Civil Engineering Reference
In-Depth Information
of 0.01, as shown in Equation (7.9), to prevent excessive cracking and possible loss of
concrete integrity. ACI 440.2R-08 also limits this strength improvement technique
to concrete having
f
c
less than 10 ksi (69 MPa) due to the lack of experimental stud-
ies with concrete of higher values. It is important to state here that in the case where
Īµ
ccu
is controlled by Equation (7.9), the slope of the second hardening line (
E
2
) in
Equation (7.4) will be kept constant, and the value of (
f
cc
) will be reduced accord-
ingly (ACI 440.2R-08).
7.2.2 C
onSiDeration
oF
r
eCtangular
S
eCtionS
ACI 440.2R-08 confirmed the findings of several researchers that confining square
and rectangular columns with FRP jackets may provide a marginal increase in
the maximum confined axial compressive strength (
f
cc
). The provisions of ACI
440.2R-08 ignore the contribution of conventional steel transverse reinforcement
in confining the columns, and they limit this application to columns having
h
/
b
aspect ratio less than or equal to 2.0, with both
h
and
b
less than 36 in. (914 mm).
ACI 440.2R-08 adopts the theoretical model proposed by Lam and Teng (2003b) that
accounts for the reduced area of the confined rectangular column. The rectangular
section is converted into an equivalent circular section for the purpose of calculat-
ing the maximum confining pressure (
f
l
) using Equation (7.7), with the diameter
D
replaced to be the diagonal of the rectangular section (FigureĀ 7.3).
2 2
=+
(7.10)
Dbh
To determine the area of the cross section that is effectively confined by the FRP
jacket, four parabolas are drawn inside the rectangular section to isolate the inner
confined area from the outer unconfined area. The parameters controlling the sizes
of the parabolas are the column dimensions (
b
,
h
); the radius of the corners of the
r
c
b
D
h
FIGURE 7.3
Effective area of confined rectangular section per ACI 440.2R-08. (Courtesy
of Lam and Teng [2003b].)
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