Experimental And Theory Study On Unstiffened And Typical Stiffened Steel Tubular Joints Of Large-Span-Weight Structures

Along with the development of large-span-weight spatial structures, stiffenedjoints are more and more applied to actual projects. The outside-stiffened methods areoften used, and more complete formulae of loading-carrying capacity and explanationof construction about these methods are presented in the domestic and foreign steeltubular structure codes and design guides. Until now complete formulae ofloading-carrying capacity about the inner-stiffened methods are not presented, whoseapplication depends on experiment and FEM analysis of actual projects. Hollowstructural section (HSS) structures include circular hollow section (CHS) structures,rectangular hollow section (RHS) structures and combined CHS-RHS structures. Theinner-stiffened joint is one of the stiffened joints. The research results of internalring-stiffened CHS joints were presented in domestic and foreign, but the research ofinner-stiffened RHS joints in spatial structures were seldom presented. The main jointmodel of this paper is made up of CHS brace and RHS chord combined with actualprojects. Through the analysis of loading-carrying capacity of simple T-shapedinner-stiffened joints under each kind of load condition, a series of methods used tostructural design are quantitatively summarized. Based on the analysis of T-shapedjoints, loading-carrying capacity formulae of K-shaped inner-stiffened joints arepresented. At last application of inner-stiffened joints in two projects are introduced.Despite complexity of inner-stiffened joints in the actual projects, quantitative resultsare acquired in this paper contributed to the basic research of inner-stiffened joints.Experimental method is applied to the research of HSS structures,correspondingly model analysis is important which includes simplified model andFEM. Loading-carrying capacity formulae of simple T-shaped joints were gainedbased on virtual work method and yield line theory which are efficient methods forjoint analysis. FEA is one of more important method along with the development ofcomputer and numerical analysis. Based on FEA mainly, secondly theory analysis,combined with joint tests, parametric analysis influencing loading-carrying capacity is carried out to acquire loading-carrying capacity formulae of unstiffened andstiffened joints.Failure modes of joints have been described based on experimental research asfollows mainly: flexural failure of the chord face, local bucking of the chord walls,punching shear failure of the chord face, weld failure between the chord and the brace.The failure mode of unstiffened and stiffened joints in this paper is flexural failure ofthe chord face by the control of the joint parameters. The failure mode in actualprojects is often combined, for example, the failure mode of full-width stiffened jointssubjected to bending includes bending distortion of the chord face, weld crack at thecomer of the brace welded to the chord and local buckling of the brace wall.The determination of ultimate limit is always important part of HSS joints. Theload-deformation modes are summarized as three main modes. About the connectionsthat do not exhibit a clearly defined yield or peak load, the concept of ultimatedeformation being complementary to the ultimate load, is accepted widely. The jointsbased on flexural failure of the chord face in this paper do not exhibit a apparent peakor yield load. The load at 1%b_o(d_o) deformation is suggested as the serviceabilitydeformation limit, and the load at 3%b_o(d_o) deformation is suggested as the ultimatedeformation limit. Although this standard is not absolute, the above concept isadopted widely. The upper proposal was mainly based on tests of hot-rolled andcold-formed sections, and it is also suitable for welded box section. The results ofT-shaped joint tests show that the ultimate capacity is close to code formula. Thisproposal is adopted for inner-stiffened joints in this paper, based on the samedistribution of Von-Mises stress on the chord face at 3%d_o deformation.Axial load, in-plane bending load and out-plane bending load are considered inthe loading-carrying capacity of T-shaped unstiffened and inner-stiffened RHS-CHSjoints in this paper. The design formula of these connections under axial load waspresented in many codes and guides, and the formula was gained for the connectionsunder in-plane bending load. Firstly, a formula is presented in this paper, for theultimate strength of T-shaped unstiffened CHS-RHS joint under out-plane bendingload. The yield line model based on flexural failure of the chord face, is assumed onthe basis of Von-Mises stress distribution on the chord face. There is a good agreement between the ultimate strength using this formula and the result of FEA.Secondly, a formula of T-shaped inner-stiffened CHS-RHS joint under axial load isacquired using yield theory and virtual work method. Subsequently, three formulae ofT-shaped inner-stiffened CHS-RHS joint under axial load, in-plane bending load andout-plane bending load are concluded based on the numerical calculation. Lastly aseries of formulae included the concept of efficient width are presented.Load-carrying capacity research of K-shaped unstiffened and inner-stiffenedconnections under axial load is carried out in this paper. It's found that the formulaeof T-shaped inner-stiffened joints are also suitable for K-shaped gap joints, and thereis a good agreement between the ultimate strength using the formulae and the resultsof FEA.The application of inner-stiffened joints to actual projects is more complicated.In this paper, one or two stiffener is laid internally on T-shaped or K-shapedconnections. But in actual projects, more than two stiffeners are used for reducingstress concentration, which brings difficulty to confirm ultimate capacity andquantitative analysis is difficult. The application of inner-stiffened joints in themegatrusses of Shanghai International Circuit and the Vierendeel trusses ofShanghai's Nanhui Administration Center, successfully resolved the strength lack ofunstiffened joints. Along with the increasing application of inner-stiffenedconnections, the correlative research will gain more and more fruit.