Ultimate Strength of Steel Pile Top Filled with Concrete Using Reduced Scale Model Test Considering Effects of Inner Ribs and Concrete-Pile Bonds

In the latest version of Japanese design standards for foundations, the contribution of inner ribs on the ultimate strength is calculated by substituting them with the equivalent concrete bonding effect. However, stress transmission via contact force between inner ribs and infill concrete and that via bonding force between a steel pile surface and infill concrete are not exactly the same mechanism, which may cause different mechanical characteristics especially at the ultimate state. A series of bi-axial loading tests were conducted to clarify stress transmission mechanism of concrete-filled parts of steel piles, using reduced-scale specimens. The results show that the stress transfer from the steel pipe pile to the infill concrete through the bond effect begins to decrease immediately after the steel pipe yields, and then the stress transfer through the inner ribs increases. Therefore, these two mechanisms did not work simultaneously, and inner ribs became more effective at the ultimate state.