By Joe P Hasler | Popular Mechanics | November 2010 |
When the Dallas Cowboys’ practice facility caved in, it was the perfect storm of bad weather and shoddy engineering.
On the stormy afternoon of May 2, 2009, 27 newly minted Dallas Cowboys were finishing a workout in the team’s indoor practice facility when the hanging lights 86 feet above them began swaying. On the sidelines of the 83,000-square-foot field house—a tent-like steel-framed building with a tensioned fabric exterior—beat writers and photographers stopped watching the minicamp and stared at the fabric walls, which were snapping like a flag in brisk winds.
“I heard something long before I saw anything,” says photojournalist Arnold Payne. “It was this huge crash of metal and steel. You knew it was time to run. Just from the sound.”
It was the sound of a massive structural failure. Buckling under the winds, the facility’s western wall collapsed on top of the field. The roof and eastern wall soon followed suit. As people dashed for the exits, the fabric covering fell, enveloping them in darkness. Most of the 70 inside the building managed to escape safely. But 12 people were injured, some seriously.
The practice facility had been erected in 2003 at the behest of new head coach Bill Parcells. Summit Structures, an Allentown, Pa.–based company owned by Canadian firm Cover-All Building Systems, put up a $4 million, 406-foot-long field house over one of the fields at the Cowboys’ Valley Ranch campus. But in 2008, after the collapses of four similar Summit- or Cover-All-built facilities over a six-year period, the Cowboys requested structural reinforcements. Summit put in additional horizontal bracing, buttressed some critical structural points and re-covered the field house in a new exterior fabric, all to make the building more robust. None of it worked. So who—or what—was to blame for the collapse?
First to weigh in was the National Weather Service. Studying Doppler radar from May 2, it concluded that a phenomenon known as a microburst—a localized, concentric wall of air capable of traveling at speeds of up to 150 mph—was a major factor. But investigators at the National Institute of Standards and Technology (NIST) soon determined that weather wasn’t the sole culprit in the cave-in. “There was nothing unusual about this wind event in terms of its relationship to structural design,” says Fahim Sadek, who co-authored the NIST report.
Investigators determined that the building, designed to withstand 90-mph winds, broke apart in gusts between just 55 and 65 mph. In its final report, published eight months after the collapse, NIST detailed a series of engineering failures—a progression of flawed calculations, incorrect assumptions and unexplained deviations from original plans. Wind load, the force applied to an object by blowing wind, was calculated for an average building height of 60 feet; the structure actually had a mean height of 67 feet. As a result, the load on the building was more than double what Summit had assumed. Engineers also assumed that the building was “fully enclosed,” but it had vents and several large openings for doors, affecting internal pressure—and wind resistance.
When the facility was upgraded in 2008, Summit calculated the new roof’s slope at 11 degrees; the actual slope was 21 degrees—so demands on the building were 68 percent greater. And Summit assumed the building’s fabric covering provided lateral bracing for the structure. NIST’s computer models showed that it did not.
The NIST report noted that tented structures like the field house are relatively unregulated, and the practice of counting the fabric as lateral bracing is a point of disagreement among engineers. Summit may not be around to weigh in on the debate: Cover-All, Summit’s parent, filed for bankruptcy in March. (Attempts to contact Summit were unsuccessful.)
In the wake of the collapse, other organizations with similar structures, including Texas A&M and the University of New Mexico, had engineers evaluate the safety of their facilities. But it’s not known how many buildings of this type exist—or how many could be suffering from structural deficiencies. But the takeaway from this disaster is clear: Engineering errors and inclement weather can be a tragic combination. The Cowboys’ structure was built to fail; it just needed a good push. And on May 2, it got one.
INSIDE THE TENT COLLAPSE
The Cowboys’ practice facility was built by Summit Structures in 2003. Twenty-eight steel-truss gable frames, reinforced with steel webbing, created the 86-foot-tall facility’s structure. Sheets of fabric hanging from the frame formed the interior walls and ceiling, and a layer of tensioned fabric over the frame created the exterior walls and roof. In 2008, Summit upgraded the facility, purportedly to make it safer.
At 2:57 pm on May 2, a localized pocket of cool air in the midst of a spring storm drops from the clouds to the ground. The cool air pushes outward, creating a “stagnation point” from which a concentric wall of wind—essentially an inverted tornado—travels at speeds between 58 and 62 mph, blowing northeast toward Valley Ranch, about 1 mile away.
At 3:24 pm, the wind smashes into the field house (1). On the western wall of the structure, its critical joints—the so-called “knees”—strain under the wind load, which outweighs the load-bearing capacity of these vital members; they buckle (2). Failures at the knee and ridge (3) cause the north end of the facility to collapse onto the field. The fabric that created the walls and ceiling also falls, cloaking 70 people trying to escape in darkness.
As the north end collapses (1), the near-tornado winds blow the east side of the southern end of the structure over onto the neighboring outdoor practice field (2). Most of the players, staff and local media inside the building managed to escape safely, but 12 people were injured. Special teams coach Joe DeCamillis sustained a broken neck, and Rich Behm, a scouting assistant for the Cowboys, was paralyzed from the waist down.
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