Seismic behavior and modeling of concrete partially filled spirally welded pipes (CPF-SWP)

Hollow spirally welded pipes (SWP) have been used widely as piles for the foundations of buildings, bridges, and flood wall structures. These members have limited ductility due to extensive local buckling and denting when subjected to seismic loading. In cases where improved seismic performance and ductility is required, the SWP are filled, fully or partially, with concrete. Fully filled SWP are called CF-SWP, while partially filled SWP are referred as CPF-SWP. Researchers have experimentally investigated the structural behavior of SWP and CF-SWP. However, there is lack of benchmarked numerical models that can be used to investigate the seismic behavior of SWP, CF-SWP or CPF-SWP, and lack of research regarding the seismic behavior and ductility of CPF-SWP. This paper develops and benchmarks detailed 3D finite element method (FEM) models for evaluating the seismic behavior of SWP, CF-SWP, and CPF-SWP. The FEM models account for behavioral complexities such as steel yielding, pipe local buckling and fracture, concrete cracking and crushing, cyclic stiffness degradation and recovery, and effects of concrete confinement. The FEM models for both the hollow SWP and filled CF-SWP are benchmarked using experimental data available in the literature. The benchmarked models are used to investigate the seismic behavior and ductility of the partially filled CPF-SWP, which include the transition from hollow SWP to filled CF-SWP. These investigations show that the infill ratio (Lc/L) has a significant influence on the seismic behavior of CPF-SWP. Increasing the infill ratio delays the steel pipe local buckling and fracture, and improves the stiffness, strength, ductility, and energy dissipation capacity. Fully filling the plastic hinge region with concrete can significantly improve the seismic behavior of CPF-SWP.