Strengthening of Columns for Confinement - Strengthening Design of Reinforced Concrete with FRP - page 169

f ´ cc

Heavily conned

f c

Moderately conned

Lightly conned

Unconned

ε c ε cu ε ccu

Axial Strain ε c

FIGURE 7.1 Stress-strain curves for unconfined and FRP-confined reinforced concrete
columns.

where the strength-reduction factor ϕ = 0.75 for spiral-reinforced members and ϕ =
0.65 for tie-reinforced members (according to Figure 2.1) for compression-controlled
sections, f cc is the maximum confined concrete strength due to FRP only (as per
ACI 440.2R-08), A g is the gross cross-sectional area of concrete, A st is the area of
longitudinal steel reinforcement in the column, and f y is the yield strength of the
longitudinal steel reinforcement.
ACI 440.2R-08 adopted the Lam and Teng (2003a) model, which takes into
account FRP confinement only. Abd El Fattah (2012) modified the maximum con-
finement pressure ( f l ) expression to account for both transverse steel and FRP inside
the core ( f le ) and only FRP in the cover ( f lf ). The Lam and Teng (2003a) model is
schematically presented in Figure 7.2.

f cc

E 2

1

f c

Unconned Concrete

Conned Concrete

E c

1

ε´ c ε´ t 0.003 ε ccu

Axial Strain ε c

FIGURE 7.2 Lam and Teng (2003a) model for FRP-confined concrete under pure axial
compression.