Civil Engineering Reference
In-Depth Information
SOLUTION
w
u
(1.2)(1.2)
(1.6)(2.1)
4.8 k/ft
V
u
(10)(4.8)
(4)(4.8)
28.8 k
V
c
(0.75)(50,684)
38,013 lb
V
u
28,800 lb
V
c
2
V
u
19,006.5 lb
V
c
A minimum amount of reinforcement is needed.
Since effective prestress is greater than 40% of t
ensile
strength of reinforcing,
A
ps
80
f
pu
f
y
s
d
d
b
w
A
v
(ACI Equation 11-14)
250,000
40,000
1.0
80
s
18
18
12
(2)(0.11)
3
4
s
41.38
, but maximum
s
(24)
18
Use 18
19.12
ADDITIONAL TOPICS
This chapter has presented a brief discussion of prestressed concrete. A number of other
important topics have been omitted from this introductory material. Several of these items
are briefly mentioned in the paragraphs that follow.
Stresses in End Blocks
The part of a prestressed member around the end anchorages of the steel tendons is called
the
end block
. In this region the prestress forces are transferred from very concentrated areas
out into the whole beam cross section. It has been found that the length of transfer for post-
tensioned members is less than the height of the beam and in fact is probably much less.
For posttensioned members, there is direct-bearing compression at the end anchor-
age; therefore, solid end blocks are usually used there to spread out the concentrated pre-
stress forces. To prevent bursting of the block, either wire mesh or a grid of vertical and
horizontal reinforcing bars is placed near the end face of the beam. In addition, both verti-
cal and horizontal reinforcing is placed throughout the block.
For pretensioned members where the prestress is transferred to the concrete by bond
over a distance approximately equal to the beam depth, a solid end block is probably not
necessary, but spaced stirrups are needed. A great deal of information on the subject of
end block stresses for posttensioned and pretensioned members is available.
4
Composite Construction
Precast prestressed sections are frequently used in buildings and bridges in combination
with cast-in-place concrete. Should such members be properly designed for shear transfer
4
Nawy,
Prestressed Concrete
, p. 173.
