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文献、资料中文题目：预应力混凝土
文献、资料英文题目：
Prestressed
Concrete
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翻译日期：
2017.02.14
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The
Concrete structure
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、外文资料原文
Prestressed Concrete
Concrete is strong in compression, but
weak in tension: Its tensile strength varies
from 8 to 14 percent of its compressive
strength. Due tosuch a Iow tensile capacity,
fiexural cracks develop at early stages
ofloading. In order to reduce or prevent such
cracks from developing, aconcentric or
eccentric force is imposed in the longitudinal
direction of the structural element.
This force prevents the cracks from developing by
eliminating
or
considerably
reducing
the
tensile
stresses
at
thecritical
midspan
and
support
sections
at
service
load,
thereby
raising
the
bending,
shear,
and
torsional
capacities of the
sections. The sections are then able to behave
elastically, and almost
the full
capacity of the concrete in compression can be
efficiently utilized across the
entire
depth of the concrete sections when all loads act
on the structure.
Such
an
imposed
longitudinal
force
is
called
a
prestressing
force,i.e.,
a
compressive force that
prestresses the sections along the span ofthe
structural element
3
prior
to
the
application
of
the
transverse
gravitydead
and
live
loads
or
transient
horizontal
live
loads.
The
type
ofprestressing
force
involved,
together
with
its
magnitude,
are
determined
mainly
on
the
basis
of
the
type
of
system
to
be
constructed and the span
length and slenderness desired.~ Since the
prestressing force
is applied
longitudinally along or parallel to the axis of
the member, the prestressing
principle
involved is commonly known as linear prestressing.
Circular
prestressing,
used
in
liquid
containment
tanks,
pipes,and
pressure
reactor
vessels,
essentially
follows
the
same
basic
principles
as
does
linear
prestressing.
The
circumferential
hoop,
or
stress
on
the
cylindrical
or
spherical structure, neutralizes the
tensile stresses at the outer fibers of the
curvilinear
surface caused by the
internal contained pressure.
Figure 1.2.1
illustrates, in a basic fashion, the prestressing
action in both types
of structural
systems and the resulting stress response. In(a),
the individual concrete
blocks
act
together
as
a
beam
due
to
the
large
compressive
prestressing
force
P.
Although
it might
appear that the blocks will
slip and vertically simulate shear slip
failure, in fact they will not because
of the longitudinal force P. Similarly, the wooden
staves in (c) might appear to be
capable of separating as a result of the high
internal
radial
pressure
exerted
on
them.
But
again,
because
of
the
compressive
prestress
imposed
by
the
metal
bands
as
a
form
of
circular
prestressing,
they
will
remain
in
place.
From
the
preceding
discussion,
it
is
plain
that
permanent
stresses
in
the
prestressed
structural
member
are
created
before
the
full
dead
and
live
loads
are
applied in order to
eliminate or considerably reduce the net tensile
stresses caused by
these loads. With
reinforced concrete,
it
is
assumed
that
the
tensile
strength
of
the
concrete
is
negligible
and
disregarded.
This is because
the tensile forces resulting from the bending
moments are resisted by
4
the bond created in the reinforcement
process. Cracking and deflection are therefore
essentially irrecoverable in reinforced
concrete once the member has reached its limit
state at service load.
The
reinforcement in the reinforced concrete member
does not exert any force of
its own on
the member, contrary to the action of prestressing
steel. The steel required
to
produce
the
prestressing
force
in
the
prestressed
member
actively
preloads
the
member, permitting a
relatively high controlled recovery of cracking
and deflection.
Once the flexural
tensile strength of the concrete is exceeded, the
prestressed member
starts to act like a
reinforced concrete element.
Prestressed
members
are
shallower
in
depth
than
their
reinforced
concrete
counterparts
for
the
same
span
and
loading
conditions.
In
general,
the
depth
of
a
prestressed
concrete
member
is
usually
about
65
to
80
percent
of
the
depth
of
the
equivalent reinforced
concrete member. Hence, the prestressed member
requires less
concrete, and,about 20 to
35 percent of the amount of reinforcement.
Unfortunately,
this
saving
in
material
weight
is
balanced
by
the
higher
cost
of
the
higher
quality
materials
needed
in
prestressing.
Also,
regardless
of
the
system
used,
prestressing
operations
themselves result in an added cost: Formwork is
more complex, since the
geometry
of
prestressed
sections
is
usually
composed
of.
flanged
sections
with
thin-webs.
In
spite of these additional costs, if a large enough
number of precast units are
manufactured,
the
difference
between
at
least
the
initial
costs
of
prestressed
and
reinforced
concrete
systems
is
usually
not
very
large.~
And
the
indirect
long-term
savings are quite substantial,
because
less maintenance is
needed;
a longer working
life is possible due to better quality
control of the concrete, and lighter foundations
are
achieved due to the smaller
cumulative weight of the superstructure.
Once
the
beam
span
of
reinforced
concrete
exceeds
70
to
90
feet
(21.3
to
27.4m),
the
dead
weight
of
the
beam
becomes
excessive,
resulting
in
heavier
members
and,
consequently,
greater
long-term
deflection
and
cracking.
Thus,
for
larger
spans, prestressed concrete becomes mandatory
since arches are expensive to
construct
and do not perform as well due to the severe long-
term shrinkage and creep
they
undergo.~
Very
large
spans
such
as
segmental
bridges
or
cable-
stayed
bridges
can only be
constructed through the use of prestressing.
Prestressd
concrete
is
not
a
new
concept,
dating
back
to
1872,
when
P.
H.
Jackson, an engineer from
California, patented a prestressing
system that used a tie
rod to construct
beams or arches from individual blocks [see Figure
1.2.1 (a)]. After a
long lapse of time
during which little progress was made because of
the unavailability
of high-strength
steel to overcome prestress losses, R. E. Dill of
Alexandria, Nebraska,
recognized the
effect of the shrinkage and creep (transverse
material flow) of concrete
on
the
loss
of
prestress.
He
subsequently
developed
the
idea
that
successive
post-tensioning
of unbonded rods would
compensate for
the time-dependent
loss of
stress in the rods due to the decrease
in the length of the member because of creep and
shrinkage. In the early 1920s,W. H.
Hewett of Minneapolis developed the principles
of
circular
prestressing.
He
hoop-stressed
horizontal
reinforcement
around
walls
of
concrete
tanks through the use of turnbuckles to prevent
cracking
due
to
internal