Research On Mechanical Behavior Of Steel Tubes In Concrete Filled Steel Tubular Columns During Concrete Pouring

Concrete-Filled Steel Tube(CFST) not only has excellent mechanical properties,but also can be acted as formwork which makes the construction of CFST structures very convenient, meeting the requirements of industrialization of modern construction technology, is widely used in engineering fields. During the construction of CFST structures, hollow steel tubes are installed first in several floors generally, followed by floor structures, and then pouring core-concrete at one time, so that the steel tubes subjected to vertical construction loads in addition to horizontal loads of inner wet concrete will suffer initial stresses and deformations before working together with concrete infilled, causing construction problems easily. The research on the mechanical behavior of steel tubes during concrete pouring is very important to quality and security of construction.The square and rectangular steel tube is taken as the research object due to poor performance compared with circular steel tubes. A mechanical analysis model are established with reasonable model assumptions after analyzed the force conditions of steel tubes during pouring, and it is verified in good agreement by an practical example.Based on the finite element model, this paper mainly completed the following tasks:(1)By comparing the effects of position relation on interior columns, side columns and corner columns in concrete-filled steel tubular frame structure system, it found that position relation has a great influence on the vertical stress of steel tubes and has less on other mechanical behavior with different width-thickness ratios, so it is not necessary to take position relation into account in construction if maximum stress and deformation in the steel tubes are chosen as main security control items.(2)The influencing factors on the mechanical behavior of rectangular steel tubes are also analyzed, the results show that maximum stress and deformation in the steel tubes increases linearly with the increase of the number of floors which can be lifted-up pouring at one time under the condition that the maximum stress does not exceed the proportional limit, but not with length-width ratio; the inner ring plate effectively reduces the stresses and deformations in steel tubes. With the larger length-width ratio,and the better, the closer to the position of maximum stress and deformation, the effect is more significant. The stresses and deformations can also be limited obviously due to vertical inner plate.(3)Lastly, this paper analyses and discusses the construction organization scheme for square and rectangular steel tubes with main aspects of the number of floors which can be lifted-up pouring, pumping pressure and setting up later braces. The research found that the maximum number of floors can be lifted-up pouring without reinforcement measures decreases with the increase of width-thickness ratio and pumping pressure, and to rectangular steel tubes with length-width ratio and pumping pressure, the changes are nonlinear. Setting up braces according to the method presented in this paper, for the square steel tubes, the main arrangement spacings and section size are equal basically or proportional with strong regularity, but for rectangular steel tubes with small section size and length-width ratio changed in the range of 1.2~1.6, the main arrangement spacings vary less. The position and quantity of braces for square steel tubes is mainly controlled by maximum stress, while rectangular steel tubes controlled by maximum deformation.The analysis mentality methods and research conclusions in the paper could be references for the formulation and optimization of the construction organization scheme for concrete filled steel tubular columns.