Study Of Effect Of Vibration-control Device On The Wind-induced Fatigue Performance Of Welded Joints In High-rise Steel Structures

Welding connection is the main connection type in steel structures,which is widely used in high-rise buildings.Due to the local stress concentration effect and the welding defects,the welded beam-column joints of high-rise steel structures may suffer from fatigue failure under the action of strong wind.It will bring a vital threat to the safety of the structures.Therefore,it is of great significance to predict the fatigue life of weak joints and extend the fatigue life of welded joints through some vibration control methods.In this study,the fatigue crack initiation life of welded beam-column joints before and after vibration control of high-rise steel structures are studied by means of theoretical calculation,numerical simulation and experimental study,with the nominal stress and hot spot stress being the damage parameters.The main conclusions are summarized as follows:1.The finite element numerical simulation of welded beam-column joints and their counterpart with viscous fluid damper is carried out through ANSYS.According to the distribution of local stress along the width direction of the flange,the stress along the outside toe line is approximately inverted V-shaped,while that in the inside toe line and the root line is approximately W-shaped,so the location of fatigue crack initiation in the joint can be deduced.Through ANSYS transient response analysis,the rationality and validity of improving fatigue performance of welded joints by means of vibration control are verified.The high cycle fatigue lives of the two kinds of joints are predicted based on the hot spot stress method.The results show that the fatigue life decreases nonlinearly with the increase of load,and the use of viscous fluid damper can remarkably enlarge the fatigue life of the joints.Under different load amplitudes,the life extension multiples are in the range of 2.5 to 15.3.2.The axial high cycle fatigue tests were carried out on the standard cross butt joint specimens with Q345 steel and the characteristics of high cycle fatigue failure were obtained.The characteristic parameter C=3.98×10¹¹ under 97.7%guarantee rate was acquired,which is more conservative compared with the parameters recommended by IIW.The nominal stress method and hot spot stress method are used to evaluate the high cycle fatigue life of the specimens.Compared with the test results,the predicted lives are shorter than the measured ones,which indicates that the design of components based on the life prediction results is more conservative and safer.Six full-scale welded beam-column joints were tested,and the fatigue failure characteristics of the weld were obtained.There are two main sources of flange fatigue cracks:one is located in the side of the beam flange weld,and the other is near the weld hole in the middle of the beam flange weld.This phenomenon is consistent with the results of numerical simulation,and conforms to the mechanism of fatigue crack initiation.Vibration control of viscous fluid dampers improves the ultimate bearing capacity of the joint,prolongs crack initiation life and crack propagation life.Under the same displacement amplitude,the dynamic extreme value of joints with dampers increases by 10%¹5%.The stress level in the weld zone of joints decreases significantly.With the increase of loading rate,the effect is more significant,which is consistent with the results of finite element simulation.It conforms to the energy dissipation mechanism of viscous fluid damper and reflects its velocity dependence.3.The wind-induced fatigue analysis of high-rise steel structure is carried out respectively based on the full-scale model and the multi-scale model with consideration of the welded beam-column joints.The results show that the shortest fatigue crack initiation life of welded beam-column joints is 93 years under the action of good wind,which is shorter than the service life period of 100 years.The nominal stress prediction method considering the FAT36 makes the engineering design more conservative.Under the action of super strong typhoon,the shortest fatigue crack initiation life of welded beam-column joints is only 6 days.There is a tendency of low cycle fatigue crack initiation,which should be paid attention to in design.4.Wind-induced fatigue of high-rise steel structures under three vibration control methods（viscous fluid damper,tuned mass damper and metal shear damper）is studied.The results show that different connection modes of viscous fluid damper have different effects on the fatigue performance of welded joints.Damping index is the main factor affecting the longevity of viscous fluid damper under good wind conditions.The effect of life extension tends to decrease with the increase of damping index.If the fatigue life of joints connected with dampers under inclined bar support type connections is expected to be increased,the design of damping index should be more than 0.45.When mass ratio is fixed as 5%,the frequency ratio of TMD is the main factor affecting the life extension.With the increase of frequency ratio,the life extension ratio of TMD increases first and then decreases.When the frequency ratio reaches 0.9,the fatigue life of each joint reaches its maximum.Using metal shear dampers for vibration control,the life-span ratio of each joint is more than 13,but excessive additional stiffness is not conducive to the overall seismic performance of the structure,which should be paid attention to in engineering design.The research work and results in this study can provide references and suggestions for fatigue life prediction and structural fatigue resistance design of welded joints in high-rise steel structures.