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

Abstract

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 (Xcr) 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 (τmax), concrete compressive strength in the CFFT (fct’) and footing (fc’), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasing D, L, τmax, fc’ of the footing, or the axial compression load, reduce Xcr, whereas increasing t, fct’ of the CFFT, the fraction of longitudinal fibers in the tube, or the axial tension load, increase Xcr. As the axial load eccentricity increases, Xcr reduces for tension loads and increases for compression loads, until both cases converge asymptotically to the same Xcr value, essentially that at pure bending.