Wood, steel, concrete … Mother Nature can bring em all down

April C. Murelio

Associate Editor

Whether made of wood, light-duty steel, concrete, fiberglass or composite materials like resins and glass, all utility poles and towers can bend, break and bob when Mother Nature kicks up her heels.

Utilities generally assign wood poles a useful life of 30 years, and alternative pole manufacturers tout a useful life of 80 years or more for their products. But a high wind, lightning bolt, tree limb or ice and snow can bring them down in seconds.

– In February, tornadoes ripped through central Florida, bringing down a 69 kV line in Kissimmee when a 100-foot spun concrete transmission pole-stress tested to withstand 250 mph winds-twisted and snapped.

– Niagara Mohawk, which serves about 1.5 million customers across 24,000 square miles of upstate New York, reported steel towers folding like card houses during an ice storm earlier this year.

– During an April 1997 blizzard, now referred to as “the big one”, North Dakota`s Cass County Electric and Minnkota Power Cooperative saw the wind twist metal towers like paper clips and 2,000 wood poles succumb to ice and snow.

– And as John Howarth, engineering manager for Tillamook People`s Utility District in Oregon, said about watching high water carry away 25 poles in a single swoosh, “If you have a flood, it probably doesn`t matter what kind of pole you`re using-it`s coming down.”

According to those who put the pieces back together after these disasters, facing the forces of nature and retaining some semblance of infrastructure requires an openness to innovation.

Traditions wane

No dependable numbers exist, but wood poles still appear to be the most widely used. In many ways, wood poles are like a favorite recliner-it may not be the prettiest piece of furniture in the house, but it is familiar.

And according to a study by Engineering Data Management Inc. (EDMI) of Fort Collins, Colo., they remain a cost-effective choice for most overhead line applications when compared to steel, fiberglass or concrete.

Still, traditional materials are increasingly challenged, with steel pole usage growing in the North and West and concrete poles appearing more frequently now in the Southeast and Southwest.

Design habits also have changed, with utilities switching from horizontal, all-wood H-frames to vertical, self-supporting steel poles or wood/steel structures that take up less room in rights- of-way.

Environmental and aesthetic concerns drive much of this move from tradition. Wood poles, treated with chemicals like creosote and pentachloro- phenol, face tough scrutiny. A Southwest utility deliberately installed concrete poles in one area because of a customer`s allergies to pole chemicals. Fiberglass poles, considered more pleasing to the eye, are an attractive option when lines must travel through a customer`s property.

A good storm also spurs research into new approaches. For example, West Oregon Electric Cooperative, which experienced extensive damage during the Inauguration Day Storm of 1993, replaced many of their crossarms with a “trim-line” construction design that clustered their lines on a steel pin insulator. These lines survived a windstorm in 1995, and the utility saved about 70 percent on maintenance and repair costs.

Brad Schmidt, Cass County`s transmission and distribution services senior vice president, said after its `97 blizzard, which caused $4.5 million in damages, the cooperative tacked on $2 million more for system improvements.

To increase the strength of its lines, he said Cass County shortened the space between poles to about 250 feet by replacing the 2,000 it lost to high winds and ice with about 4,800 new wood poles. The cooperative also put up 10 light-duty steel poles, and is currently evaluating their ability to “stand against the wind.”

So far, so good, said Marc Kress, construction and maintenance engineering manager. But Cass County is not ready to junk the wood poles yet. “We have wood poles that are 50 to 60 years old, and they don`t have any problems. Wood poles are readily available, and they`re a good, reliable product. But as the price of steel goes down and we get more familiar with them, we may make a switch.”

Tillamook PUD, Howarth said, also continues to evaluate the alternatives, making some room in its system for about 20 fiberglass poles and 60 steel. The PUD`s service area encompasses a mixture of terrains, including mountains and coastline where 80 to 90 mph winds and high water are common.

Fiberglass poles, which are lighter than wood, are good for hard-to-reach areas where vehicle access is a problem. Poles placed by helicopter in mountainous areas are generally fiberglass.

