Civil Engineering Reference
In-Depth Information
7
Strengthening of Columns
for Confinement
7.1 OVERVIEW
FRP jackets are used to provide confinement to reinforced concrete columns, which
improves both the strength and ductility. Confinement using FRP contributes to
enhanced peak load resistance and enhanced rotation and drift ratio without a sig-
nificant reduction in strength.
7.2 ENHANCEMENT OF PURE AXIAL COMPRESSION
The highest level of strength enhancement is obtained in the case of pure axial com-
pression, as confinement activates a three-dimensional (3-D) state of stress under com-
pression, which yields a higher axial capacity than that of unconfined compression.
FRP jackets are known to offer passive confinement to columns. This means that
such confinement is not activated until dilation and cracking take place. Accordingly,
intimate contact between the FRP wrapping and concrete column is important. This
application is, therefore, called contact-critical application.
As the level of confinement increases, the uniaxial stress-strain behavior along
the column axis changes from unconfined to lightly confined to a moderately con-
fined softening curve to a heavily confined hardening curve (Figure 7.1).
Confinement of columns is accomplished by orienting the fibers transverse to the
axis of the column such that it will act in a similar way as that of conventional spiral
or tie steel reinforcement. The contribution of axially aligned fibers is beyond the
scope of this chapter.
The peak strength in the axial direction (
f
cc
) is the stress of the concrete section
corresponding to the peak load after subtracting the contribution of the longitudinal
steel reinforcement.
For columns with existing spiral steel reinforcement, ACI 440.2R-08 specifies the
ultimate axial load as
(
)
φ= φ
P
0.85
0.85
f
AA Af
−
+
(7.1)
n
cc
g
st
st
y
For columns with existing tie steel reinforcement, ACI 440.2R-08 specifies the
ultimate axial load as
(
)
φ= φ
P
0.8
0.85
f
AA Af
−
+
(7.2)
n
cc
g
st
st
y
169
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