Thirteen years after a tornado ripped through Joplin, Missouri, killing 161, destroying a major medical center and leveling schools, homes and businesses, the International Code Council (ICC) has for the first time added a section on tornado resilience in 2024 . version of the international building code.
The new provisions of ASCE 7, Chapter 32, Tornado Loads, include requirements for critical structures such as hospitals and schools located in tornado-prone regions primarily east of the Continental Divide to withstand wind speeds that correspond to approximately in the most frequent EFO to EF2. – Classified tornadoes.
“These requirements apply to certain types of important community structures, what the building code identifies as risk categories 3 and 4,” which includes schools, nursing homes, theaters and essential facilities such as fire and police stations and hospitals, says Marc Levitan. , principal research engineer for the National Windstorm Impact Reduction Program for the National Institute of Standards and Technology.
Within two days of the devastating 2011 tornado, the city of Joplin was working with NIST to provide data and other insights to an initial research team of three engineers and a social scientist who were on the ground conducting interviews, looking at data of mobile radar, studying building failures and evaluating design documents that showed none of the city’s buildings had been designed specifically for tornadoes.
Joplin, what saw more than 7,400 residences damaged by the tornado, about 40% of which were completely destroyed, moved quickly to adopt stricter requirements for single-family homes, but has not changed its requirements for risk 3 and 4 commercial buildings.
Communities are not required to follow the IBC’s minimum recommendations for designing safe structures, but the ICC is widely used to create local building codes.
“We felt the 2006 IBC Code standards were sufficient,” says Bryan Wicklund, Joplin’s construction manager. “Any commercial building built under the 2006 code held up as well as could be expected in 250 mph winds.”
Design for Tornado Winds
To make recommendations about how to design buildings more resistant to tornadoes, NIST researchers also used wind tunnels, studied historical data and examined debris to learn how tornadic winds push and pull buildings.
“Tornadic winds are very different from straight-line winds,” explains Levitan, who was honored as an ENR Newsmaker in 2021 for his research on tornado damage to structures. “Tornada winds have strong updrafts, so the air is sucked up along the surface of the Earth and then turns and turns. Turn the corner and you get these strong updrafts that accentuate the roof elevation. You don’t get these strong updrafts near the ground surface in other storms.”
While buildings are typically designed to prevent the roof from falling in, engineers designing for tornadoes must also prevent the roof from coming off, he says.
The new provision contains instructions for structural engineers to learn what tornado intensity they should prepare for in their area and what pressures those tornadic winds will have on their buildings. It details procedures for such topics as determining design tornado loads and selecting the types of building enclosures and windows needed to best withstand tornadoes, along with maps of tornado-prone areas.
Under the code addition, NIST notes that a new hospital facility being built in Dallas, Texas, with a footprint of 1 million square feet would have to withstand tornado wind speeds of 124 mph. Without the new tornado code, the hospital itself would only have to be built for straight-line winds of 117 mph.
“It may seem like a small difference, but wind pressures are a function of the square of the velocity,” states NIST. “Combined with other factors such as increased roof lift, tornado loads on some elements of a building can increase by a factor of two or more compared to wind loads. Once they use the code to account for wind loads, engineers must adjust their design to meet the increased requirement.
Levitan says solutions to increase tornado resistance can include using thicker roof decking, larger roof joist sizes and more connectors or anchors.
While NIST doesn’t track state and municipal code adoptions, Levitan says tornado load provisions have already been adopted in Florida state and city of Houston building codes.
“Most commonly, states, counties and cities will end tornado load provisions as they adopt the 2024 IBC, which incorporates ASCE 7 tornado loads by reference,” says Levitan.
The addition of the code was based on proposals from the American Society of Civil Engineers (ASCE) and the Federal Emergency Management Association (FEMA), along with NIST.
“As severe weather threats continue to increase in frequency and severity, it’s critical to keep building codes continually updated to reflect the latest research in building science and weather science—that’s how we save lives and preserve communities,” he says Dane Rankin, Chief Structural Engineer. by the ICC.
Considering that most of the commercial buildings that were damaged by the tornado were older buildings. including the St. John’s Medical Center, a 10-story building that had to be completely rebuilt, Wicklund says “everyone has a vested interest in building codes. But you have to consider what’s feasible.”
