Note: Descriptions are shown in the official language in which they were submitted.
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STEEL PIPE POLE BASE AND REINFORCING METHOD THEREOF
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a steel pipe
pole base and a reinforcing method of the steel pipe pole
base for, for example, fixing a steel pipe pole such as a
street light support pole, a road sign support pole and
the like to a skeleton such as road and the like.
2. Description of the Related Art
As a steel pipe pole base for fixing a steel
pipe pole such as a street light support pole, a road
sign support pole and the like to a skeleton made of
concrete and the like, a structure constructed by welding
a base plate 11 to the lower end portion of a steel pipe
pole 10 and reinforcing the joint between the steel pipe
pole 10 and the base plate 11 with a plurality of ribs
12, as shown in Figure 10, is generally employed. Each
of the ribs is a tabular triangular rib the upper end
portion of which is cut obliquely and is welded to the
steel pipe pole 10 in the form of a T-joint. Then, the
steel pipe pole 10 is vertically supported by fixing the
base plate 11 to the skeleton with anchor bolts 15.
However, in a conventional steel pipe pole base
as described above, there has been a danger that, when a
bending moment is imposed on a steei pipe poie 10 uue to
wind, vibration or the like, a large stress concentrates
on the steel pipe pole 10 near the weld toes 16 of ribs
12 and, as a consequence, the strength at the portions
deteriorates due to the repeated stress. Another problem
has been that structural defects are likely to occur in
the boxing welded portions at the upper end portions of
the ribs 12 as a result of the combined effect of the
residual tensile stress and the material degradation of
the heat-affected zones caused by welding heat and to
cause the proof stress and the fatigue property to
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deteriorate.
Those problems are common to joint structures
in which reinforcing ribs are welded to structural
members in the form of a T-joint and, in view of this,
the Japanese Society of Steel Construction points out, in
"Guidelines for Fatigue Design of Steel Structures and
Its Interpretation", that a joint in which a gusset is
welded by fillet welding adversely affects the proof
stress and fatigue property of a steel member and
therefore attention has to be paid to the design of
structures.
SUMMARY OF THE INVENTION
The present invention has been established for
solving the aforementioned conventional problems and
providing a steel pipe pole base and a reinforcing method
of the steel pipe pole base, those making it possible to:
suppress the deterioration of strength in the vicinity of
the weld toe of a rib even when a repeated bending moment
is imposed on the steel pipe pole; and prevent the
deterioration of the proof stress and fatigue property of
a boxing welded portion at the upper end portion of the
rib. The gist of the present invention is as follows:
(1) A steel pipe pole base reinforced with ribs
welded to said steel pipe pole base in the form of a T-
joint, characterized by forming peening processed
portions at weld toes by ultrasonic vibration.
(2) A steel pipe pole base according to the item
(1), characterized by said ribs being tabular ribs.
(3) A steel pipe pole base according to the item
(1), characterized by said ribs being inverted-U or
inverted-V shaped ribs bent at the upper end portions.
(4) A method for reinforcing a steel pipe pole base
according to any one of the items (1) to (3),
characterized by applying peening treatment to weld toes
by ultrasonic vibration after said tabular ribs,
inverted-U shaped ribs or inverted-V shaped ribs are
welded to said steel pipe pole base in the form of a T-
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joint.
(5) A method for reinforcing a steel pipe pole base
according to the item (4), characterized by applying
peening treatment to said weld toes by ultrasonic
vibration while a load is imposed on said steel pipe pole
base so as to impose a tensile stress in the direction of
the steel pipe axis on the base material in the region
subjected to said peening treatment.
(6) A method for reinforcing a steel pipe pole base
according to the item (4) or (5), characterized by
applying said peening treatment by ultrasonic vibration
under the conditions of 20 to 50 m in amplitude and
several tens of kHz in frequency.
As mentioned above, in the present invention,
peening treatment is applied by ultrasonic vibration to
the weld toes of inverted-U or inverted-V shaped ribs
formed by bending the upper end portions of tabular ribs
welded to a steel pipe pole base in the form of a T-
joint. The method employed for the peening treatment is
a method wherein a cylindrical tool is ultrasonically
vibrated in the axis direction, the tip of the vibrating
cylindrical tool is applied to the surface of an
objective metal and, by so doing, the surface is made
concave. This method makes it possible to strengthen a
steel pipe pole base by imposing a high level energy on a
metal surface, thus producing plastic deformation,
relaxing stress concentration, and imposing residual
compressive stress on a weld toe.
