Nonlinear tensile behavior of ±55˚ concrete-filled GFRP tubes: Experimental and theoretical studies

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) with angle-ply configurations, such as ±55˚ glass-FRP (GFRP) tubes, show highly nonlinear behavior that is not captured by conventional cross-ply tubes. A key challenge has been capturing their failure strains under tension, since large deformations have prevented accurate measurements with conventional instrumentation. This study addresses this challenge by introducing a novel test configuration capable of capturing the tensile response of ±55˚ GFRP tube filled with concrete (±55˚ CFFTs) under pure axial tension. A well-instrumented ±55˚ CFFT beam test was conducted, which recorded ultimate tensile strain of 0.077 mm/mm for the first time. To complement the experiments, nonlinear finite element (FE) models were developed to simulate ±55˚ CFFTs tensile behavior. The models successfully reproduced the nonlinear response with only 3%-4% variance from experimental results. Building on these findings, a design-oriented constitutive tension equation was formulated specifically for ±55˚ GFRP tubes, and an analytical model incorporating this equation was then established to predict the moment capacity of ±55˚ CFFT beams, and validation against multiple experimental beam tests demonstrated strong predictive accuracy (85%-100% agreement in ultimate load and deflection), representing a substantial improvement over existing models.