At 400 ft, a flagpole that is being erected by ACUITY, a property and casualty insurance company, at its headquarters in Sheboygan, Wisconsin, will be the tallest in North America, visible to drivers at a considerable distance. Mortenson Construction
Raising a 400 ft tall flagpole in Sheboygan, Wisconsin, requires expertise from multiple engineering disciplines, including structural, mechanical, and geotechnical.
May 6, 2014—Following several unsuccessful attempts to erect an unusually tall flagpole at its headquarters in Sheboygan, Wisconsin, one firm isn’t giving up. Determined to fly a large American flag on its campus, the company has hired a full engineering and construction team to fine-tune every aspect of the design and successfully raise a new flagpole on its property. At 400 ft, the flagpole will be the tallest the company has constructed—and the tallest in North America.
ACUITY, a property and casualty insurance company that operates in 22 states, is erecting the flagpole at its headquarters along Interstate 43 in Sheboygan—between Milwaukee and Green Bay on Lake Michigan. The flagpole will be the fifth the company has hoisted on its campus since 2001, when it erected its first flagpole as a “symbol of freedom” in the wake of the September 11, 2001, terrorist attacks. “We put the flagpole up ... as a tribute to all of the people in our country who worked so hard and sacrificed so much to protect the freedom that we have,” says Sheri Murphy, the vice president of services and administration for ACUITY.
The company’s first flagpole was just 80 ft tall and was eventually replaced with one that was 150 ft tall. But when the company expanded its headquarters and made its building taller, it again increased the size of its flagpole—extending the existing pole to 199 ft—to complement the building’s scale. Unfortunately, the taller pole fell down in high winds. ACUITY made two more attempts to erect an even taller, 338 ft flagpole, but those poles also swayed in the wind, so the company proactively took both down. “At that point we thought maybe we needed to be done with this because maybe it was not meant to be for us to have a flagpole,” Murphy says.
The flagpole will be founded on an octagonal 6 ft thick spread
footing, the bottom of which will be approximately 10 ft below
grade.Hans-Erik Blomgren, Arup
But since the last flagpole came down in 2008, the company has received many emails and phone calls from its customers and passersby requesting that it replace the flagpole, which had become a landmark for drivers of the 50,000 vehicles that pass ACUITY’s campus on Interstate 43 each day. “The flagpole has stirred a very strong and emotional response from people who connected to it in different ways,” Murphy says. “So that’s what really prompted us to give it yet another attempt.”
Intent on getting it right, ACUITY advertised a request for proposals for a design/build team and hired the Seattle office of Arup, an engineering firm headquartered in London, as its technical advisor on the project. Arup reviewed the technical merits of the bidders’ proposals, and on the basis of that analysis ACUITY selected Minneapolis-based Mortenson Construction as the design/build contractor on the project. U.S. Flag and Flagpole Supply, a firm based in Wimberley, Texas, that specializes in manufacturing and installing flagpoles that are greater than 100 ft tall, is the subcontractor to Mortenson and is responsible for delivering an engineered flagpole and flag-and-winch system. Arsene Professional Engineers (APE), Inc., an engineering firm based in Laguna Niguel, California, is the engineer of record.
ACUITY wanted its flagpole to be the tallest in North America—surpassing the current record holder, which stands at 308 ft in Laredo, Texas—so it settled on an even height of 400 ft. Even so, the flagpole will be significantly shorter than the 541 ft tall flagpole that will hold the world record when it is completed later this year in Jeddah, Saudi Arabia. “We hear about all of the world records that are being set overseas, and we have no desire to try to compete with that,” Murphy says. “But we did want to make our own personal statement and tribute by having the tallest flagpole in the United States and North America. It just goes back to appreciating the country in which we live and operate and really wanting to pay tribute to those who make that all possible.”
