Research On Hot Spot Stress Characteristics Of Steel Truss Bird-beak Welded Tubular Cross-joints

Bird-beak square hollow section(SHS)joint is a new type of welded tubular joints,which can be obtained by rotating the members of a conventional SHS joint at 45° about their longitudinal axes.Correspondingly,two types of bird-beak SHS joints,that is,square birdbeak joint with chord rotated only and diamond bird-beak joint with chord and brace both rotated,are commonly considered.The rigidities and strengths of bird-beak joints under static loads have been revealed to be higher than those of conventional joints with identical dimensions.However,very limited references are available for the fatigue behavior of bird-beak joints,which do prevent the applications of bird-beak joints in bridge structures.This study focuses on the stress concentration characteristics of both square and diamond bird-beak cross-joints by employing testing and numerical methods.The contents and conclusions are as follows:(1)Static tests are conducted to reveal the stress concentration characteristics of bird-beak cross-joints.Two groups of bird-beak crossjoints,two specimens in each group,were fabricated and statically tested.The hot spot stresses at weld toes were extrapolated by using the measured strains within the extrapolation regions,followed by the calculation of strain concentration factors(SNCF).Results show that the non-linear strain variations perpendicular to the weld are real,which means quadratic extrapolation is needed for such joints.Comparatively,the maximum SNCF of square bird-beak joints are smaller than diamond bird-beak joints,which indicates better fatigue resistance of the former.(2)Parametric FE modeling and analysis of bird-beak cross-joints were implemented.An advanced finite element modeling technique for hot spot stress analysis is proposed by using ANSYS parametric design language(APDL),where the charasteristic bird-beak configurations were especially considered.Tetrahedral-shaped solid elements were used so that the refined FE meshes generated from the map meshing match the requirements of extrapolation region that have been specified in IIW and CIDECT fatigue design guides.All extrapolating works were automatically implemented by pre-coded programs using Perl language.The accuracy of constructed FE models has been validated by comparing with existing experimental data.The proposed modeling approaches are applicable to the bird-beak joints under various load cases and boundary conditions.(3)Numerous parametric studies of stress concentration factors(SCF)were successively carried out by using the validated finite element models,with the aims of deriving SCF formulation.Both diamond and square birdbeak cross-joints under brace axial forces and brace in-plane bending were employed for the analysis by considering common value ranges of geometrical sizes.The influences of three major non-dimensional parameters(β、2γ、τ)on SCFs are obtained,and the SCF formulations were numerically fitted,which will provide the references to the calculation of hot spot stress of bird-beak cross-joints.The achievements of this study will provide theoretical basis and method references for the fatigue assessment of bird-beak welded crossjoints,and will contribute to the applications of such new-type joints in steel truss bridges.