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Tornadoes Are Deadly. These New Building Codes Will Save Lives.

  • Tornadoes are deadly, but until recently there were no building codes designed to protect communities from tornadoes.
  • Tornado winds push and pull on buildings in unique ways that require special safety designs.
  • NIST research led to the first building code provision for tornado resilience.
A collage of images related to the investigation of tornadoes and the development of new tornado building codes.

Decades of research have gone into understanding tornadoes and how they impact buildings. Now that new understanding is making its way into building codes.

Credit: B. Hayes/NIST

Sirens blared across Joplin, Missouri, on a spring afternoon in 2011, just 23 minutes before the tip of the deadliest tornado on official records descended from the clouds and touched the ground.

This tornado maxed out the scale — a five out of five on the Enhanced Fujita (EF) scale, according to the National Weather Service.

Directly in the path of the vortex was St. John’s Regional Medical Center, a huge hospital complex that employed more than 1,000 people. The staff had done tornado drills before and quickly jumped into action, ushering patients and visitors away from the windows as dark clouds blotted out the sun. Then the lights went out. Ventilators stopped working. The storm had torn apart an electrical substation and the hospital’s emergency generator building, causing a total loss of power. The occupants were in complete darkness when the doors and windows blasted open, shooting broken glass and furniture through the hallways.

It took less than a minute for the mile-wide tornado to pass through St. John’s before moving on to tear through public schools, retirement homes, grocery stores, and suburban neighborhoods. The entire tornado lasted about 30 minutes, but by the end of it, a quarter of Joplin was destroyed and 161 people had lost their lives.

Just hours after the tornado hit Joplin, the National Institute of Standards and Technology (NIST) began a long-term study of what went wrong that day. After more than a decade of work, that research is making its way into standards and codes to make new buildings safer.

Tornadoes Are a Serious Problem

Because of its unique geography, the United States has more tornadoes, and more intense tornadoes, than any other country. Cool, dry air from the Rocky Mountains collides with warm, moist air from the Gulf of Mexico to twist up into more than 1,200 tornadoes per year.

And they’re deadly.

Tornadoes have killed more people in the United States than hurricanes and earthquakes combined. Part of that is because of the short warning times — on average 10 to 15 minutes. If you’re caught in a tornado, there is no time to evacuate. You must rely on the walls around you. Your local building code could be the difference between life and death.

But back in 2011 it was rare for engineers to factor tornado resiliency into their building designs. Joplin didn’t have regulations for helping hospitals resist tornadoes. After the disaster, some buildings survived while others were demolished. It was important to figure out why.

Learning From Disaster

Within 48 hours of the Joplin tornado, a small team from NIST was in Missouri to study the damage. Marc Levitan, who had been hired only a few months before, was part of the team. He was shocked by the devastation. 

“You try to keep your emotions in check because people lost their lives,” he said. “They lost their livelihoods. They lost their property. They lost everything. Everybody’s belongings are just littered all around.”  He knew that this disaster would be the start of years of study to figure out what went wrong and how to prevent it from happening again.

Tornadoes 101 with Marc Levitan
Tornadoes 101 with Marc Levitan
Mark Levitan, a wind and structural engineer at NIST, delves into the impact of wind on buildings and the characteristics of tornadoes. He explains the Enhanced Fujita scale and the importance of designing buildings to withstand tornadoes. He also discusses load path, building codes, and the necessity of adopting tornado load requirements.

The NIST report on the Joplin tornado, published in 2014, set out a plan for how to make communities more resilient to tornadoes. Levitan and his colleagues have been following through on that plan ever since.   For example, they developed new tornado wind-speed maps that show what wind speeds different buildings should be designed to withstand. These maps, which cover the contiguous United States, are based on a statistical analysis of more than 60 years of tornado records.

These hazard maps were the first to account for the size of the building. The bigger a building is, the more likely it is to be struck by a tornado, and so including building size significantly improves the accuracy of the tornado risk assessment.    

But for all that work to make a difference, it needs to get incorporated into local building codes.

The process of updating a building code takes four main steps: research the issue, develop a standard, update the model building code, and pass local laws. 

Finding Out What Makes Tornadoes So Deadly 

To perform the research step, NIST and its collaborators used wind tunnels, historical tornado data, and carefully examined debris to figure out how to design buildings to resist tornadoes. One thing they discovered is that not all wind is the same. “Tornadic winds are very different from straight-line winds,” explains Levitan.

For one thing, tornadic winds have higher windspeeds close to the ground and rapidly change direction. These winds can pull and push on a building in ways it wasn’t designed for. For example, wind flowing quickly over a building can decrease the air pressure above the roof and create a strong upward force. 

It’s the same principle that lifts airplanes into the air. At high wind speeds, aerodynamics are as important for buildings as they are for airplane wings.

That uplift force can be made even stronger by updrafts. Near the center of a tornado, winds blow at an upward angle. A NIST-funded team simulated updrafts using a wind tunnel to blow air over miniature houses and found that updrafts tend to suck roofs upwards with more force than straight winds. 

“Normally, buildings are designed to keep the roof from falling down due to gravity,” said Levitan. “Engineers designing for tornadoes should also keep the roof from ‘falling up.’” If a roof peels off, it could cause the entire structure to collapse.

