Analysis oriented model for hybrid filament wound FRP-concrete-steel double skin tubular columns (DSTC) under axial compression

There exists no analytical model in the literature capable of predicting the stress-strain behavior of hybrid filament wound fiber reinforced polymer (FRP)-concrete-steel double skin tubular columns (DSTC) under axial compression. To address this, an analytical model is first developed to incorporate bi-directional fibers and predict the non-linear hoop stress-hoop strain response in filament wound FRP laminated cylindrical tube under internal pressure. This model is then used to predict the confinement pressure exerted by the FRP tube on the sandwiched concrete in DSTC when subjected to uniaxial compression. Next, an analysis-oriented model is used to predict the complete stress-strain response of DSTC confined by bi-directional FRP tube. The model's predictions are compared with the test results collected from literature, and based on performance parameters, it is concluded that the proposed model significantly reduces the error in predicting the global axial stress-strain behavior of DSTC with bidirectional FRPs. Parametric studies are then performed to understand the influence of geometrical parameters and material properties on the axial behavior of DSTC. It was found that the variation of effective confinement pressure across the concrete section is the key factor influencing the global axial stress-strain behavior. Finally based on these parametric studies, a critical confining stiffness necessary to supress the softening response in DSTC was proposed, which is dependent on the void ratio of these columns.