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This paper presents an experimental study on the mechanical behavior, particularly the tensile stiffness, of square concrete-filled steel tubes (SCFTs) under axial tension, from both the force/displacement-level and strain-level perspectives. A group of SCFT and hollow tube specimens are tested with extensive data of the steel tube strains measured. From the force/displacement-level perspective, it is found that the tensile stiffness of SCFTs is 31.8% higher than that of hollow tubes on average. It is proposed that tensile stiffness enhancement is a combined result of the confine-stiffening effect and tension-stiffening effect . The confine-stiffening effect is because of the biaxial tensile stress state of the steel tube due to the concrete-to-steel tube confinement of SCFTs. From the strain-level perspective, the existence of the concrete-to-steel tube confinement is proved by comparing the measured transverse-to-longitudinal strain ratio of the steel tube between SCFT and hollow tube specimens. Besides, the evidence for the tension-stiffening effect in SCFTs is also provided by the non-uniformly distributed longitudinal steel strain of the steel tube. In addition, the fractal cracking phenomenon is experimentally observed and quantitatively measured, indicating the interaction of the concrete-to-steel tube confinement and tension-stiffening effect. It is concluded that the tensile stiffness of SCFTs will be underestimated if the SCFTs under tension are simply treated as equal to the hollow tubes by neglecting the concrete-to-steel tube confinement and tension-stiffening effect; this underestimation may cause non-conservative inaccuracies in calculating the component internal forces, structural natural frequencies, and seismic effects in the design and analysis of structures. [ABSTRACT FROM AUTHOR]

Abstract: When subjected to the horizontal earthquake load, compression–bending–torsion combined actions often exist in the piers of curved bridges because the center of stiffness and the center of mass of curved bridges do not locate at the same position. A new fiber beam-column element considering the nonlinear torsion behavior for concrete filled steel tubes with circular section was developed based on the traditional fiber beam-column model, which had widely been used. The calculation and solution procedures of the proposed fiber beam-column model were also illustrated. The reasonable geometrical relations, material constitutive law and force equilibrium equations were all applied. Based on the principles of fiber beam-column element considering torsion effect, a nonlinear solution program was developed using FORTRAN language. The entire loading history of concrete filled steel tube columns subjected to combined compression–bending–torsion load could be predicted with good agreement with test results. Using the proposed fiber beam-column model for the time-history analysis of curved steel–concrete composite girder bridges, the high solution precision and efficiency could be obtained, compared with the traditional finite element model. [Copyright &y& Elsevier]

Abstract: In this work, experiments on composite connections consisting of steel girders framing into concrete-filled steel tube (CFST) beam-columns conducted in recent years were investigated and the test results were compared with the calculation results of the AIJ specification, the calculation method proposed by Fukumoto and Morita, and the calculation method verified by Nishiyama et al. Through data analysis, the study confirmed the applicability of each of the three methods. The ultimate shear strength calculation results of Fukumoto and Nishiyama were found to be more accurate and reliable than AIJ specification procedure, and the shear yield strength calculation results of the two methods were also shown to be appropriate. Through a parametric analysis, this study confirmed that the three methods have a broad applicability for joints of various axial compression ratios, connection formations and column cross sections. The analysis results suggested that the applicable steel tensile strength and concrete compression strength for the AIJ specification should not exceed 450MPa and 70MPa, respectively, and the applicable ratio of steel tensile yield strength to concrete compression strength should not exceed 8 for the AIJ procedure to yield accurate results. This paper also provides proposed amendments to these procedures for joints having special connection details. [Copyright &y& Elsevier]

Abstract: In order to improve the ductility of the core wall in super high-rise buildings subjected to high axial compressive force and seismic effect, a new detailed concrete filled double-steel-plate (CFDSP) composite wall using high-strength concrete is proposed. This CFDSP composite wall is composed of concrete filled steel tubular columns at the two boundaries and concrete filled double-steel-plate wall body which is divided into several compartments by vertical stiffeners transversely connected by distributed batten plates. In order to intensively investigate the structural mechanism of this new type of CFDSP composite walls, twelve specimens are tested under large axial compressive force and reversed cyclic lateral load. No evident buckling of surface steel plates can be observed due to reasonable width-to-thickness ratios of steel plates and properly arranged batten plates, so that the surface steel plates and infill high-strength concrete can work compatibly in the whole loading process. All the specimens exhibited good energy dissipation ability and deformation capacity with full hysteretic curves and large ultimate drift ratios, thereby indicating that high-strength concrete can be used in seismic-resistant structures when the proposed new detailed walls are adopted. Based on the test results, the stiffness and strength degradations are analyzed, and the deformation characteristics of all the specimens are discussed in detail. Finally, a strength prediction approach based on the section analysis method is presented, and some detailing requirements for routine design practice are recommended. [Copyright &y& Elsevier]

