Concrete-filled steel tube refers to the structural members formed by filling concrete in the steel tube, and the steel tube and its core concrete can bear external loads. According to the different cross-sectional forms, it can be divided into round steel tube concrete, square, rectangular steel tube concrete and polygonal steel tube concrete, etc. .
It is well known that concrete has high compressive strength. However, the resistance to bending is very weak, and the steel, especially the steel, has strong resistance to bending and good elasticity, but it easily loses its stability and loses its axial compression resistance when it is under pressure. The structure of concrete-filled steel tube can combine the advantages of the two, can make the concrete in the lateral pressure state, its compressive strength can be doubled. At the same time, due to the presence of concrete, the rigidity of the steel tube is increased, both are common. Play a role, thereby greatly improving the carrying capacity. As a newly emerging composite structure, the concrete-filled steel tube (CFST) is mainly used for compressive members with relatively low axial stress and eccentric force and is widely used in frame structures (such as factory buildings and high-rise buildings). The rapid development of the concrete-filled steel tube structure is due to its good mechanical properties and construction performance, which are manifested in the following aspects:
High bearing capacity, good ductility, excellent seismic performance
In the concrete-filled steel tube column, the confinement effect of the steel tube on its internal concrete makes the concrete in the three-direction compression state, which improves the compressive strength of the concrete. The concrete inside the steel tube can effectively prevent local buckling of the steel tube. Studies have shown that the bearing capacity of concrete-filled steel tubular columns is higher than the sum of the corresponding bearing capacity of steel tubular columns and the bearing capacity of concrete columns. The interaction between the steel pipe and the concrete causes the failure of the concrete inside the steel pipe to change from brittle failure to plastic failure. The ductility of the component is obviously improved, the energy dissipation capacity is greatly improved, and the seismic performance is superior.
Plasticity refers to the plastic deformation ability under static load. The axial compression test of the short column of concrete-filled steel tubular columns shows that when the specimen is compressed to 2/3 of the original length and the longitudinal strain reaches 30% or more, the specimen still has bearing capacity. After peeling off the steel pipe, although the internal concrete has a large bulge and fold, it remains intact, not loose, and still has about 5% of the bearing capacity. It is crushed and shed after being hammered with a hammer. Seismic performance refers to good ductility and energy absorption under dynamic load or earthquake. In this regard, concrete-filled steel tube members are much stronger than reinforced concrete members. The hysteresis curve of bending moment curvature under cyclic bending loads indicates that the energy absorption performance of the structure is particularly good, no stiffness is degraded, and there is no falling section, which is the same as the steel column without losing local stability, but in some buildings, Steel columns often use very thick steel plates to ensure local stability. However, local stability is also lost after plastic bending. Therefore, the seismic behavior of concrete-filled steel tubular columns is also superior to that of steel columns.
Construction is convenient and the construction period is greatly shortened
In the construction of concrete-filled steel tube structures, the steel tubes can be used as stiff skeletons to bear the construction load and structural weight during the construction phase. The construction is not affected by the concrete curing time. As there is no steel inside the concrete-filled steel tube, the concrete can be poured and compacted easily; In the construction, no template is required, which saves the cost of materials and labor for mold and mold removal, and saves time.
Good for fire resistance and fire prevention of steel pipes
Because the steel pipe is filled with concrete, it can absorb a large amount of thermal energy. Therefore, the distribution of the temperature field of the pipe section is very uneven during a fire, increasing the refractory time of the column, slowing down the temperature rise of the steel column, and once the steel column yields, the concrete Can withstand most of the axial load and prevent structural collapse. The fire resistance of the composite beam will also increase because the temperature of the steel beam will be reduced by transferring heat from the top flange to the concrete. The experimental statistics show that the fire-retardant coating can save 1/3 to 2/3 or even more when compared with the steel column in the first-class fire-resistance for 3 hours. With the increase in the diameter of the steel pipe, more paint is saved.
Corrosion resistance is superior to steel structure
The pouring of concrete in the steel pipe reduces the exposed area of the steel pipe, and the area of corrosion by the outside air is much less than that of the steel structure, and the cost for corrosion resistance and corrosion resistance is also saved compared with the steel structure. The cross-section form of concrete-filled steel tubular members has a great influence on the mechanical properties, ease of construction, construction period and project cost of concrete-filled steel tubular structures. The steel pipe concrete compressive component is effectively restrained by the round steel pipe to its internal concrete, so that the concrete inside the steel pipe is in the three-direction compression state, so that the concrete has a higher compressive strength. However, the construction of round steel tube concrete structures is difficult and the construction cost is high. In contrast, the construction of square concrete-filled steel tubular structures is more convenient, but the concrete-filled steel tubular concrete is less constrained and the structural bearing capacity is lower.
There are few parts of concrete-filled steel tube columns, and there are few welds and the structure is simple. The column foot often uses insert-type column feet that are reserved on the concrete foundation. Therefore, the factory manufacture is relatively simple, and the weight of the components is small, and the transportation and lifting are also relatively low. Easy construction is very simple, and the concrete-filled steel tube columns are made of sheet metal. The thickness of the sheets is not large, generally within 40mm. No matter whether the factory welding or on-site docking, there is no difficulty. At the same time, compared with reinforced concrete columns, the steel pipe of the concrete-filled steel tubular columns has the function of reinforcing steel bars, and has the functions of longitudinal steel bars and lateral stirrups. Therefore, there are no steel bars in the pipes, and a series of processes such as steel bar cutting and tying steel bars are saved. Since the column outer skin tube itself is a template for lateral pressure resistance, it also saves the process of mold support and mold removal. In recent years, pumping rafts are quite common, and there is no difficulty in watering at the site. The construction method of concrete that has been created and widely used in China and can't be easily thrown down has simplified the process of concrete filling on site, and the construction has been simplified. There is also a temporary pouring hole in the lower part of the column, and the method of pouring the concrete from the bottom up with a concrete pump is fast and ensures the quality of the watering. Moreover, after pouring, the steel tube is in a relatively stable humidity condition, the moisture is not easy to evaporate, the water-saving maintenance process is omitted, and the curing process of the concrete is simplified.