Reliability Bases for Tornado Load Criteria for ASCE Standard 7-22

Tornadoes are intense localized convective windstorms that are among the most devastating natural hazards that occur in the United States. Although the probability of any tornado striking one particular building in any given year is quite low because of its small footprint, when strong tornadoes strike densely populated areas, the results can be catastrophic; the Joplin, MO tornado of 2011 caused 161 fatalities and nearly $3 billion in damages. Such losses are projected to increase in the future as a result of urbanization and economic development. ASCE Standard 7-22 on Minimum Design Loads has, for the first time, included new reliability-based design criteria for tornado loads. The objective of this paper is to provide archival documentation of the basis for these tornado-resistant load provisions in Chapter 32 of ASCE 7-22. This paper summarizes the significant challenges that were overcome in addressing the fundamental differences between effects of tornado and nontornadic winds and the higher uncertainties associated with tornado wind pressures that must be accommodated in the risk-informed framework of ASCE 7-22. These challenges were addressed through reliability analyses that led to new tornado load criteria that provide reasonable consistency with the reliability delivered by existing criteria for nontornadic winds.