Note: Descriptions are shown in the official language in which they were submitted.
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,
STRUCTURAL ASSEMBLY FORMED
OF COMPOSITE MATERIALS
FIELD OF THE INVENTION
The present invention relates to structural assemblies and more particularly,
to the
assembly of parts made from composite materials which may then form towers for
the
telecommunications and electrical transmission towers.
BACKGROUND OF THE INVENTION
Transmission towers are widely used around the world for supporting overhead
power lines. Transmission towers are also known by other names such as
electricity pylon
and hydro towers. Basically, the transmission tower is usually a steel lattice
tower and are
used in high voltage AC and DC systems as well as for communication systems.
They come
in a wide variety of shapes and sizes. Thus, typical heights range from 15 to
55 meters
although the tallest have extended up to approximately 350 meters.
Typically, there are a variety of ways which they can be assembled and
erected.
Thus, the tower may be assembled horizontally on the ground and erected by
push-pull
cable. This method suffers from the disadvantage of requiring a large assembly
area. It is
also known to assemble the towers vertically in their final upright location.
An alternative is
the use of a gin-pull crane. In remote locations, helicopters may serve as
aerial cranes for
their assembly in such locations. This method is somewhat expensive and the
weight of the
tower must be taken into consideration.
While most towers are formed of a lattice steel construction, sometimes
aluminum is
used for remote location. The extra cost of aluminum towers will generally be
offset by a
lower installation cost. The design of the aluminum lattice towers is similar
to that for steel,
but must take into account aluminum's lower young's modulus.
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1 =
4
%
Other materials which have been used include wood and concrete. However, these
are only suitable materials for certain locations and uses.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a transmission tower which
is both
light weight and relatively inexpensive.
According to one aspect of the present invention, there is provided a
transmission
tower comprising a plurality of legs, each leg being formed of a composite
material formed
as a pultrusion of reinforcing fibers, a plurality of diagonal members
extending between the
legs, a connection plate extending between an end of one of the diagonal
members and the
leg, the connecting member being formed of a plurality of layers of fibers and
resin, the
layers of fiber having differing fiber directions with respect to each other,
the connecting
member being adhesively secured to the leg and being connected to the diagonal
member.
The legs of the transmission tower are formed of a composite material in the
form of
a pultrusion. Pultrusion is a process for the manufacture of composite
materials having a
constant cross section. Reinforcing fibers are pulled through a resin,
possibly followed by
separate preforming system, and into a heated die where the resin undergoes
polymerization.
Many resin types can be used in pultrusion including polyester, polyurethane,
vinylester and
epoxy. Pultrusion may also be successfully used with thermoplastic matrices
such as
polybutylene, terephthalate (PETE), TET either by powder impregnation of the
glass fiber or
by surrounding it with sheet material of the thermoplastic matrix which is
then heated.
Different fibers may be utilized with fiberglass being a preferred material.
Naturally, other
fibers such as KevlarTM could also be utilized.
Die heating is one of the critical process control parameters that determines
the rate
of reaction, the position of reaction within the die and the magnitude of the
peak exotherm.
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A puller is usually separated from the die exit in order to allow the hot
pultruded product to
cool in the atmosphere or in a forced water or air cooling stream.
As mentioned above, one can use a wide variety of fiber reinforcement and
resin
systems to get a composite material. Thus, one may design the tower to have
various
mechanical properties such as stiffness, tension and impact strength while the
resin system
provides physical properties such as resistance to fire, weather, ultraviolet
light, corrosive
chemicals, etc. The choice of resin can depend upon the final property
desired. Polyester
resins are popular since they exhibit good corrosion resistance and the
electrical properties
of polyesters make them suitable for use as primary insulators in high voltage
applications.
Vinylester resins show better corrosion resistance and mechanical properties
at elevated
temperatures but are more expensive than polyesters. Epoxy resins are suitable
for high
temperature use but are relatively expensive. As is known in the art, glass,
carbon and
aramid fibers can be substituted one for the other depending upon the final
physical
properties required.
As used herein, a "pultruded product" refers to a product which has been
formed by
the pultrusion process. The transmission tower according to the present
invention may
include any conventional type of tower. Generally, a transmission tower will
include
communication towers which are structures used in establishing line of sight
for microwave
or UHF links. The towers' height may vary greatly but will typically be
between 25 meters
and 100 meters. The towers are lightweight (generally 25% the weight of
steel), have
corrosion resistance, weather resistance, radar transparencies, a high impact
strength and are
low maintenance.
