Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02688195 2009-12-11
Cementitious Fibre Reinforced Composite Cross Arm
Field of the Invention
The present invention relates to the field of cross arms in the electrical
utility industry.
Background of the Invention
Cross arms are used throughout the world as structural elements to support
electrical power
transmission lines above the ground. These transmission cross arms, normally
between 6 to 14
m in length, can be made of a variety of materials, the most common of which
is timber.
The service life of cross arms is a very important factor. Given the
difficulties of reaching and
replacing the cross arms (which may be in very remote locations), the cost of
replacing a cross
arm exceeds that of the cost of the cross arm, itself.
The use of timber cross arms, as shown in Figures 1 and 2, poses certain
challenges. Good
quality timber for use in the cross arm is becoming increasing difficult to
obtain given
diminishing old growth forests which is the prime timber source, as well as
the impact of modern
environmental laws.
Timber cross arms also have a limited life span (typically about 25 years) and
decay naturally.
Moreover, it can be difficult to detect cracks in timber through visual
inspection. Also the
moistness and/or temperature of the ambient surroundings of the timber may
hide or exaggerate
such cracks.
Timber cross are combustible and propagate fire rapidly in forest fires; they
are attractive to
woodpeckers; and, under certain weather conditions, they can initiate a pole
top fire. Timber
cross arms also creep (e.g. deflect) under heavy loads sustained for long
periods of time.
There have been several attempts to overcome these difficulties by
substituting timber with other
materials. Despite these attempts, timber remains the primary source of cross
arms in the power
transmission industry.
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Metal, particularly galvanized steel, cross arms have been used. The primary
disadvantage of
using a metal cross arm is its electrical conductivity, which makes the cross
arm very dangerous
for transmission line technicians (or linemen) to work with on energized live
lines. The
galvanized coating of such cross arms has a life expectancy of about 25 years,
after which the
cross arm is susceptible to corrosion. For these reasons, metal cross arms are
not widely used.
Laminated timber has also been used for cross arms, wherein the timber has
been coated with a
protective coating to prevent moisture penetration to increase the life
expectancy of the cross
arm. Some coatings are environmentally unfriendly, and may leach into the
surrounding
environment. Moisture and cracks may cause delamination of the timber. Under
many
circumstances, such cross arms may have a lower life expectancy than untreated
timber.
Fibre reinforced polymer has also been used to make cross arms. These have a
glass fibre
interior coated with a polymer matrix. Such cross arms lack fire resistance
and suffer from
delamination if not protected from ultraviolet light.
Concrete, while commonly used as a building material, has not proven suitable
for use as a cross
arm. Concrete has large capillarity porosity, which allows water to penetrate
and can cause the
concrete to crack in freezing and thawing cycles. Unreinforced concrete will
crack under tension
stress. Regular concrete without reinforcement is quite brittle, and lacks
ductility, which is a
problem when used as a long cross arm. Given the different load conditions in
electrical
transmission lines (load due to the weight of conductors, insulators, radial
ice on conductors,
wind on conductors) the cross arm requires ductility, i.e. the ability of the
material to plastically
deform while continuing to carry loads without fracture, even after micro
cracking. Also,
concrete is not easily usable with thin sections of a cross arm. A cross arm
made of concrete
would be large, bulky, heavy and would require steel reinforcement for
structural bending
capacity and stirrups for shear reinforcement. For the above reasons concrete
has not been used
for cross arms across the transmission industry.
Summary of the Invention
A cross arm is provided including a back member having a top edge and a bottom
edge; a top
extension extending from the top edge; a bottom extension extending from said
bottom edge; and
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wherein the cross arm is composed of a cementitious composite. The top
extension and bottom
extension extend generally perpendicular to the back member, which has a
generally flat back
portion. The cross arm may include a number of metal or glass reinforced
polymer bars running
positioned on the longitudinal axis along the cross arm within the cross arm.
The cross arm is
attachable to a utility pole opposite a second cross arm, and supports a
conductor.
