Vibes on Harmonic Resonance

On April 4, the North Platte (NE) Bulletin newspaper reported, “A Chambers, Nebraska woman was killed Friday afternoon in North Platte after a 75-foot Taco Bell sign fell on top of the pickup she and her husband were in. . . The pole broke at a welded joint about 15 feet above the ground due to strong winds. . . The National Weather Center Office in North Platte recorded wind speeds of 30-35 miles an hour at the time at the North Platte Regional Airport, with an occasional gust of 40 miles per hour.”

On April 16, the Navajo Times (Window Rock, AZ) newspaper reported, “No one was killed when a large metal golden arches McDonald’s sign fell on an SUV in Window Rock yesterday. . . According to the National Climactic Data Center, in Window Rock at 12 pm, winds were at 24 miles per hour, gusting up to 39 miles per hour. . . ‘I’ve been working here two years, and I’ve seen it blow back and forth in the wind,’ [Richardson] Laughing said. ‘I’ve been saying to my wife it’s going to blow over some time’. . . Laughing pointed out that the weld that showed at the end of the pole where the sign used to be does not look very substantial.”

A North Platte attorney described the incident as “a one-in-a-million freak accident,” the newspaper reported. A police representative called the McDonald’s incident a “freak accident.”

On April 19, Memphis TV station WMC reported on its website, “A Memphis woman was critically injured when a two-story wall at a Denny’s Restaurant in downtown Memphis collapsed Sunday morning. . . A large Denny’s sign peeled away from the building’s [Benchmark Hotel] façade, causing the wall to crumble onto the sidewalk in a cloud of dust and debris. Faye Hall was buried under the rubble.”

Are these indicative of an increase in shoddy work by the sign industry; is the economy and/or slow/non-existent remuneration to installers from national sign companies causing shortcuts? Are bids on jobs so low that only incompetent people can win certain contracts? Is it simply that instant communication via Twitter, YouTube, etc. is causing “megawareness”? Or, most importantly, are some new engineering standards needed?

Apparently, it’s not just the sign industry. Finger pointing currently abounds with regard to the May 2 collapse of the Dallas Cowboys’ fabric-covered practice facility that paralyzed scouting assistant Rich Behm and injured 11 others. There, the National Weather Service reported winds of 64 mph, with possible microbursts of more than 70 mph.

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In between the McDonald’s and Denny’s incidents, the International Sign Assn.’s (ISA) Sign Expo opened in Las Vegas. That first morning, ISA offered a Discovery Seminar entitled “Sign Structure Integrity.” ISA had previously communicated the essentials of this issue to its members. To reach a wider audience in the wake of these incidents, however, ISA added the seminar only two weeks before the Sign Expo. Speakers included Bill Dundas, ISA’s director of technical and regulatory affairs; Roy Flahive, CNP Signs & Graphics (National City, CA); and Wes Wilkens, Persona Inc. (Watertown, SD), chairman of ISA’s Mechanical and Structural Subcommittee. An estimated 40 to 50 people attended.

This didn’t happen overnight. ISA began investigating sign failures in 2006 via a study conducted with the University of California San Diego. Roughly a dozen case histories were sent for evaluation.

In some cases, these failures may result from improper technique or incompetence. Of much greater interest were signs that appeared to have been engineered and erected correctly, but failed nonetheless. Although the study was completed in 2007, more research and testing will be needed before concrete solutions can be found.

However, “harmonic resonance” has emerged as the primary, identifiable topic for further discussion/investigation. Although the term is most often associated with music, it also pertains to unusual wind effects that apparently affect slender, single-pole structures: signs, as well as lightpoles and smokestacks. The ISA seminar touched on how this phenomenon has possibly caused fatigue in some signs.

At this point, additional questions greatly outnumber the answers. ISA would rather wait to have more definitive conclusions/results than offer possible explanations that might later prove incorrect. An article on this may occur as soon as next month.