Confined concrete shows much larger strength and ductility
than unconfined concrete. Fig. 1 shows the difference of stress–
strain relations between confined and unconfined concrete. The
enhanced strength and ductility of confined concrete depend on
the confining pressure. Therefore many researchers have studied
how to effectively confine concrete. For a reinforced concrete
(RC) column, Roy and Sozen [1] tested rectangular concrete members
confined by transverse reinforcements. They presented that
there was no increase in concrete strength, but a significant increase
in ductility could be achieved. Iyengar et al. [2] and Desayi
et al. [3] showed that there was a significant increase in both
strength and ductility of concrete members including circular steel
spirals. Vallenas et al. [4], Sheikh and Uzumeri [5], and Scott et al.
[6] tested more realistically scaled specimens of real building columns.
They presented the stress–strain relations of the specimens.
The test results for confined square sections with rectangular and
octagonal shaped transverse reinforcements indicated that the
strength and the ductility of the specimens were significantly
enhanced. Recently, many researchers ([7–10]) made analytical
models for the explanation of the effect of concrete confinement.
The analytical results showed widely divergent opinions about
the increase in strength and ductility of concrete as the shape of