Previous studies summarized the application of concrete-filled steel tube (CFT) in architecture, the importance of interfacial bond stress state, and analyzed its combined effect. The experimental study shows that shrinkage is very harmful to bond stress, and the degree of shrinkage is determined by concrete characteristics, steel pipe diameter and inner surface state of steel pipe. The larger the diameter of steel tube and d/t value, the smaller the bond stress, which is closely related to the sliding between steel bar and concrete surface.
1 overview
The application of CFT column in Japan is ahead of the United States, and most of the research results come from Japan. In practice, the performance requirements of the interaction surface have been analyzed, and there are reports in the United States to prove this deficiency, but some test results have also been evaluated, and the bond stress and shear connection of CFT at different levels have been studied. The experimental data can be used for our reference.
Most CFT columns in the United States are supporting members, and the axial stress under vertical load needs the continuous action of bond stress. Its diameter often exceeds 1000mm, even as high as 3000 mm, and the d/t ratio reaches 100, and some structures even reach 200. Because the weak axial stiffness will affect the overall function of CFT, high strength concrete is often used.
CFT columns are widely used in Japanese seismic structures. Both circular and rectangular tubes have been popularized. The diameter of a circular tube is usually less than 700 mm and the d/t ratio is less than 50. Shear connection mode of members: the shockproof partition is embedded in the steel pipe, and then concrete is poured. This fixed connection reduces the load of bonding stress. At the same time, innovations are being made to strengthen the bonding ability, such as setting ribs in steel pipes. The research and development of CFT in China began in the mid-1960s, and it was first applied to the Beijing subway project, and was successfully used to build the platform columns of "Beijing Station" and "Qianmen Station". After that, the platform columns of the circular subway project all adopted concrete-filled steel tube structures. Since 1970s, it has been gradually applied to single-storey and multi-storey industrial workshops, blast furnace and boiler frames, power transmission and transformation frames and various supporting structures. More than 100 construction projects have been completed, and the diameter of steel pipes used is increasing.
2 Bond load of concrete filled steel tube
Firstly, the structural model, six-story resisting frame and twelve-story supporting frame are designed, and the load acts on the same center to carry out the test. For different structural systems and different positions of the structure, the bond stress load is different, and the load is the largest in discontinuous areas such as foundation and connection. Filling concrete in the connection requires less connection force than direct steel connection, and the bond stress on the flexural member is much smaller than that on the supporting frame. Braced frame system under horizontal load. At each node, the axial force on the bearing is transformed into vertical load, and at the position, the tensile force is transferred to the column foot, and the bonding stress is at the maximum load.
3 Determination of interface state
The mutual transmission of adhesive force between steel pipe and concrete depends on the pressure generated by plastic concrete in the shell, the radial displacement generated by the shrinkage of concrete core and the irregularity degree inside the steel pipe. The radial displacement is limited to the range where the interaction between concrete and reinforcement has not yet appeared, and the strains of reinforcement and concrete are not the same at this time.
Because the cross section of CFT column is axisymmetric and the pressure is p, it can lead to the radiation expansion of cross section e 1. For steel pipes:
( 1)
D= diameter, t= wall thickness, Es= elastic modulus, and c= linear shrinkage strain of concrete.
The pressure range depends on the viscosity of concrete, the humidity at both ends, the pressure borne by concrete itself and the diameter of steel pipe; Transverse shrinkage value e2 of concrete:
(2)
The influence of shrinkage comes from the concrete itself, curing conditions and diameter, and there may be three states on the interacting surface:
State a: (3)
State b: (4)
Country c: (5)
Gap value caused by irregular inner surface of e3 steel pipe.
State A: There is always concrete pressure on the interaction surface, and the internal bond strength after shrinkage is provided by the bond force between steel bars and concrete. This state is called chemical bond force. When the increase of shear force exceeds the bearing capacity formed by this bond, the load is mainly borne by the mechanical bite force of the surface. At this time, there are two characteristics: the friction caused by the interaction of surface pressure and the bond stress caused by the bite of steel bar and concrete. This paper does not distinguish these two different mechanical bonding stresses;
B state: when shrinking, there is a gap between the two materials and the rigid body moves. When one of the materials is pushed, it has only a small bonding strength and resistance;
State c: it is an intermediate state, with huge bond stress loss, and the mechanical bite force is accompanied by unpredictable behavior of reaching state b ... which is beneficial to test the transition of CFT members from one state to another. The default values of this data are usually c=0.003 and d/t= 100. The figure shows that when the pressure on concrete reaches 1.2MPa, a state appears, and when the value of d/t is smaller, more pressure is needed, which is difficult to obtain under practical conditions. However, the contraction displacement e2 is generally greater than e 1, so it is difficult to reach the state A in CFT.
If the value of c is 0.003 and the value of d is 2540mm, the roughness in the pipe should reach 0.38mm to prevent the appearance of B state. If the pipe diameter is small, it should be smaller accordingly. In traditional practice, the roughness inside the actual pipeline can generally exceed 0.25 mm This shows that the surface conditions of most CFT tend to be C state. In addition, these comparisons also show that CFT members with large shrinkage and large diameter may have B state.
