Structural assembly formed of composite materials

Abstract

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, and a connection plate extending between an end of the diagonal member and the leg, the connecting member being formed of a plurality of layers of fibers and resin, the layers of fibers 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 transmission tower is very light and has the requisite strength.

Description

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.

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 Kevlar™ 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. 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. 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, 45°, 60° and 90° 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.

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:

FIG. 1 is a perspective view of a tower according to the present invention; FIG. 2 is an enlarged view of a portion of a leg with a plurality of reinforcing members and a connecting plate; and

FIG. 3 is a sectional view along the lines 3-3 of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings in greater detail and by reference characters thereto, there is illustrated in FIG. 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 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.

Claims

1. 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 said legs;

a connection plate extending between an end of one of said diagonal members and said leg, said connecting member being formed of a plurality of layers of fibers and resin, said layers of fiber having differing fiber directions with respect to each other, said connecting member being adhesively secured to said leg and being connected to said diagonal member.

2. The transmission tower of claim 1 wherein said diagonal members are connected mechanically to said connection plate.

3. The transmission tower of claim 2 wherein said mechanical connection comprises nuts and bolts.

4. The transmission tower of claim 1 wherein said reinforcing fibers are glass fibers.

5. The transmission tower of claim 4 wherein said resin is a polyester resin.