The American Society of Civil Engineers (ASCE) has introduced the first-ever design criteria for tornado-resistant buildings in the revised ASCE/SEI 7-22 code, titled “Minimum Design Loads and Associated Criteria for Buildings and Other Structures.” ASCE standards are widely used in the U.S. and globally, playing a critical role in shaping building codes. The inclusion of tornado-specific provisions marks a significant step toward enhancing building resilience against extreme weather events.
ASCE/SEI 7-22 and Tornado Provisions
The ASCE/SEI 7-22 revision includes a newly established Chapter 32, which provides specific guidelines for tornado-resistant construction. Additionally, this updated edition incorporates digital data to assess and design structures against various hazards, making the process more precise and data-driven.
Background and Motivation
The urgency for tornado-resistant design standards became apparent after the devastating 2011 Joplin, Missouri tornado, which resulted in 162 fatalities. In response, the National Institute of Standards and Technology (NIST) emphasized the need for a structured wind-load standard for tornado-prone areas. ASCE’s Structural Engineering Institute took this recommendation forward, leading to the inclusion of tornado provisions in the 2022 edition of ASCE/SEI 7.
Scope of the Tornado-Resistant Standards
The tornado-resistant design provisions apply exclusively to buildings and structures classified under Risk Category (RC) III and IV and located in tornado-prone regions, as per Figure 32.1-1 of ASCE/SEI 7-22. These categories include structures critical to public safety and emergency response:
- Risk Category III: Schools, community shelters, and large gathering spaces.
- Risk Category IV: Hospitals, fire stations, police stations, emergency shelters.
Notably, the current standard does not include apartment buildings, warehouses, or manufacturing plants. However, there is a possibility that these requirements will be expanded in future revisions.
Process for Determining Tornado Load Requirements
To determine whether a building requires tornado-resistant design, engineers must follow a structured evaluation process:
- Identifying Wind-Resistant Systems: Main Wind Force Resisting Systems (MWFRS), cladding, and components (C&C) in RC III and IV structures must be designed to withstand tornado forces.
- Determining Load Combinations: Tornado loads are calculated using specific tornado provisions in Chapter 32 or wind load combinations from Chapters 26–31.
- Assessing Vulnerability: Engineers assess the effective area of the building and consult hazard maps to determine its risk exposure.
- Differentiating Tornado and Wind Loads: Unlike general wind loads, tornado loads require unique calculations based on regional tornado speed and exposure categories.
- Following the Design Flowchart: The decision-making process for tornado load requirements is visually outlined in Figure 32.1-2 of ASCE/SEI 7-22.
Exemptions and Special Considerations
Tornado loads may not be required in certain cases. However, they are critical for buildings that:
- Are located in tornado-prone areas such as “Tornado Alley,” the Midwest, and the Southeast.
- Have large effective plan areas, making them more susceptible to wind damage.
- Serve as essential facilities, including emergency services and shelters.
Future Implications
The permitted procedures and methodologies for determining tornado loads are outlined in Figure 32.1-3 of ASCE/SEI 7-22. As these provisions evolve, there is potential for expanded inclusion of additional building types, such as apartment complexes and industrial structures, in future code revisions. These updates could significantly enhance the resilience of a broader range of buildings against tornado events.
Conclusion
The introduction of tornado-resistant design standards in ASCE/SEI 7-22 represents a landmark shift in structural engineering and safety regulations. By prioritizing essential facilities and high-risk structures, these provisions aim to reduce loss of life and property damage caused by tornadoes. As building codes continue to advance, wider adoption of these standards is expected to play a crucial role in safeguarding communities against extreme weather events.