sustained high stress/strain, the temperature has almost noimpact on risk. It can be suggested that the important components of steel structure should bemade in Q345 steel.