Stress-Strain Model of Concrete Columns Externally Collared with Steel Angles Subjected to Axial Compression

RC structures with poorly confined columns have been known to have poor performance during earthquake events. Brittle failures of columns with low ductility can lead to progressive overall building collapses. Many confining techniques have been proven to be successful in retrofitting circular columns. However, for square or rectangular columns, providing effective confining stress by the external retrofitting method is not a simple task due to the high stress concentration occurring at the column’s corners. In this paper, an analytical model for the axial stress-strain relationship of square concrete columns confined only by external steel collars has been developed. The combined effect of conventional internal stirrups and the external steel collars on the improved stress-strain relationship has been also developed analytically and proposed as the retrofit design approach. Comparison with experimental results has shown that the axial stress-strain relationship of square RC columns could be well predicted by the proposed retrofit design approach.