Study On Mechanical Behavior Of Prestressed Concrete Underpinning Joints

With the rapid development of urban construction, the problem between the important building demolition and protection in the urban planning and old city reconstruction is increasingly prominent. Building moving offers a good way and technical support to solve this problem. Underpinning is one of the key technologies in the building moving, which affects stability, security and economy directly of the process.Along with the increase of large span buildings, complex structure and the high rise buildings, column spacing increase lead underpinning load bigger. Existing studies have shown that the design between the old and new concrete surface is more significant for the increase of underpinning load. Therefore, how to ensure the underpinning node on the surface of the beam combination between the old and the new security, is the key to solve high-rise building big tonnage underpinning problems.To solve the problem of large tonnage underpinning, research group proposed the prestressed underpinning joints, and have carried out the relevant experimental study. The experimental study showed that the prestressed underpinning joint can improve the anti sliding bearing capacity of the underpinning joint significantly. Based on the existing experiment research, this paper study on the influence of prestress on the anti sliding bearing capacity of the underpinning joint and the relationship between the prestress and the bond slip stiffness of the new and old concrete contact surface of underpinning beam and frame column, and analyze the interfacial shear distribution proportion relationship of underpinning beam and underpinning coupling beam with different prestress applying way by the finite element analysis of the underpinning joint. Specifically:1. The influence of direction of prestressing on interfacial bond-slip stiffness between underpinning beam and frame column is significant, and the bond slip stiffness is in good linear relationship with the amount of the prestressing, and through experimental data, the formula of bond-slip stiffness and prestress (preloading stress) is obtained.2. The amount of the prestress influences a little on the distribution ratio of the friction shear which works on the underpinning beam and the underpinning coupling beam under the same direction of prestress condition. The interfacial friction of underpinning beam of the four-side wrapped underpinning is less than the underpinning coupling beam’s, indicating that more than a half of the underpinning load is transmitted by the underpinning coupling beam interface. We can conclude that the four-side wrapped underpinning node is superior to the double beam underpinning joint(without coupling beam), and the necessity of setting the underpinning coupling beam.3. The anti sliding ultimate bearing capacity of the prestressed reinforced concrete underpinning joints including adhesion after injury, friction provided by all around "hoop force" and interfacial friction provided by prestress, and the modified formula of anti slip bearing capacity of prestressed underpinning joints is obtained by finite element siumulation.4. For the two-way prestressed underpinning joints, the ultimate bearing capacity of underpinning joint is obtained by the underpinning component, with the safety of the bearing capacity of underpinning beam or coupling beam.