Jacketing of the concrete leads to the increase of the strength and ductility of jacketed columns. A significant amount of research has been devoted to round columns that have been retrofitted with fiber-reinforced polymer (FRP), but much less is known about FRP-confined rectangular/square columns in which the concrete is non-uniformly confined and the effectiveness of confinement is much reduced. Results of an experimental investigation of the compressive behavior of square concrete columns confined by carbon FRP are presented. It was found that a simple formula which involves the definition of the equivalent round column and the shape factor can well estimate the compressive strength of the confined square columns
