Increased knowledge of tornado risks leads to better building codes

Extreme wind events such as tornadoes pose serious challenges for building designers striving to balance safety with affordability. According to the National Institute of Standards and Technology (NIST), roughly 1,300 tornadoes are reported nationwide every year. Between 1950 and 2011, tornadoes caused an estimated 5,600 fatalities in the United States — more per year than were killed by hurricanes and earthquakes combined.

The effects of tornadoes spread far beyond the immediate measures of loss in lives and property. One of their most pervasive effects is to drive up insurance rates. The Insurance Information Institute estimates that the top 10 costliest tornadoes — half of which occurred within the last decade — caused nearly $52 million in insured losses in 2021 dollars.

Many in the code enforcement community see extreme wind events like tornadoes as a risk that hasn’t been sufficiently addressed in current building codes. But many in the construction industry are concerned that stricter standards designed to better protect buildings against such events could significantly raise construction costs and, in turn, hurt new home sales.

“There’s a lot of common misperceptions out there about tornadoes,” said Dr. Mark Levitan, lead research engineer at NIST’s National Windstorm Impact Reduction Program. “For example, that tornadoes are too rare, or that the losses from tornadoes are small compared to those in other hazards, or that there’s nothing we can do about them. People think we have inadequate knowledge about them, or that we’d all have to live in concrete bunkers or underground and that would be too expensive.”

Discussion of improved tornado resilience standards and building codes

To help bring clarity to the situation, in February the International Code Council hosted an online panel discussion that reviewed recent efforts to develop new and improved tornado resilience standards. The panel, moderated by Code Council Vice President of Fire and Disaster Mitigation Karl Fippinger, brought together some of the world’s most knowledgeable experts in wind science and tornado risk mitigation:

• Dr. Mark Levitan, lead research engineer at NIST’s National Windstorm Impact Reduction Program
• Dr. Pataya Scott, a civil engineer with the Federal Emergency Management Agency (FEMA) Earthquake and Wind Programs Branch
• Donald Scott, P.E., S.E., F.SEI, F.ASCE, the senior principal at PCS Structural Solutions
• Lawrence Novak, SE, F.SEI, CERT, LEED AP, the Code Council’s chief structural engineer

Building codes and tornado loads in ASCE 7

Levitan and Donald Scott both participated in the preparation of a new chapter for the 2022 edition of ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, which is incorporated by reference into the International Building Code (IBC). The new chapter is dedicated specifically to tornado loads — the first time the standard has included specifications for such extreme wind events as part of design criteria. Levitan chaired the committee tasked with drafting the chapter, while Scott co-chaired its wind load subcommittee.

Currently, there are two proposals to adopt the ASCE 7 tornado loads chapter into the 2024 IBC, which could, in turn, lead to its incorporation into other future International Codes such as the International Residential Code — one by the ASCE Structural Engineering Institute, and the other coauthored by Levitan and FEMA’s Dr. Pataya Scott.

Data used to improve extreme-wind standards used in building codes

Last December, Fippinger, Novak and Dr. Scott participated in a post-disaster assessment of storm damage in Kentucky as members of a FEMA Mitigation Assessment Team, or MAT. The MATs travel to disaster areas to assess building performance and evaluate design, construction and code criteria in order to make recommendations for improving building performance. Over the course of four days, the Kentucky MAT examined and assessed the performance of 600 properties and structures — invaluable data for “ground-truthing” the next generation of wind standards.

“We saw every possible failure mechanism, from roofs uplifting to the entire building lifting off the foundation,” Novak told webinar participants. “But we have some positive stories to tell, too. For example, we saw one home that had a walkout basement, which the owner had added when he built his house; his neighbors without basements left their homes and joined him. Every one of those people made it out safe and sound.”

Fippinger said that the mix of anecdotal and quantitative data collected by the MAT was a good example of the kind of information being used to improve extreme-wind standards used in building codes. “We wanted our experience with the MAT to serve as a springboard to discuss the research that’s going on right now and what we can expect in terms of next steps to make buildings safer against extreme winds generally and tornadoes specifically,” Fippinger said.

Establishing procedures for determining tornado loads

Levitan explained that while it’s inappropriate to completely ignore tornado hazards in designing the built environment, it’s also not necessary to require buildings to withstand the most severe tornadoes. That’s where the ASCE 7-22 tornado chapter comes in. Of the 1,200-plus tornadoes that occurred between 1995 and 2016, Levitan pointed out, more than 97 percent generated winds of 157 mph or less, corresponding to EF-0 through EF-2 ratings on the Enhanced Fujita scale of tornado damage intensity.

The new chapter on tornado loads, which Levitan oversaw, lays out the procedures for determining tornado loads on buildings as well as components, cladding, appurtenances and other structures, and for assessing variables such as wind speed and direction, topography and ground elevation, and internal and external pressure coefficients.

“ASCE 7 is the only standard in the world that has design provisions for tornadoes,” said Donald Scott. “The knowledge that we have gained over the last 10 years from Mark [Levitan]’s research has been a hundredfold over what we knew about tornadoes before. And as he continues his research, that knowledge will just increase.”

Code professionals and wind scientists collaborating for better building codes and standards

The design criteria for tornadoes in ASCE 7-22 does not cover the most severe tornadoes, classed as EF-4 and EF-5; life safety protection for those tornadoes requires dedicated safe rooms and storm shelters. Standards for their construction are covered in ICC 500, the ICC/National Storm Shelter Association Standard for the Design and Construction of Storm Shelters. The ICC 500 standard is used by FEMA as the basis for its guidance on residential and community safe rooms — FEMA P-320, Taking Shelter from the Storm: Building or Installing a Safe Room for Your Home, and FEMA P-361, Safe Rooms for Tornadoes and Hurricanes: Guidance for Community and Residential Safe Rooms — and updates those documents following every revision of ICC 500. Additionally, the International Building Code requires safe rooms for schools and critical infrastructure.

Dr. Scott said FEMA also relies on data collected in the field by the MAT teams, like the one that traveled around Kentucky last December, to fine-tune safe room guidance, creating a virtuous cycle of ever-improving standards. “It’s really helpful for us to be able to say, ‘Look, we found this in the field, and that’s why we want to incorporate these updates into the codes and standards.”

Donald Scott believes that as our knowledge of extreme wind events continues to improve, and performance-based codes continue to evolve, the universe of structure-specific data relied on by code officials will gradually converge with the macro-scale data collected by climate scientists. “We have to get those two groups working together,” Scott said.

“Forty years ago, when I first started on the ASCE 7 committee, people said, ‘Tornadoes are too big, we can’t design for them.’ Well, people used to say the same thing about earthquakes, and now look where we are with regard to earthquake design standards. When it comes to our knowledge of tornadoes, we’re only just starting to scratch the surface.”