Big-Box Stores’ Construction Design May Not Hold Up in Tornadoes
A building design used in thousands of warehouses, schools, and big-box stores across the U.S. is vulnerable to catastrophic damage in tornadoes and similar high-wind weather conditions, according to engineers who studied the aftermath of the tornado that occurred in Joplin, Missouri, on May 22.
The engineers delivered their conclusions after examining the ruins of a Joplin Home Depot store in which at least seven people were killed after taking refuge in the store to escape the tornado, The Kansas City Star reported last weekend in a special investigative piece. At least 28 other people survived, huddled in an un-reinforced training room in the back of the building.
The construction design in the Joplin Home Depot store, known as “tilt-up wall,” met city building codes but proved deadly when the tornado lifted the roof off and the walls pancaked down, largely intact, crushing victims under a 100,000 pound concrete panel. The death toll from the Joplin tornado was last reported as 156, making it the most deadly U.S. tornado in 60 years. It destroyed roughly 8,000 homes and more than 500 businesses.
As investigative reporter Mike McGraw wrote in The Kansas City Star:
There aren’t many safe havens in such ferocious, 200-mph winds. Most building codes in ‘tornado alley’ require that commercial structures withstand only 90-mph winds, slower than many major league pitchers’ fastballs.
Some engineers believe that tilt-up wall construction, which Home Depot uses in many of its stores, has weak links that often fail, even in winds much less ferocious than those that hit Joplin on May 22. In fact, a report on the “Super Tuesday” tornadoes that struck Alabama, Arkansas, Kentucky, and Tennessee in February 2008, found that tilt-up wall buildings failed under much lower wind speeds — 110 to 167 mph — than those that slammed into Joplin’s Home Depot, said Larry Tanner, one of the team of engineers that traveled to Joplin to study the collapsed Home Depot and other building failures for the Federal Emergency Management Agency, and that also investigated the Super Tuesday tornadoes.
Tanner said that when the roof of tilt-up design buildings is exposed to high winds, it can become compromised and the heavy wall panels can fall like dominos. He and other engineers told The Star that if the building code had required the Home Depot store to have stronger roof-to-wall connections, it might have sustained less damage, even in such intense wind.
The Star article says:
Noting that they had never before lost a store to high winds, Home Depot officials defended the tilt-up wall design, which they’ve used in hundreds of their nearly 2,000 stores across the country, including several in the Kansas City area.
They said their engineers ‘fundamentally disagree’ with Tanner and his colleagues.
Home Depot officials told The Star they were so confident in the tilt-up wall design that they would use it again when they rebuild in Joplin.
Several days later, Home Depot officials said they would use a different design — smaller precast walls made elsewhere and trucked to the site — similar to tilt-up construction, but allowing quicker completion while still meeting local building codes. They also said they will install a storm shelter, making the Joplin store the only Home Depot in the country to have such a feature.
According to The Star:
In defending tilt-up construction, Home Depot officials said that any big-box store would sustain the same damage their store did, no matter how it was constructed.
But that doesn’t appear to be the case in Joplin.
‘The Home Depot was all on the ground, except for one corner,’ said Joplin Fire Chief Mitch Randles.
Two other big-box retailers on the same corner, both of which were built of concrete blocks rather than tilt-up walls, were not as heavily damaged, he noted.
Joplin city officials have talked about requiring inexpensive changes to the local building code, such as metal reinforcing to better hold a building’s roof to its walls and beefed-up concrete block foundations. But they said they will not ask for any major code changes, such as compulsory storm shelters, because they do not want to institute requirements that would be costly to homeowners and drive businesses away.
Image of The Home Depot logo, used under Fair Use: Reporting.