Table 14 demonstrates that failure of the confined concrete
is the controlling limit state at [P.sub.b] axial load level, whereas failure in the lap splice zone is the controlling limit state at all other axial load levels.
the increase in ductility of confined concrete
is related to the stiffness of the confining device.
The lateral expansion of concrete core is confined by the lateral pressure produced by the lateral confinement steel such that the slope of the descending postpeak branch of the stress-strain curve of confined concrete
decreases with the increase of confinement degree.
(2011)  studied the freeze-thaw effects on the stress-strain relationship for plain concrete and confined concrete
. In Duan et al.'s study, the number of freeze-thaw cycles and the concrete strength were changed; then a general constitutive model was proposed but the damage of the concrete was assumed to be uniformly distributed.
The behaviour of such uniformly confined concrete
has been extensively studied, leading to many models for both the compressive strength and the stress-strain behaviour.
As can be seen in Figure 1, steel tubes together with the confined concrete
can resist the axial compression remarkably, while the CFRP cylinders can provide the lateral confinement to the steel tube or concrete directly and make the composite column behave better indirectly.
Despite these efforts, research on the size effect of confined concrete
did not begin until the 1990s.
The variation of the maximum strength of confined concrete
for different volumetric ratios of lateral reinforcements are shown in the Figs 5 and 6.
Bertero, "Influence of high strain rate and cyclic loading of unconfined and confined concrete
in compression C," in Proceedings of 2nd Canadian Conference on Earthquake Engineering, pp.
Reddar, Stress-strain model of Unconfined and Confined Concrete
and Stress-Block Parameters, Master Thesis [Master, thesis], University of Texas A & M, College Station, Tex, USA, 2009.
Cai, Modern Steel Tube Confined Concrete
Structures, China Communications Press, China, 2007.
The model is based on a cross-sectional analysis satisfying strain compatibility and equilibrium conditions and can be utilized assuming any constitutive law for the confined concrete
. The model is able to take into account several mechanical and geometrical parameters, such as section aspect ratio, longitudinal reinforcement amount and distribution, confinement effect on the concrete core, reinforcing steel hardening, and/or softening postbuckling behaviour.