Transverse confining stress in concrete members, commonly provided by transverse
reinforcement, has been recognized to enhance strength and ductility. Nowadays, the confining
method has been further developed to external confinement approach. This type of confinement
can be used for retrofitting existing concrete columns. Many external confining techniques have
been proven to be successful in retrofitting circular columns. However, for square or rectangular
columns, providing effective confining stress by external retrofitting method is not a simple task
due to high stress concentration at column’s corners. This paper proposes an analytical model to
predict the peak strength of square concrete columns confined by external steel collars.
Comparison with the experimental results showed that the model can predict the peak strength
reasonably well. However, it should be noted that relatively larger amount of steel is needed to
achieve comparable column strength enhancement when it is compared with those of
conventional internally-confined columns.