Introduction - Strengthening Design of Reinforced Concrete with FRP - page 6

comparisons with experimental results are made to study the conservatism in the
code procedure.
1.5 BEHAVIOR OF REINFORCED CONCRETE
COLUMNS WRAPPED WITH FRP
Columns in seismic regions must behave in a ductile manner in flexure, shear, and
axial directions. One very efficient way to increase this ductility is through the
use of FRP wrapping of sheets or fabric such that the main load-carrying fibers
are oriented in the hoop direction. This hoop wrapping restricts concrete radial
expansion under axial load, leading to columns subjected to confining pressure
or a triaxial state of stress, which is known to increase the strength and improve
the deformability, as demonstrated in Figure 1.5. Column wrapping with hoop
FRP jackets enhances axial behavior. Experimental results and analytical find-
ings confirmed the effectiveness of this technique, especially for seismic upgrade
and structural performance of columns subjected to impact (Lam and Teng 2003).
Failure modes of wrapped FRP jackets are primarily the FRP fiber rupture at
premature levels, debonding, or wrap-unwinding at some point during the loading
(Hart 2008), as shown in Figure 1.6. Accordingly, an effective axial strain needs
to be established experimentally, at which point circumferential strain is critical
(ACI 440.2R-08).
Another application similar to column retrofitting but used for new construction
is the utilization of concrete-filled FRP tubes. In this application, flexural perfor-
mance, shear capacity, compressive strength, and strain performance are enhanced
due to the significant stiffness of the tube in the longitudinal (axial) direction, con-
tributing to the composite action of the section. Extra stiffness in the hoop direction
contributes to the confinement and additional shear capacity of the concrete-filled
FRP tube.
FIGURE 1.5 Confinement in FRP-wrapped columns: (a) unconfined column and (b) con-
crete column confined with wrapped FRP.