The connecting member is also a composite material, but differs in that it is
formed
of a plurality of layers of fibers extending at different angles with respect
to each other.
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Thus, for example, a first layer may extend parallel to the fibers in the
pultruded leg, with
other layers being at angles of, for example, 450, 600 and 900 with respect to
the first layer.
The connecting member is connected to the leg of the transmission tower by
means
of a suitable adhesive. It is important that the connecting member maintain
continuous
contact with the transmission tower leg.
The connecting member can also be adhesively secured to the diagonal member or
alternatively, may be connected by mechanical means such as nuts and bolts.
The loading of braced members of composite materials is a situation wherein
the
loads must be distributed so that no part of the connection is over stressed
and not able to
carry and disperse the load in a continuous fashion. In the present invention,
the connection
acts as a reinforcement by bonding the main materials to the secondary
structure in a manner
wherein the laminated layers distribute the loads with a symmetrical geometry.
The loading
related to the tension through the legs is reinforced by sealing the cut fiber
by means of
impregnating a bored section with resin and so allowing the loads to flow
through the
perimeter of this section in a linear fashion.
The fabrication of the connecting member is done by locating the fibers in a
symmetrical predetermined direction which allows the connection plate to carry
loads
related to the tension and compression of the tower. At the same time, the
structure is able
to carry torsion, compression and tension from the bracing members to the
primary structure
which is the legs. The material forming the connecting plate will be
impregnating with the
resin layer by layer and preferably transferred into a mold with a vacuum
capacity which
will compress the different layers of fiber. Once compressed, the resin is
introduced.
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Because of the vacuum, the connecting plate will be less susceptible to
delamination and
more capable of carrying the desired load.
When drilling for the mechanical connections, the bored opening should not
have any
jagged edges which would create a local failure. Accordingly, it is preferred
that the area
around the bored hole or opening be sealed with a suitable material such as a
mixture of
resin and epoxy.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the invention, reference will be made to the
accompanying drawings illustrating an embodiment thereof, in which:
Figure 1 is a perspective view of a tower according to the present invention;
Figure 2 is an enlarged view of a portion of a leg with a plurality of
reinforcing
members and a connecting plate; and
Figure 3 is a sectional view along the lines 3-3 of Figure 2.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings in greater detail and by reference characters
thereto, there
is illustrated in Figure 1 a typical transmission tower which is generally
designated by
reference numeral 10. Transmission tower 10 may be of the type to carry
overhead power
lines. Alternatively, they may be used for many different types of antennas or
the like. For
these illustrations, neither of the antennas or transmission lines are
illustrated. It is,
however, understood that many different types of arrangements may be used at
the upper
part of the tower for supporting transmission lines or antennas.
Tower 10 has a plurality of generally vertically extending legs 12. Extending
between legs 12 are a plurality of horizontal reinforcing members 14 and
diagonal
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reinforcing members 16. Again, it will be understood that many different
arrangements of
the reinforcing members are possible as is well known in the art.
A connecting plate 18 is designed to interconnect at least some of reinforcing
members 14, 16 with legs 12. As illustrated, connecting plate 18 is adhesively
22 secured to
leg 12 while mechanical connections such as nuts and bolts 20 may be utilized
to connect to
reinforcing members 14, 16. Naturally, adhesive connections could also be
utilized for
interconnecting plate 18 and reinforcing members 14, 16.
The arrangement is one wherein connecting plate 18 is formed of a plurality of
layers
of fibers, each layer having the fibers oriented in a different direction.
Various
combinations of angles may be utilized as is known in the art.
The use of connecting plate 18 transmits any torsion of forces to the
reinforcing
members 14, 16.
A single connecting plate has been described herein. The number of connecting
plates employed can be varied depending upon the location and use of the
tower. In other
words, some of the connections between the leg and reinforcing members may be
utilized.
It suffices to say that one skilled in the art could design the tower and
determine the
placement of the reinforcing plates depending on the final tower requirements.
It will be understood that the above described embodiment is for purposes of
illustration only and that changes and modifications may be made thereto
without departing
from the spirit and scope of the invention.
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