Civil Engineering Reference
In-Depth Information
3.
As previously described, stirrups cannot resist appreciable shear unless they are
crossed by an inclined crack. Thus to make sure that each 45
crack is intercepted by at
least one stirrup, the maximum spacing of vertical stirrups permitted by the Code
(11.5.4.1) is the lesser of
d
/2 or 24 in. for nonprestressed members and
h
for prestressed
members
o
r 24 in. where
h
is the overall thickness of a member. Should, however,
V
s
ex-
ceed ,* these maximum spacings are to be reduced by one-half (ACI 11.5.4.3).
These closer spacings will lead to narrower inclined cracks.
Another advantage of limiting maximum spacing values for stirrups is that closely
spaced stirrups will hold the longitudinal bars in the beam. They reduce the chance that
the steel may tear or buckle through the concrete cover or poss
ib
ly slip on the concrete.
Under no circumstances may
V
s
be allowed to exceed (Code 11.5.6.9).
†
The
shear strength of a beam cannot be increased indefinitely by adding more and more shear
reinforcing because the concrete will eventually disintegrate no matter how much shear
reinforcing is added. The reader can understand the presence of an upper limit if he or she
will think for a little while about the concrete above the crack. The greater the shear in the
member that is transferred by the shear reinforcing to the concrete above, the greater will
be the chance of a combination shear and compression failur
e o
f that concrete.
4.
Section 11.1.2 of the Code states that the values of used for the design of web
reinforcing may not exceed 100 psi
‡
except for certain cases listed in Section 11.1.2.1. In
that section, permission is given to use a larger value for members having the minimum
reinforcing specified in ACI Sections 11.5.5.3, 11.5.5.4, or 11.6.5.2. Members meeting
these requirements for extra shear reinforcing have sufficient post-crack capacities to pre-
vent diagonal tension failures.
5.
Section 12.13 of the Code provides requirements about dimensions, development
lengths, and so forth. For stirrups to develop their design strengths they must be ade-
quately anchored. Stirrups may be crossed by diagonal tension cracks at various points
along their depths. Since these cracks may cross very close to the tension or compression
edges of the members, the stirrups must be able to develop their yield strengths along the
full extent of their lengths. It can then be seen why they should be bent around longitudi-
nal bars of greater diameters than their own and extended beyond by adequate develop-
ment lengths. Should there be compression reinforcing, the hooking of the stirrups around
them will help prevent them from buckling.
Stirrups should be carried as close as possible to the compression and tension faces of
beams as the specified cover and longitudinal reinforcing will permit. The ends of stirrup
legs should ideally have 135
3
4
c
b
w
d
4
f
c
b
w
d
8
f
c
f
hooks bent around longitudinal bars, with develop-
ment lengths as specified in ACI Sections 8.1 and 12.13. Detailed information on stirrups
follows:
or 180
(a)
Stirrups with 90
bends and 6
d
b
extensions at their free ends may be used for #5
and smaller bars as shown in Figure 8.7(a). Tests have shown that 90
bends with
6
d
b
extensions should not be used for #6 or larger bars (unless
f
y
is 40,000 psi or
less) because they tend to “pop out” under high loads.
1
3
f
c
b
w
d
*In SI
V
s
.
†
It's
2
3
f
c
b
w
d
in SI units.
‡
It's
25
3
MPa in SI.
