Strengthening hollow reinforced concrete columns with fibre reinforced polymers

Hollow Reinforced Concrete (RC) columns have been preferred to solid columns to reduce the cost of structures, and to decrease their self-weight where it is technically necessary. In spite of their widespread use, even modern design codes do not address any specific problems related to hollow section columns. This study aims to predict the behaviour of hollow RC columns confined with FRP wraps. A stress-strain model valid for both solid and hollow confined cylinders was developed. A modelling study was undertaken to generate the axial load- bending moment diagrams and the loading lines which give the axial load versus bending moment values under axial compression from early service loads up to the failure of FRP wrapped hollow RC columns using a moment magnification approach. To validate the modelling study, an experimental study was carried out to see the effect of column height, FRP wrapping configuration and loading eccentricity. A total of 18 hollow RC columns with the same cross section geometry and internal steel reinforcement were cast and tested under axial compressive loading in the Structural Engineering Laboratories of the University of Wollongong. Nine of these sample columns were 500 mm in height (short columns) and the other nine were 885 mm tall (tall columns). Each group of sample columns were divided into three sub-groups of three and a different FRP wrapping configuration was applied on each group. The sample columns were then tested under axial compressive load with 0, 25 and 50 mm eccentricities. The application of the eccentric load was achieved by special loading heads and knifeedges designed and manufactured for this study. The test results and the modelling study were seen to be in good agreement showing that the theoretical model leads to reliable results.