Testing and Modeling of a New Moment Connection of Concrete-Filled FRP Tubes to Footings under Monotonic and Cyclic Loadings

This study explores a new moment connection of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) to concrete footing. The tube is tightly fitted and adhesively bonded to a short reinforced concrete stub protruding from the footing, which facilitates concrete filling of the tube without the need for shoring. To establish the critical stub length ($X_{cr}$), specimens with heavily steel-reinforced stubs varying in length from $0.5D$ to $2.0D$, where $D$ = diameter of the CFFT, were fabricated and tested in flexure by using a cantilever setup. The $X_{cr}$ required to achieve flexural failure of the CFFT was $1.05D$. Additional specimens with a sufficient stub length of $1.5D$ were then fabricated to examine the effect on strength and ductility of the steel reinforcement ratio (${\rho }_s$) in the stub. The optimal ${\rho }_s$ of the stub required for the CFFT to reach flexural failure was 3.2%. Finally, the effect of low-cycle reversed bending fatigue was studied with and without an axial compression load. Remarkable ductility associated with the formation of a plastic hinge was observed at ${\rho }_s$ of 2%. An analytical model capable of predicting (1) moment capacity of the connection, (2) whether failure is governed by flexure or bond, and (3) $X_{cr}$ was developed and validated. It was then used in a parametric study to explore the effects of CFFT mechanical and geometric properties on $X_{cr}$.