JOPLIN — A new engineering study of the damage caused by the May 2011 tornado that struck Joplin found no evidence that it was an EF-5, as the National Weather Service found, because the city's homes and businesses weren't built to withstand wind speeds that strong, making such a determination 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, which is equal to the maximum wind speed of an EF-2 tornado, and that about 13 percent of the damage was caused by winds of 138-167 mph, consistent with an EF-3 tornado. Only 4 percent of the damage was indicative that it had been an EF-4 tornado, which can have winds speeds ranging from 168 to 199 mph, the report said.
The ASCE 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 ASCE investigators concluded it was impossible to find evidence of EF-5 ratings in the damage because none of the buildings met the high construction quality threshold required for determining that level of wind speed, The Joplin Globe reported Saturday.
The findings of the ASCE damage-assessment team 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 ASCE findings, however, do not change the National Weather Service's classification of the Joplin tornado as an EF-5, with peak winds of 200-208 mph.
"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 and that it would take winds of 200 mph to do that damage."
The ASCE team also concluded that because the structures were so poorly built to withstand wind, flying debris from houses made damage worse and if the houses in the tornado zone had been built with hurricane ties the damage would have been less. Hurricane ties are metal clips that fasten the rafters and trusses to the exterior walls of a house.
"The study team believes that a relatively large number of buildings could have survived in Joplin if they had been built to withstand hurricane winds," said Bill Coulbourne, a member of the ASCE engineering team that went to Joplin. "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."
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. The ASCE study recommends that practice be implemented throughout tornado-prone areas.
"You have to keep the roof on the house. If you don't, it won't hold together," Cope said.
He said the report builds a case for stronger construction codes. If the recommendations are incorporated into building codes, building design for wind resistance could be strengthened from the current maximum of 90 mph to 150 mph.
"It will cost more money to make these changes," said Davis, with the NWS. "But what's your life worth?"