The 2023 Florida Building Code (Eighth Edition) as well as the 2024 International Building Code recently added provisions and design requirements for tornado loads, in accordance with ASCE/SEI 7-22. The following article attempts to answer some questions pertaining to what it means for construction and how structural design will be affected.
What are tornado loads?
Tornado loads refer to the forces exerted on structures by the high winds and pressure changes associated with a tornado. These loads can include wind pressure on the building surfaces, debris impact, and changes in atmospheric pressure.
Why does the building code need provisions for tornado loads? Don’t all structures have to be designed for hurricane level winds anyway?
One of the differences between hurricanes and tornadoes is their region of formation. Typically, hurricanes develop over water; tornadoes develop over land. Hence, while an inland region might be less susceptible to hurricanes, tornadoes are much more likely to develop, and the structural design must consider that.
Example: Consider the city of Liberal, Kansas.
MRI (mean recurrence interval) is an indirect measure of the intensity of an event. The above chart has been developed using the data obtained from https://ascehazardtool.org/ for a Risk Category III structure in Liberal, KS, USA.
From the above chart, it can be observed that a hurricane with an MRI of 10,000 years in Liberal, KS, has a wind speed of 131 mph. By comparison, a tornado with an MRI of 10,000 years in Liberal, KS, has a wind speed of 125 mph. Hence, logically, hurricanes would govern the structural design.
However, a hurricane with an MRI of 100,000 years in Liberal, KS, has a wind speed of 149 mph. By comparison, a tornado with an MRI of 100,000 years in Liberal, KS, has a wind speed of 174 mph. In this case, tornadoes would govern the structural design.
In summation: we must check for both the hurricane wind speed and the tornado wind speed, based on the MRI of each and the area under consideration, to decide which effect will govern the structural design.
Which structures will the tornado design provisions affect?
Per Sec 32.1.1 of ASCE/SEI 7-22, Sec 1609.5 of the 2024 IBC and Sec 1609.5 of the 2023 FBC, tornado design only applies to the design and construction of
- Risk Category III (high occupancy) structures such as theaters, lecture halls and similar assembly structures, elementary schools, prisons, and small health-care facilities, etc.
- Risk Category IV (essential facilities) structures such as hospitals, police stations, fire stations, emergency communication centers, and buildings with similar uses.
Most low-risk structures (such as barns and screened enclosures) and moderate-risk structures (such as residences, small offices, storage spaces) are not affected by tornado design provisions in the building code.
Please see our blog about Risk Categories for more details.
What areas are tornado-prone? How do I know if I am in a tornado-prone region?
Summary:
The inclusion of tornado loads in the 2023 FBC and 2024 IBC aims to ensure the structural integrity of buildings and minimize danger to human life. These stringent provisions are mainly directed at larger and more critical structures, while most residences, accessory structures, and small offices will remain unaffected by these changes.
By incorporating these new requirements, the Building Codes are taking a proactive approach to enhancing safety and resilience in the face of tornadoes.
At Florida Engineering LLC, our team of highly qualified engineers is experienced in designing structures to withstand tornado loads, ensuring their safety, structural integrity, and compliance with the latest building codes. Please contact orders@fleng.com for a quote and expert assistance with your project.
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