Further, in the present invention, by employing
inverted-U or inverted-V shaped ribs as mentioned above,
the upper end portions of the ribs are liberated from the
principal stress direction of a steel pipe pole to a
direction perpendicular to the principal stress direction
and the rigidity of the rib upper end portions is
lowered. As a result of this, it is possible to
considerably relax stress concentration produced at weld
toes, when bending stress is imposed on a steel pipe
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pole, and also the residual tensile stress caused by
welding heat.
Furthermore, in the present invention, a peening
treatment is applied to the weld toes of ribs by
ultrasonic vibration. The method employed for the
peening treatment is a method wherein a cylindrical tool
is ultrasonically vibrated in the axis direction, the tip
of the vibrating cylindrical tool is applied to the
surface of an objective metal, and by so doing the
surface is concaved. In consequence, a_high level energy
is imposed on the weld toes, plastic deformation is
produced, and residual compressive stress is imposed.
For this reason, the weld toes that have been the weak
points of a steel pipe pole base are further strengthened
and therefore it becomes possible to suppress the
deterioration of strength at the weld toes of ribs and
prevent the deterioration of the proof stress and the
fatigue property, of boxing welded portions at the upper
end portions of the ribs, even when a repeated bending
moment is imposed on a steel pipe pole.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view showing the first
embodiment according to the present invention.
Figure 2 is views explaining a portion subjected to
peening treatment by ultrasonic vibration in the first
embodiment shown in Figure 1; Figure 2(a) is a side view,
2(b) a front view and 2(c) a perspective view.
Figure 3 is a front view showing the second
embodiment according to the present invention.
Figure 4 is a perspective view showing the third
embodiment according to the present invention.
Figure 5 is a stress concentration profile obtained
by subjecting a steel pipe pole base to which an
inverted-U shaped rib is attached to FEM analysis.
Figure 6 is a stress concentration profile obtained
by subjecting a steel pipe pole base having a
conventional construction to FEM analysis.
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Figure 7 is S-N curves showing the results of
fatigue strength tests in the case of Example 1 on page 10.
Figure 8 is S-N curves showing the results of
fatigue strength tests in the case of Example 2 on page 10.
Figure 9 is a side view of an ultrasonic impact
device.
Figure 10 is a perspective view showing a
conventional example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Firstly, a steel pipe pole base reinforced with
tabular ribs according to the present invention is
explained hereunder.
In Figure 1, reference numeral 10 denotes a steel
pipe pole used as a street light support pole, a road
sign support pole or the like, 11 a base plate welded to
the lower end portion of the steel pipe pole 10, 12 a
plurality of ribs welded in the form of a T-joint for the
reinforcement between the steel pipe pole 10 and the base
plate 11. Each of the ribs 12 is tabular and cut
obliquely-at the upper end portion and forms a triangular
rib. Then, in the case of a road pole, the steel pipe
pole 10 is vertically supported by fixing the base plate
11 to a skeleton made of concrete by using anchor bolts
15. The above configuration is the same as a
conventional one.
In the present invention, a peening processed
portion 20 subjected to ultrasonic vibration is formeu at
the weld toe 16 of each tabular rib 12 as shown in Figure
2. The peening treatment by ultrasonic vibration is
applied by the method as shown in Figure 9 wherein the
tip of a cylindrical tool 22 of an ultrasonic impact
device 21 is applied to the surface of an objective
metal, is ultrasonicalls vibrated in the axis direction,
and by so doing makes the surface concave.
The tip of the cylindrical tool 22 generally has a
round section and a preferable diameter thereof is about
1 to 6 mm. The reason is that, when a diameter is less
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than 1 mm, the strength is insufficient and enough impact
cannot be imposed, and, in contrast, when a diameter
exceeds 6 mm, the mass is too large and therefore
ultrasonic vibration is hardly generated.
A preferable frequency of the cylindrical tool 22 is
in the range from 10 to 50 kHz and a preferable value of
amplitude thereof is in the range from 20 to 50 m. The
reason for regulating the frequency as above is that a
large impact energy can be imposed efficiently on a steel
material in that frequency range. When a value of
amplitude is less than 20 ~Lm, a sufficient impact cannot
be imposed. On the other hand, when a value of amplitude
exceeds 50 m, the plastic deformation of a steel
material undesirably increases excessively in some cases.