Abstract: Based on the force–displacement mixed control quasi-static test on eight CFST columns subjected to combined compression, bending and torsion cyclic load, the mechanical behavior of CFST columns with various section types, bending moment to torsion moment ratios and axial load levels was studied. The test results showed that the hysteretic curves of CFST columns under combined compression, flexure and torsion are plump due to the good seismic behavior and the ductility was also good. But for rectangular CFST columns with high bending moment to torsion moment ratio, the strength degradation was observed due to the local buckling of the steel plate at the bottom. The torsion capacity of CFST columns would be reduced by the bending moment. The plane section assumption of axial strain of CFST columns could be satisfied. The shear strain of the steel tube has good linear relationship with the rotation angle of the section when CFST columns subjected to combined compression, flexure and torsion. Based on the test results and literatures available, the mechanism of CFST columns was qualitatively analyzed. [Copyright &y& Elsevier]

Abstract: Based on the quasi-static test on eight CFST columns subjected to pure torsion and compression–torsion cyclic load, the torsion behavior of CFST columns with various section types, steel ratios and axial load levels was studied. The test results showed that the hysteretic curves of CFST columns under pure torsion and low compression–torsion cyclic load are very plump, which indicate that CFST columns have good seismic capacity. The unloading stiffness of CFST columns was equal to the initial elastic stiffness. The torsion capacity of CFST columns could be improved by the low compressive load, and the ductility was also good. But the torsion capacity of CFST columns would be reduced by the high compressive load. When CFST columns subjected to pure torsion, spiral diagonal compressive struts will be created in the in-filled concrete, and the axial components of the diagonal compressive force of the in-filled concrete was equal to the axial tensile force of the steel tube in order to satisfy the axial load equilibrium condition on the section, so the axial strain will be produced in the steel tube. The shear strain has good linear relationship with the rotation angle of the section when CFST columns subjected to pure torsion and compression–torsion combined action. Based on the test results and literatures available, the torsion mechanism of CFST columns was preliminarily analyzed. [Copyright &y& Elsevier]

The concrete-filled double-skin steel tubular (CFDST) structures show high strength, large stiffness and convenient construction, which has been adopted in engineering practice more and more widely. However, the nonlinear shear behaviour of CFDST is still lack of research when the shear span ratio is low. This study conducts shear tests on four CFDST deep beams with large diameter of 1500 mm as well as low shear span ratio of 0.33 and 0.83, based on the application of CFDST in a typical nuclear engineering. The failure modes, load-displacement relations, concrete cracks and strains on steel tubes are carefully analysed. Then a refined nonlinear finite element model is built and the shear mechanism of the CFDST is revealed. The shear contribution of various components is obtained including the outer and inner steel tubes, the in-filled concrete, the stiffeners. Parametric analysis is further conducted to find the influence of key design parameters including hollow ratio, material strength and thickness of steel plate. Finally, based on test results and finite element analysis, a shear capacity formula considering the superposition of concrete, steel tube and stiffeners is proposed. It is demonstrated that the CFDST beams show typical shear failure for shear span ratio of 0.33 while shear-flexure failure for shear span ratio of 0.83. The shear strength is high with satisfied ductility. The proposed formula gives accurate prediction which is verified by test results and finite element analysis. • Shear tests are conducted on four CFDST deep beams with large diameter and low shear span ratio. • CFDST beams show shear and shear-flexure failure for shear span ratio of 0.33 and 0.83, respectively. • Shear contribution of various components is obtained including steel tubes, in-filled concrete and stiffeners. • Parametric analysis is conducted to find the influence of key design parameters. • A shear capacity formula considering superposition of concrete, steel tube and stiffeners is proposed. [ABSTRACT FROM AUTHOR]

