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How Much Stress Can a Flagpole Take?

FullTextImage/img/@altCameron Weatherholtz '14 and Lt. Col. Chuck Newhouse perform tests on a flagpole in front of barracks. -- VMI Photo by H. Lockwood McLaughlin.

 Cadet Research Project Finds Out

Lexington, Va., July 29, 2013 – A Summer Undergraduate Research Institute project currently under way at VMI has as its ultimate goal giving the Institute more information about two of its oldest and tallest assets: the flagpoles in front of barracks.

Cadet Cameron Weatherholtz ’14, a civil engineering major, has been testing the strain that the gigantic garrison flags, 20 x 40 feet in size, put on the two flagpoles in front of barracks.

The garrison flags are flown over post on parade days and holidays.

For controls, he’s also testing the flagpoles’ strain with no flag, and with the smaller storm flags, which are used in bad weather.

The ultimate goal of the flagpole project, explained Weatherholtz and his faculty mentor, Lt. Col. Chuck Newhouse, is to come up with a series of recommended wind speeds for each type of flag, and to get an idea of how well the flagpoles would hold up in an extreme wind event such as a hurricane.

To conduct his tests, Weatherholtz first measures the wind speed at ground level, using a handheld gauge. As he does this, though, he’s aware that the wind speed at ground level is lower than the wind speed 75 feet in the air, the approximate height of the flagpoles. Typically, he said, a .5 miles per hour wind speed on the ground translates into a four to five miles per hour wind speed at the top of the poles.

With the wind speed established, Weatherholtz then uses a strain gauge to measure the force put on the poles by the wind.

“[The strain gauge] measures the strain at the bottom of the flagpole,” explained Weatherholtz. “From that, from various equations, we can calculate the forces that are being applied to the flagpole due to wind.”

The test for each situation – no flag, storm flag and garrison flag – runs for 10 minutes, for a total of 30 minutes of testing.

During the tests, a box attached to the gauge collects data every .33 seconds. “If the flagpole is swaying, it sways back and forth about once every second,” Weatherholtz said. “So you’re getting three data points. What you end up seeing is like a sine curve.”

To no one’s surprise, Weatherholtz has found that the garrison flags put the greatest amount of strain on the flagpoles, and that the strain is greater at the bottom of the pole than it is at the top.

Newhouse, a structural engineer by training, came up with the idea of measuring the strength of the flagpoles. “Wind loads are very complicated,” he noted. “We knew if we got involved with them it would be more than enough work to do a SURI project.”

To begin their research, Weatherholtz and Newhouse tried to find out the age of the flagpoles, but that proved to be difficult. In the end, they determined that the poles were put in place sometime between 1920 and 1934, but no one knows exactly when. Col. Keith Gibson, executive director of the VMI Museum System, told them that the poles were a gift from the class of 1920.

“I don’t think it’s written anywhere how long a flagpole’s lifespan should be,” commented Newhouse. Maj. Matt Swenty, assistant professor of civil and environmental engineering, added that if flagpoles are well taken care of with regular painting, as VMI’s appear to be, they could last for more than 100 years.

Newhouse and Swenty explained that design of flagpoles falls into a gray area, somewhere between the purview of an architect and that of a structural engineer.

“A lot of times they’re just put up,” said Newhouse, without regard to codes or design standards. In his research, Weatherholtz was able to find a 2007 design code for flagpoles, but the two professors said that VMI’s poles date from a time when flagpoles simply weren’t addressed by building codes – thus the need to get more information about their structural integrity and maximum load-bearing capability.

Projects such as the one Weatherholtz has undertaken this summer are of special importance to Newhouse and Swenty, because the two are working to establish a civil engineering structures lab at VMI.

Newhouse explained that such a lab would allow cadets to test their civil engineering skills in a greater variety of ways, especially regarding load-bearing capacity. The equipment that Weatherholtz has been using would go in the structures lab, if such a lab were to be established.

“Cameron’s at the forefront of applying [this equipment] to a really neat project,” said Swenty. “We’re making progress. I think we’ll have [the structures lab] in the long term.”

–Mary Price