FAYETTEVILLE, Ark. - Current building codes incorrectly estimate the effect of thunderstorm winds on tall buildings, according to University of Arkansas wind engineer Panneer Selvam. This may cause codes to be too conservative, increasing the cost of construction without adding benefits.

"Most building codes are based on a developed boundary layer (DBL)," said Selvam, professor of civil engineering. "But a DBL wind is a straight-line wind and has a different wind profile from a downdraft, which is the prevalent wind in thunderstorms."

Selvam has developed a computer model that will account for this difference, as well accounting for terrain features such as hills. He will present his work on thunderstorm and tornado wind models, "Computational Wind Engineering for Extreme Wind Effects on Structures," at the 2001 American Society of Civil Engineers Conference in Houston on Oct. 13.

Thunderstorms account for about one-third of all extreme winds in the United States. Downdrafts, not straight-line winds, are responsible for most of the property damage from thunderstorms. Depending on the type, a downdraft can last from 2 to 30 minutes and have damaging winds as high as 75 meters/second (168 mph).

"The thunderstorm downdraft is a very complex phenomenon. The dynamics are controlled largely by events in the region between the cloud base," Selvam explained. "As a thunderstorm reaches maturity, rain-cooled (heavier) air within the cloud produces a strong downdraft that impacts the ground and spreads radially in all directions. The weight of the precipitation and the cooling act to accelerate the air downwards."

Current building codes are calculated for straight-line DBL winds and use a wind speed average from 10 meters (33 feet) above the ground. For structures taller than 3 stories, current models use a multiplier based on DBL data to determine the wind force. Unlike DBL winds, which increase in force the higher they are from the ground, downdrafts have maximum winds near the ground. Because the maximum force for a downdraft typically occurs around 50 meters (164 feet) above ground, the multiplier gets more incorrect as the structure gets taller

"Design procedures based on DBL winds may be conservative for buildings taller than 50 meters (164 feet) in areas where thunderstorms are the primary extreme wind condition," Selvam explained. "And over-design usually means increased costs."

Selvam began studying thunderstorm phenomena in 1990. His model was validated from with data from the United States and Australia.