Tornado Loads Sweeping through ASCE 7-22 Design Requirements

By Jennifer Keegan AAIA 07-25-2022
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Growing up in the northeast, tornadoes were these random events that occurred in the movies, and far off places like Kansas. But according to the National Institute of Standards and Technology (NIST), roughly 1 200 tornadoes occur in the US each year! AND they occur in all 50 states, but primarily in the lower 48 and east of the Rocky Mountains. Given that tornadoes kill more people per year in the U.S. than hurricanes and earthquakes combined, ASCE's Structural Engineering Institute revised the ASCE 7 Standard for Minimum Design Loads and Associated Criteria for Buildings and Other Structures in hopes that it will be adopted into the 2024 International Building Code.

While it's the right thing to do to save lives, the implications involved with designing a building to withstand the catastrophic tornadoes we see on the news feels almost unattainable! Until you break it down.

ASCE created Chapter 32 to address the calculation of tornado loads. However, it is important to recognize that wind loads still must be calculated following the requirements in Chapter 30. Chapter 32 is required for Risk Category III and IV buildings only! This covers essential facilities - such as hospitals and emergency response facilities - that need to remain operational in the event of extreme environmental loading. These buildings are required to maintain functionality after a design-level tornado. Note that functionality is not synonymous with a Safe Room or Storm Shelter, which are intended to provide near absolute protection in extreme wind events. FEMA developed a separate document for Safe Rooms and Storm Shelters, which rise well above the level of requirements of ASCE 7-22.

Understanding Tornadoes

So what exactly are tornadoes? According to Marc Levitan of NIST and the chair of the ASCE 7-22 task committee that developed the tornado provisions, a tornado is a violently rotating column of air that forms inside thunderclouds when warm, humid air rises and cool air falls, touching the ground in a spiral of air currents.

Tornadoes can significantly vary in size and intensity. The Enhanced Fujita Scale (EF) was introduced in 2007 to provide estimates of tornado strength based on damage surveys. The higher the number on the EF scale, the more devastating a tornado's potential. Figure 1 summarizes the wind speed range and resulting damage with each level of the EF scale.

Figure 1: EF Scale, related wind speeds, and resulting damage

While EF levels 3, 4, and 5 can be devastating, the good news is that we see significantly more EF 0, 1, and 2 tornadoes. The map in Figure 2 aggregates the total of tornadoes by EF category and shows that tornadoes are not limited to "tornado alley" like those of us who didn't grow up with tornado drills and sirens may believe. Most of the country can end up at risk for a tornado under the right conditions. Interestingly, Dr. Robert Rohde assembled the historic data from NIST and animated the tornadoes over time, noting that April has had more violent F4 & F5 tornadoes than any other month; while May has had the largest quantity of tornadoes.

Figure 2: NIST map illustrating the location of tornadoes, EF intensity, and number of tornadoes over a 67 year period.

Similarly, the National Oceanic and Atmospheric Administration (NOAA) tracks reported tornadoes. According to data from NOAA's Storm Prediction Center, during December 2021, "there were 193 confirmed tornado reports. This is approximately eight times the 1991-2010 average of 24 tornadoes for the month of December, as shown in Figure 3. This was the highest U.S. count of December tornadoes on record - double the previous final record of 97 from 2002."

Figure 3 Courtesy of NOAA's Storm Prediction Center - Reported Tornadoes

Cost of Tornadoes

According to NIST, tornadoes kill more people per year in the United States than hurricanes and earthquakes combined. In the past 25 years, nearly 1 700 lives were lost to tornadoes. Following the most catastrophic tornado on record, the Joplin Tornado in 2011, NIST reports that 5 600 lives were lost from 1995 to 2011 from tornadoes. Deaths per year in the U.S. from these events average:

  • Tornadoes 91,6

  • Hurricanes 50,8

  • Earthquakes 7,5

The Insurance Information Institute, Inc. reports that the United States experiences more tornadoes than any other country. Tornadoes accounted for nearly 40 % of insured catastrophe losses from 1997 to 2016, according to Verisk's Property Claim Services (PCS). Hurricanes and tropical storms were a close second largest cause of catastrophe losses, accounting for 38,2 % of losses.

