Performance Of Concrete-filled Double Skin Steel Tube To Steel Tube K-joints

Concrete-filled double skin steel tube (CFDST) is a new type of compositemember, which has an increasing utilization in infrastructures, e.g. electricitytransmission towers. When CFDST member is adopted as the chord member intrusses and towers, composite K-joints are formed by the CFDST chord andhollow steel tubular brace. In the past, seldom research have been reported onthis kind of joints, both experimental and theoretical investigations are thusneeded in this area. This paper mainly focused on the performances of K-jointsformed by CFDST chord and hollow steel tubular brace with circularcross-sections (referred as CFDST K-joint for short). The main research workand the achieved outcomes include:1. Investigations were carried out on the behavior of lateral local loadedCFDST members. The influencing of important parameters such as hollowration, loading angle and etc. were investigated through experiments. It wasobtained from the test results that the loading capacity as well as ductility ofCFDST members subjected to local bearing forces is much better than that of thereference hollow steel tubes. Finite element analysis was then carried out, basedon which the typical failure modes, load-deformation relationship andload-transfer mechanism of the composite members under local bearingcondition were studied.2. A set of experiments on the CFDST K-joints subjected to static loadswere conducted. The tests were carried out in two types of typical boundaryconditions, from which the load carrying capacity, deformation ability as well asstress development of the composite joints were studied whilst the influencingof important parameters on the capacity and failure modes were investigated. Itwas observed that the CFDST K-joints show favorable capacities under staticloading, failure modes of which are different with that of the hollow steeltubular truss joints. Plastification of the chords as well as buckling of the braceswere obtained when the hollow ratio of the chord ranges from0to0.6whilst thediameter ratio between the brace and the chord ranges from0.4to0.6. 3. A finite element analysis (FEA) modeling was established to study thebehavior of the CFDST K-joints under static loading. Both material andgeometric nonlinear, as well as the interaction between the steel tube andsandwished concrete were considered in the FEA modeling, the accuracy of themodeling was verified using the experimental results in this paper and otherliteratures. The FEA modeling was then used to carry out analysis on the typicalfailure modes and full-range load-deformation relation of the composite joints.The stress distribution study of the chord and brace members were alsoachieved.4. Simplified design formulae for the CFDST K-joints were proposed.Parametric analysis was conducted to study the influencing of importantparameters that affect the loading capacities of the CFDST K-joints, includingthe hollow ratio of the CFDST chord member, the diameter ratio between thebrace and the chord, the axial loading ratio of the chord, material properties, andso on. Based on this, typical failure modes of the CFDST K-joints under loadingwere summarized and the corresponding design formulae for the plastificationof the chord, as well as bearing failure of the sandwished concrete wereproposed. The calculated results show good consistency with these of themeasured results and calculations of the FEA modeling.