CSI Tornado: Decoding – and chasing – supercells with the experts
CSI Tornado: Chasing supercells, interviewing a homeowner sucked off his front porch in an Oklahoma tornado outbreak, and examining the path of a destructive funnel, an expert expedition shows how science is close to decoding the way a tornado works.
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In January, he received a National Science Foundation grant to explore ways to design new homes and retrofit old ones to resist tornadoes.Skip to next paragraph
For all the attention paid to designing buildings to resist earthquakes and hurricanes, no generally accepted building standards yet exist in the US for tornado resistance, suggests a report by a team of researchers, including Prevatt, who analyzed damage from a tornado rated as a 4 on the Enhanced Fujita (EF) scale in Tuscaloosa and Birmingham, Ala., last spring.
One design, foam-and-concrete domes – built small as backyard shelters, mid-size as houses, or large for use as school gyms or churches – have proved robust in the face of even powerful tornadoes. Windows and doors may get blown in, but the structure remains intact.
LaDue testifies to the dome effect. "You're preaching to the person that surveyed one in Blanchard last May," he said when the subject came up during the drive to Woodward.
On May 24, 2011, an EF4 tornado plowed 32 miles across Oklahoma, injuring 15 people and killing one. During his survey of the damaged town of Blanchard, he came across an elderly couple's dome home that "performed remarkably well." Like water flowing around a rounded boulder in a stream, tornado winds easily flowed over and around the building, leaving the overall structure intact.
But most building departments have little or no experience with domed structures, so permitting can be as difficult as developing the dome's aesthetic appeal, advocates acknowledge.
Current codes don't address tornadoes well, Prevatt explains. Design standards account for horizontal winds, but tornadoes include vertical winds. Only recently have researchers developed laboratory tools that allow engineers to effectively estimate what those stresses may be on a full-scale building, he says. For now, engineers have no idea of how strong vertical stresses are at key points in a structure, he adds.
Moreover, the twister's winds hit like a boxer's punch, compared with the slow, steady increase in winds that come with an approaching tropical storm or hurricane.
Cost versus risk is an issue, too, says Prevatt. "It's not as much of an engineering problem as it is a societal one, accepting what its risk is and what it's willing to pay for its safety."
Last year 1,700 tornadoes may have touched down in the US, but the region at most risk covers 2 million square miles: The likelihood of a tornado hitting any single spot in a given year is tiny. But when that spot is shared by homes, schools, and businesses, the effect on individual lives and the community can be shattering.
Even in the absence of specific tornado standards, much more could be done to improve structures' resilience – at least with new construction, says Timothy Marshall, a meteorologist and principal engineer at Dallas-based Haag Engineering, a forensic engineering firm.
Today's codes are minimum standards. But the goal, Mr. Marshall argues, should be to exceed code. He notes that homes everywhere could benefit from the strapping and anchoring to secure roofs and walls that hurricane-prone states now require.
"It's going to cost more, but not a lot more – $500 to $1,000 more," he says.
He sounds a note of frustration at poor construction practices he found during surveys following severe tornadoes in Greensburg, Kan., in 2007 and the outbreak in the South last spring.
"It's like an offering to a tornado," he says.
He's heartened, though, by success stories in these events – people who'd installed safe rooms or shelters that allowed them to survive.
Race to investigate