JOPLIN, Mo. — As the monster tornado bore down on them, Rusty Howard and his two small children sought refuge in a Home Depot store.
But the young father, the children and four other people died when the roof came off and the walls came down, crushing them beneath a 100,000-pound concrete panel.
Within seconds the entire structure collapsed in a heap of concrete slabs, metal trusses and roofing. At least 28 other people survived, huddled in an unreinforced training room in the back of the building.
Rescue workers found Howard with an arm wrapped around each child.
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.
But while all big-box stores are vulnerable to high winds, the Joplin Home Depot — even though it met local building codes — was especially at risk, according to engineers who study tornado destruction.
The Joplin Home Depot and many of the company's other stores used a popular construction method called tilt-up wall that the Kansas City Star found can be deadly under certain conditions.
The design is used in thousands of warehouses, stores and schools across the country, and some engineers think it has weak links that often fail — even in winds much less ferocious than those that hit Joplin on May 22.
Some engineers interviewed by the Star said building codes for big-box stores need to be strengthened, or the stores should have internal storm shelters when they're built.
"Unfortunately, the code has become the lowest common denominator," said Tim Reinhold, chief engineer at the Insurance Institute for Business & Home Safety, which does research for the insurance industry. "Engineers know they are competing for the minimum building code, and so they design right to them."
In tilt-up wall buildings, the structures are erected with concrete walls that are poured on site and lifted into place with cranes. The walls are held upright by critical connections to a relatively lightweight roof system.
Under high winds, the roof can become compromised, and the panels in tilt-up wall buildings can fall like dominoes, said Larry Tanner, a tornado expert and part of a team of engineers who traveled to Joplin to study the Home Depot collapse and other building failures for the Federal Emergency Management Agency.
"Once you lose one (wall) panel, then the dominoes all start to fall, so the failure becomes increasingly more catastrophic," Tanner said. "I was aghast at how extensive Home Depot's failure was."
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 heavy winds, said Tanner and other engineers interviewed by the Star.
"It's an efficient and low-cost building system, but it's only stable when all the connections are working," said Perry Adebar, an engineer who studies tilt-up wall designs at the University of British Columbia. "It's a bit of a house of cards."
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.
They said their engineers "fundamentally disagree" with Tanner and his colleagues.
"The structural engineer that designed the buildings designed them to code, and the connections are determined by wind load and the local requirements," said David Oshinski, director of construction for Home Depot. "You have to remember this was an F5 tornado. There isn't anything you can build to withstand an F5 tornado."
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.
A few days later they said they'd use a different design: smaller precast walls made elsewhere and trucked to the site. The new design is similar to tilt-up construction but will allow quicker completion and still meet local building codes.
But many local codes are not good enough, said Tim Marshall, a meteorologist and engineer with Haag Engineering in Irving, Texas.
"This year I have seen so much carnage, and so many people who died who did not need to die. These strong tornadoes are no match for those kinds of safety rules," Marshall said.
The new Home Depot will exceed Joplin codes in at least one respect: Company officials told the Star that the new store will have an added feature that some engineers have long recommended in such buildings.
They said it will be the only Home Depot in the nation with a storm shelter.
Anatomy of a collapse
Wayne Lischka, a Leawood architectural engineer hired by the Star, said the tornado first hit the south corner of the west-facing front wall of Home Depot. That put inward pressure on the front wall panels. The tornado lifted the roof off, causing the walls to pancake, largely intact.
Lischka said the roof was designed to withstand uplift pressures of about 28 pounds per square foot, strong enough for 90-mph winds. But he said the 200-mph tornado exerted upward forces of up to 90 pounds per square foot.
At that point, Lischka said, the inside bar joists supporting the roof began to bend and pull away from the concrete walls. In fact, he said, the forces were so strong that 3/4-inch-diameter metal anchors for the joist connections — which were embedded inside the concrete walls — bent and were pulled out of the hardened concrete.
"This type of construction would perform very well under normal loading," Lischka said. "Tornadoes are not normal."
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.
"Academy Sports' roof is missing, but their walls are still standing, and most of what was missing at the Walmart was the roof," Randles said.
Randles added, however, that he believes Home Depot took a direct hit from the tornado. He is not sure whether the other buildings did.
Some engineers think that concrete block structures may be safer in a collapse than tilt-up wall buildings. If masonry block walls fail, they can come apart as they are falling, some engineers noted, leaving voids beneath them where some people could survive.
But when tilt-up wall buildings fail, the walls come down in huge sections that leave little chance of survival, unless customers are inside a hardened structure.
Tilt-up wall buildings
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 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."
But Tanner noted that there is no need to condemn the entire concept of tilt-up wall construction.
"We just need to do a better job of supporting that (roof-to-wall) system," he said.
In a report on the tornadoes that struck Alabama, Arkansas, Kentucky and Tennessee in February 2008, Tanner and his colleagues 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.
FEMA also found several cases in which roof-to-wall connections in tilt-up buildings failed in Kansas and Oklahoma following tornadoes in the late 1990s.
"There are lots of different ways buildings can fail, but a consistent problem (with tilt-up failures) is the loss of roof connections, and we see that time and time again," said John Snow, a meteorologist and dean of the college of atmospheric and geographic sciences at the University of Oklahoma.
Several engineers who were part of a different team sent by the American Society of Civil Engineers to assess the damage in Joplin also agreed with Tanner's assessments on tilt-up structures.
"Generally speaking, I agree with the wind engineers 100 percent," said Bill Coulbourne, a consulting structural engineer who specializes in buildings designed to withstand hurricanes and tornadoes. When tilt-up buildings collapse, Coulbourne said, "they come apart easier."
But tilt-up wall structures have strong defenders.
"There is no huge smoking gun here," said Dave Weber, an engineer with Allstate Consultants in Columbia, Mo., who was in Joplin to advise rescue workers.
Weber, who has designed several tilt-up buildings, said, "Obviously a more resilient connection would help at the wall-to-joist location."
But he said nothing unreasonable was done in the design of the Home Depot building, and making the roof-to-wall connection stronger simply would move the weak link of the structure to some other area.