Developments of Concrete-Filled Steel Tube Structures Stiffened by Diagonal Ribs

Concrete-filled steel tubular (CFST) columns have high strength, favorable seismic performance, reasonable cost, and are widely used as the main structural types in high-rise buildings and heavily-loaded structures. In comparison to circular CFST columns, square CFST columns have the advantages of relatively wide section, large flexural stiffness, high flexural capacity, simple joint details, convenient manufacturing and construction, convenient layout of building space, and easy decoration, and thus are widely used in practical engineering. However, the previous research indicated that the composite action between a rectangular steel tube and the in-filled concrete is relatively weak and can be improved by setting stiffening schemes. Meanwhile, shear connectors are usually welded to the steel tube to improve the interfacial longitudinal shear transfer capacity. The existing stiffening forms are summarized as point-open, point-closed and line-open confinement. The diagonal stiffener, welded on two adjacent sides of a steel tube is referred to as the diagonal rib; it is a new efficient stiffener developed recently and can also be used as the shear connector. This paper systematically summarizes the mechanical behavior and design methods of members, joints, and frames of concrete-filled steel tube stiffened by diagonal ribs. At the level of members, the concentric compression, eccentric compression and seismic behavior of columns were analyzed; the details of diagonal ribs, width-to-thickness ratio limits, thickness matching relationship between the diagonal rib and steel tube, and axial load ratio limits were recommended; the modified plastic stress distribution method was proposed to calculate the strength of columns under combined compression and bending; the shear mechanism of ultra-short columns was figured out, and the shear model and shear force versus shear deformation relationships were proposed. At the level of joint, the axial compression and seismic behavior of CFST column to RC beam joints were studied; the axial compression and shear mechanism were analyzed, and the rational details of the joint zone were suggested to satisfy the requirement of strong-joint/weak-component; the modified equations, which considered the effect of axial load and compression zone height of the column, were proposed to more accurately predict the bond performance of beam reinforcements in the joint zone. At the level of frame system, the pushover analyses, IDA analyses and seismic fragility analyses of CFST column stiffened by diagonal ribs to RC beam frame were conducted, and the yielding mechanism and collapse mechanism were analyzed. The results showed that the diagonal ribs combine the advantages of existing stiffeners, efficiently transferring the interfacial shear force, constraining the concrete and avoiding or postponing the local buckling of steel tube, significantly improve the load capacity, deformation capacity, and seismic performance of square concrete-filled steel tubular columns, and have great application prospect in practical engineering. Finally, the engineering application scenarios for diagonal-rib stiffened CFST structures are listed; the development direction and problems which need further investigation of the structure are outlined.