Work began on Chapter 32 in 2014, about the time NIST published a report on its investigation of the Joplin tornado, conducted under the National Construction Safety Team Act. Among its 16 recommendations, NIST called for improved measurement and characterization of tornado hazards and new methods for tornado-resistant design of buildings.

Somewhat Controversial

The idea of tornado design provisions was “somewhat controversial,” says Ronald O. Hamburger, a senior principal of Simpson Gumpertz & Heger and the chair of the ASCE/SEI 7-22 committee. Initially, the committee of about 50 voting members was split 50-50 on the subject, he adds.

Many members “felt it did not make sense to require tornado-resistant design as a routine requirement because most structures, even in the Midwest and Southeast where tornados frequently occur, would never be affected by one,” says Hamburger.

Levitan and others at NIST defended the inclusion of the new chapter, saying NIST’s Joplin report made a strong case for tornado load provisions. Historical data, they argued, indicates tornadoes kill more people per year in the U.S. than hurricanes and earthquakes combined. In addition, the average annual insured catastrophe losses for events involving tornadoes exceed those for hurricanes and tropical storms combined, according to 2020 data from the Insurance Information Institute.

Compromise

Ultimately, the committee and NIST came to a compromise. The provisions use the same probability of structural failure because of a tornado as the standard uses as the basis for structural failure from non-tornadic winds. 

“We don’t design for the very worst tornadoes,” says Levitan. Much of the damage, on an annual basis, is done by weaker tornadoes because there are so many more of them, he explains. 

The standard also does not apply to all buildings and other structures, only those classified as Risk Category (RC) III or IV and located in the tornado-prone region of the U.S., which is mostly the area of the contiguous U.S. east of the continental divide.

RC III buildings and structures represent a substantial hazard to human life in the event of failure. Among these are occupancies for public assembly or large groups, schools and certain infrastructure. RC IV buildings are those designated as essential structures, such as hospitals.

The provisions do not apply to RC I buildings, which represent a low hazard to human life in the event of failure, such as agricultural facilities, certain temporary facilities and minor storage facilities. And it does not apply to most RC II buildings, including residential and office buildings, except those that include places of public assembly.

All Components and Cladding Included

For those buildings that fall under the standard’s criteria, the main wind force resisting system and all components and cladding “shall be designed and constructed to resist the greater of the tornado loads determined in accordance with the provisions” of Chapter 32 or the wind loads determined in accordance with Chapters 26 through 31, using the load combinations provided..., says the standard.

Buildings with larger footprints are more vulnerable to tornado damage because they are larger targets, says Levitan. Under the provisions, to determine tornado loads, engineers must first measure the effective area of the building, which often goes beyond its actual footprint if it is not a rectilinear shape. They then use hazard maps to determine the specific vulnerability to tornadoes of the building's location.

The tornado speeds to use in the calculations are provided in the standard. NIST is in the process of developing maps that will help show where it is more likely that tornado loads will control various aspects of the design compared to wind loads, as there are many variables at play, more than just comparing tornado speed to basic wind speed, says Levitan. 

“Tornadic winds have significant vertical components; rapid atmospheric pressure changes that can induce loads; and load combinations including tornadoes are not always the same as those including wind,” states Chapter 32. “Hence, tornado loads are treated separately from wind loads, not as a subset of wind loads. This is analogous in some ways to the separate treatment of flood loads and tsunami loads.”

Editor's note: This article was updated on Dec. 3.