Atlanta Tornado Loss Shows Hurricanes Aren't The Only Big Windstorm Threat

Classified as an EF2 event on the Enhanced Fujita scale, it brought gusts up to 135 miles-per-hour. The severe thunderstorm system that spawned it also brought hail and torrential rains.

Within days of the event, an AIR survey team arrived to assess the damage. Based on published reports and a tornado track map prepared by the National Weather Service, the team targeted four regions of interest: the touchdown point in Vine City, Atlanta's downtown, Cabbagetown and the Cotton Mills lofts, and the end point near Braeburn Circle.

The NWS tornado track included two "red zones," or sections of heavy damage, separated by approximately one mile. This variation in damage is not untypical, as tornadoes usually undergo periods of weakening and reintensification along their path.

In the case of the Atlanta tornado, another factor was that the damage was enhanced in neighborhoods where the built environment was densest–that is, where more debris could be accumulated by the tornado, increasing its damage potential.

To fully understand the damage patterns that resulted, the AIR team surveyed not only structures in the tornado's path, but also sites outside its perimeter.

The team began surveying damage at the touchdown point–a residential area west of the city's business district. The area is dominated by one- and two-story single or multifamily homes with wood frame construction and asphalt shingle roofs. Most are old, and many are not well maintained.

AIR engineers noted that damage at the touchdown point was only slight–generally in the form of windows broken by flying debris. Few houses had roof damage–the ones that did were damaged by fallen tree limbs, rather than by wind, indicating that wind speeds were relatively low here.

While damage was most severe within the proposed tornado track, damage was also observed as far as 500 yards from the track. This was likely caused by straight-line winds that typically wrap into the developing cyclonic winds. The substandard condition of many of the homes in the neighborhood undoubtedly exacerbated the extent of the damage.

Downtown Atlanta is home to large convention centers and hotels that generate significant revenue for the city. The heaviest losses were incurred at the 3.9 million-square-foot Georgia World Congress Center and the CNN/Omni Hotel complex.

The Congress Center–which, according to local officials, accounted for two-thirds of the overall losses from the storm–had substantial roof damage. Breaches in the roof permitted water to enter the building's atrium and damage its contents. Nearby, the CNN/Omni Hotel Complex suffered extensive roof and window damage.

Unfortunately for downtown Atlanta's business community, the storm struck at the start of the convention season–resulting in substantial business interruption losses. The Georgia World Congress Center, forced to close for more than a week, lost the Atlanta Home Show, a dental convention and much of the Southeastern Conference basketball tournament as a result.

Next door at the CNN Center Complex, nearly 500 of the 1,067 available rooms at the Omni Hotel were damaged, temporarily downsizing the facility by 50 percent. Consequently, some hotel guests were sent to the Westin Peachtree Plaza Hotel north of Centennial Park.

Even though the Westin–a 735-foot cylinder of glass and steel–was outside of the NWS tornado track, surveyors observed considerable damage to its windows.

Elsewhere downtown, AIR engineers found minor damage–mostly to windows–in more than 10 high-rise buildings, including the 36-story Centennial Tower, the 35-story Equitable Building and the 52-story Georgia Pacific Building.

However, as would be expected in well-engineered steel and reinforced concrete structures like these, no significant structural damage was observed.

Of the high-rise buildings in which AIR engineers observed damage, six had much greater window damage to their upper stories than to their lower sections.

This was true even in areas where the damaged high-rise buildings were not in close proximity to other tall structures, suggesting that the window damage at the upper floors may have been caused by high wind pressures or debris carried upward by the vertical wind component of the tornado.

Managers of these structures told the AIR team that business interruption was not expected to be significant. Since the storm hit on a Friday, building owners had the weekend to clean up and repair shattered window panes by Monday.

After the tornado exited downtown Atlanta, it caused lower levels of damage en route to Cabbagetown, a neighborhood to the east of the city center.

On the edge of Cabbagetown–the second red zone in the tornado track–the storm smashed into a mid-rise building called the Cotton Mill Lofts. Built in the late 1800s and renovated 100 years later, the structure is of heavy-timber construction with load-bearing brick walls and concrete floors. A wood frame structure supports the roof. Masonry is extremely vulnerable to tornado-force winds.

Not unexpectedly, the building's exterior masonry walls on the top floor collapsed inward. In two of the sections, the collapse progressed all the way to the basement. Additionally, the wood-frame roofs of two westward facing sides of the building were completely ripped away.

The tornado then moved into Cabbagetown, where roof damage to single-family wood-frame homes was widespread. While the homes in Cabbagetown were similar in construction to those in Vine City, Cabbagetown has a large number of old trees, which were uprooted by tornadic winds, causing significant structural damage.

A quick survey in East Atlanta revealed a building inventory similar to Cabbagetown's and similar damage patterns, though damage was confined to a more narrow area as the tornado neared the end of its life. Even so, AIR engineers again found homes with missing windows or roof damage outside the NWS-estimated tornado track.

The preliminary insured loss estimate for this tornado was $340 million. Not surprisingly, older masonry buildings and wood-frame houses in the tornado's path sustained significant damage–in many cases due to uprooted trees and broken branches landing on homes.

Well-engineered commercial buildings–such as those in Atlanta's downtown–fared better, with the bulk of the damage confined to plate glass windows.

AIR engineers found that the tornado track and width as estimated by the National Weather Service did not capture all damaged property. In particular, where the tornado first touched down in Vine City, the spread of damage extended well beyond the width of the tornado's path. Similar patterns were observed in both Cabbagetown and Braeburn Circle.

As expected, the damage was not uniform along the track of the tornado. Lower property damage at either end of the track and higher damage in the midsection dramatically illustrate the natural life cycle of tornadoes.

Miraculously, there were no reports of fatalities as a result of the Atlanta tornado. However, the severe weather that spawned it continued into the next day, when the Storm Prediction Center received more than 40 tornado reports, 160 reports of hail and 70 reports of straight-line winds across north Georgia and South Carolina. Two deaths were reported in Northwest Georgia counties.

The Atlanta tornado and the severe thunderstorm system that spawned it serve as yet another reminder of the importance of actively managing severe thunderstorm risk.

Cagdas Kafali and Vineet Katiyar are research engineers at Boston-based AIR Worldwide.