Research On Mechanical Behavior And Design Method Of CHS-to-SHS N-joints

Constructions with directly welded steel tubes have been widely used in largespan truss structures for artistic appearance and superior mechanical behaviour.However, internal forces of the joints are quite complicated because of the differenceof the quantity, angle, cross section shape and size of steel tubes intersecting atdirectly welded tubular joints. Thus far, although each country has establishedbearing capacity formulas for joints of various steel tubes in accordance with theircross section shapes (CHS or RHS), there is no bearing capacity formulas especiallyfor tubular joints with circular hollow section braces and square hollow section chord(CHS-to-SHS). Instead, bearing capacity formulas for rectangular tube joints is, afterlittle modification, applied directly into bearing capacity computation of tubularjoints with square chords and circular braces, which is lack of experimentalverification. In addition, this method is simply based on local buckling failure ofbraces without taking the following factors into consideration, such as axial force ofchords on the sides of joints, hidden weld absent or not, axial force properties ofbraces, etc.The dessertation, on the basis of experiments and non-linear finite elementanalysis, investigates the mechanical behavior, failure modes and ultimate bearingcapacity of CHS-to-SHS N-joints while focusing on the influence of the followingfactors on sturctural behavior of joints, e.g. axial force of chord on the sides of joints,hidden weld absent or not, axial force properties of braces, geometry parameters, etc.The dissertation studies the following aspects:Firstly, eight overlap CHS-to-SHS N-joints and the corresponding two zero-gapjoints were tested. The mechanical behavior and failure modes of joints under theinfluence of axial force, the stress and deformation distribution of each key parts ofjoints were investigatd, and the influence of the following factors on sturcturalbehavior of joints, e.g. hidden weld absent or not, axial force properties of braces,axial force of chords on the sides of joints, braces overlap or not, were discussed.Secondly, the dissertation studies the applicability of finite element model forultimate bearing capability of CHS-to-SHS N-joints. Considering various factors thatmay affect ultimate bearing capacity of joints such as element type, weld modeling,constraint conditions, loading modes, length of chords and braces, non linear analysis, etc., an effective and elaborate finite element model is established as the basis ofparameter analysis and is employed in the analysis of experimental joints. Theanalysis results are in good agreement with experimental results, which indicates thatthis model can well simulate the bearing capacity of CHS-to-SHS N-joints.Thirdly, the dissertation points out deficiencies of code formulas by the contrastbetween experimental results and computational results according to code formulas ofbearing capacity of CHS-to-SHS N-joints, and then systematically carries outvariable parameter analysis on the bearing capacity of CHS-to-SHS N-joints. Theauthor, focusing on the influence of the following factors on failure mode andultimate bearing capacity of joints, e.g. geometry parameters, axial force of chords onthe sides of joints, hidden weld absent or not, axial force properties of braces, etc.,finds out the regular pattern of influence of the above parameter variation onstructural behavior of joints.Fourthly, on the basis of formulas of China Code for Design of Steel Structures（GB50017-2003）, the dissertation, by means of parameter regression analysis,obtains the correction coefficient for bearing capability of partially overlapCHS-to-SHS N-joints when considering factors such as geometry parameter, hiddenweld absent or not, axial force properties of braces, etc., puts forward influencecoefficient for axial force of chords, and thus proposes the computational formula forbearing capacity of partially overlap CHS-to-SHS N-joints. In accordance with thefinite element analysis results the author simplifies the computational formulas forbearing capacity of gap CHS-to-SHS N-joints in the code so as to make its formulathe same as that of overlap joint, and puts forward correction coefficient of theformula. The comparison and statistic analysis between experimental data and finiteelement data indicates that the formula proposed by this dissertation has higherprecision.Finally, through establishing appropriate space coordinates, the dissertation putsforward intersecting line equations for overlap CHS-to-SHS N-joints by means ofcurve surface equations and thus proposes a practical computational formula forlength of each constituent part of joint intersecting line employing infinitesimalmethod on matter the hidden weld is absent or not. Finally, the author obtainssimplified computational formula for intersecting line length of overlap CHS-to-SHSN-joints by means of linear regression so as to calculate the length of welding seam.