Closed-Form Model and Parametric Study on Connection of Concrete-Filled FRP Tubes to Concrete Footings by Direct Embedment

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been introduced as a new system for piles, columns, and poles. A simple moment connection based on direct embedment of the CFFT into concrete footings or pile caps, without using dowel-bar reinforcement, has been proposed by the authors. Robust analytical models to predict the critical embedment length ($X_{cr}$) were also developed and experimentally validated. In this paper, a comprehensive parametric study is carried out using the models developed earlier along with a newly developed closed-form model for the general case of axial loading, bending, and shear applied to the CFFT member. The parameters studied are the diameter ($D$), thickness ($t$), length outside the footing ($L$), and laminate structure of the FRP tube, as well as the tube-concrete interface bond strength (${\tau }_{\max }$), concrete compressive strength in the CFFT ($f_{ct}^{\prime }$) and footing ($f_c^{\prime }$), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasing $D$, $L/D$, ${\tau }_{\max }$, and $f_c^{\prime }$ of the footing, or the axial compression load, reduces $(X/D{)}_{cr}$, whereas increasing $t$ and $f_{ct}^{\prime }$ of the CFFT, the fraction of longitudinal fibers in the tube, or the axial tension load, increases $X_{cr}$. As the axial load eccentricity increases, $X_{cr}$ reduces for tension loads and increases for compression loads until both cases converge asymptotically to the same $X_{cr}$ value, essentially that of pure bending.