Design of Interaction Curves for RC Columns with FRP Materials under the Effect of Axial Force and Biaxial Bending Moment

Fiber Reinforced Polymer (FRP) composites are widely used in retrofitting and strengthening of Reinforced Concrete (RC) structures. FRP composites are applicable for strengthening various structural elements including beams, columns and plane elements such as floor slabs and shear walls. Despite their broad usage, available guidelines typically do not contain a comprehensive procedure for retrofitting design of RC elements under general loading conditions. As RC columns are essentially subjected to simultaneous axial force and bending moment in monolithic construction, their evaluation and retrofit under such combined effects are of major importance in the retrofit design process. It appears that available expressions in most design guidelines merely accounts for the possible increase in the compressive strength of concrete due to the enhanced confinement on the concrete core provided by the FRP jackets. Theoretical and experimental studies on the behavior of columns confined by FRP composites subjected to axial force and bending moment are available in the literature. Those studies have demonstrated that FRP jacketing enhances the elements behavior in term of both strength and ductility. Most of studies are primarily devoted to the strengthening of RC elements under pure axial force. In recent years, however, studies have been conducted on response of RC elements under combined effects of axial load and uniaxial bending. Rocca (2009), among others, conducted a very precise and valuable practical study on the effects of bending moments on retrofitting of RC columns using FRP composite. He introduced the interaction curves of retrofitted RC columns with FRP jackets based on the principles of equilibrium and strain compatibility. Moreover, results of various experimental tests are found in the literature emphasizing the behavior of RC elements under eccentric loading conditions. Such tests have investigated the effects of various parameters including the strength of FRP composites, number of FRP layers, orientation of FRP layers, etc. on both strength and ductility of the retrofitted members.
The present paper deals with the retrofitting and strengthening of RC columns under combined effects of axial force and biaxial bending moment using FRP composites. The study is essentially the extension of the work conducted by Rocca (2009) to the more general biaxial case. Longitudinal, transverse and combined fiber-sheets are considered as alternatives of retrofitting RC columns using FRP composites. The proposed design procedure is then explained via a case study by which advantages and disadvantages of longitudinal, transverse and combined fiber composite sheets are investigated and discussed according to the interaction curve of the retrofitted column. Results demonstrate that the proposed procedure is appropriate for practical retrofitting applications, and different fiber-sheet layouts are effective in enhancing the load bearing capacity of both the compression- and tension-controlled columns.