State C provides variable performance as long as the concrete irregularly engages with the steel surface and the concrete itself shrinks. As mentioned earlier, this biting force is smaller in steel pipes with larger diameters. In the steel pipe with large diameter, it may not exist, and the shrinkage state is uncertain, and the uncertainty of long CFT members and diameter size may reach state C. The value of d/t is very meaningful, because only by keeping the irregular shape in the pipe, the biting force can effectively prevent the concrete from sliding, and the radial stiffness of the steel pipe with large d/t value will be slightly lower. Therefore, it is easy to twist, which leads to the reduction of bonding stress caused by occlusion.
State C is an unpredictable interface state, which not only changes spatially along the length direction, but also changes spatially at the intersection edge of two materials in the steel pipe. This ability of local bond stress is unreliable. In order to get useful design values, the average value must be taken in a limited area.
Computer analysis; In order to better understand the bond stress between steel and concrete, the simulation state of steel and concrete in CFT columns is analyzed by three-dimensional grid. ANSYS and SAP are usually used for this analysis. Firstly, axial load or bending moment is applied to steel or concrete, and then elastic redistribution is carried out to achieve the mixing effect. The calculation is based on the pipe diameter, wall thickness and column length in practical application. When the state of the interaction surface allows 0 to slide, the bond stress will be normalized according to the true curve of exponential distribution, and the compressive stress on concrete will reach the maximum at the interaction surface closest to the action point (end), but it will be approximately 0 at the end d/2. This calculated distribution is not affected by the length, unless the length of the sample is close to d/2. For steel pipes with a d/t value greater than 100, the distance between the bond stress and the zero point is slightly less than d/2, while for steel pipes with a d/t value less than 50, it is slightly greater than d/2. The limit value of normalized bond stress can reach 1.0. In the range of pressure point 0.2d, the sliding length regenerates the friction force, and the friction force replicates and overlaps to absorb the constant bond stress.
The load conversion characteristics of sliding section and the exponential distribution characteristics of bond stress in non-sliding section indicate that the bond stress demand is concentrated in a certain area. If sliding is to be avoided, the unbalanced load between steel and concrete must be distributed in a small area.
4 Research on the corresponding tests Most of the tests to evaluate the bonding ability are thrust tests. Bond stress is defined as the average value of surface stress, which is related to the sliding of concrete core rigid body relative to steel tube. The load causing sliding is p, and the maximum average bond stress is, which is expressed as:
(6)
L= concrete surface length;
Another test scheme is to remove the air isolation and replace it with a combination of concrete and steel, so P in formula (6) is the maximum load provided by the stress converted (transferred) to the steel-concrete combination ratio at the foundation. Verdi, Dowling (1975), Chakir-Carlyle (199 1, 1993a, b) and morrissey Tower. (1979a, b) has been tested on this kind of equipment.
1, under eccentric load, the numerical growth rate is greater than the axial force;
2. With the increase of steel-concrete interface roughness, the value increases;
3. This value remains unchanged regardless of whether the connecting member is installed or not, and the connecting key only starts to work after the sliding starts.
The maximum diameter of CFT bond stress test specimens is 300mm, most of which are less than 200mm, and the d/t value is less than 60, which is mainly distributed in the range of 15-35. The diameter and d/t value of these specimens are smaller than the actual components, so there is a problem whether the test results are applicable to practice.
5 Summary and generalization
By determining the bond stress requirements of buildings, two structural prototypes are designed and analyzed. The analysis results show that the demand for bond stress of bracing members is greater than that of flexural members. Among them, the most important part of bond stress demand is the connection between CFT column and foundation, but the connection between support beam and CFT column is equally important, because support plays the role of transmitting vertical component. The details of the connection are very important for the requirements of bond stress. If members with shear keys are inserted into concrete fillers to resist the sliding between steel bars and concrete, the requirements for bond stress can be greatly reduced.
The analysis of the contact surface between steel pipe and concrete filler shows the importance of concrete shrinkage and the influence of column diameter on bond stress performance. If the sliding of the interface between steel and concrete is prevented, the bond stress requirement will be reduced and will not exceed 1/2 along the interface diameter. For steel pipes with large d/t value, the conveying length will be shorter, the d/t value will be smaller and the conversion length will be longer. When slip occurs, the bond stress is distributed almost uniformly along the slip zone.
The previous test results for testing the factors affecting bond stress clearly show three trends, although the results are very scattered:
1. The bond stress of rectangular CFT columns is lower than that of circular columns;
2. The relationship between bond stress and concrete strength is not obvious;
3. Bond stress decreases with the increase of pipe diameter and d/t value.
The last phenomenon is worthy of attention, because in practical application, the larger the diameter of CFT column means the larger the d/t value, and there is no data record in this field in the past, which needs further research and practice.
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