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In this study, a total of 18 cold-formed built-up T-section (CFBUT) columns with two sectional dimensions and three length types were tested under uniaxial compression. The failure mode, ultimate strength and load-uniaxial displacement curve of the specimens were investigated. The test result shows that the main failure modes of long columns (LC), short columns (SC) and intermediate long column (MC) were flexural-torsional buckling mode, distortional buckling mode as well as flexure-torsional buckling mode, respectively. Subsequently, parametric analyses were conducted on the investigated specimens using the finite element analysis. The influences of the slenderness ratio and the web depth to plate thickness ratio on the mechanical behavior of the CFBUT columns were studied. Finally, the validation and accuracy of the effective width method in both Chinese design code GB50018-2002 and American code AISI-2007 and the direct strength method in AISI-2007 were evaluated for the CFBUT columns by using both the test and numerical results. The comparison results show that the prediction results obtained by adopting the two methods in both Chinese and American specification for LC, MC and SC-90 type specimen were rather conservative. However, for the SC-140 type specimen, the prediction difference between the experimental and calculation results obtained by adopting the two methods in both Chinese and American specifications can be controlled within 15%.

Multi-limbs built-up cold-formed steel stub column is a main structural member consisting of a single C-shaped and U-shaped basic component connected by self-drilling screws. Axial compression bearing capacity tests and finite element analysis of nine multi-limbs built–up cold–formed steel stub columns with three different section forms were conducted in this paper. The results of finite element analysis are close to the test results, which verify the accuracy of the finite element analysis. In order to study the influence of width–thickness ratio and screw spacing on the bearing capacity of the build-up column, the parametric analysis was carried out on the maximum width–thickness ratio of the plates and the screw spacing. The results show that the failure modes of all specimens are local buckling and distortional buckling. Multi-limbs built–up cold–formed steel stub columns consisting of a few basic components can work in harmony, the integral behavior is desirable. The axial load bearing capacity of the multi-limbs built-up section stub column increases when the maximum width–thickness ratio of the plates decreases; the screw spacing has a little impact on the ultimate axial compressive capacity and the buckling capacity of the multi-limbs built-up cold-formed steel stub columns.

The ductile fracture characteristics of Chinese Q460 high strength structural steel under quasi-static condition were studied by using mechanical tests of four types of notched specimens. The influence of stress state on fracture mechanism of the material was investigated by observing the fracture surfaces of all test specimens using the Scanning Electron Microscope. Meanwhile, corresponding numerical simulations were conducted to collect the critical stress and strain at notch for all test specimens. The effects of stress triaxiality and Lode angle parameter, which were found to be the key parameters governing the ductile fracture of metallic material in many studies, on fracture strain of the Q460 structural steel were investigated. The analysis results show that different fracture mechanisms were observed in different stress triaxiality regions. At high stress triaxialities, Q460 steel exhibits a typical mechanism of “void nucleation, growth and coalescence”. When stress triaxiality equals to zero, a shear fracture mechanism was observed. At low stress triaxiality values, fracture develops as a combination of shear and void growth modes. In addition, the ductility of Q460 structural steel under pure shear or plane strain is lower than that under axisymmetric tension, especially at low stress triaxiality.

The infilled CFS shear walls with FGD gypsum were developed in the paper. For lightweight design, the FGD gypsum was mixed with glazed hollow beads and air-entraining agents. Five full-scale specimens were investigated under cyclic horizontal loads. The configuration of the specimens varied in compressive strength of infilled gypsum and sheathing types. The shear performance of the walls can be improved obviously due to the participation of infilled material. The failure modes are characterized by the loss of stressed skin provided by sheathings together with compressive failure of infilled gypsum. Besides, a series of tests concerning fastener responses and material characteristics of gypsum was conducted. Furthermore, the numerical models based on OpenSEES were proposed. After calibration of parameters, a combination of beam elements with pinned joints, zero-Length elements with Pinching-4 material and equivalent truss elements with Concrete-01 material was employed to describe the nonlinear behaviour of the infilled walls. The comparison between tests and simulations was then carried out. The results of numerical models have an agreement on test results. The developed numerical model can be used for predicting cycle performance of the infilled CFS walls and related analysis of structural systems in further studies.

In the current AISI standard, the modified slenderness method (MSM) is proposed for the design of cold-formed steel (CFS) built-up columns. According to the available literature, such a method has been extensively adopted in hot-rolled steel research, and usually generates very conservative strength estimates. Nonetheless, there are few corresponding theoretical studies into CFS built-up columns. In this situation, this paper presents a new analytical approach to establish a computing method for the flexural buckling bearing capacity of the CFS built-up back-to-back section column. To achieve this goal, a new flexural buckling model is proposed to establish the kinematic relationship of single profiles. In addition, shear panels are employed at the location of screws to consider the discrete shear deformation restraint effect. The shear rigidity of the shear panels is determined using the cross-sectional shear stress transfer path. Then, based on the energy method, a computing method is derived. Further, simplifications are made to the formula to allow easier use in practical engineering situations. To verify the derived formula, a finite-element model (FEM) is developed and validated using the available test results. Then, based on the developed FEM, parametric studies are conducted to verify the derived formula. In addition, the performance of the derived formula is further verified by comparing the corresponding ultimate strength based on the direct strength method (DSM) expressions using available test results and FEM parametric studies. The comparison and validation results show the following: (i) the derived formula can predict the critical flexural buckling load as well as the corresponding buckling strength based on the DSM expression, and (ii) with an increase in screw spacing, the MSM will result in very conservative strength estimates for CFS built-up back-to-back section columns.