Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
REINFORCED CONCRETE COLUMN OR BRIDGE PIER
BACKGROUND OF THE INVENTION
The present invention relates to a reinforced
concrete column or bridge pier.
In a conventional reinforced concrete column or
bridge consisting of a long concrete block, a plurality
of deformed steel bars are arranged in the longitudinal
direction, and hoops arranged with desired intervals
around the deformed steel bars along the longitudinal
direction of the bars. The concrete directly adheres
to the deformed steel bars.
In countries with frequent earthquakes, such as
Japan, the problem to be solved for reinforced concrete
columns or bridge piers constructed as above is to
prevent destruction due to the abrupt and excessive
shear stress caused by earthquakes. To this end,
according to the current design, the reinforced
concrete column or bridge pier has a number of hoops
resistant to a shear stress.
However, production of a reinforced concrete
column or bridge pier having a number of hoops requires
a large number of steel bars, and is very expensive.
In addition, construction work to erect the steel bars
and casting of concrete are difficult.
Unbonded prestressed concrete (unbonded PC) is
known as a concrete. An unbonded PC structure is
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formed by inserting a PC structural material into
a sheath located in concrete, thereby pre-stressing
the concrete. The space between the sheath and the PC
structural material is left unfilled or filled with oil
to prevent corrosion. This structure is frequently
used in beams for buildings. It has advantages that
the span can be longer and flexing or cracking can be
prevented. The reason why unbonded PC is employed is
that work can be simplified, since no grouting is
performed. Thus, unbonded PC is clearly distinguished
from the reinforced concrete (RC) structure.
BRIEF SUMMARY OF THE INVENTION
The present inventor found the following.
When abrupt and excessive shear stress is exerted on
a reinforced concrete column or bridge pier in
an earthquake, if a plurality of deformed steel bars
are arranged in direct contact with a long concrete
block, the shear stress causes cracks due to high
adhesion between the concrete and the deformed steel
bars, if the number of hoops is small. Further, the
shear stress is transmitted through the deformed steel
bars in the longitudinal direction (height direction),
resulting in cracks in a wide area of the column or
bridge pier. As a result, shear failure in diagonal
directions occurs.
Based on the above findings, the inventor arrived
at the present invention for a reinforced concrete
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column or bridge pier with high toughness, wherein
a sheath covers at least a portion of each deformed
steel ba r on which shear stress is exerted, so that the
deformed steel bars are not in direct contact with the
concrete. Surprisingly, even if the number of hoops
was small, cracks due to shear stress occurred only
in a limited portion and could be prevented from
propagating. As a result, shear failure in diagonal
directions was effectively avoided. The sheath is
formed of a thin steel or plastic plate, which has
flexibility. Therefore, even if it directly adheres
to the concrete block, the propagation of the shear
strength, which occurred in the deformed steel bars,
can be prevented from occurring.
The inventor also arrived at the present invention
for a reinforced concrete column or bridge pier with
high toughness, in which smooth round bars with coated
with a lubricant, such as grease, are used instead of
the deformed steel bars to lessen adhesion to the
concrete. Surprisingly, even if the number of hoops
was small, cracks due to shear stress occurred only
in a limited portion and could be prevented from
propagating. As a result, shear failure in diagonal
directions was effectively avoided.
The inventor conducted experiments using concrete
specimens in which a deformed steel bar, a deformed
steel bar covered with a sheath, a smooth round bar,
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and a smooth round bar coated with grease are arranged.
These bars were extracted from the concrete blocks to
examine the relationship between slip and adhesive
stress. FIG. 3 shows the results. In FIG. 3, lines
A-D represent the slip-adhesive stress characteristic
of a deformed steel bar, a deformed steel bar covered
with a sheath, a smooth round bar, and a smooth round
bar coated with grease, respectively.
As seen from FIG. 3, the adhesive stress of the
deformed steel bar covered with a sheath is always
zero. Further, the adhesive stress of the smooth
round bar coated with grease is smaller than those of
both the deformed steel bar and the smooth round bar.