A support structure for a conductor is provided, including an utility pole; a
first cross arm
composed of a cementitious composite, attached to the utility pole; a second
cross arm composed
of the cementitious composite, attached to the utility pole, on the opposite
side of the pole,
parallel to the first cross arm, wherein the conductor is supported between
the first and second
cross arm. The first and second cross arms are attached to the utility pole by
a threaded rod
passing between the cross arms and the utility pole. An insulator, supported
by a hardware
member, holds the conductor.
Description of the Figures
Figure 1 is a perspective view of a timber cross arm as known in the prior
art;
Figure 2 is a cross sectional view thereof;
Figure 3 is a perspective view of a cementitious composite cross arm according
to the invention
Figure 4 is a cross sectional view of a cross arm;
Figure 5 is a cross sectional view of an alternative embodiment of a cross
arm;
Figure 6 is a front view of cross arms in place in place on two utility poles;
Figure 7 is a side view of a portion thereof, showing two cross arms secured
to a utility pole;
Figure 8 is a cross sectional view showing how the cross arm supports a
conductor;
Figure 9 is a cross sectional view of single cross arm attached to a pole; and
Figure 10 is a cross sectional view of a single cross arm supporting a
conductor.
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Description of the Invention
Recent improvements to concrete, including the use of organic or metal fibres
have provided a
concrete composite that offers advantages when used to form cross arms. Such
fibres and
composites are disclosed in U.S. Patent Nos. 6,478,867; 6,881,256; and
6,723,162. The concrete
compositions used in cross arm 1, as seen in Figure 3, include a hardened
cement matrix in
which organic or metal fibres are dispersed, which can be obtained by blending
with water, a
composition also containing cement; granular elements (having a maximum grain
size (D) of 2
mm); fine elements with a pozzolanic reaction (having a particle size of no
more than 20 Jim);
and a dispersing agent. The weight percentage of water to the weight of the
cement and fine
pozzolanic elements is between 8% and 25%. The organic fibres have a minimum
individual
length (I) of 2 mm and an I/(1) ratio of at least 20, 4:136 being the fibre
diameter. The volume of
fibre represents no more than 8% of the concrete volume, and the ratio between
the average fibre
length and the maximum grain size D is at least 5.
An alternative concrete composition includes a hardened cement matrix in which
organic fibres
are dispersed, which is obtained by blending with water, a composition also
containing cement;
granular elements; fine elements with a pozzolanic reaction (having a particle
size of no more
than 1 m); and a dispersing agent. The weight percentage of water to the
weight of the cement
and fine pozzolanic elements is between 8% and 24%. The organic fibres have a
minimum
individual length (I) of 2 mm and an I/41:1 ratio of at least 20, (1) being
the fibre diameter. The
volume of fibre represents no more than 8% of the concrete volume. The cement,
granular
elements and fine elements have a grain size D75 of at most 2 mm, and a grain
size D50 of at
most 150 m. The ratio between the average fibre length and the grain size D75
is at least 5.
Yet another alternative concrete composition includes a hardened cementitious
matrix including
cement; aggregate particles having a particle size (Dmax) of no more than 2
mm; pozzolanic-
reaction particles having an elementary particle size of no more than 1 inn;
constituents capable
of improving the toughness of the matrix selected from the group consisting of
acicular and flaky
particles, wherein the particles have an average size of at most 1 mm and
which are present in a
proportion by volume of between 2.5 and 35% of the combined volume of the
aggregate particles
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and of the pozzolanic-reaction particles; at least one dispersing agent; metal
fibres dispersed in
the hardened cementitious matrix, wherein the fibres have an individual length
(1) of at least 2
mm and an 1/d ratio of at least 20, d being the diameter of the fibres, the
ratio 8 of the average
length (L) of the fibres to the maximum particle size (Dmax) of the aggregate
particles is at least
and the amount of fibres is such that their volume is less than 4% of the
volume of the
concrete after it has set; and water, wherein the percentage by weight of
water W with respect to
the combined weight of the cement and of the particles is in the range 8-24%.
