Civil Engineering Reference
In-Depth Information
6
d
b
but not
less than 2
2
"
12
d
b
6
d
b
6
d
b
12
d
b
or
(a)
90
° bends for #5
and smaller stirrups
(also for #6, #7, and
#8 stirrups with
f
y
≤
40,000 psi)
(b)
90
° bends for #6,
#7, and #8 stirrups
with
f
y
> 40,000 psi
(135
(c) 135
bends for
#8 and smaller
stirrups
°
hooks preferred)
°
or 180
°
Note:
Fit stirrups as close to compression and tension surfaces as cover and other reinforcing permits.
Figure 8.7
Stirrup details.
(b)
If
f
y
is greater than 40,000 psi, #6, #7, and #8 bars with 90
bends may be used if the
extensions are 12
d
b
[see Figure 8.7(b)]. The reason for this specification is that it is
not possible to bend these higher strength bars tightly around the longitudinal bars.
(c)
Stirrups with 135
bends and 6
d
b
extensions may be used for #8 and smaller bars,
as shown in Figure 8.7(c).
6.
When a beam reaction causes compression in the end of a member in the same di-
rection as the external shear, the shearing strength of that part of the member is increased.
Tests of such reinforced concrete members have shown that in general, as long as a gradu-
ally varying shear is present (as with a uniformly loaded member), the first crack will
occur at a distance
d
from the face of the support. It is therefore permissible, according to
the Code (11.1.3.1), to decrease somewhat the calculated shearing force for a distance
d
from the face of the support. This is done by using a
V
u
in that range equal to the calcu-
lated
V
u
at a distance
d
from the face of the support. Should a concentrated load be applied
in this region, no such shear reduction is permitted. Such loads will be transmitted directly
to the support above the 45
cracks, with the result that we are not permitted a reduction in
the end shear for design purposes.
Should the reaction tend to produce tension in this zone, no shear stress reduction is
permitted because tests have shown that cracking may occur at the face of the support or
even inside it. Figure 8.8 shows two cases where the end shear reduction is not permitted.
In the situation shown in Figure 8.8(a) the critical section will be at the face of the sup-
port. In Figure 8.8(b) an I-shaped section is shown, with the load applied to its tension
flange. The loads have to be transferred across the inclined crack before they reach the
support. Another crack problem like this one occurs in retaining wall footings and is dis-
cussed in Section 13.10 of this text.
