Civil Engineering Reference
In-Depth Information
Figure 6.3
Effects of cracks on deflections.
Section 9.5.2.3 of the Code gives a moment of inertia expression that is to be used for
deflection calculations. This moment of inertia is an average value and is to be used at any
point in a simple beam where the deflection is desired. It is referred to as
I
e
, the effective
moment of inertia, and is based on an estimation of the probable amount of cracking
caused by the varying moment throughout the span:
2
M
cr
M
a
3
M
cr
M
a
3
I
e
(
I
g
)
1
I
cr
(ACI Equation 9-8)
In this expression,
I
g
is the gross amount of inertia (without c
on
sidering the steel) of the
section,
M
cr
is the cracking moment
c
*
f
r
I
g
/
y
t
, with
f
r
7.5
f
for normal-weight con-
2
Branson, D. E., 1965, “Instantaneous and Time-Dependent Deflections on Simple Continuous Reinforced Concrete
Beams,”
HP
R Report No. 7, Part 1, Alabama Highway Department, Bureau of Public Roads, August 1963, pp. 1-78.
*or
c
0.7
f
in SI.