A metal surface processed under the aforementioned
conditions undergoes plastic deformation by a high level
energy, is made concave to a depth of about 0.1 to 0.5
mm, and a tensile stress can be introduced up to the
depth of 10 mm or more from the surface. Further, the
metallographic structure changes largely up to the depth
of about 100 ~tm from the surface, a texture layer called
a white layer is formed, and good corrosion resistance,
good wear resistance and the reduction of friction
resistance can be obtained.
In the present invention, such a peening processed
portion 20 subjected to ultrasonic vibration as merit.ioiied
above is formed at the weld toe 16 of each tabular rib 12
as shown in Figure 2. It is preferable to form the
peening processed portion 20 at a portion from the upper
end portion of the rib 12 to at least about 10 mm
downward. As a result, the stress concentration at the
weld toe 16 is relaxed, a tensile stress is imposed on
the weld toe 16, and the fatigue strength improves
remarkably. Further, though the weld toe 16 of a rib 12
is a portion where structural detects are likely to occur
by the combined effect of residual tensile stress and the
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degraded heat-affected zone caused by welding heat as
mentioned above, even structural defects such as fine
cracks can be remedied by changing the microstructure by
applying peening treatment by ultrasonic vibration.
In another embodiment, a steel pipe pole base reinforced
with inverted-U or inverted-V shaped ribs bent at the upper
end portions according to the present invention is explained
hereunder.
In Figure 3, reference numeral 10 denotes a steel
pipe pole used as a street light support pole, a road
sigh support pole or the like, 11 a base plate welded to
the lower end portion of the steel pipe pole 10, 13 a
plurality of inverted-U shaped ribs welded in the form of
a T-joint for the reinforcement between the steel pipe
pole 10 and the base plate 11. Here, the inverted-U
shaped ribs 13 may be replaced with inverted-V shaped
ribs 14 as shown in Figure 4. The base plate 11 is fixed
to a skeleton such as a road by using anchor bolts 15 and
the steel pipe pole 10 is supported vertically.
The steel pipe pole 10 undergoes the principal
stress in the vertical direction and the ribs 13 or 14
also stretch as a whole in the principal stress direction
of the steel pipe pole 10. However, the upper portion of
each of the ribs 13 is bent gradually in the shape of a
circular arc and the upper end portion 16 of each of the
ribs 13 that forms a weld toe is bent to the extent of
forming a right atigle with the direction oi the principal
stress of the steel pipe pole 10.
In this way, by gradually bending the upper end
portion 16 of each rib 13 in such a direction as to be
liberated from the principal stress direction of a steel
pipe pole 10, the upper end portion 16 of each rib 13 can
be formed into a structure having a low rigidity. As a
result, stress concentration at the upper end portion 16
of each rib 13 is relaxed, residual weld thermal stress
at a weld is also relaxed greatly, and the proof stress
and the fatigue property as a welded structure are
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improved considerably.
In order for these effects to be achieved
sufficiently, it is preferable that the radius of
curvature at the upper end portion 16 of each rib 13 is
set at not less than three times the thickness of the rib
13. If the radius of curvature is less than the above
value, the material quality tends to deteriorate when a
rib 13 is bent and also the effect of lowering the
rigidity is lessened.
Here, Figures 5 and 6 show stress concentration
profiles obtained by subjecting a steel pipe pole base
shown in Figure 3 and a conventional steel pipe pole base
shown in Figure 10 to FEM analysis. Each of those
profiles shows by contour lines the distribution of
stress generated in the vicinity of a rib 13 or 14 when a
horizontal load is equally applied to the upper end
portion of a steel pipe pole 10 and the unit of the
numerals in each profile is MPa. It can be understood,
from a comparison between Figures 5 and 6, the maximum
value of stress concentration is reduced up to half that
of a conventional structure by bending the upper end
portion 16 of a rib 13.
Note that the arrows in the upper right direction
shown in Figures 7 and 8 mean that the loading was
terminated since no change in the specimens was observed
at the time. Further, the expression 'n = 2" means that
the number of the specimens is two. In this regard, the
weld quality of the partial specimens used in the tests
is of a very high grade and therefore the fatigue life
may possibly be shortened to some extent at the ordinary
industrial production level.