A further alternative concrete composition includes a hardened cementitious
matrix including
cement; aggregate particles; pozzolanic-reaction particles having an
elementary particle size of at
most 1 gm; constituents capable of improving the toughness of the matrix
selected from the
group consisting of acicular and flaky particles, wherein the particles have
an average size of at
most 1 mm and are present in a proportion by volume of between 2.5 and 35% of
the combined
volume of the aggregate particles and of the pozzolanic-reaction particles;
and at least one
dispersing agent, wherein the combination of the cement, aggregate particles,
pozzolanic-
reaction particles and constituents has a D75 particle size of at most 2 mm
and a D50 particle
size of at most 200 tim; metal fibres dispersed in the hardened cementitious
matrix, wherein the
fibres have an individual length 1 of at least 2 mm and an 1/d ratio of at
least 20, d being the
diameter of the fibres, and the ratio (R) of the average length (L) of the
fibres to the D75 particle
size of the combination of the cement, aggregate particles, pozzolanic-
reaction particles and
constituents is at least 5, and the amount of fibres is such that their volume
is less than 4% of the
volume of the concrete after it has set; and water, wherein the percentage by
weight of water W
with respect to the combined weight of the cement and of the pozzolanic-
reaction particles is in
the range 8-24%.
Another alternative concrete composition includes a hardened cementitious
matrix in which
metal fibres are dispersed and represent a volume (V1) of the concrete after
setting, which is
obtained by mixing, with water, a composition which includes, apart from the
metal fibres:
cement; aggregate particles having a particle size D90 of at most 10 mm;
pozzolanic-reaction
particles having an elementary size ranging between 0.1 and 100 gm; at least
one dispersing
agent; and satisfying the following conditions: (1) the percentage by weight
of water with respect
to the combined weight of the cement and of the pozzolanic-reaction particles
lies within the 8-
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24% range; (2) the metal fibres have an average length ii of at least 2 mm and
an 11/01 ratio of at
least 20, 01 being the diameter of the fibres; (3) a ratio, V1 /V, of the
volume Vi of the metal
fibres to the volume V of the organic fibres is greater than 1, and a ratio,
11/1, of the length of the
metal fibres to the length of the organic fibres is greater than 1; (4) a
ratio R of the average
length 11 of the metal fibres to the size D90 of the aggregate particles is at
least 3; and (5) the
amount of metal fibres is such that their volume is less than 4% of the volume
of the concrete
after setting. The above is improved by adding to the concrete, organic fibres
having a melting
point of less than 300 C., an average length 1 of greater than 1 mm and a
diameter 0 of at most
200 gm; the amount of organic fibres being such that their volume V ranges
between 0.1 and 3%
of the volume of the concrete after setting; the concrete having a
characteristic 28-day
compressive strength of at least 120 MPa, a flexural strength of at least 20
MPa, and a spread
value in the unhardened state of at least 150 mm; the compressive strength,
flexural strength and
spread value being given for a concrete stored and maintained at 20 C.
Yet another alternative concrete composition is a fire-resistant ultrahigh-
performance concrete
having a 28-day compressive strength of at least 120 MPa, a flexural strength
of at least 20 MPa,
and a spread value in the unhardened state of at least 150 mm; the compressive
strength, the
flexural strength, and the spread value being given for a concrete stored and
maintained at 20 C;
the concrete including a hardened cementitious matrix in which metal fibres
are dispersed and
represent a volume V1 of the concrete after setting, which is obtained by
mixing, with water, a
composition which includes, apart from the metal fibres: cement; aggregate
particles having a
particle size D90 of at most 10 mm; pozzolanic-reaction particles having an
elementary size
ranging between 0.1 and 100 11M; at least one dispersing agent; organic fibres
having a volume
V; and satisfying the following conditions: (1) the percentage by weight of
water with respect to
the combined weight of the cement and of the pozzolanic-reaction particles
lies within the 8-24%
range; (2) the metal fibres have an average length 11 of at least 2 mm and an
11/01 ratio of at least
20, 01 being the diameter of the fibres; (3) the organic fibres have a melting
point of less than
200 C, an average length 1 of greater than 1 mm, and a diameter 0 of at most
200 gm; (4) a
ratio, VI/V, of the volume V1 of the metal fibres to the volume V of the
organic fibres is greater
than 1, and a ratio, li/l, of the length 11 of the metal fibres to the length
1 of the organic fibres is
greater than 1; (5) a ratio R of the average length 11 of the metal fibres to
the size D90 of the
aggregate particles is at least 3; (6) the amount of metal fibres is such that
their volume is less
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than 4% of the volume of the concrete after setting; and (7) the amount of
organic fibres is such
that their volume ranges between 0.1 and 3% of the volume of the concrete
after setting.
