A Hard Wind Is Gonna Blow

FAYETTEVILLE, Ark. - This year 800 tornadoes will strike in the United States, resulting in deaths, injuries, and millions of dollars in property damage. Is this a psychic prediction? No. According to the National Severe Storm Laboratory, these statistics are the average for the U.S. But they are statistics that Paneer Selvam, a civil engineering professor at the University of Arkansas, wants to change.

Although Selvam is an expert on wind dynamics, he can do little to predict or prevent tornadoes. However, he is working to minimize the destruction that they can cause. He has studied tornadoes and collected data on their impact for 15 years. As a result, he has developed a computer model that can accurately predict what will happen when a tornado slams into a building. He is currently working on a model that runs on a desktop computer and can be used to design affordable tornado-safe buildings.

There has been a lot of misunderstanding about how tornadoes do their damage, according to Selvam. At one time, people thought that tornadoes created a vacuum that caused houses to explode. People were advised to open their windows before seeking shelter. Researchers now know that this has no effect on the destruction caused by tornadoes.

"Most buildings, particularly houses, stand by the grace of God and gravity," Selvam explains. "Gravity must be taken into account for the structure to be sound. But many designers rely on gravity for structural stability."

This works well until the structure encounters a force that can counteract gravity. And that is exactly what tornadoes do. Selvam’s research has shown that a tornado exerts an upward force on a building up to 10 times as strong as the force of gravity.

"The key is that this is an upward force. When that happens, the entire structure is lifted up," Selvam explains. "Anchor bolts are sheared off. Then the roof is lifted from the walls, the walls drop back down, and everything falls apart."

Tornadoes are classified from F0 to F5, according to their destructive force. A Category F0 tornado has winds of 41-72 mph and produces light damage, such as breaking limbs off of trees or tearing down signs. A Category F3 is a severe tornado with winds of 158-206 mph. It causes severe damage - tearing off roofs, uprooting trees and lifting heavy cars. In a Category F5 tornado winds reach 261-318 mph and incredible damage results. Strong houses are lifted and carried considerable distances and automobile-sized missiles fly through the air for more than 100 yards.

In recent years, weather forecasters have concentrated on prediction of storms. New technologies have greatly improved their accuracy and have resulted in far lower death and injury rates, but many people are still killed and injured each year. And property damage costs continue to escalate. Selvam’s approach focuses on building structures to withstand the force of the tornado, reducing deaths, injuries and property damage.

On just two days in 1999, for example, three large tornado outbreaks occurred, causing hundreds of injuries and deaths and well over $1 billion in damage. On Jan. 21, an F3 tornado struck Little Rock, Ark., resulting in three fatalities. There were 63 tornadoes in Arkansas that day - three times the average for a year. On May 3, and F3 tornado struck Wichita, Kan., causing 6 deaths, 150 injuries and over $140 million in damage. The same day, an F5 tornado plowed through Oklahoma City, Okla., leaving 42 people dead, several hundred injured and over $1 billion in damage. Almost 70 tornadoes of F3 or stronger were reported in Oklahoma and southern Kansas on that day.

To decrease deaths, injuries to humans and damage to property, it is important that builders understand how to construct houses that are more resistant to tornadoes, says Selvam. By determining exactly what is needed, builders can provide safer housing without dramatically increasing its cost.

"If you just throw everything you know at it, you may produce a building that will withstand a tornado," Selvam adds. "But will anyone be able to afford to live in it? For each structure, there is an optimum point where the house is as safe as possible and still affordable."

Although constructing tornado-safe buildings is possible, Selvam believes that cost is a limiting factor for builders. And because governing entities lack the political will to require tornado-safety elements in building codes, society must bear the cost in human injuries, increased insurance rates and loss of lives and property.

To help builders, designers and homebuyers find the optimum balance between cost and safety, Selvam is working on an enhanced computer model that will show how a specific structure will react to a tornado. This model will allow the user to make changes in a structure to improve its resistance to tornadoes and then calculate the costs of those improvements. Although Selvam uses multi-million dollar massively parallel computers to develop his model, the end result will be a package that can run on a desktop computer.

"Traditionally, this type of work has been done using wind tunnels," explains Selvam. "But you can’t really model a tornado in a wind tunnel because a tornado both rotates and moves forward at the same time. So testing a model in a wind tunnel would produce unreliable results. The only way to truly model a tornado is with a computer."

An internationally recognized expert in computational wind engineering, Selvam has wide-ranging expertise in modeling of wind behavior. He has collaborated with researchers from Australia, Denmark, France, Germany and South Africa on problems ranging from bridges to solar panels to microchips. He is currently studying aerodynamic forces on airplane wings for the U.S. Department of Defense and the Air Force Office of Scientific Research.

His interest in tornadoes began in 1983 when he was a student at Texas Tech University in Lubbock, Texas. His work with the Institute for Disaster Research there resulted in an invitation from the National Research Council to study the tornado that struck West Memphis, Ark., in 1987. He has been invited to present his results at numerous technical conferences and they have been published in the Journal of Wind Engineering and Industrial Aerodynamics.