Moreover, in the present invention, the weld toe of
each of the bent ribs 13 is further subjected to peening
treatment by ultrasonic vibration. A peening processed
portion 20 is defined by the region extending at the
central angle a on both sides of the center line of the
rib 13 as shown extendedly in Figure 3 and a preferable
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angle a is generally in the range from 30 to 60 degrees.
The angle a in Figure 3 is about 45 degrees.
In this way, in the steel pipe pole base according
to the method of the present invention, as the weld toe
of each of the inverted-U shaped ribs 13 or the inverted-
V shaped ribs 14 bent at the upper end portions 16 is
subjected to peening treatment by ultrasonic vibration,
the effects of those constituents are combined together.
As a result, it is possible to considerably relaxation
stress concentration generated in the vicinity of the
weld toe of each of the ribs 13 or 14 when a bending
moment is imposed on a steel pipe pole 10 due to wind,
vibration or the like, and to conspicuously improve the
fatigue strength at the portion as shown in the data of
the examples described later.
Further, it is generally acceptable to apply peening
treatment by applying the tip of a cylindrical tool 22 of
an ultrasonic impact device 21 to the weld toe of each of
ribs 12, 13 or 14 welded to the base of a steel pipe pole
10. However, it is also acceptable to apply peening
treatment by ultrasonic vibration while a load (a bending
load for example) is imposed on a steel pipe pole base so
that a tensile stress in the steel pipe axis direction is
applied to the base material in the treatment region. In
this way, by applying peening treatment to and imposing a
compressive stress on a weld toe while a tensile stress
is imposed by externally given force, it becomes possible
to make a far larger compressive stress remain at the
weld toe 16 when the externally given force is removed.
Consequently, a far more excellent reinforcing effect can
be obtained.
Though peening treatment is applied by ultrasonic
vibration to only the weld toe of each of the ribs 12, 13
or 14 in the above explanations, needless to say, it is
also acceptable to apply peening treatment to other
welded portions. However, it is estimated that the
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application of peening treatment to the lower portions or
the like of the ribs 12, 13 or 14 is not practically
beneficial because the portions do not directly affect
the fatigue strength of the steel pipe pole base.
Example 1
Fatigue strength tests were carried out by imposing
repeated tensile stress on partial specimens around steel
pipe ribs having the construction shown in Figure 1. The
material used for both the steel pipes and the ribs was
SM 490. The fatigue property of the conventional
construction having as-welded ribs as shown in Figure 10
corresponded to the E to D grades of the design life
curves in the "Railway Bridge Design Guideline" as shown
by the black round marks in Figure 7. On the contrary,
the fatigue property of the invention construction
wherein peening treatment was applied to weld toes by
ultrasonic vibration markedly improved up to the grade B
or higher of the design life curves as shown by the white
round marks in the figure. Here, the amplitude of the
tool tip was 40 m and the frequency thereof was 30 kHz.
Further, when peening treatment was applied by
ultrasonic vibration while a load for applying tensile
stress was imposed on a steel pipe pole base, the fatigue
property improved up to the grade A of the design life
curves as shown by the black triangular mark. Moreover,
even when peening treatment was applied to the weld toe
where fatigue cracks were generated by ultrasonic
vibration, the fatigue property improved up to the grade
A of the design life curves as shown by the white
triangular marks. The data show that peening treatment
by ultrasonic vibration has the function of remedying
fatigue cracks.
Example 2
Fatigue strength tests were carried out by using a
test device and imposing repeated tensile stress on the
steel pipe pole bases according to the present invention
as shown in Figures 3 and 4. The material used for both
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the steel pipes and the ribs was SM 490. In comparison,
fatigue strength tests were carried out by imposing
repeated tensile stress on partial specimens around steel
pipe base ribs having the construction shown in Figure
10. As a result, the fatigue property of the
conventional construction shown in Figure 1.0 corresponded
to the E to D grades of the design life curves in the
"Railway Bridge Design Guideline" as shown by the black
round marks in Figure 8. On the contrary, the fatigue
property of the invention construction markedly improved
up to the grade A of the design life curves as shown by
the white round marks in the figure. Here, the amplitude
of the tool tip was 40 m and the frequency thereof was
30 kHz.
Further, when peening treatment was applied by
ultrasonic vibration while a load for applying tensile
stress was imposed on a steel pipe pole base, the fatigue
property improved up to the A grade or higher of the
design life curves as shown by the white triangular
marks.