The above concrete compositions are herein each referred to as "cementitious
composites" and
are available from Lafarge North America under the trade-mark DUCTAL.
The use of cementitious composites provide a cross arm 1 that is not
combustible and is
environmentally benign (i.e. it has no negative impact on the local
environment). Cross arm 1
has a long life expectancy, of at least 75 years, and therefore a lower life
cycle cost when
compared to timber or steel, given the cost of replacement. Cross arm 1 can be
installed using
installation equipment and methods commonly used with timber cross arms. Cross
arm 1 is
electrically non conductive, and can resist harsh weather conditions, for
example, cross arm 1 is
freeze and thaw resistant, ultra violet light resistant, corrosion resistant,
and does not rot or
decompose.
Cross arm 1 also provides several advantages when compared to concrete.
Cementitious
composites do not have capillarity porosity. The fibres cause the cementitious
composite to
provide ductility to cross arm 1, and allow deflection without fracture.
Another feature of cross arm 1 is that it self heals small cracks as the
fibres engage the cracks
and unhydrated cementitious particles react with air and moisture to further
increase the
mechanical strength of the material. Moreover, in an extreme overload
condition, large visible
cracks allow for an efficient visual inspection of cross arm 1, including the
potential for a future
structural problem. In the same manner, ductility in cross arm 1 allows for
deformations and
cracks without structural failure, hence providing an opportunity for
effective replacement.
Cross arm 1 fits into the existing electrical grid. Internal storage of cross
arm 1 is not required,
and it can be stored externally. Cross arm 1 may colored as the user selects,
and may be colored
to match the colour of the utility pole (e.g. a color close to wood) to which
they are attached, or
the environment in which they are placed, which may be preferable for
marketing and public
acceptance reasons. Dirt and other contaminants on cross arms can be a source
of electrical
conductivity. The cross section and surface of cross arm 1 are designed to
provide self cleaning
benefits using rain.
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Given the ease of storage, cross arms 1 can be purchased and maintained in
bulk. Replaced cross
arms 1 can be crushed and then recycled and used as a road base or other
construction. Also,
industrial by-products are used in the making of cross arm 1.
Installation, repair or replacement of cross arm 1 can be done on an energized
electrical line, as
cross arm 1 is not conductive. Cross arm 1 will not risk electrical
interference causing partial
discharges and is more able to withstand a lightning strike. Also as the
cementitious composite
is not combustible, it will not propagate fire in forest fires. For this
reason, the risk from
vandalism and fire is minimal. Cross arm 1 will have a weight similar to or
less than a timber
cross ann.
The function of cross arm 1 is to support conductors in the air within a
frame, such as an "H
frame", i.e. supported by two utility poles 100, as seen in Figure 6. H frames
may be further
supported by cross braces 101, 102, placed on opposite sides of utility poles
100. Cross arms 1
are attached to a utility poles 100 by a galvanized steel threaded rods 120,
as seen in Figure 7,
passing through cross arms 1 and utility pole 100. Typically two cross arms 1
are mounted back
to back on opposite sides of utility poles 100. A bracket 110 may be in place
between cross arm
1 and pole 100 to help support cross arms 1. Threaded rods 120 extend through
cross arm 1
through aperture 155 and through both sides of pole 100, and are held in place
with washer 130
and nut 140.
