Abstract
Modern construction projects such as skyscrapers, bridge with a large span, residential structures, towers for transmission, warehouses and other industrial structures have all incorporated concrete-filled steel tube (CFST) columns. Fire resistance, excellent ductility and high bearing capacity are all the advantages of CFST columns. Among the different shapes of CFST columns, uses of rectangular and square CFST columns are becoming more favored in modern construction projects for their simple beam-to-column connections design. This study tested 10 concentrically loaded CFST columns and 8 eccentrically loaded CFST columns. One of the most effective techniques to delay local buckling and enhance the ultimate strength of the CFST columns is to use longitudinal inner stiffeners on these steel tubes. Cold-formed steel tube sections are vulnerable due to local buckling; however, a concrete core is able to restrict such early local buckling in the cold-formed steel tube. The test specimens consisted of unstiffened sections and longitudinal inner-stiffened sections. In total, the experimental program tested 18 CFST stub columns with both compact and slender sections, without longitudinal stiffener, with 2 longitudinal inner stiffeners and with 4 longitudinal stiffeners. The test strengths have been as compared with predictions based on the existing design code AISC 360-16 for calculating the ultimate capacities of the CFST stub columns subjected to both concentric and eccentric loading. The focus was mainly on the effect of vertical stiffener square concrete-filled steel tubes on the overall concentric and eccentric compression capacity. The results identified that the longitudinal inner stiffeners had a significant impact on the ultimate strength, deformability and mode of failure in the CFST stub columns. To prevent local buckling of such steel tube, a longitudinal inner stiffener could be applied. In addition, the compression capacity was increased by 12–13.5%.
Highlights
In the past decade, especially in Asia, concrete-filled steel tubes (CFST) were becoming more popular in skyscrapers, bridges, other industrial structures and pile constructions because CFST has advantages of high ductility, high energy absorption capacity, good strength and stiffness and saves cost in construction
The CFST stub column with a 3.2 mm thick steel tube (C1 and C2) exhibited vertical deformation during loading where the vertical deformation slowly developed with the increment of loads, and the CFST stub columns were classified as compact sections, with local buckling occurring originally in one of the CFST stub columns just prior to the peak load was developed and other deformations due to buckling occurred in the CFST stub column after the maximum load
For the CFST stub columns C6 and C7, local steel tube buckling failure arouse upon reaching the peak load, which happened much later than for the CFST stub columns C4 and C5. This was attributed to the fact that the buckling mode shape of the 4 longitudinal stiffener steel tubes could change from single curvature to double curvature, as shown in Figure 4 because the concrete core was able to delay and prevent local buckling of the longitudinal inner stiffener
Summary
Especially in Asia, concrete-filled steel tubes (CFST) were becoming more popular in skyscrapers, bridges, other industrial structures and pile constructions because CFST has advantages of high ductility, high energy absorption capacity, good strength and stiffness and saves cost in construction. The rectangular/ square shapes exhibit reduction on both the lateral confining pressure when compared to circular CFST and with the enhancement of high strength steel; this becomes very noticeable in cold-formed steel tubes as local buckling is delayed in failure mode. Ge and Usami [1] developed longitudinal inner stiffeners to improve the performances of square/rectangular CFST stub columns In their experiments, local failure mode of the steel tube was hampered by longitudinal stiffeners. Yang [5] investigated 2 non-stiffened counterparts and 8 stiffened square CFST stub columns with high slenderness ratio of enclosing steel to study the stiffeners affected the tube's local buckling.
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