Loading tests and numerical analyses using the finite element method were carried out to clarify the deformation characteristics of a concrete-filled double steel pipe pile subjected to a horizontal load. The diameter and length of the concrete-filled double steel pipe pile were about two times and ten times the inner pipe diameter, respectively. The results indicated that the steel pipe and concrete behaved as a single unit from the pile head to the central part of the concrete-filled double steel pipe pile. On the other hand, this unity was lost in the length range of 3 to 4 times the diameter of the inner steel pipe from the bottom end of the concrete-filled double steel pipe pile. This is due to slippage at the boundary between the steel pipe and the concrete, and local compressive deformation of the concrete. We attempted to simply calculate the deformation of this pile when subjected to a horizontal load by constructing a beam spring model that combines the interface springs between the steel pipe and concrete and Timoshenko beams. The applicability of this calculation method was verified by comparing the results with those of loading tests.
