The 2011 tornado spring was one for the record books, tying 1936 as the second deadliest tornado season in history, with 550 tornado deaths nationwide. April 2011 saw the highest monthly occurrence of tornadoes on record (758), far surpassing the previous record of 542 tornadoes, set in May 2004. The April 27 outbreak, which hammered the deep South and wreaked havoc in Tuscaloosa, Alabama, also chalked up the highest daily total on record, with 200 storms reported over 24 hours. By itself, the Joplin tornado in May, with 161 fatalities, was the seventh deadliest single tornado in U.S. recorded history.
Severe catastrophes always trigger a discussion about what could have been done to reduce the death and destruction. And the Joplin tornado, on the heels of the deadly April outbreak in Alabama, has set that discussion off again.
David Prevatt, a professor at the University of Florida, led a National Science Foundation team that investigated the damage pattern in both the Tuscaloosa and the Joplin tornadoes. The Joplin study produced an interactive map of Joplin showing the tornado path and describing the damage to individual buildings, ranging from minor to catastrophic. The Tuscaloosa team released a full written report, " Damage Study and Future Direction for Structural Design Following the Tuscaloosa Tornado of 2011."
As in Joplin, the Tuscaloosa tornado struck mostly old, established neighborhoods of smaller homes built with traditional construction under older generations of the building code. Typically, walls had diagonal board sheathing, not plywood, and the sheathing did not overlap the sole plates. Walls were not bolted to foundations, but attached with cut nails. And roof framing was attached to wall top plates with a few toe-nails, which resisted any uplift force only in withdrawal. Not surprisingly, the giant Alabama tornado splintered many of those houses into matchsticks. Luck seems to have been all that separated the partially destroyed houses from the clean-swept slabs.
The commonly accepted conclusion seems to be that home designs could be upgraded, cost-effectively, to provide better protection against the weakest tornadoes — storms graded F0, F1, or F2 on the Enhanced Fujita scale of tornado damage. But in the face of stronger storms, researchers have generally concluded that building entire homes strong enough to save lives and property is not cost-effective, and may not even be possible. "Light-frame wood buildings do not, and will not, have the ability to resist EF4 or EF5 tornadoes," Prevatt’s team wrote.
If you want to be ready for the really strong tornadoes, the team argues, the best idea is to focus on storm shelters and safe rooms: small, specialized structures within a house, or larger public shelters at public and commercial facilities, where people can seek refuge when a storm hits — as long as they have sufficient warning. "Interior closets and bathrooms provide shelter at lower wind speeds on the edges of the tornado, but were no guarantee of survival," said the report: "The concept of a ‘safe spot’ should still be taught, but a safe spot is not a substitute for a safe room or tornado shelter."
But the report points out that there’s no way to know whether a house will get hit by a weak tornado or a strong tornado — or whether it will be in the vortex of the storm, where turbulent wind force is the strongest, or only on the fringe, facing relatively less powerful inflow winds. So they suggest, the design community should apply both philosophies at once — improving the load path from foundation to ridge in order to reduce property damage in the weaker tornadoes, and providing shelter such as a tornado safe room to protect against death or injury in any tornado.
But when it comes to strengthening wood buildings, engineers face many unknowns. The Tuscaloosa report says, "The current understanding of tornado loading on structures is not comprehensive or even comparable to that for straight strong winds because of the high level of turbulence and debris in a tornado. This is partially due to the lack of experimental procedures to accurately represent tornado loading. Unlike widely adopted scaled wind tunnel testing for wind loading on structures and components, it is very difficult to experimentally investigate the spatial characteristics of the loading on buildings within a tornado path."
David Prevatt plans on learning more. In January, he was awarded a grant from the National Science Foundation to study "tornado-resilient structural retrofits for sustainable housing communities." With computer modeling and full-scale testing, he’ll try to shed more light on the way tornado winds affect buildings, and develop a design method that will identify affordable ways to make buildings stronger. The results will be made available to designers, engineers, and builders on the Web.
But the bottom line remains that surviving a tornado is largely a matter of luck — and the biggest factor is whether the tornado hits you or not. As Prevatt himself told the ASCE News: "When you think about it, if this tornado had made a path two miles south, no one would have known about it because it would have passed through farmland."