This was a fascinating article about a Texas-based firm that engineered and installed 80-foot-plus metal lighting poles all over the nation, mostly in high schools. The picture above is what they looked like a year later.
What happened? As always, a combination of things. The key thing to me is the lessons that should be learned by the design engineer, and highlights the industry problem that too many contractors are looking for "the man with the span" - the cheapest engineer with the tallest, highest, largest capacities, regardless if they're right or not.
Reports of more falling stadium poles surface
From this article.
The Texas Association of School Business Officials sent an e-newsletter to 5,000 members four weeks ago after a March 6 incident in which a 125-foot Whitco pole at the Hays school district's Bob Shelton Stadium in Buda toppled and slammed onto a gymnasium. At least 60 people were in the stadium at the time. The gym was empty.
In April 2007, a 90-foot Whitco pole fell to the ground at Applebee's Park, home to the minor-league Lexington (Ky.) Legends. The pole, which had been installed in 2001, fell away from the stadium, into an empty parking lot.
Although it was a gusty day, "the wind was certainly well less than the approved threshold" that guaranteed the poles' ability to withstand 90 mph winds, team President Alan Stein said. A subsequent examination revealed cracks in three other poles, so the team replaced all four, he said. "We decided it wasn't worth taking a chance."
The new poles and lights cost the Legends about $200,000, he said.
Possible causes
Forensic engineers called in to examine individual cases of the falling Whitco poles have reached different conclusions.
A report on a 90-foot pole that crashed to the ground at Worcester Technical High School in Massachusetts in February 2007 said a faulty weld holding the tall tube to the base plate was to blame. "Fatigue cracking caused by wind-induced vibrations" toppled a 5-year-old, 130-foot Whitco pole at the Round Rock school district's athletic complex in December, according to a January report by an Austin engineering firm.
A review of six of the toppled poles conducted by a Texas lighting pole manufacturer, however, has uncovered a common thread. CHM Industries Inc. of Saginaw evaluated the Whitco failures by examining the design specifications of each of the projects.
The first thing CHM noticed was that based on the failed poles' size and design, the stresses on the structures were extremely severe, according to a company engineer who said he didn't want his name printed because of company policy.
In a letter sent to its customers, CHM explained that the engineering safety margin of error on the poles was virtually zero.
That alone is not necessarily significant, several engineers said. Pole companies looking to bid competitively often shave the amount of metal needed to withstand the stress of wind and weight extremely close to their engineering tolerances.
Makers Co. president Greg Haskin, who said his company only welded the shafts to baseplates according to Whitco's specifications, said that in about 2000, when Whitco was purchased, some of the company's designs changed. In failed Whitco poles Haskin has since examined, he said, "some of them basically were overloaded. The base plate should have been a thicker base plate. The tube should've been thicker for the load. It appears that some of these poles were underdesigned."
CHM's analysis, summarized in the letter sent to customers, found that Whitco appears to have made mathematical errors in its design. In several instances, for example, the company's engineers seem to have based their stress calculations on the height of the pole, according to CHM. While such calculations are sufficient for street lighting, they can be inaccurate for tall athletic lighting towers where the light fixtures generally are attached to the top of the pole, effectively adding another six feet or so to the height.
By failing to account for the extra height, CHM's analysis found, several of the Whitco poles appear to have been underdesigned — the metal at the base was too thin to withstand the stress for the pole's actual size. In other instances, CHM determined that Whitco appeared to have miscalculated the area of the lights and fixtures attached to the poles, which also would add unaccounted-for stress to the base.
The CHM engineer said that in one instance, lighting and other fixtures added to the pole later caused the structure to exceed its engineered capacity.
"We do not know how many poles have been supplied by Whitco during this time frame or what percentage of their towers exhibit the same design flaw," the letter, dated March 31, concluded. "What we do know is that there is a serious cause for concern in regard to installations of this nature as produced by Whitco in or around the year 2002."
Posts
