June 27, 2011
JOPLIN, Mo. | As the monster tornado bore down on them, Rusty Howard and his two small children sought refuge in a Home Depot store.
Tanner, the engineer and tornado expert from Texas Tech University, has been recording tornado-related failures of tilt-up wall buildings since at least 1999. Based on what he has learned, he said there were numerous issues that should be addressed.
He said that most tilt-up wall structures have relatively lightweight roofs and questionable lateral support systems for the side walls. Tanner also said better tilt-up structures have strong panel-to-panel connections.
Tanner also said that tilt-up wall panels were usually not connected to one another, meaning they tend to fail progressively.
“Walmart only had a catastrophic failure in one part of their store, but Home Depot was bad,” said Tanner, who studied the debris in Joplin as part of a mitigation assessment team sent by FEMA.
Tanner said his team’s final report probably would rate the Joplin tornado at less intensity than earlier estimates. They think it was an EF-4 (wind speeds of 168 to 199 mph), not an EF-5 (200 to 234 mph).
“What happens in these various storms, especially in Home Depot,” Tanner said, “is that the storm removes the roofing and roof insulation and light metal decking. And once you lose your (roof) decking, you lose the lateral support of your joist and beam system, and they become less supportive.”
Tanner and other engineers said that a concrete roof — which some Home Depot stores in hurricane country have — “would add weight to the roof system and could prevent roof failures.”