The flagpole is being raised as six sections for a total of 400 ft,
making it the tallest flagpole in North America. Mortenson
While most people don’t think of flagpoles as engineering marvels, erecting one of such height in a place where the temperature frequently dips well below freezing requires a great deal of design work. Arup has relied on engineering experts from throughout its company—including structural, materials, moveable structures, corrosion protection, mechanical, electrical, and geotechnical engineers—to meticulously scrutinize the design to ensure that the flagpole cam be sustained for a 50-year design life. “It’s an operating piece of equipment, so you really have to be diligent and not get sucked into the idea that it’s just a flagpole,” says Hans-Erik Blomgren, P.E., S.E., P.Eng., Struct.Eng., M.ASCE, an associate and structural engineer for Arup. “We really considered every aspect we could [in] reviewing the design strategy.”
The steel flagpole will be founded on an octagonal, 6 ft thick, spread footing, the bottom of which will be approximately 10 ft below grade. Rising from there, the flagpole will be 11 ft in diameter and have a plate thickness of 1 3/8 in. at its base. The flagpole will taper to 5 1/2 ft in diameter as it ascends skyward, and its top will have a plate thickness of 1/2 in. It will be fabricated in six sections, its forged flanges measuring up to 4 1/2 in. thick and 16 in. wide. The flanges will be field-bolted together on the inside. Broadwind Energy, a firm headquartered in Cicero, Illinois, that fabricates components for the energy and infrastructure markets, is drawing on its expertise in fabricating wind turbines for the energy sector to fabricate the flagpole, Blomgren notes.
A site-specific wind and climate report determined that the flagpole should be designed for a low service temperature of -41 degrees Celsius. The analysis also considered minimum and maximum wind loading using criteria established by ASCE 7: Minimum Design Loads for Buildings and Other Structures (ASCE, 2013) for icy and nonicy weather conditions. A subsequent wind analysis by APE, Inc., revealed that the flagpole would be subject to significant cross-wind response, causing it to oscillate substantially—a phenomenon known as vortex shedding. To reduce the movement and suppress the first three modes of vibration on the pole, three pendulum-style tuned-mass dampers will be suspended at various heights within the flagpole. “A slender piece of steel cantilevered out of the ground like a flagpole will oscillate with very low inherent dampening—well below one percent of critical,” Blomgren explains. “With the dampers, we are increasing it to two to three percent dampening.”
The dampers will also reduce fatigue on the flagpole. As a flagpole moves in the wind, it becomes stressed repeatedly, and its steel plates, bolts, and flanges become fatigued over time. By suppressing the oscillations, the dampers will also reduce the fatigue stresses to achieve the desired longevity. “This idea of a 50-year design life is not present in a lot of day-to-day engineering work—at least not on the building side, where I do most of my work,” Blomgren says. “You really have to think harder and more in-depth about your strategy to achieve that objective.” Working outside the code, the team relied on documents from the American Wind Alliance as well as European, Japanese, and United States standards to develop a consensus for the design.
Cranes are being used to install the 11 ft diameter flagpole on
ACUITY's property in Sheboygan, Wisconsin. Paul Miller, ACUITY
The flagpole will support a 60 ft wide by 120 ft long American flag, which will be weighted at the bottom to prevent it from wrapping around the pole. The flag will be on a winch and cable system that will extend through the center of the pole, the cable looping through a rotating slew-bearing truck assembly at the top of the pole to allow the flag to be raised and lowered and rotate freely in the wind. Like the rest of the flagpole, that system has been designed to withstand icing and meet the design life with proper maintenance. “This is a bearing system that like any moving-part system could wear itself out if it’s not maintained or designed properly,” Blomgren explains.
The flagpole will be raised near a pond at the northern end of ACUITY’s property during the next couple of weeks. The company plans to hold a ceremony to dedicate the flagpole in the coming weeks, and eventually remove from storage and reinstall the veterans’ memorial—a brick paver patio featuring the names of Sheboygan County residents killed in active duty—at the flagpole’s base. Murphy says the company is confident this flagpole will be a success thanks to the strong engineering and construction team involved in the project, and it looks forward to presenting the flagpole to the community. “We’re going to have a marvelous monumental structure that is going to answer the prayers of a lot of the people who have been asking for it to come back,” Murphy says. “It’s going to become a beacon for many.”