It can be seen that with the increase of [r.sub.sf] the SFCB has a larger energy-dissipation capacity at the same peak strain.
The reasons for this are mainly as follows: (1) FRP is assumed to be completely straight in the calculation and the steel bar had no initial bending, while in the experiment the surface of the SFCB was formed by a plastic band; that is, the longitudinal fibers were partially curved, resulting in an FRP strain slightly behind that of the inner steel bar; (2) the inner steel bar was assumed to work together with the outer FRP, while in reality a relative slip occurred at the steel/FRP interface after the inner steel bar yielded.
To avoid the influence of the effect of the loading path on the compressive behavior of SFCB, the performance of SFCB under monotonous compressive loading is presented.
The Compressive Behavior of SFCB. FRP is wrapped on the outside of the SFCB, so it is necessary to analyze the compressive behavior of an FRP bar.
The stress-strain relationship of SFCB under compressive loading (see (4)) can be obtained according to the constitutive relation under tensile loading :
where "--" means that the SFCB is under compressive loading.
It can be found that [r.sub.sf.sup.-] of an SFCB is 0 when the tensile postyield stiffness ratio [r.sub.sf] is 0, and the [r.sub.sf.sup.-] of a pure FRP bar is 1 when the corresponding [r.sub.sf] is 1.
Since there is a large difference between the compressive modulus of a steel bar and an FRP bar, the equivalent flexural rigidity of an SFCB is defined as [E.sub.s][I.sub.sf_Is.sup.e] = [E.sub.s][I.sub.s] + [E.sub.f][I.sub.f] (Figure 8), and the corresponding equivalent diameter ([d.sub.sf_ls.sup.e]) and equivalent compressive strength ([f.sub.sf_Is.sup.e]) of SFCB can be calculated using (6) and (7), respectively:
where [d.sub.s] and [d.sub.sf] are the diameter of the inner steel bar and the diameter of the SFCB, respectively.
The corresponding ELDR of SFCB is defined as [R.sub.sf_Is.sup.e] = [L.sub.u]/[d.sub.sf_Is.sup.e].
Parametric Analysis of SFCB. The experimental study of the compressive behavior of BFRP bar under monotonic compressive loading has been conducted by Zhou , who found that the compressive elastic modulus of a BFRP bar is approximately 80% of the tensile elastic modulus, and the compressive strength is approximately 50% of the tensile strength.
The load-strain curves and equivalent stress-strain curves of SFCB, with the same [L.sub.u]/[d.sub.sf_Is.sup.e] and different postyield stiffness ratios, are shown in Figure 10.