Property damage and resulting financial loss per tornado is analyzed by NIST in the Joplin Investigation Report. The average loss per tornado and total loss by EF number (in red) was calculated for reported tornadoes during the 1995 to 2011 timeframe. As expected, the per tornado loss is significantly higher for stronger tornadoes, especially the EF4 and EF5 category tornadoes. Interestingly, the cumulative losses of EF1 through EF5 tornadoes are similar. The higher volume of EF1 and EF2 level tornadoes aggregate to similar financial losses as the EF3 to EF5 tornadoes.

Figure 4: Property damage and resulting financial loss per EF category analyzed by NIST in the Joplin Investigation Report

Designing for Tornado Loads

These statistics are all drivers behind the addition of this chapter in ASCE 7-22. We have an opportunity to reduce the loss associated with tornadoes, in terms of human life as well as economic impacts.

The good news is that it's not necessary to design buildings to withstand the most violent tornadoes in order to significantly reduce tornado damage. Over the past twenty-plus years, 97 % of the tornadoes have been classified as EF0 to EF2 (see Figure 5), and the ASCE 7-22 tornado provisions are geared towards reducing loss and damage during these events. Storm shelters and safe rooms are required for critical emergency operations and education facilities designed for EF3+ tornadoes, and governed by IBC Section 423 and ICC 500.

Figure 5: EF Tornado Frequency

The ASCE 7-22, Chapter 32, Tornado Loads, requires buildings be designed for tornados of approximately EF2 intensity or less, with wind speeds ranging from 60 to 138 mph, depending on geographic location and other factors, for risk category III and IV buildings located in tornado prone regions per Figure 32-1-1. Components and cladding must resist the greater of tornado loads or wind loads, using load combinations in Chapter 2.

Chapter 32 provides a design flowchart to guide designers through the design process, as shown in Figure 6. Steps 1 and 2 identify the risk category and the tornado prone regions per section 32.1.1. Steps 3 and 4 determine the wind speed per section 32.5.2. Most of the wind load coefficients and equations have been modified to account for differences in tornadic wind (VT), and the return periods are the same as used for wind loads (risk category III = 1 700 years; IV = 3 000 years). The tests on VT represent approximate threshold tornado speeds at which tornado loads might begin to control some aspect of the wind load design. The Basic Wind Speed (V) and exposure category are determined in accordance with Ch 26, based on the exposure resulting in the greatest wind loads for any wind direction at the site.

Figure 6 - Flowchart at the beginning of Chapter 32 identifying the process to determine where design for tornado loads are and are not required.

The enclosure classification is a bit more prescriptive for tornado loads than wind loads. Schools typically do not require impact resistant glazing (although highly recommended), while essential facilities such as fire stations and hospitals do require impact resistant glazing in tornado prone regions. When impact resistant glazing is utilized, the building can be designed as an "enclosed" structure. However, when projects utilize non-impact resistant glazing, the building must be considered "partially enclosed" to account for the additional interior pressures when the windows are broken from flying debris.

Tornado loads are likely to govern when:

  • Located in central or southeast US (coastal areas typically governed by hurricanes)

  • Risk Category IV; Designated as Essential Facilities

  • Have large effective plan areas

  • Low mean roof heights (demonstrated to result in higher peak pressures)

  • Classified as an enclosed buildings for wind loads

Tornado loads can control over wind loads when tornado speeds are as little as half of the basic wind speeds. Where tornado loads control, design uplift pressures on the roof will typically increase. Examples where wind and tornado loads govern will be analyzed in a future blog article. In the meantime, look for the adoption of ASCE 7-22 into the 2024 IBC. And remember, Code includes minimum design requirements. Enhanced design considerations, such as the requirements outlined in ASCE 7-22, are prudent and encouraged, regardless of Code adoption and mandates. After all, as designers we have taken an oath to protect the health, safety, and welfare of the general public.

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