Development of Pipe-in-Pipe Filled with Granular Material for Flexible and Ductile Bending Performance

The pipe-in-pipe which the authors propose in this paper is an innovative structural material to exert a more flexible and ductile bending performance than usual steel pipes. It consists of double thin wall pipes with granular materials such as sand between the outer and inner pipes to transmit their interactive stresses within a cross section. If a single hollow pipe is subjected to bending moment, the curvature of the pipe increases with the growth in magnitude of bending moment. Upon increasing of the bending moment, the pipe finally failed by local failure, and an oval-shaped deformation of cross section was observed. This failure is known as the Brazier Effect, a kind of local buckling. However, the major parts of the pipe still keep its original strength except for the failure portion. The sand filled between the outer and inner pipes of the pipe-in-pipe prevent this local failure by transmitting the interactive stresses between the pipes. As a result, smooth and flexible bending deformation is realized even under large bending moments. The purposes of our study are to clarify the mechanism of this system and to construct the model for design of this structure.