Civil Engineering Reference
In-Depth Information
Prestressed concrete Runnymede Bridge over Thames River near Egham, Surrey, England.
(Courtesy of Cement and Concrete Association.)
As in conventional reinforced concrete design, a minimum area of shear reinforcing
is required at all points where
V
u
is greater than This minimum area is to be deter-
mined from the expression to follow if the effective prestress is less than 40% of the ten-
sile strength of the flexural reinforcement (ACI Code 11.5.3):
1
2
V
c
.
c
b
w
s
but shall not be less than
50
b
w
s
f
y
A
v
0.75
f
(ACI Equation 11-13)
f
y
where
b
w
and
s
are in inches.
If the effective prestress is equal to or greater than 40% of the tensile strength of the
flexural reinforcement, the following expression, in which
A
ps
is the area of prestressed re-
inforcement in the tensile zone, is to be used to calculate
A
v
:
A
ps
80
f
pu
f
y
s
d
d
b
w
A
v
(ACI Equation 11-14)
Section 11.5.4.1 of the ACI Code states that in no case may the maximum spacing
exceed 0.75
h
or 24 in. Examples 19.5 and 19.6 illustrate the calculations necessary for de-
termining the shear strength and for selecting the stirrups for a prestressed beam.
EXAMPLE 19.5
Calculate the shearing strength of the section shown in Figure 19.13 at 4 ft from the supports,
using both the approximate method and the more detailed method allowed by the ACI Code. As-
