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May 15, 2014

Arup, Mortenson go to great lengths for tallest flagpole in North America

  • It took a year to design the 400-foot-tall structure that will be finished next month in Wisconsin.
  • By LYNN PORTER
    Journal Staff Reporter

    The pole consists of 60-foot-long segments that taper from 11 feet at the base to 5.5 feet at the top. Segments are bolted together at their flanges.

    It seems like a simple thing: You put up a flagpole and hang a flag on it.

    But not if the flagpole is 400 feet tall and the flag is 60 by 120 feet and weighs 500 pounds.

    Acuity Insurance is constructing that size flagpole at its Sheboygan, Wis., headquarters, where it will fly the giant American flag.

    More than a dozen engineers and contractors are working on the project, including the Seattle office of international engineering firm Arup as technical adviser.

    “They're in the insurance industry so they don't know much about engineering flagpoles,” said Hans-Erik Blomgren, who is coordinating Arup's work on the project. “They wanted to have somebody (with Arup's expertise) at their table.”

    That's not to say Acuity is new to the flagpole business.

    This is the fourth — and tallest — flagpole it has erected at the headquarters.

    Acuity first erected a 150-foot-tall flagpole in 2003 in response to the 9/11 terrorists attacks, according to Sheboyganpress.com. The pole was raised to 200 feet in 2004, but toppled during a windstorm on New Year's Eve that year.

    Acuity then put up two more flagpoles, each taller than 300 feet, the news site reported. One pole, dedicated on July 1, 2005, was taken down in 2007, and the other, a 338-foot pole erected in April 2008, was removed just months later. Blomgren said Acuity was worried about the last pole swaying in the wind.

    Blomgren said Acuity brought in the team of specialists for its latest flagpole as it will be taller than any of the company's other flagpoles, and Acuity wanted the assurance that state-of-the-art practices were being used so the pole will stand for decades.

    Mortenson Construction is head of the design-build team on the project. Florin Arsene is the structural engineer of record and U.S. Flag & Flagpole Supply is overseeing delivery of the flagpole.

    Acuity's flagpole will open in mid-June. It will be North America's largest flagpole although there are others in the Middle East and elsewhere abroad that are taller.

    The base of the Acuity flagpole will be 11 feet in diameter and the pole will taper to roughly 5.5 feet at the top, Blomgren said.

    The flagpole is designed to withstand the elements, from bitter cold to high winds.

    “It took a year to design this structure, and I think that kind of speaks for itself,” Blomgren said. “You can't fall into the trap that it's just a flagpole. You have to treat it as a working, operating piece of equipment with a 50-year design life.”

    Photos courtesy of Acuity Insurance [enlarge]
    A massive concrete base was poured to support the pole and its 500-pound flag.

    One reason Arup got the job is it has wind engineers that have designed other tall slender projects. Specifically Cormac Deavy, Arup's Seattle office leader, was project manager on The Spire of Dublin — a large, stainless steel, pin-like monument designed by Ian Ritchie Architects for downtown Dublin. Arup is a technical adviser on the world's tallest flagpole, which is nearing completion in Jeddah, Saudi Arabia. It will be 557 feet tall.

    “It's a specialty that's not addressed in building codes directly,” said Blomgren. “When the structures are this shape and proportion (building codes say) to seek authoritative documents that are outside the code. It's literally that vague.”

    Arup reviewed the structural engineer's specifications for the Acuity flagpole, along with the design drawings. It also did its own computer analysis of how the pole would perform.

    Over time, wind can cause stress that bends the metal and eventually causes it to fracture and the pole to fall. “It's not the 100-year wind that comes and blows the pole over,” Blomgren said. “It's winds that happen on a monthly basis. The analogy we use is bending a paper clip 10 times will snap it.”

    To try to account for the elements, a study was done to assess historical data on wind speeds and directions and temperatures at the exact address where the pole is going.

    That helped the team pick the right grade of metal, welds and bolts for the pole, said Blomgren.

    The pole was fabricated at Broadwind Towers and Heavy Industries in Manitowoc, Wis.

    Arup toured the Broadwind plant to ensure it was capable of doing the work, Blomgren said.

    Broadwind's core business is fabricating wind turbine tower masts. “There's a lot of parallels with the way the big 300-foot-tall masts have to perform,” Blomgren said. “It's not too far off from a (large) flagpole.”

    The pole comes in 60-foot-long segments bolted together in the field, with flanges on each segment.

    Blomgren said it took awhile to get the steel because a 400-foot-tall flagpole “is not something most people are building.”

    To lessen the effect of wind on the pole, three steel pendulums were hung from bars inside the pole. Blomgren said the force created by the pendulums will cancel out the force created by wind, causing the pole to swing in the opposite direction of the wind hitting it, which means less stress on the steel. “You want them to be perfectly out of phase with each other,” he said.

    The Acuity flagpole has a cylinder at its top that sits on a ball-bearing assembly. The cylinder rotates with the flag so the flag is always flying in line with the wind and won't wrap around the pole.

    The flag is raised by a winch system that goes up the middle of the pole. The pole was painted with three coats of high-performance paint, so it only needs to be repainted once more in its lifetime.

    Acuity said the project is in response to a public outcry since the last pole came down. It said a memorial at the base of the flagpole will have the names of Sheboygan County veterans killed in the line of duty.

    According to Sheboyganpress.com, the insurance company has said the pole will cost less than a penny per insurance policy per year.
     


    Lynn Porter can be reached by email or by phone at (206) 622-8272.


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