Experimental Study on the Mechanical Performance of Column Underpinning Joints with Prestressed U-Shaped Steel Bars

To meet the requirements of highly efficient force transmission, easy operation and low damage in RC column underpinning engineering, a new underpinning joint with prestressed U-shaped steel bars was proposed. Static testing of seven specimens was carried out, and the number of the U-shaped steel bars, their arrangement and the pre-pressure acting on the joint were considered as the main parameters. The results demonstrated that the U-shaped steel bars might greatly increase the load-bearing capacity of the underpinning joint, and the more the number, the higher the capacity; the arrangement of the U-shaped steel bars also had a significant effect on the load-bearing capacity. Two pairs of the U-shaped steel bars arranged on the opposite sides of the column was better than that arranged on four sides. And the horizontal part of the UsSBs embedded in the column grooves was better than in the underpinning beams; the pre-pressure could effectively improve the sliding stiffness of the underpinning joint, while the effects on the load-bearing capacity was not obvious. Based on the curve of load-displacement, force process of the underpinning joint was divided into four phases: no slip, initial slip, interfacial slip expansion and punch slip, consequently a constitutive force-slip model was constructed. As a result, a simple mechanical model of the underpinning joint was established, and the calculation formula for the load-bearing capacity of the new joint was proposed. Finally, the engineering design expression was prepared according to the characteristics of the real underpinning project.