To support utility lines, for example, 345 KV power lines, it is conventional to use single pole structures, Y structures, H frame structures, and/or lattice steel towers.
Conventional utility line support structures may be made of tubular metal sections, wood, angle iron, or concrete. The concrete may be solid or tubular, and may be reinforced, post-tensioned, or pre-stressed.
Many conventional Y structures and H frame structures are made of tubular pole sections. Tubular pole sections are expensive.
One reason that closed sections such as tubes are expensive is because welding adjacent tube sections together to make a long pole requires using circumference welds that are difficult to make and require careful inspection and a high degree of quality control.
However, a closed section is desirable in utility line supports because they have a great deal of strength and torsional stability.
On the other hand, an open section, such as a channel, is easier and less expensive to manufacture, and is easier to work with in the field when installing it to form utility line support structures. Also, channel sections are easy to butt weld together with easily inspected welds.
Tubular steel poles compete with wooden poles as support structures for utility lines. While tubular steel poles are comparatively more expensive than wooden poles, tubular steel poles cost less to erect than the wooden ones.
Lattice steel towers are also used to support utility lines. Lattice steel also is more expensive to erect due to the many pieces and also they require four foundations precisely placed with respect to each other. Moreover, lattice steel towers are very rigid and do not deflect under load. If something happens to a utility line to suddenly increase the load, such as a wire breaking or ice falling off of one span but not the other, the tower either supports the load or it fails. On the other hand, tubular support structures have the advantage of deflecting when subjected to an increased load, so they continue to support the power transmission lines without falling to the ground.
In a transmission line supported by rigid steel towers, when a sudden load causes one of the towers to collapse, a number of other towers may also fall down because they cannot withstand the sudden increased load caused by the fallen tower. However, if the same imbalance occurs in a flexible support structure, the other flexible support structures deflect and by deflecting reduce the imbalanced load thus reducing the possibility of their falling down.
In at least one instance, 60 miles of power line and its supporting lattice steel towers cascaded down, one after the other, because the rigid towers could not deflect and withstand the increased load.
The various rolled shapes available as structural elements for utility line support structures, such as rolled "I" beams and channels, have not been acceptable to the utility companies because they are not economical or efficient from a strength viewpoint. Accordingly, there is a need for a utility line support structure that is economical and has a high strength to weight ratio.