Stupid Question ™
May 23, 2002
By John Ruch
Q: What would happen if an F5 tornado hit a skyscraper?
Q: Or if a tornado hit a nuclear power plant?
A: Conveniently enough, tornadoes have hit both kinds of buildings.
But first, a word about understanding and using the Fujita Intensity Scale, or F-Scale. It rates tornadoes from F0, with winds 40-72 mph, to F5, with winds 261-318 mph. (It actually goes up to F12 at about 750 mph, but nobody thinks tornadoes get anywhere near such speeds.)
But really the F-Scale measures tornado damage and estimates wind speed from it, not vice versa. That’s because accurate direct measurements of tornado winds have proven impossible to obtain.
So by definition, an F5 tornado would do F5 damage. F0 is “minimal damage.” F5 is “incredible damage”—houses totally gone, cars hurled 300 feet or more, asphalt and topsoil completely scoured away.
Also, the F-Scale is based on damage to typical well-built houses. It doesn’t cover highly engineered buildings such as skyscrapers and nuke plants, which have never been hit by an F5-class tornado.
The F-Scale is very subjective and not absolute; a tornado strong enough to do F4 damage to a town might be rated F2 if it only snaps a few telephone poles on the prairie. Like the experts, we can only look at historical damage and extrapolate.
On March 28, 2000, an F2 (“major damage”) tornado plowed through downtown Forth Worth, Texas.
It made a direct hit on a 37-story, 454-foot skyscraper then called the Bank One Tower. It blew out 80 percent of the 3,540 windows, mostly from flying debris, and destroyed many interior walls. However, the building remained sound, with a ground-floor bank and a top-floor restaurant reopened within six weeks.
The real damage was economic. It proved too expensive to repair the building, and also to tear it down. For over a year, boarded-up windows earned it the nickname Plywood Skyscraper; it now stands totally abandoned.
Skyscrapers are typically built to withstand sustained hurricane-force winds (c. 100-150 mph); the World Trade Center towers swayed only 8 inches in 100-mph winds.
It’s possible an extremely strong tornado hurling many cars or other large missiles could deform a skyscraper’s frame. The office spaces and exterior walls of a glass-walled skyscraper would almost certainly be torn away and turned into debris.
On June 24, 1998, the Davis-Besse Nuclear Station in Oak Harbor, Ohio took a direct hit from an F2 tornado about 100 yards wide. It sucked water and debris from the cooling tower and toppled all phone and power lines. But the actual reactor shut down automatically and was unscathed.
The Nuclear Regulatory Commission requires all nuclear plants to be tornado-proof. The reactor is typically surrounded by steel-reinforced concrete walls 2 to 7 feet thick, lined with steel and situated underground.
Basically, it’s an ideal storm shelter. An F4 or F5 tornado might be able to damage the outside of such a structure, but the reactor would almost certainly be untouched.