Civil Engineering Reference
In-Depth Information
The development lengths used for deformed bars or wires in tension may not be less
than the values computed with ACI Equation 12-1 or 12 in. If the equation is written as
d
d
b
, the results obtained will be in terms of bar diameters. This form of answer is very
convenient to use as, say, 30 bar diameters, 40 bar diameters, and so on.
f
y
3
d
40
d
b
(ACI Equation 12-1)
c
c
K
tr
d
b
f
or
f
y
d
3
d
b
40
c
c
K
tr
d
b
f
Or in SI units,
f
y
9
d
10
d
b
c
K
tr
d
b
c
f
This expression, which seems to include almost half of the Greek alphabet, is much
easier to use than it might at first appear because several of the terms are usually equal to
1.0. Even if not equal to 1.0, the factors can be quickly obtained.
In the following paragraphs all of the terms in the equation that have not previously
been introduced are described. Then their values for different situations are given in
Table 7.1.
1.
Location of reinforcement.
Horizontal bars that have a least 12 in.* of fresh con-
crete placed beneath them do not bond as well to concrete as do bars placed nearer
the bottom of the concrete. These bars are referred to as
top bars
. During the plac-
ing and vibration of the concrete, some air and excess water tend to rise toward
the top of the concrete, and some portion may be caught under the higher bars. In
addition, there may be some settlement of the concrete below. As a result, the re-
inforcement does not bond as well to the concrete underneath, and increased de-
velopment lengths will be needed. To account for this effect the
reinforcement
location factor
(alpha) is used.
2.
Coating of bars.
Epoxy-coated reinforcing bars are frequently used today to pro-
tect the steel from severe corrosive situations, such as where de-icing chemicals
are used. Bridge decks and parking garage slabs in the colder states fit into this
class. When bar coatings are used, bonding is reduced and development lengths
must be increased. To account for this fact, the term
(beta)—the
coating
factor
—is used in the equation.
* 300 mm in SI.
