Wind Engineering Important for Safety

Spring isn’t just known for allergies, but also for the threat of severe weather. Even though nothing can be done to predict Mother Nature’s force, civil engineers are constantly working to improve structural stability in buildings to withstand high winds and tornadoes.

Panneer Selvam, the James T. Womble professor of computational mechanics and nanotechnology modeling at the University of Arkansas, has been working on a computer model since 1990 that studies wind effects on buildings. By understanding the effects of different types of winds’ — straight-line winds or tornadoes — engineers can use technology to build safer and sturdier buildings.

Selvam, who works in the UA’s civil engineering department, and his team use computer models to measure wind effects instead of using a wind tunnel, which can cost about $100,000 and take about three months to test. Also, the nearest wind tunnel is located in Texas. But by using Selvam’s model, it can take only a couple weeks for engineers to test different wind speeds on structures. That helps engineers know what kind of load structures can take before actually being built.

Wind vs. Man

Tornadoes and straight-line winds have different effects on buildings, Selvam said. Tornadoes provide twice as much pressure on roofs of buildings and 50 percent more on walls than straight-line winds do at the same speed.

The reason is that straight-line winds hit a structure at a horizontal level, going around the sides of the building and over the top of it, he said. But a tornado hits a structure with circular winds, which means the winds are already going around the building and on top of it as it passes through. So, the only place for the winds to go is up. That’s why roofs are ripped off buildings.

“For tornado-type winds, it is very difficult to know what kind of forces are created on the building,” Selvam said. That’s because it depends on the angle, if the tornado is on the ground or above the ground, and at what speed it is going.

But the vulnerability of buildings depends on the type of structure, Selvam said. An engineered structure — like office buildings and schools made with concrete and reinforced steel — is built using an engineer. Those structures are built with code regulations for wind speed.

Different parts of the U.S. have different regulations. For the Midwest, including Arkansas and Oklahoma, an engineered structure should be built to withstand 90 mph straight winds. In comparison, the Gulf and Atlantic coasts have a 140 mph structure load because of hurricanes.

For structures that are built to withstand tornadoes, studies show that engineers should build buildings in the Midwest to withstand 240 mph winds from tornadoes, he said.

Not all engineered structures are built with tornado-force winds in mind. But if someone were interested in doing so, it would cost more money to do because it would increase materials used on the structure.

“If you know what kind of load is applied [to a structure], we can design the building in such a way that materials can take that amount of load,” Selvam said. “We know how to do that.”

For non-engineered buildings, such as houses and older brick buildings, most structures have no chance against tornadoes. A tornado that hit Fort Smith and Van Buren in April 1996 demolished houses and historic buildings. But in Van Buren, surrounded by destroyed houses, a water tank erected out of steel was still standing. That’s because it was an engineered structure, Selvam said.