Civil Engineering Reference
In-Depth Information
in the development length expression. It is called the
transverse reinforcement
factor
. In the equation
c
represents the smaller of the distance from the center of
the tension bar or wire to the nearest concrete surface, or one-half the center-to-
center spacing of the reinforcement.
In this expression,
K
tr
is a factor called the
transverse reinforcement index
. It is used
to account for the contribution of confining reinforcing (stirrups or ties) across possible
splitting planes.
A
tr
f
y
t
1500
sn
K
tr
where
A
tr
the total cross-sectional area of all transverse reinforcement having the center-
to-center spacing
s
and a yield strength
f
yt
n
the number of bars or wires being developed along the plane of splitting
0 to simplify the
calculations even if transverse reinforcing is present. ACI 12.2.3 limits the value of
used in the equation to a maximum value of 2.5. (It has been found that if val-
ues larger than 2.5 are used, the shorter development lengths resulting will increase the
danger of pullout-type failures.)
The calculations involved in applying ACI Equation 12-1 are quite simple, as is illus-
trated in Example 7.2.
The Code in Section 12.2.4 conservatively permits the use of
K
tr
c
K
tr
/
d
b
A
tr
f
y
t
10
sn
In SI units
K
tr
EXAMPLE 7.2
Determine the development length required for the #8 uncoated bottom bars shown in Figure 7.8.
(a)
assume
K
tr
0 and
(b)
use the computed value of
K
tr
.
ƒ
y
= 60,000 psi
ƒ
c
= 3000 psi
15
"
#3 stirrups
@ 8
"
18
"
3 #8
3
"
1
2
1
2
2
"
2
"
2@3=6
"
11
"
Figure 7.8
