The twister that killed 46 people in Tuscaloosa has provided a national lesson on how to build and reinforce homes, often with just relatively minor changes, a new study found. Things like $1 straps, stronger shingles, more anchors and thicker vinyl siding could have prevented much of the damage to houses on the fringes of the twister.
Researchers said nothing could have saved structures that were in the direct path of the EF-4 tornado, which skirted the University of Alabama campus with winds up to 190 mph on its way through the city. Forecasters said it was one of the strongest twisters to hit the state during the severe weather outbreak that killed more than 240 people statewide on April 27.
"The winds are so high that a wood-frame structure is not going to withstand them. In those cases, you need a safe room," said researcher Andy Graettinger of the University of Alabama. "But the vast majority of the area (experienced) lower wind speeds that you can engineer for. You need to have the roof tied to the walls and the walls tied to the foundation to prevent major damage."
Homeowners on the fringes of the tornado would have been spared at least some damage with different construction methods or improvements to existing homes, said Graettinger, who has also studied twister damage at other sites, including Joplin, Mo.
In some cases, he said, homes could have been saved from catastrophic damage by metal clips or straps that cost about $1 each.
"You're looking at a few thousand dollars for these clips that hold everything together," Graettinger said. "It's a very small amount compared to the cost of the house."
A research team funded by the National Science Foundation assessed more than 150 homes along the nearly six-mile path of the tornado in Tuscaloosa, looking at everything from homes that were leveled to those that weren't damaged at all. Their report was released Monday.
The most serious damage was at the center of the path of destruction, where the swirling winds were strongest and the most debris flew through the air. Homes on the edges of the storm received far less damage because the storm was weaker on its edges, and the team focused on how to reduce damage in that zone.
Researchers found that basic changes like using wind-rated shingles; additional anchors at the bottom of porch columns; metal straps to link roofs with walls; and higher-quality vinyl siding could have made a huge difference in how well some homes survived the storm.
"We would have saved a lot of rebuilding cost if that had been done," said Graettinger, who worked with Alabama engineering professor John W. van de Lindt and other researchers on the project.
Researchers didn't determine how much the needed changes would add to the cost of a new home, or the cost of retrofitting existing homes to make them stronger.
But they hope the findings will lead to better construction techniques in tornado-prone areas, much as coastal areas have adapted building codes and methods to help structures withstand hurricanes.
In Joplin, Graettinger told NBC News, he and colleagues used 3D image scans of homes and a destroyed hospital to gather data for researching better building techniques.
"We'll get details of the hospital without ever having to physically get in the building," he said.
Graettinger said he'd also been to see buildings destroyed by Hurricane Katrina in 2005. "We saw similar issues with load path where the load that's pulling up on the roof is not connected to the walls and the foundations," he said.
David Prevatt, a University of Florida assistant engineering professor who was on the Tuscaloosa team, told NBC that Florida's experience with hurricanes might help tornado-vulnerable communities build better structures.
"The more difficult part — and this affects particularly not just tornadoes, hurricanes, anything in the country — is what do you do about the existing structures? Do we place a saferoom in all these structures? Do we reinforce or refit those structures or do we sort of break down and start again?
"It's an economic question, it's a social question," he added. "But we have the engineering knowledge now to add to that question. We want to be at the table when a city or town like Joplin is discussing what they want to look like in the next 50 years in terms of their residential structures."