In high-rise and super-high-rise buildings, steel outrigger trusses are commonly used as key components for improving the structural seismic performance. The joints between the trusses and core concrete walls enable reliable transfer of large axial loads from the former to the latter and therefore deserve special attention. In the present study, an equivalent fishbone-shaped beam–column model is developed for the analysis and design of the composite joint. The joint is composed of concrete-filled steel tube (CFT) boundary columns and double-skin steel plates, which work together to transfer the axial loads. In the proposed model, elastic beam–column elements are used to represent the CFT boundary columns, double-skin steel plates, horizontal rebar, and diagonal concrete struts in the wall. Two important geometric parameters of the model, namely the length of the double-skin steel plates and the height of the CFT boundary columns, are investigated. Comprehensive parametric analyses based on elaborate finite element models with solid and shell elements are conducted to reveal the main factors that determine the two above-mentioned parameters, and to propose calculating formulas. Then the model is further verified by finite element models with shell and solid elements in terms of the axial force distribution and horizontal displacement. Finally, a method for designing the truss–wall joint using the proposed beam–column model is developed and a typical design example is presented. It is demonstrated that the design of the joint is convenient and reliable once the axial load demand is determined by seismic analysis of the entire building. [ABSTRACT FROM AUTHOR]

This paper studies the mechanical behavior of the circular concrete filled steel tubes (CCFTs) under axial tension. A group of three CCFTs with different steel ratios and in-filled concrete diameters are tested and compared with hollow steel tubes (HTs). Based on the experimental results in the present and previous studies, three key issues of CCFTs under tension, i.e. the confining-strengthening effect , the confining-stiffening effect , and the tension-stiffening effect , are studied at both the stress-resultant level (i.e., force-displacement relationship) and stress levels (i.e., stress-strain relationship). Due to the tendency of the shrinking and slipping of the steel tube in relation to the infilled concrete, the confining contact stress and bonding shear stress, in the vertical and tangential direction respectively, are distributed in the interface between the steel and concrete, which results in the confining and bonding effect of the CCFTs. At the stress-resultant level, the test results demonstrate that the strength and stiffness of CCFTs are average 10% and 29% larger than those of HTs, respectively. The strength increase is contributed by the confining-strengthening effect, and the stiffness enhancement is a combined result of the confining-stiffening and tension-stiffening effects. At the stress-strain level, the measured strains show that the steel tube of the CCFT is in the state of the bi-axial tensile stress state because of the difference of the Poisson's ratio between steel tube and infilled concrete, which contributes to the confining-strengthening and confining-stiffening effects of the CCFTs. In addition, a fractal cracking phenomenon of the core concrete of tensile CCFTs is observed and discussed. [ABSTRACT FROM AUTHOR]

• Six innovative shear critical composite shear walls without tie bars were tested. • The shear compression failure for each specimen was observed with notable capacity and ductility. • The influence of axial compression ratio and faceplate slenderness ratio was elaborately investigated. • A novel database of 38 composite shear walls was established for shear capacity evaluation. Many skyscrapers have used the Reinforced Concrete (RC) filled Composite Plate Shear Walls (CPSW). Based on in-plane shear tests on six shear-critical CPSW specimens and a RC shear wall specimen, the behavior of CPSW systems with stud or tie bar connectors, various connector spacings, and various axial compression ratios were reported. In the test program, the Concrete Filled Steel Tubes (CFSTs) are applied for each specimen as boundary elements to simulate the engineering design of CPSW in skyscrapers. This paper presents the test observations, the ultimate capacity, the lateral stiffness, the ductility and the energy dissipation results. Based on the test results, the CPSW specimens with CFST boundary element failed in shear compression failure and the reference RC shear wall failed in shear tension failure. The weld fracture was generally found at boundary element (CFST) in the descending branch. Compared to the RC shear wall, the shear capacity of the CPSW systems were enhanced by 104–129%. The difference between CPSW systems with stud connectors and those with tie bars was insignificant in terms of ultimate capacity. Subsequently, a new database including 38 test specimens was established from the aforementioned test program and available literatures. The database results indicate the compressive capacity of infilled concrete is the dominating factor influencing the shear capacity, instead of the yield capacity of steel. In addition, a design formula for the in-plane ultimate capacity of shear-critical CPSW systems is proposed. Comparisons demonstrated that the proposed model was consistent and exhibited a reasonable level of consistency. [ABSTRACT FROM AUTHOR]