Load-Strain Model for Steel-Concrete-FRP-Concrete Columns in Axial Compression

A load-strain model for a steel-concrete-FRP-concrete (SCFC) hybrid column section in compression is proposed. The section layout has a square steel tube as the outer layer and a circular fiber-reinforced polymer (FRP) tube as the inner layer, and concrete is filled between these two layers and inside the FRP tube. Thus the section can be regarded as a concrete-filled steel tube (CFST) with a FRP-confined concrete core (FCCC), in which the FCCC is essentially a concrete-filled FRP tube (CFFT) in sectional configuration. However, the mechanical behavior of a SCFC is superior to the simple superposition of CFST and CFFT without consideration of the interaction mechanisms among the different materials. The load-strain behavior of a SCFC differs from that of a CFST or CFFT in that it includes an initial parabola portion, a second linear portion, and a postpeak portion. The model is established by superposing four load-strain models of the constituent layers and attempting to reveal the mechanical responses of the SCFC sections under axial compression. In the modeling, several mechanical characteristics, namely yielding point, peak strain, peak load, and postpeak residual bearing portion, are investigated and the effects of three parameters, FRP thickness, steel thickness, and concrete strength, are examined. Comparisons between the modeling results and experimental results show good agreement in terms of yielding strain, yielding load, peak strain, and peak load. Furthermore, a set of predictions for peak load covering a greater range of parameters than the experiments is developed according to this load-strain model.