FAYETTEVILLE, Ark. - Everyone knows that tornadoes affect buildings, but University of Arkansas researcher Panneer Selvam has found that buildings also have an effect on tornadoes. And he has built a three-dimensional computer model that shows how a building affects a tornado’s path.

"Tornadoes cause millions of dollars in property damage every year in the United States," explained Selvam. "If we are to mitigate this damage, we must design buildings that are more resistant to tornadoes. The first requirement for accomplishing this goal is a better understanding of the way that a tornado interacts with a structure."

Selvam, professor of civil engineering, was invited to present his findings recently at the Second International Symposium on Advances in Wind and Structures in Busan, Korea.

Rapidly changing wind speed and direction make a tornado difficult to study in a wind tunnel. Previous attempts have been made to make a computer model of a tornado, but the limited computational power available resulted in major drawbacks. They either had to ignore turbulence, assuming that the wind is straight-line, or they discounted the effects of viscosity. But in reality, tornadic winds have both high turbulence and viscosity.

"Tornado-structure interaction is a complex phenomenon," Selvam said. "For example, the details of tornado wind speed and turbulence from ground level up to 100 meters are needed. In addition, the tornado is moving with respect to the building and the details of tornado wind speed at every instant of time are essential to impose the proper boundary conditions."

Selvam’s research may also shed light on why tornadoes seem to take such an unpredictable path. They may move down one side of a street for blocks, then, inexplicably, hop to the other side or veer around a corner. Selvam’s model shows that the dimensions of a structure can affect the path of a tornado.

"When we set up the tornado to move directly toward a building, we were surprised to find that the vortex of winds around the building actually caused the tornado to veer to the side of the structure," Selvam said. "Because it is a 3-D model, we can change the height and shape of the building. Although we are primarily interested in exploring the tordano’s impact on buildings, we can also see that the building has an impact on the tornado’s path."

With so many variables, it is impossible to study tornadoes experimentally. However, as computers have become more powerful and computational processes more streamlined, it has become possible to model tornadoes by using data acquired from actual events.

Early researchers tried to determine the tornado’s force by investigating storm damage and a calculating the maximum wind speed, according to Selvam. This process was used to produce the Fujita Scale, which is currently used to classify tornadoes. However, it assumes that the wind forces are constant when in fact they change over time.

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.

However, Theodore Fujita, who developed the scale, also pointed out its limitations. He advised that scientists "not use F-scale winds literally. These precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar-looking damage from place to place -- even from building to building."

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.

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 buildings that are more resistant to tornadoes, says Selvam. By determining exactly how structures interact with the tornado, the researchers hope to give designers and planners tools to minimize the destruction caused by tornadoes.