As seen in Figure 8, cross arms 1 holds conductors 170 by hardware component
150 which has
flanges on both sides allowing hardware component 150 to be supported by both
cross arms 1.
Insulator 160 hangs from hardware component 150, and conductor 170 is held
below insulator
160 by clamp 175.
As seen in Figures 1 and 2, cross arms in the prior art are solid with a
rectangular cross section,
or may have a solid rectangular exterior with a hollow interior. Cross arm 1,
made of
cementitious composite, need not have a rectangular cross section. As the
cementitious
composite is stronger than timber, a rough "C" cross section shape, as seen in
Figures 3 and 5,
may be used wherein rectangular back member 10 is positionable adjacent to
utility pole 100.
Conductors 170 are typically supported at approximately either end 105, 115 of
cross arm I and
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at middle 125. Top and bottom extensions 20, 30, which extend along the length
of cross arm 1,
provide additional strength.
Top extension 20 may extend from a top edge of back member 10. Likewise,
bottom extension
30 may extend from a bottom edge of back member 10. Top and bottom extensions
20, 30 may
extend generally perpendicularly from back member 10. Top and bottom
extensions 20, 30 may
have curved interior flanges 185 as seen in Figure 5. Figure 4 shows an
embodiment of cross
arm 1 wherein curved flanges 185 are not present.
In addition, metal or glass fibre reinforced polymer bars 50, 60, as seen in
Figures 4 and 5, may
be placed within cross arm 1 along the longitudinal axis of cross bar 1, which
allows for
additional reinforcement, thereby increasing the ductility of cross arm 1.
Bars 50, 60, if made of
a conductive material, may be electrically connected by a number of embedded
wires 55 to
eliminate voltage differential between the bars to prevent electrical
interference. If bars 50, 60
are made of glass reinforced polymer, two additional bars 51, 52, as seen in
Figure 5, may also
be used.
In an embodiment of the invention, extensions 20, 30 will extend about 10 cm
from the near
edge of back member 10 and each have about 1 cm of height at the farthest
point from back
member 10. Back member 10 may be about 30 cm high. Cross arm 1 may have a
length of
about 30 m, but may be of any length appropriate. Cross arm 1 may, in fact,
have a wide variety
of sizes, for example the length of extension 20, 30 may range from 5 cm to 15
cm. Likewise the
height of back member may be from 15 cm to 54 cm. The height of extensions 30,
40 may range
from 1 to 4 cm, and in the flanged embodiment shown in Figure 5, extensions
20, 30 may angle
inward 2 mm to 5 mm.
In an alternative embodiment, as seen in Figures 9 and 10, a single cross arm
1 could be attached
to two utility poles 100. In this embodiment, a plate 200, secured to cross
arm 1 using two bolt
215 and nut 220 combinations, can be used to hold insulator 160. At the bottom
of plate 200 is a
u-ring 230 held in place by bolt 241 and nuts 237. A single cross arm 1 is
secured to a utility
pole 100 in a manner similar to that in the case of two cross arms 1, as shown
in Figure 7.
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Cross arm I should be precast due to complex production processes (e.g.
forming, batching,
casting, stripping and curing). Cross arm I can be manufactured industrially
in a controlled
environment so that weather conditions do not influence the availability of
cross arm I. Cross
arm 1 is easily shipped and can be manufactured in large volumes, with minimal
environmental
impact (particularly in comparison to timber). Installation holes may be pre
drilled before
delivery. Depending on the voltage level on the transmission lines, cross arm
1 may be made of
any appropriate length.
In general, in the following claims, the terms used should not be construed to
limit the invention
to the specific embodiments disclosed in the specification and the claims, but
should be
construed to include all possible embodiments along with the full scope of
equivalents to which
such claims are entitled. Accordingly, the invention is not limited by the
disclosure, but instead
its scope is to be determined entirely by the following claims.
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