Axial Load Capacity of Concrete-Filled FRP Tube Columns: Experimental versus Theoretical Predictions

Abstract

This paper presents the experimental and theoretical results of small and medium-scale concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) columns. A total of 23 CFFT specimens were tested under axial compression load. Five different types of new FRP tubes were used as stay-in-place formwork for the columns. The effects of the following parameters were examined: the FRP-confinement ratio, the unconfined concrete compressive strength, the presence of longitudinal steel reinforcement, and the height-to-diameter ratio. Comparisons between the experimental test results and the theoretical prediction values by the three North American codes and design guidelines (ACI 440.2R-08, CSA-S6-06, and CSA-S806-02) are performed in terms of confined concrete strength and ultimate load carrying capacity. The results of this investigation indicate that the design equations of the ACI 440.2R-08, CAN/CSA-S6-06, and CAN/CSA-S806-02 overestimate the factored axial load capacity of the short CFFT columns as compared to the yield and crack load levels. Also, the CAN/CSA-S6-06 and CAN/CSA-S806-02 confinement models showed conservative predictions, while the ACI 440.2R-08 was slightly less conservative. A new confinement model is proposed for the confined concrete compressive strength of the CFFT cylinders. Also, the design equations are modified to accurately predict the ultimate and yield load capacities of internally reinforced and unreinforced short CFFT columns. Two new factors are introduced in the modified equations, (kcc) accounts for the in-place-strength of CFFT columns to CFFT cylinder strength, and (kcr) accounts for the initiation of the steel yielding and concrete cracking for the FRP-confined columns.