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The American Society of Civil Engineers (ASCE) has launched the first-ever design criteria for tornado-resistant buildings in their revised code ASCE/SEI 7-22, “Minimum Design Loads and Associated Criteria for Building and Other Structures.”
ASCE is the most widely used standard and an integral part of building codes in the United States and around the globe.
The new version ASCE/SEI 7-22 has included a new Chapter-32 for tornado provisions and digital data for all hazards.
ASCE/SEI 7-22 Standards for Tornado Resistant Construction
The plan to set up a wind-load standard exclusively for the tornado-resistant design came into serious note after the incident in Joplin, Mo., Washington, where a tornado killed 162 people in 2011. The goal was set up by the National Institute of Standards and Technology (NIST), on account of which the ASCE Structural Engineering Institute published the 2022 edition of ASCE/SEI 7.
The standard applies only for buildings and other structures categorized as Risk Category (RC) III or IV and for those located in the tornado-prone region of the U.S ( As per Fig.32.1-1, ASCE/ SEI 7-22).
The buildings and structures coming under RC III and RC IV as per ASCE 7 (Table 1.5.1 ASCE 7-10, ASCE 7-16) are explained in Figure-1 below.
Process to Determine the Requirement of Tornado Loads
- All the main wind force resisting systems (MWFRS), claddings, and all components (C & C) of the buildings coming under the risk categories (RC III and RC IV) and located in the tornado-prone regions are designed and constructed to resist the greatest of the tornado load combinations.
- The load combinations are determined as per Chapter 32 or through the wind load combinations in accordance with Chapters 26 to 31.
- The engineers must measure the effective area of the building and check the hazard maps to determine the specific vulnerability of the structure to tornados.
- Tornado load calculations are treated separately from wind load calculations.
- The figure below shows the process flowchart for determining when the design for tornado loads is required. The design process is provided as a function of risk category, location relative to the tornado-prone region, the value of tornado speed, and the value of tornado speed relative to the basic wind speed.
The figures and sections mentioned in the figure above are based on the data given in chapter 32 of ASCE 7. Here, Vt is the Tornado speed in (mph), V is the basic wind speed determined in accordance with Section 26.5, and the exposure category is determined as per Section 26.7.3.
- Tornado loads are not considered if,
Tornado loads are more likely to control at least some elements of the wind load design for building structures that are:
- located in “tornado alley,” the midwest, and the southeast (except locations near the coast dominated by hurricanes)
- have large effective plan areas
- are designated as essential facilities
The permitted procedures and the outline for determining tornado loads are given in Figure 32.1-3 of ASCE/ SEI 7/22 Chapter 32. The section also provides the guidelines to determine the tornado loads on the MWFRS and C&C systems of buildings and structures.
RC III and IV categories only include a narrow group of buildings, such as hospitals, fire stations, and police stations. Apartment buildings, warehouses, most manufacturing plants or houses are not included and are expected to have it included in the code eventually.
ASCE/SEI 7-22, “Minimum Design Loads and Associated Criteria for Building and Other Structures,” is the first-ever design criteria for tornado-resistant buildings in their revised code. Chapter 32 of this code provides the design criteria and procedure to conduct tornado-resistant design.
The standard applies only for buildings and other structures categorized as Risk Category (RC) III or IV and for those located in the tornado-prone region of the U.S (as per Fig.32.1-1, ASCE/ SEI 7-22).
RC III and IV categories only include narrow buildings, such as hospitals, fire stations, and police stations. Apartment buildings, warehouses, most manufacturing plants or houses are not included.