And it’s not just the structure engineers need to worry about. These unique wind patterns are also much more likely than straight-line wind to kick up debris. Winds from the Joplin tornado lifted all kinds of heavy objects, from 100-pound (45-kilogram) steel manhole covers to semi-trucks. And once debris is airborne, the pieces become deadly missiles.

A Better Standard for Better Buildings

After that research, Levitan and his team led the effort to incorporate what they learned into the standard. A building standard is a collection of the most up-to-date minimums for designing a safe structure put together by a committee of public and private sector experts. The standard that deals with wind loads for buildings is called ASCE 7, and the most recent version, published in 2022, is ASCE 7-22. This version — for the first time — contains rules for making critical and high-occupancy buildings resistant to tornadoes.

So what does the new standard say? A detailed breakdown of how to use the standard can be found in this design guide put together by FEMA and NIST. Essentially it contains a set of instructions for structural engineers to find out what tornado intensity they need to prepare for in their area and what pressures those tornadic winds will impart on their buildings.

For example, a new hospital facility being built in Dallas, Texas, with a footprint of 1 million square feet would need to withstand tornado wind speeds of 124 mph (92,907 square meters and 200 kilometers per hour). Without the new tornado code, the same hospital would only have to be built for 117 mph straight-line winds (188 kilometers per hour). 

It may seem like a small difference, but pressures are a function of the square of the speed. Combined with other factors such as increased roof uplift, tornado loads on some elements of a building can increase by a factor of two or more compared to wind loads, according to an analysis led by the NIST Applied Economics Office. Once they use the code to calculate the wind loads, it’s up to the engineers to adjust their design to meet the increased requirements.

That might mean using stronger materials or changing the angle of a roof to make it more aerodynamic. Levitan says one cost-effective solution for wood-frame buildings is adding specially designed metal straps that hold a roof to the walls. Small, inexpensive changes like these can have a big impact on tornado resiliency.

From the Standard to the Code

After a standard is updated, a different committee of experts decides whether to include it in the model building code. This is a huge document that is widely used as a starting point for local building regulations. Before this year, there was no minimum tornado requirement for building loads in the model code. But now, in a major milestone, the new tornado section of the ASCE 7 standard was incorporated into the 2024 version of the International Building Code (IBC).

Getting into the IBC was a huge step forward, but that model code isn’t a law just yet. “By itself, it’s just a reference book on a shelf,” says Levitan. It doesn’t start to make an impact until it gets adopted and enforced.

From Model Code to Local Laws

It’s up to local regulators to decide how much of a model code is right for their community. Every state makes that decision a little differently. For example, Florida has a single building code that applies to the entire state. Officials began implementing the new tornado resiliency standard directly at the end of December, making Florida the first state in the country to enforce the new standard. “It typically takes five to 10 years before the majority of places adopt a new code,” says Levitan. But gradually the standards he helped develop will make it into local regulations, and from there into the buildings we depend on to keep us safe.

This new code provides new protection where there was none before, but it has limitations. It will only affect new buildings, and of those, it only applies to certain structures: public utilities, facilities that are considered essential during an emergency such as hospitals and fire stations, and buildings that represent a substantial hazard to human life in the event of failure, like schools, nursing homes and places of public assembly.

It also only requires that those buildings withstand up to EF2 tornadoes. That’s about 97% of all tornadoes. Extreme tornadoes like the EF5 that hit Joplin are rare, but these standards will help even in that worst-case scenario. About 70%  of the area hit by the Joplin tornado was EF2 or lower.

A pie chart titled “US. Tornadoes by Intensity, 1995-2022.” The chart shows that 95.2% of tornadoes are EF0 to EF2, while much smaller slivers of the chart are EF3, 4, 5, and Unknown.
The vast majority of tornadoes are EF2 or below, including most of those labeled "Unknown." By focusing on these lower-intensity storms, the update to the 2024 International Building Code protects people and structures from most tornadoes.
Credit: B. Hayes/NIST

Making a structure that can withstand the very worst tornadoes is not part of the new standard. But it is possible, and there is a standard that shows how to do it. Following up on another NIST recommendation from the Joplin report, Levitan is the chair of the committee that develops and maintains the ICC 500 Standard for the Design and Construction of Storm Shelters. These storm shelters are heavily reinforced buildings or rooms that can keep people safe, even if the rest of the building collapses. One of the strict requirements of this standard is that the walls (and windows if they have them) need to be tested to withstand the impact of a two-by-four plank launched at 100 miles per hour. 

Many hundreds of storm shelters built to this standard have been hit by tornadoes, and none have failed. They provide near-absolute life safety from even EF5 tornadoes. Starting in 2015, the model building code began to require these storm shelters for certain buildings like schools and emergency response facilities in tornado-prone areas.

Ultimately, St. John’s Regional Medical Center did not survive the tornado. The damage was too great to repair. But Joplin rebuilt quickly. A new building, Mercy Hospital Joplin, opened in 2015. Long before the new code was published, the hospital decided to rebuild above and beyond the minimum code requirements so that it’s able to withstand the next tornado that comes along. Now as the new model building code spreads, the tragic lessons the tornado taught will be built into new hospitals, schools and offices across the country. The ordeal Joplin endured is starting to make us all a little safer.

Released July 9, 2024, Updated July 15, 2024