Thus, the smooth round bar with grease can reduce the
adhesion to concrete.
According to an aspect of the present invention,
there is provided a reinforced concrete column or
bridge pier comprising a plurality of deformed steel
bars arranged in a longitudinal direction, and hoops
arranged at desired intervals around the deformed steel
bars along the longitudinal direction, wherein a sheath
covers at least a portion of each deformed steel bar on
which shear stress is exerted.
According to another aspect of the present
invention, there is provided a reinforced concrete
column or bridge pier comprising a plurality of smooth
round steel bars coated with lubricant and arranged in
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a longitudinal direction, and hoops arranged at desired
intervals around the smooth round steel bars along the
longitudinal direction.
Additional objects and advantages of the invention
will be set forth in the description which follows, and
in part will be obvious from the description, or may be
learned by practice of the invention. The objects and
advantages of the invention may be realized and
obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The accompanying drawings, which are incorporated
in and constitute apart of the specification,
illustrate presently preferred embodiments of the
invention, and together with the general description
given above and the detailed description of the
preferred embodiments given below, serve to explain
the principles of the invention.
FIG. 1 is a schematic view showing a reinforced
concrete column or bridge pier according to a first
embodiment of the present invention;
FIG. 2 is a perspective view showing a sheath and
a deformed steel bar arranged in the reinforced
concrete column or bridge pier shown in FIG. 1;
FIG. 3 is a diagram showing the relationship
between slip and adhesive stress of a deformed steel
bar, a deformed steel bar covered with a sheath,
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a smooth round bar, and a smooth round bar coated
with grease;
FIG. 4A is a schematic view showing a test
specimen having a reinforced concrete column of
an example l;
FIG. 4B is a sectional view taken along line A-A
in FIG. 4A;
FIG. 5 is a schematic view showing a test machine
to inspect failure modes and crack patterns of the test
specimens having the reinforced concrete columns of the
example 1 and a comparative example 1;
FIG. 6 is a diagram showing a load-displacement
hysteresis loop of the reinforced concrete column of
the example 1;
FIG. 7 is a diagram showing a load-displacement
hysteresis loop of the reinforced concrete column of
the comparative example 1;
FIG. 8 is a diagram showing crack patterns of
the reinforced concrete column of the example l;
FIG. 9 is a diagram showing crack patterns of
the reinforced concrete column of the comparative
example 1;
FIG. 10 is a diagram showing a load-displacement
hysteresis loop of the reinforced concrete column of
a example 2; and
FIG. 11 is a diagram showing crack patterns of
the reinforced concrete column of the example 2.
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DETAILED DESCRIPTION OF THE INVENTION
A reinforced concrete column or bridge pier
according to the present invention will be described in
detail with reference to the accompanying drawings.
(First Embodiment)
FIG. 1 is a schematic view showing a reinforced
concrete column or bridge pier according to the first
embodiment of the present invention, and FIG. 2 is
a perspective view showing a sheath and a deformed
steel bar arranged in the reinforced concrete column or
bridge pier shown in FIG. 1.
A reinforced concrete column (or bridge pier) 1
has a long concrete block 2. As shown in FIG. 2,
a deformed steel bar 3 is inserted in a bottomed
cylindrical sheath 4. A plurality of deformed steel
bars 3 inserted in the sheaths 4 are arranged in the
concrete block 2 along the longitudinal direction
(height direction). Since each deformed steel bar 3
is covered by the sheath 4, it does not directly adhere
to the concrete block 2. In other words, the deformed
steel bars are arranged so as not directly adhere to
the concrete block 2. A plurality of hoops 5 are
arranged at desired intervals along the longitudinal
direction around the sheaths 4.
The sheath 4 covers at least a portion of the
deformed steel bar on which shear stress is exerted.
However, it may cover the other portion of the deformed
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steel bar.
The sheath is preferably made of material that has
substantially no resistance to tension and that is not
deformed by contact pressure in pouring of concrete.
For example, it is made of steel or plastic, such as
polypropylene, polyethylene and polyvinyl chloride.
