Investigation of FRP strengthened circular columns under biaxial bending

This paper presents the analysis and design of cylindrical reinforced concrete (RC) columns confined with fiber reinforced polymer (FRP) composites. The columns studied are under combined axial loads and biaxial bending moments. The fiber method modeling (FMM) together with finite element analysis (FEA) are adopted to investigate the behavior of such columns. The procedure finds the inclination and depth of the neutral axis that satisfy the equilibrium conditions. Moreover, the proposed analysis is capable of solving slender columns under biaxial bending through a developed computer program. The finite element derivation of a general stiffness matrix that accounts for the loss of symmetry of the cross-section due to material nonlinearity and biaxial moments is presented herein. The derivation starts from the principle of virtual work and develops an updated Lagrangian procedure for geometric nonlinear analysis. It should be noted that the stiffness for an elastic–plastic cross-section is a function of the current state and the past history of the strain of the column section as well as the stress–strain relations for each of concrete, steel and the fibers used. Therefore the cross-section is partitioned into a number of small elemental areas and a step-by-step application of the force–deformation equilibrium equation is applied. Interaction diagrams for columns under uniaxial bending are plotted and compared with experimental results conducted in the literature review. Moreover, contour lines for columns under biaxial bending are plotted. A parametric study was carried out to investigate the effect of various parameters on the strength of the column. The parameters include unconfined concrete strength, type of FRP used, thickness of FRP and the column height to the cross-sectional diameter (H/D) ratio. The developed diagrams provide the designer with an easy and reliable way to analyze and design such columns.