Design equation for concrete-filled FRP tubes in flexure including damage effects

Abstract

Nonlinear finite element models are developed to investigate the effect of tube damage on flexural strength of concrete filled fiber reinforced polymer (FRP) tubes (CFFTs). The tube, being essentially an external reinforcement, is vulnerable to accidental damage or vandalism. The tube damage is induced at the outermost tension side of the CFFT in the form of a linear narrow cut with a variable length. The model is verified using experimental results from the literature. A comprehensive parametric study, comprising 140 new models, was then performed to study the damage effects in tubes with diameter-to-thickness (D/t) ratios of 30 to 100, [±θ] angle-ply tubes with (θ) varying from 15° to 75° and cross-ply tubes with 25–75% longitudinal fiber fraction. Additionally, the effects of damage were examined for hollow FRP tubes and compared to CFFTs with the same damage. It was shown that for angle-ply tubes and cross-ply tubes with 25–50% longitudinal fiber fraction, a sharp decrease (30–50%) in flexural strength occurs at circumferential cuts of length-to-perimeter (α) ratio of 5%, at all (D/t) ratios. For larger cuts, the decrease in capacity continuous but at a reduced rate so the reduction increases to (60–75%) as α increases to 30%. For hollow tubes, the damage affects moment capacity, only when material failure governs (at small D/t), not when local buckling governs (at large D/t). Statistical analysis was used to assess results of the comprehensive parametric study. A simple empirical equation was then deduced for condition assessment purposes. It enables engineers to predict flexural strength of both intact and damaged CFFTs for any given α, D and t, for a wide range of FRP tube laminate structures.