Analytical model of concrete-filled FRP-steel composite tube columns under cyclic axial compression

The concrete-filled FRP (Fiber-reinforced Polymer)-steel composite tube (CFCT) column is a new structure that integrates the advantages of various materials. The establishment of the cyclic stress-strain relationship under axial loading is of great importance for the seismic analysis and design method of this new structure. In this study, the stress-strain behaviors of six CFCT columns with various FRP thicknesses and types under cyclic axial loading were investigated. The experimental results reveal that the failure process is homologous to that of the CFCT column under monotonic axial compression. CFCT columns that are confined by carbon FRP (CFRP) can achieve higher strength and deformation capacity. The ratio of the plastic strain to the unloading strain enhances with the number of loading cycles. When the axial strain develops to 0.02, the ratio tends to be close to 1.0, and the plastic strain becomes difficult to recover. CFRP plays a more important role in inhibiting the development of plastic deformation compared with basalt FRP (BFRP). On the basis of experimental results, a complete design-oriented stress-strain model is established to predict the cyclic stress-strain relationship of CFCT columns for the first time, and this model includes equations for predicting the returning strain, the plastic strain and the unloading and reloading loading paths. The proposed model can predict the main features of the cyclic stress-strain behaviors of CFCT columns accurately and conveniently both before and after FRP fracture.