When air crashes into a building, it’s forced to move up and around. This produces negative pressures that create suction and uplift.

This is why tornadoes can tear off roofs and rip apart houses. And it’s why building codes are important, according to Dr. Christine Alfano, a structural engineer and tornado damage expert.

“The building codes have done a lot to improve building performance over the years. There’s a little more science behind it. The issue is that tornadoes are not included in the building code,” Alfano said.

In November 2020, Metro Nashville enacted their strongest codes to date — at the time, the latest version of what’s called the International Business and Residential Codes for the region.

They require architects to design buildings to withstand 115 mph wind gusts, an upgrade from the previous 90 mph requirement, according to Metro Nashville Codes Director Bill Herbert.

But a wind gust is only one part of the story. Calculating wind load involves diverse variables, like the weight of materials, directional factors, and topography—and loads can be affected by the quality of construction.

“A house is only as strong as its weakest link,” Alfano said. “It could come down to a nail connection.”

Courtesy Christine Alfano

As winds get stronger, uplift forces can overcome the gravity load of a roof. Similarly, if debris hits a window or door—garage doors are especially vulnerable—that could internally pressurize the building and push roofs up.

The latest codes improved some aspects of structures and altered others, and the net result was a similar resistance level, according to Alfano.

Buildings under this code might be able to escape damage during an EF-1 tornado, which moves air between 73 and 112 mph. But that’s iffy given the variables.

In the past, tornadoes weren’t a priority for homebuilders. That was financially logical, given the extremely low probability of a single home ever getting impacted — even the recent EF-4 and EF-3 tornadoes that extended more than 100 miles were only about a mile and 1,000 yards wide, respectively.  

But that probability is changing in Tennessee and neighboring regions. There isn’t enough data on the connection between climate change and tornadoes to model future risks, broadly, but there has been a documented trend of tornadoes shifting eastward from the Great Plains to the more densely-populated Southeast.

Courtesy Christine Alfano

“Wind doesn’t impart a load on a building equally. So, different parts of the building are going to experience higher wind loads and that’s generally where our failures will initiate,” Alfano said.

“We’ve known for decades now how buildings react to wind, and we have just continued to fail to implement these changes and now we’re seeing these large-scale disasters,” she said. “Maybe it’s not so economical anymore.”

Nashville could require buildings to have tornado-proof concrete safe rooms, which the New York Times recently reported could cost as little as $15,000 in a commercial building. If tornadoes exceed 160 mph, shelters really become the only option, according to Alfano.

Another solution is hurricane straps, which are straps that connect the roof and the walls of a structure. They’re very common in coastal communities. After Hurricane Andrew basically bankrupted the insurance industry in Florida in 1992, the state changed the standards for many homes along the coast, according to Dr. Stephen Strader, a tornado researcher at Villanova University.

“But we don’t have anything like that for tornado zones,” Strader said.

There have been attempts to create tornado-specific codes — including ones that would require certain existing structures to be retrofitted with safe rooms, which the Federal Emergency Management Agency recommends — but these proposals have not been successful.