Because Tillamook must deal with corrosive salt deposits, Howarth said the PUD recently installed steel poles during a project to widen the Pacific Coast highway, and as long as the galvanized coatings hold, the utility expects the experiment to go well.

Maintenance costs close gap

Although the upfront cost remains higher with its steel and fiberglass poles, Howarth said Tillamook hopes to make up the difference with lower maintenance costs.

Currently, the utility spends about $85,000 a year for treating and testing its wood poles and another $250,000 a year to replace the old and rotten ones. “We would like to slow that down a bit,” Howarth said. “A pole that requires less than half that maintenance and can last 80-plus years provides some big incentives.”

According to EDMI, the calculated difference in maintenance costs between wood poles and other types of poles ranges just between 5 percent and 10 percent of initial cost, which means about $30 to $90 per pole for a distribution line and $100 to $300 for transmission lines.

However, EDMI`s economic analysis does not include composite poles that could tip the scale considerably.

Powertrusion 2000 International of Scottsdale, Ariz., boasts a new pole with zero maintenance costs, an 80-year life, and the ability to stand against high winds and ice.

These poles, which weigh about 320 pounds at 40 feet, are produced by an automated process called pultrusion. The composite materials are pulled through a heated die and come out of the machinery somewhat like sausage coming out of a meat grinder in long continuous strands.

Charlie Hendrickson, an engineer and Powertrusion`s domestic sales vice president, said the hollow, octagonal strands can be cut to any length and formed to any width. However, Powertrusion keeps its poles as close as possible to the equivalent wood pole diameters for hardware compatibility.

Hendrickson conceded that up to 50 feet, the wood pole price tag is hard to beat, but because Powertrusion`s composites do not require maintenance, the price remains competitive. “For poles at 55 feet and longer, we expect our prices to be equal to or less than the similar wood pole. At 60 feet and longer, our poles will cost less than any competitor.”

The first Powertrusion poles rolled off the assembly line in July, and according to strength tests conducted by INTEC, an independent lab that also tests products for Boeing, these poles may indeed live up to their manufacturer`s claims.

Of the poles tested, the weakest broke at 2,644 pounds of force, and the strongest broke under 2,823 pounds, representing a deviation in ultimate strength of less than 5 percent. In contrast, deviations for wood poles undergoing similar tests are all over the board. As a result of the INTEC tests, Powertrusion`s 40- to 45-foot poles meet the criteria for an ANSI (American National Standards Institute) Class 2 engineered pole.

Hendrickson said Powertrusion plans to lobby the National Electric Safety Code (NESC) for a 2.5 safety factor. NESC does not currently rate composites, but the 2.5 safety factor is used for steel and pre-stressed concrete poles.

During winter storms, ice particles attach to the fibers of wood poles and freeze, making the poles brittle and easy to break. However, Powertrusion`s poles should not have that problem since the surface is smooth and non-porous, Hendrickson said.

As the orders come in and the poles find their way into the field, the true tests-high winds, heavy rain, ice and snow, and record-breaking temperatures-begin. Until there is a pole or structure design that can withstand all nature throws its way, utilities must keep an open mind, an eye on developing technology, and, of course, keep the emergency plan handy.

Click here to enlarge image

Whether made of wood, light-duty steel or concrete, all utility poles and towers can succumb to Mother Nature. Photo courtesy of Cass County Electric.

Click here to enlarge image

During an April 1997 blizzard, Cass County Electric (North Dakota) and Minnkota Power saw the wind twist metal towers like paper clips. Photo courtesy of Cass County Electric.

Click here to enlarge image

North Dakota`s Cass County Electric lost 2,000 wood poles during a `97 blizzard but installed 4,800 new ones to shorten the span between the poles and minimize future storm damage. Photo courtesy of Cass County Electric.

Previous articlePOWERGRID_INTERNATIONAL Volume 4 Issue 1
Next articleELP Volume 77 Issue 2

No posts to display