Civil Engineering Reference
In-Depth Information
From a practical viewpoint, stirrups are usually spaced with center-to-center di-
mensions that are multiples of 3 or 4 in., to simplify the fieldwork. Although this proce-
dure may require an additional stirrup or two, total costs should be less because of
reduced labor costs. A common field procedure is to place chalk marks at 2-ft intervals
on the forms and to place the stirrups by eye in between those marks. This practice is
combined with a somewhat violent placing of the heavy concrete in the forms followed
by vigorous vibration. These field practices should clearly show the student that it is
foolish to specify odd theoretical stirrup spacings such as 4 @ in. and 6 @ in.
because such careful positioning will not be achieved in the actual members. Thus the
designer will normally specify stirrup spacings in multiples of whole inches and per-
haps in multiples of 3 or 4 in.
With available computer programs it is easily possible to obtain theoretical arrange-
ments of stirrups with which the least total amounts of shear reinforcing will be required.
The use of such programs is certainly useful to the designer, but he or she needs to take
the resulting values and revise them into simple economical patterns with simple spacing
arrangements—as in multiples of 3 in., for example.
A summary of the steps required to design vertical stirrups is presented in Table 8.1.
For each step, the applicable section number of the Code is provided. The author has
found this to be a very useful table for students to refer to while designing stirrups.
7
16
5
8
6
Table 8.1
Summary of Steps Involved in Vertical Stirrup Design
Is shear reinforcing necessary?
1.
Draw
V
u
diagram.
2.
Calculate
V
u
at a distance
d
from the support (with
11.1.3.1 and
certain exceptions).
Commentary (R11.1.3.1)
c
b
w
d
3.
Calculate
V
c
2
f
(or use the alternate method).
11.3.1.1
11.3.2.1
4.
Stirrups are needed if
V
u
V
c
(with some exceptions
1
2
11.5.5.1
for slabs, footings, shallow members, and joists).
Design of stirrups
1.
Calculate theoretical stirrup spacing
s
A
v
f
y
d
/
V
s
11.5.6.2
where
V
s
(
V
u
V
c
)/
.
2.
Determine maximum spacing to provide minimum
11.5.5.3
A
v
f
y
0.75
f
area of shear reinforcement
s
c
b
w
A
v
f
y
50
b
w
but not more than
3.
Compute
m
aximum spacing:
d
/2
24 in., if
11.5.4.1
c
b
w
d
V
s
.
4.
Compute
m
aximum spacing:
12 in., if
4
f
d
4
11.5.4.3
c
b
w
d
V
s
.
5.
V
s
may not be
4
f
c
b
w
d
8
f
11.5.6.9
6.
Minimum practical spacing
approximately 3 or 4 in.
