New Building Standard Can Map Out Tornado Threat for the First Time

The NIST team, led by Levitan, found that although the most powerful winds laid waste to buildings near the tornado center, more than 70% of the total damage path stemmed from lower wind speeds farther out. And designing for that level of wind is much more economically viable.

“Building codes and standards don’t require design for the worst possible hurricanes and earthquakes. We need to apply that same philosophy to tornadoes and design for the more modest and more common tornado design speeds,” said Levitan.

ASCE 7 already contains hazard maps for hurricanes and other windstorms that prescribe wind speeds to be considered in the design of buildings in different areas throughout the country. Engineers use these maps to identify strong winds their building might encounter and then, using other sections of the standard, calculate how much pressure that wind will apply to elements of their building, including its frame, walls and roof.

NIST researchers, working with engineers from Applied Research Associates Inc., sought to establish a similar process for tornadoes. But first they had to obtain a great deal of information about where and how often twisters occur and how much their winds push and pull on building components.

The researchers, among other efforts, analyzed and filled in gaps in more than 60 years of tornado records, studied how miniature buildings responded to small-scale vortexes in the lab and performed extensive computer-based structural modeling. And by analyzing damage from Joplin and other tornadoes they were able to back up the performance of their models with real-world data. Then they used their findings to develop tornado hazard maps and methods to help engineers determine what forces their structure would need to stand up against.

The guidance, which now appears in the draft version of ASCE 7, is only required for buildings located in the tornado-prone region of the country, east of the Rocky Mountains, and only for buildings defined as risk categories III or IV. ASCE describes structures in category III as being those that could represent a substantial hazard to human life if they fail. Category IV includes essential facilities.

For risk category III buildings, which include schools and other high-occupancy structures, the standard requires engineers to use maps with wind speeds that have about a 3% chance of being surpassed at least once in 50 years — a common target for service life in building design. That tolerance tightens to 1.7% for vital category IV buildings, such as hospitals or fire stations. The standard also provides additional maps with even higher wind speeds for communities aiming to design sturdier buildings that exceed the minimum requirements.

The maps account for building size too. Their tornado wind speeds change depending on the area of a structure’s footprint, reflecting the fact that the larger the structure is, the more likely it is to be struck by tornadoes.

Once the 2022 edition of ASCE 7 is finalized this December, tornado-prone regions, many of which may have been brutalized by violent twisters in the past, will have the guidance to design new buildings to better weather the next assault. And if the standard is adopted by the model building codes (maintained by independent codes and standards organizations) and those codes are incorporated into U.S. state and local buildings codes, the new practices will become widespread, saving lives and property from one of nature’s most vicious threats.

The draft standard is available for download from the ASCE website (free registration required)