美国国家标准与技术研究院于2011年5月22日摧毁了密苏里州的乔普林之后，开始朝着龙卷风抗性设计的防风标准的目标。当美国土木工程师协会结构工程学院出版了2022年的圣经时，目标是设计负载。ASCE/SEI 7-22首次包含有关龙卷风抵抗的一章。
“This is all brand new, even within the technical community,” says Marc L. Levitan, lead research engineer in the National Windstorm Impact Reduction Program of the National Institute of Standards & Technology (NIST) and chair of the ASCE/SEI 7-22 task committee on tornado loads, which developed Chapter 32 of ASCE/SEI 7-22: Minimum Design Loads & Associated Criteria for Buildings & Other Structures.
工作于2014年的第32章开始，大约是NIST发布了对根据《国家建筑安全团队法》进行的对乔普林龙卷风的调查的报告。在其16个建议中，NIST要求改进龙卷风危害的测量和表征，以及用于建筑物的抗龙卷风设计的新方法。
有些争议
Simpson Gumpertz＆Heger的高级负责人，ASCE/SEI 7-22委员会主席Ronald O. Hamburger说，龙卷风设计规定的想法“有些争议”。他补充说，最初，约有50个投票成员的委员会在该主题上分配了50-50。
Many members “felt it did not make sense to require tornado-resistant design as a routine requirement because most structures, even in the Midwest and Southeast where tornados frequently occur, would never be affected by one,” says Hamburger.
NIST的Levitan和其他人捍卫了新篇章的包含，称NIST的Joplin报告为龙卷风负荷规定提出了有力的理由。他们认为，历史数据表明，龙卷风每年杀死的人数比飓风和地震的总和多。此外，根据保险信息协会的2020年数据，涉及龙卷风事件的每年平均灾难损失超过了飓风和热带风暴的灾难。
Compromise
Ultimately, the committee and NIST came to a compromise. The provisions use the same probability of structural failure because of a tornado as the standard uses as the basis for structural failure from non-tornadic winds.
列维坦说：“我们不会为最糟糕的龙卷风而设计。”Much of the damage, on an annual basis, is done by较弱他解释说，龙卷风是因为还有很多。
The standard also does not apply to all buildings and other structures, only those classified as Risk Category (RC) III or IV and located in the tornado-prone region of the U.S., which is mostly the area of the contiguous U.S. east of the continental divide.
RC III buildings and structures represent a substantial hazard to human life in the event of failure. Among these are occupancies for public assembly or large groups, schools and certain infrastructure. RC IV buildings are those designated as essential structures, such as hospitals.
这些规定不适用于RC I建筑，在失败的情况下，对人类生活的危害很小，例如农业设施，某些临时设施和次要存储设施。它不适用于大多数RC II建筑物，包括住宅和办公楼，除了包括公共议会场所的建筑物。
All Components and Cladding Included
For those buildings that fall under the standard’s criteria, the main wind force resisting system and all components and cladding “shall be designed and constructed to resist the greater of the tornado loads determined in accordance with the provisions” of Chapter 32 or the wind loads determined in accordance with Chapters 26 through 31, using the load combinations provided..., says the standard.
Levitan说，具有较大占地面积的建筑物更容易受到龙卷风破坏的影响，因为它们是更大的目标。根据规定，要确定龙卷风负荷，工程师必须首先测量建筑物的有效区域，如果它不是直线形状，则通常超出其实际占地面积。然后，他们使用危险图来确定建筑物位置龙卷风的特定脆弱性。
The tornado speeds to use in the calculations are provided in the standard. NIST is in the process of developing maps that will help show where it is more likely that tornado loads will control various aspects of the design compared to wind loads, as there are many variables at play, more than just comparing tornado speed to basic wind speed, says Levitan.
“Tornadic winds have significant vertical components; rapid atmospheric pressure changes that can induce loads; and load combinations including tornadoes are not always the same as those including wind,” states Chapter 32. “Hence, tornado loads are treated separately from wind loads, not as a subset of wind loads. This is analogous in some ways to the separate treatment of flood loads and tsunami loads.”
Editor's note: This article was updated on Dec. 3.
