Fatigue Behaviors Of Bird-beak Welded X- And K-joints In Steel Truss Bridges

Bird-beak joint is a new type of welded square hollow section(SHS)joint as created by rotating chord or brace members at 45° about their longitudinal axes.According to the fact that whether brace members rotate or not,bird-beak joints can be divided into two types: square and diamond.The static behaviors of bird-beak joint have been proved to be higher than the conventional ones in existing literatures.However,limited references are available concerning the fatigue behaviors of bird-beak joint which has become a main obstacle for its application in steel truss bridge structures.This study focused on the stress concentration characteristics and fatigue behaviors of square and diamond SHS X-and K-joints under three load cases by experimental method and numerical simulation method.The main research achievements are as follows:(1)Static tests were carried out to obtain the hot spot stress characteristics of bird-beak joints.Eight X specimens divided into two groups were statically tested under brace axial force and in-plane bending moment load cases respectively,while four K specimens with gap were statically tested under brace in-plane multiple forces.Each group of specimens consisted of square and diamond bird-beak joints.Strain distributions along the direction perpendicular to the weld toe in the hot spot extrapolation areas were obtained.Results show that quadratic variations of strain distributions near the weld toe are evident.Therefore,the strain concentration factors(SNCFs)and stress concentration factors(SCFs)at the weld toe could be calculated by the quadratic extrapolation method.The maximum SNCFs of bird-beak joints appear on saddle areas under brace axial forces while on crown areas under the other two load conditions.Compared with diamond bird-beak joints,square bird-beak joints have lower maximum SNCFs under brace axial forces,but have higher maximum SNCFs under the other two load conditions.The numerical results from finite element models by ANSYS software match well with the experimental results,and the conversion relations of SCFs and SNCFs for bird-beak joints are also obtained for engineering application.(2)High-cycle fatigue tests based on experimental SNCFs were carried out for fatigue behaviors of bird-beak joints.Each specimen was subjected to the corresponding load case,and the fatigue characteristics,such as fatigue failure modes,fatigue crack initiation and propagation,stiffness changes and characteristic fatigue life were all revealed.The results show that:(1)Under brace axial forces,the fatigue cracks of X-joints initiated at the weld toes in the chord saddle area where the maximum hot spot stress was found in static tests.Two cracks,respectively initiating from two saddle areas on one chord wall of square bird-beak X-joints,propagated toward each other until they met and got combined into one,and further propagated away from the chord weld toes.During this period,the crack length extended rapidly and the in-plane stiffness of the joint degraded rapidly.Additionally,the crack reached the limit length before it penetrated the chord wall.As for diamond bird-beak X-joints,the crack initiating from the only one saddle area on one chord wall,propagated along the chord weld toes to the crown areas or away from the chord weld toes.When the cracks approached the chord crown areas,the in-plane stiffness of the joint degraded rapidly.The crack reached the limit length after it penetrated the chord wall.(2)Under brace in-plane bending moment,the fatigue cracks of X-joints originated from the weld toes in the brace crown areas,and then extended along the weld toes to the brace saddle areas or to brace wall surface.Once the crack of square bird-beak X-joints bypassed the brace angle,the in-plane stiffness of the joints decreased sharply,and the crack penetrated the brace wall before it reached the limit length.As for diamond bird-beak X-joints,the crack development period was relatively short.Once the crack penetrated the wall thickness,the crack length increased rapidly and the joint stiffness reduced sharply.(3)Under brace in-plane multiple forces,the main cracks in the tension gap zones of K-joints initiated at the weld toes on the brace crown areas,and penetrated the brace wall after a short time.The crack extending from the brace to the chord weld toes resulted in the sharply increasing displacement amplitude of some specimens and even the brace brittle fracture.The above achievements provide experimental basis and methodological references for fatigue behavior evaluations of bird-beak SHS X-and K-joints,and are conducive to the application of this new-type of joints in steel truss bridge structures.