The concrete-filled steel tube is a building member which is formed after filling the concrete in the steel pipe, which can be divided into square steel pipe concrete, circular steel tube concrete and polygonal steel pipe concrete according to the section shape.
It utilizes the combination of two materials of steel pipe and concrete in the process of stress, fully exert the advantages of these two materials, so that the plastic and toughness of concrete improved greatly, and can avoid or delay the local buckling of steel tube, so that the overall steel tube concrete has a high bearing capacity, plasticity and toughness, good economic benefits and construction convenience. Although the steel tubular concrete is composed of two kinds of materials, it is regarded as a new material in the component industry, that is, the so-called "composite material" (no longer distinguish between steel pipe and concrete). Moreover, the performance change of concrete-filled steel tubular members under different load combination is continuous and uniform. The performance of concrete-filled steel tubular members is continuous and uniform with geometrical parameters, such as steel ratio, slenderness ratio and hollow rate. The performance of CFST members is continuous and uniform with the physical parameters, such as the strength of steel and concrete.
The properties of CFST members are continuous and uniform with their cross-section shape, such as circle, polygon, octagon, hexagonal and square.
Under the guidance of "Unified theory of concrete-filled steel tubular", the mechanical properties and design methods of high-strength concrete filled steel tube and the fire resistance and fireproof design method of steel tube concrete are studied in the industry. Study on properties of high strength concrete filled steel tube high strength concrete (the definition of high-strength concrete is different in the world, in our country, it is generally thought that the strength grade is C60 and above the concrete is high-strength concrete) is a hot topic in recent years both at home and abroad. This kind of high-strength concrete advantage is high strength, can save cement, reduce component section size, reduce the weight of the structure, so often used in large loads of structure, such as high-rise buildings, underground works and large-span structure of the pillars. However, the weakness of high strength concrete is brittleness and ductility difference, which hinders its application in practical engineering, especially under the condition of complex stress, the structure is controlled by brittle failure, and its reliability of work is greatly reduced. If high strength concrete is poured into the high-strength concrete of steel tube, the ductility of high-strength concrete is greatly enhanced by the effective restriction of steel pipe. In addition, the steel pipe has great shearing and torsional ability under the condition of complex stress. In this way, through the combination of the two, it can effectively overcome the weakness of high strength concrete brittleness and ductility, so that the engineering application of high strength concrete is realized and the economic benefit is brought into full play.
A large number of examples prove that compared with the concrete filled steel tubular Hang of ordinary strength concrete, steel tube high-strength concrete can save about 50% of steel and reduce the cost; Compared with reinforced concrete column, it does not need formwork, and can save more than 50% concrete, reduce the weight of more than 50%, and the steel consumption and cost are slightly more or about equal.
The steel tubular concrete formed by filling the steel pipe with high-strength concrete, besides having other advantages of ordinary strength concrete of steel pipe, can save more than 60% of concrete and reduce the structure deadweight by more than 60%. Because the concrete-filled steel tube is a new type of structure, the standard of fire prevention of this kind of structure has not been established at present, which not only restricts the popularization of the structure, but also lacks the necessary scientific basis for the fire resistance assessment of the constructed structure. Therefore, it is very important to study the fire resistance of concrete filled steel tubular and to determine its fire prevention design methods and standards.