Steel Monopoles Leave Light Footprint on the Environment

Steel Monopoles Leave Light Footprint on the Environment
The need for a new line was clear. A large portion of northwestern Wisconsin’s transmission grid
was built to meet the electricity needs of the 1940s and 1950s. By the late 1990s, the state’s
demand for electricity was growing 2% – 3% each year. The existing line could not reliably
accommodate this kind of growth and threatened, under certain load conditions, to cause
blackouts across large, populated areas. The solution: Improve the reliability of the grid by
building a new 345kV line, 220 miles long from Wausau, Wisconsin to Duluth, Minnesota – the
area’s most challenging infrastructure project in decades.
Additional challenges reached far beyond engineering and construction concerns. Aggressive
anti-line groups fought against the project. Land owners and environmentalists formed a unified
front to complicate right-of-way issues for the line, which would cross over 80 miles of
wetlands. Protecting the environment quickly became a top priority, especially at the scenic
Namekgon River crossing. Public and government groups demanded an almost unseen structure
in this area, which created extreme design obstacles to balance the aesthetics with the physics of
the loads and materials involved.
A key factor in resolving many of the environmental and aesthetic concerns was the decision to
use weathering, tubular-steel monopoles, supplied by Trinity Meyer Utility Structures. The
flexibility, reliability and strength of steel structures provided the design team with the most
efficient solutions for resolving the special design needs of the crossing’s monumental 1,500-
foot span, for building across difficult terrain along the line’s path, and for satisfying tough
public demands. Leaving a smaller footprint on the environment than the existing H-frame wood
poles, most of the structures were single-shaft poles with drilled pier foundations, essential for
the narrower rights-of-way in both states.
Steel structures also allowed designers to customize special configurations, which included
single-circuit, single-shaft structures; double-circuit, single-shaft structures with four different
voltage levels; double-circuit, single-shaft structures with 46kV underbuild; and two-pole, heavy
angle deadend structures.
Arrowhead-Weston Case Study Page 2 of 2
This flexibility of customization played a vital role in meeting the Public Service Commission of
Wisconsin’s requirement to share corridors with as many existing utility easements as possible
and made it possible to replace portions of 12 existing lines, owned by four different utilities,
with double-circuit lines.
To add to the complexity of this project, the system was designed to survive a 400-year return
period weather event. This meant using two distinct loading criteria – one to handle the weather
effects of north central Wisconsin and another to accommodate severe icing conditions near
Lake Superior, which can cause nearly two inches of radial ice loads. In both cases, the system
was designed for winds over 100 mph.
Accomplishing such extreme environmental and design challenges required more than 50 million
pounds of steel and nearly 1,600 steel structures, some weighing in at more than 100,000 pounds
each and reaching 180 feet in height, designed to carry thousands of miles of cable across the
vast transmission line.
The success of this complex project required the decades of experience, creativity, dependability
and “Let’s make it happen” attitude of the Trinity Meyer Steel Structures team, who worked
hand-in-hand with construction teams to ensure structures were delivered when and where they
were needed to seamlessly synchronize with the installation timeline. The dedicated
collaboration of all involved enabled the project to be completed seven months ahead of
schedule. Energized in January 2008, the Arrowhead-Weston line carries enough electricity to
power 250,000 homes and provides a significant upgrade to the reliability of regional grid.

 

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