Discussion of "Retrofit of Reinforced Concrete Columns Using Partially Stiffened Steel Jackets" by Yan Xiao and Hui Wu

The authors have produced an interesting paper that describes jacketing methods for retrofitting square and rectangular reinforced concrete columns. In so doing, they have appreciated the beneficial effect of the flexural stiffness of the jackets and stiffeners on the development of confining pressure. The discussers have shown in a numerical study (Hussain and Driver 2001) of the behavior of reinforced concrete columns confined externally by "collars" made from steel hollow structural sections (HSS) that both the axial and flexural stiffnesses of the confinement collars have a significant effect on the enhancement in strength of the confined concrete. That is, by increasing the flexural stiffness of the confining elements without increasing their axial stiffness, a significant increase in the strength of the confined concrete is observed, although there exists a limit beyond which little benefit is gained. It was also observed that increasing the axial stiffness independently produces a significant increase in strength. Therefore, both the axial and the flexural stiffnesses of the confining -elements are important for confining the column concrete.