It is preferable that the sheath made of the steel is
0.25 to 0.32 mm thick and the sheath made of plastic is
0.5 to 1.0 mm thick.
(Second Embodiment)
In a reinforced concrete column (or bridge pier)
according to the second embodiment, a plurality of
smooth round bars coated with lubricant are arranged
in a concrete block in the longitudinal direction.
A plurality of hoops are arranged at desired intervals
around the deformed steel bars along the longitudinal
direction. The lubricant may be, for example, grease.
[Examples]
Examples of the present invention will be
described with reference to the drawings.
(Example 1)
As shown in FIGS. 4A and 4B, a reinforced concrete
hooting 11 and a reinforced concrete column 12
integrally constitutes a test body 13. The reinforced
concrete hooting 11 is 300 mm in width, 1200 mm in
length and 500 mm in height. The reinforced concrete
column 12 is a square prism of 300 mm. in width and
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length and 1200 mm in height. The reinforced concrete
hooting 11 has deformed steel bars 14 having a diameter
of 13 mm and arranged in the height direction, and
hoops 15 having a diameter of 10 mm and arranged around
the deformed steel bars 15. As shown in FIG. 4B, the
reinforced concrete column 12 contains twelve deformed
steel bars l6 having a diameter of 16 mm and arranged
along the height direction. The deformed steel bars 16
are compliant with Japanese Industrial Standards SD345.
The reinforced concrete column 12 also contains eight
steel hoops 17 having a diameter of 6 mm and arranged
around the twelve deformed steel bars 16 at regular
intervals along the longitudinal direction of the
deformed steel bars 16. The steel hoops 17 are
compliant with Japanese Industrial Standards SD300.
Each deformed steel bar 16 is covered with a 0.25 mm
thick steel sheath (not shown) over a length of 800 mm
from the bottom.
(Comparative Example 1)
A test specimen of the comparative example 1 has
the same reinforced concrete column as that of the
example 1 except that the deformed steel bars are not
covered with sheathes.
The test specimen was tested by a test machine
shown in FIG. 5. Mare specifically, a load-
displacement characteristic of the reinforced concrete
column and a failure mode of the test piece were
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inspected and a crack pattern was observed through the
procedures described below.
The test machine has a base member 21.
A flat-shaped base 22 is fixed to the base member 21
with bolts and nuts. A first wall 23 is fixed to
a left end portion of the base 22 with bolts and nuts.
A displacement transducer 29 is attached to the first
wall 23 and extends horizontally rightward. A second
wall 25 is fixed to a right end portion of the base
member 21 with bolts and nuts. An actuator 26
extending horizontally toward the first wall 23 is
attached to the second wall 25 so as to face the
displacement transducer 24. A beam 27 extending
horizontally is attached to the second wall 25 above
the actuator 26. A loading member 28 is mounted on
the lower surface of the beam 27.
First, the test specimen 13 was fixed to the base
22 of the test machine with bolts and nuts. The
displacement transducer 24 attached to the first wall
23 was brought into contact with the left side surface
of the reinforced concrete column 12 of the test body
13 via a spring 29: The actuator 26 attached to the
second wall 25 was fixed to the right side surface of
the reinforced concrete column 12 so as to face the
displacement transducer 24 with the column 12
interposed therebetween. The loading member 28
attached to the beam 27 was brought into contact with
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the upper end of the reinforced concrete column 12.
In this manner, the test specimen 13 was placed in
the test machine as shown in FIG. 5. The loading
member 28 applied a normal load to the reinforced
concrete column 12 under the conditions indicated
in Table 1. In this state, the actuator 26 was
reciprocated in the horizontal direction, thereby
applying a horizontal load to the reinforced concrete
column 12. At this time, yield displacement (~y)
and ultimate displacement (~u) of the example 1 and
the comparative example 1 were measured with the
displacement transducer 24. Further, under these
displacement conditions, the failure modes of the
reinforced concrete columns of the example 1 and the
comparative example 1 were inspected. The results of
measurement and inspection are indicated in Table 1.
