The power industry has used wooden structural elements such as cross arms in power distribution systems for more than 100 years because wood was plentiful and inexpensive. However, in the last 15 years preservatives have been removed from wood structures due to environmental concerns and, as a result, there has been a drastic reduction in the performance and service life for wood structures. In most areas of the United States, the expected lifetime of a wood structure has been reduced to about 5-12 years. The reduced life expectancy of wood structures has resulted in increased costs for replacement. In recent years, power distribution companies have searched for alternative materials for fabrication of power poles and cross arms. Steel was considered as an alternative to wood, but increased costs for addressing safety concerns associated with electrical conductivity lead the industry away from widespread adoption of steel structures.
Composite materials have also been investigated as an alternative to wood structures. While providing distinct advantages, composite structures such as cross arms have several disadvantages in terms of cost, structural and mechanical limitations, and installation problems. For example, in order to meet structural requirements for use in various industrial applications, composite arms must be provided in a variety of rectangular shapes to accommodate different loading needs. The requirement for structural design variation of a composite arm is accompanied by a comparable requirement for a multiplicity of hardware for ancillary devices and wire attachments. In many cases the arms must be doubled to carry anticipated loads, and other components and appliances may be needed to allow the arm to be field-drilled, and to protect the arm from excessive attachment loads.
Notwithstanding the foregoing concerns, composite materials hold out several advantages over wood including corrosion resistance, strength, longer life, and the possibility for savings in installation and replacement costs.
U.S. Pat. No. 5,004,574 discloses a method of making an elongated composite structural element having fiber reinforcement using a wet pultrusion method around a rigid pipe casing. U.S. Pat. No. 5,576,081, which is a continuation-in-part of U.S. Pat. No. 5,004,574, discloses an elongated structural element having fiber reinforcement and a hollow central region. U.S. Pat. No. 5,858,493 discloses a tapered fiber-reinforced pole for supporting electric power lines.
The present invention relates to a new composite structure that exceeds the mechanical strength to weight ratio of other designs, reduces cost, and overcomes installation shortcomings of currently available composite arms. The ensuing disclosure describes certain advantages and benefits of preferred embodiments of a structure of the present invention.