Economic analysis of ASCE 7-22 tornado load requirements
Authors: Kneifel Joshua , Levitan Marc , Harris Benchmark , Haney Blake , Smith Tom , Butry David , Crawford Shane , de Toledo Nico , Thomas Douglas 
Published: March 2022

This study analyzes the potential economic impacts from implementation of the new tornado load requirements in the ASCE 7-22 Standard: Minimum Design Loads and Associated Criteria for Buildings and Other Structures, by incorporation into the International Building Code (Proposal S63-22 in Ref. [3]) and/or direct adoption by Federal, state, and local governments. The Standard requires that Risk Category III and IV buildings located in the tornado-prone region (approximately equal to the area of the conterminous U.S. east of the Continental Divide) be designed to resist tornado loads in addition to wind loads from other types of storms. Risk Category III includes buildings that represent a substantial hazard to human life in the event of failure (e.g., theaters and other assembly occupancies, schools, nursing homes), while Risk Category IV is for essential facilities (e.g., hospitals, fire and police stations, emergency operations centers). The approach in this study is to (1) identify the potential numbers of buildings that may be impacted, (2) compare tornado loads with existing wind load requirements to understand when and where tornado loads will control design, (3) determine what building elements will require changes in construction design when tornado loads control, and (4) estimate the cost of these changes in construction design. The adoption of the new tornado load requirements in the ASCE 7-22 standard will impact a small fraction of new buildings in the United States. When excluding residential occupancies with less than 50 units, the building stock occupancy types for Risk Category III and IV buildings in the tornado-prone region represents 15.0 % of the entire U.S. building stock and 18.3 % of the building stock in the tornado-prone region. These results are effectively upper bound estimates as tornado loads will not control over wind loads for all Risk Category III and IV buildings in the tornado-prone region. Whether tornado loads control any aspect of the building design over wind loads depends on many different climatological and building characteristics. Geospatial analyses are used identify the impacts of several of these variables. In general, the tornado design requirements will have the most impact in the central and southeast U.S. Case studies are used to estimate the relative magnitude of the potential cost impacts of five building elements (roof systems, roof diaphragm, roof joists and wide flange beams and girders, exterior wall framing, and foundation anchorages) for two building types (elementary school and high school) for wind Exposure Categories B and C baseline building designs across nine locations to provide a range of potential load requirements and resulting cost implications. The results of the case studies show that tornado loads can vary significantly, from less than wind loads to more than double the wind loads. Tornado loads will not control for many locations and building types, particularly those on the periphery of the tornado-prone region. For scenarios in which the tornado loads do control, the construction design is often not influenced. Of the nine cities considered in this study, three realize cost increases for at least one exposure category for the elementary school and six for the high school. Of all the building type location exposure combinations (36), only six (three for each building type) realize cost impacts greater than 0.07 % of the project budget.