Table 1 also shows yield loads (Py), maximum loads
(Pmax) and bu/5y.
Table 1
Example Comparative
1 Example 1
Load (kN) Py 119.4 117.02
Pmax 120.17 118.21
Displacement by 13.7 9.66
(~) bu 46.8 15.74
bury 3.42 1. 63
Failure mode flexure shear
FIGS. 6 and 7 show load-displacement hysteresis
loops of the reinforced concrete columns of the
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example 1 and the comparative example 1, respectively.
The reinforced concrete columns of the example 1
and the comparative example 1 were reciprocated three
times with displacement of 5.2 mm, 10.4 mm and 15.6 mm,
and the exteriors thereof were observed. FIG. 8 shows
a crack pattern of the reinforced concrete column of
the example 1, and FIG. 9 shows a crack pattern of
the reinforced concrete column of the comparative
example 1.
As seen from Table 1 and FIGS. 6 and 7, both the
yield displacement (by) and the ultimate displacement
(~u) of the reinforced concrete column of the example 1
are greater than those of the comparative example 1.
In addition, the value of ~u/by of the example 1 is
considerably greater than that of the comparative
example 1. Moreover, the failure mode of the
reinforced concrete column of the example 1 is flexure,
while that of the comparative example is shear.
Further, as seen from FIGS. 8 and 9, in the
example 1 (FIG. 8), cracks appear only in a bottom
portion of the column when the displacement is 5.2 mm,
10.4 mm and 15.6 mm. In the comparative example 1
(FIG. 9), cracking occurs in diagonal directions from
the bottom of the column when the displacement is
10.4 mm, and diagonal shear cracks extend all over
the column when the displacement is 15.6 mm.
From the above results; it is understood that the
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reinforced concrete column of the example 1 has high
toughness; that is, even if abrupt and strong shear
stress caused by earthquakes is exerted on the column,
shear failure in diagonal directions does not occur.
(Example 2)
A test specimen of the example 2 has the same
reinforced concrete column as that of the example 1
except that the deformed steel bar covered with the
sheath is replaced by a smooth round bar made of steel,
compliant with Japanese Industrial Standards SR295,
having a diameter of 16 mm and coated with grease.
The test specimen was subjected to a loading test
of the reinforced concrete column by the test machine
shown in FIG. 5 through the same procedures as in the
case of the example 1. Also, as in the example 1, the
failure mode was flexure.
FIG. 10 shows a load-displacement hysteresis
loop of the reinforced concrete column as a result of
the test. Like the example 1, as seen from FIG. 10,
both the yield displacement (c5y: about 12 mm) and
the ultimate displacement (bu: about 58 mm) of the
reinforced concrete column of the example 2 are greater
than those of the comparative example 1. In addition,
like the example 1, the value of ~u/~y of the example 2
is considerably greater than that of the comparative
example 1.
As in the example 1 and the comparative example 1,
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the reinforced concrete column of the example 2 was
reciprocated three times with displacement of 5.2 mm,
10.4 mm and 15.6 mm, and the exterior thereof was
observed. FIG. 11 shows the results.
As seen from FIG. 11, in the example 2, a little
amount of cracks appear only in a bottom portion of the
column and slightly above the bottom portion, when the
displacement is 5.2 mm, 10.4 mm and 15.6 mm. When the
displacement is 10.4 mm, cracking that occurs in
diagonal directions from the bottom of the column in
the comparative example 1, as shown in FIG. 9, is
prevented from occurring in the example 2.
From the above results, it is understood that the
reinforced concrete column of the example 2 has high
toughness; that is, even if abrupt and strong shear
stress caused by earthquakes is exerted on the column,
shear failure in diagonal directions does not occur.
Additional advantages and modifications will
readily occur to those skilled in the art. Therefore,
the invention in its broader aspects is not limited to
the specific details and representative embodiments
shown and described herein. Accordingly, various
modifications may be made without departing from the
spirit or scope of the general inventive concept as
defined by the appended claims and their equivalents.
