Research On Mechanical Behavior Of Square Steel Tube Columns Filled With Steel-Reinforced Self-Consolidating High-Strength Concrete

Adapting to the development of modern structures toward big-span, high-rise, heavy-load and the need of industrialized production and construction, a new design model of heavy-loaded columns is proposed by combining the steel-reinforced concrete, concrete-filled steel tube and high-strength high-performance technology together, that is square steel tube columns filled with steel-reinforced self-consolidating high-strength concrete. In this type of composite columns, steel section is inserted into steel tube and self-consolidating high-strength concrete is filled into the void between them. According to the project of National Natural Science Foundation named "a new design model of steel-concrete composite columns" (No. 50078008), investigations are carried out as follows:1. Two classes of self-consolidating high-strength concrete with cubic compressive strength of 73.2MPa and 103.8MPa respectively are successfully compounded, which achieve the cast of specimens without vibration.2. Fourteen composite stub columns are tested under axial compression. The experimental results show that failure mode of the new type of composite columns is quite different from that of composite columns without steel section; the ultimate strength and deformation behavior of self-consolidated columns and vibrated columns are almost the same; concrete strength, width-to-thickness ratio and the area of encased steel section have significant effect on the bearing-capacity and ductility of the columns; the existence of the core concrete changes the local buckling mode of the square tubes, which contributes to improve the width-to-thickness ratio limit of the composite columns to be 1.6 times that of the empty square steel tubes.3. Based on the experimental results, the axial stress-strain models for steel section, square steel tube and confined core concrete are proposed, in which the model of square steel tube considers the biaxial effect and the model of the core concrete considers the confining effect from the tube and the effect of steel section on its ductility. The models are used to calculate the load-deformation relationship for composite stub columns under axial load by fiber model method. The calculated ultimate strength and postpeak response agree well with the test results. Finally, the formula for predicting the axial bearing-capacity of the composite stub columns is proposed.4. Eight composite long columns are tested under axial compression. Before loading, the ultrasonic method is used to test the quality of the core self-consolidating high-strength concrete.It shows that the quality of the core concrete is uniform and no segregation occurs during casting and placing. The experimental results indicate that the slenderness ratio (Lq/B) has the most significant effect on the behavior of the composite long columns. Maximum axial load and its corresponding strain of specimens decrease with the increase oiLJB.5. The stability capacity of the composite long columns under axial load is analyzed by tangent modulus theory. Comparison between the theoretical and experiemtal results shows that this method can give a favorable prediction for the stability capacity of the composite long columns. Based on the experimental and theoretical results, formulas for estimating the axial stability capacity of the composite long columns are proposed.6. Three composite flexural specimens are tested under pure bending. The test results indicate that the deformations of square steel tube and steel section during bending are approximately consistent. Superposition method and ultimate state design method are used to derive the flexural capacity of the composite members. Both methods can give a conservative prediction for flexural capacity of the the composite members.7. Fourteen composite column specimens are tested under constant compressive axial load and cyclic lateral load. Test results show that steel section and concrete filled in it compose a core column which prevents the formation of fracture plane in concrete, thus improves the collapse-resistance ability of the newly proposed columns; axial load ratio, width-to-thickness ratio, strength of concrete, and the content of steel section have significant effect on strength, ductility, energy dissipation and stiffness of the composite columns, among which axial load ratio is the most important factor; to ensure that the composite columns have good ductility, it is necessary to limit the axial load ratio.8. The calculation formulas for cross-section bearing-capacity of the composite columns are derived by superposition method. The calculated results agree well with the results obatian by fiber model method, and tend to safety when compared with the test results. The proposed method is suitable to be used in engineering design.