Elastic materials and fiber reinforced polymers are predominantly used today to strengthen concrete members due to their strength and other advanced material properties. Existing stress-strain relationships of concrete columns confined by these materials major on concentric loading as compared to eccentric loading. For eccentric loading, these models mainly focuse on the load carrying capacity rather than stress-strain relationship of the concrete columns.
The stress-strain relationship in such columns under concentric loading is however different from that under eccentric loading. Eccentric loading has different load paths such as increasing axial load at constant load eccentricity and vice versa. The stress-strain characteristics of eccentrically loaded columns are affected by the different load paths which have led to a lot of research work with the aim of getting more insight on the behaviors of such columns.
Eccentrically loaded columns exhibit three typical load paths. The load path I comprise constant load eccentricity with increasing axial load and is mainly prevalent in structures under gravitational loading. Load Path II exhibits increasing load eccentricity or bending moment with constant axial force and occurs when the structures loaded gravitationally gets subjected to additional horizontal loading like a seismic load. Lastly, load path III is associated with increasing axial force with constant bending moment. It is rarely experienced in columns.
In a recent collaboration Professor Yu-Fei Wu from RMIT University in Australia with Dr Yugui Cao from the Wuhan University of Technology and Dr. Cheng Jiang from The Hong Kong Polytechnic University developed a stress-strain model for concrete columns confined with fiber reinforced polymers under increasing load eccentricity with constant axial loading. This was in a bid of validating the assumption drawn from the previously constructed models that the stress-strain behavior of concrete columns confined with FRP is independent of the different load paths. Additionally, the authors investigated the effects of different load paths on the stress-strain behaviors of the FRP confined concrete columns. Their research work is published in the research journal, Composites Part B.
The authors successfully observed that the stress-strain behaviors of the FRP confined concrete column vary depending on the different load paths. For instance, different load paths resulted in differences in stress-strain behavior which occurred in the nonlinear deformation stage. Small stress in the elastic deformation range resulted in small difference of stress-strain curves under different load paths. The difference increases when strain increases. This is because small dilation, and hence confinement, was experienced in the linear deformation stage as compared to the non-linear deformation stage.
According to the authors, columns under increasing bending and constant axial force exhibited post-peak stress-strain slopes that increased with increase in the axial force ratio, and decreased with increase in load eccentricity. However under another load path of increasing axial force and constant load eccentricity, load eccentricity hardens the stress-strain curve and axial force ratio has insignificant effect on the stress-strain curve. The differences in the dilation of the concrete were the cause of the differences observed in the stress-strain curves under different load paths. Generally, the dilation property depended on the load path. The confinement pressure largely determined the resistance of the concrete.
Cao, Y., Wu, Y., & Jiang, C. (2018). Stress-strain relationship of FRP confined concrete columns under combined axial load and bending moment. Composites Part B: Engineering, 134, 207-217.Go To Composites Part B: Engineering