A new engineering study of the damage caused by the 2011 tornado that struck Joplin found no evidence that it was an EF-5, as the National Weather Service found, because homes and businesses weren’t built to withstand wind speeds that strong.
Consequently, making such a determination was impossible.
The study by the American Society of Civil Engineers found that more than 83 percent of the damage on May 22, 2011, was caused by winds of 135 mph or less, equal to the maximum wind speed of an EF-2 tornado. About 13 percent of damage was caused by winds of 138-167 mph, consistent with an EF-3.
Only 4 percent of the damage was indicative of an EF-4 tornado, which can have winds speeds ranging from 168 to 199 mph, the report said.
The team also found that while the tornado’s maximum wind speed was around 200 mph, there was no evidence of building damage from winds at 200 mph or greater, the minimum threshold for an EF-5.
The investigators concluded it was impossible to find evidence of EF-5 damage because none of the buildings met the high construction quality threshold required for determining that level of wind speed, The Joplin Globe reported.
The findings are based on five days of surveying damage in more than 150 buildings in a six-mile segment of the tornado’s Joplin path. The total tornado path was 22 miles. More than 7,000 structures were destroyed or badly damaged by the tornado, and 161 people were killed.
The civil engineers’ findings do not change the National Weather Service’s EF-5 classification.
“This does not surprise me at all,” said Bill Davis, head of the NWS station at Springfield. “There was only a very small area of EF-5 damage in Joplin. It is not easy to put a rating on these things. There is a bit of subjectivity.
“But we knew right off the bat that there was EF-4 damage. It took us longer to identify the EF-5 damage.”
The engineering team also concluded that because the structures were so poorly built to withstand wind, flying debris from houses made damage worse. And the damage would have been less had houses been built with hurricane — metal clips that fasten the rafters and trusses to the exterior walls.
“They would not survive the winds of an EF-4 or EF-5 tornado, but they could survive lower wind speeds by using hurricane ties and by strapping the house to the foundation,” said Bill Coulbourne, a member of the engineering team.
Steve Cope, the city’s building inspector, recommended – and the Joplin City Council approved – the use of hurricane-clips on every rafter and truss in new post-tornado construction.
“You have to keep the roof on the house. If you don’t, it won’t hold together,” Cope said.