NIST Calls for Performance-Based Standards for Tornado-Resistant Building DesignApril 10th, 2014 by DWM Magazine
The National Institute of Standards and Technology (NIST) released the final report on its technical investigation into the impacts of the May 22, 2011, tornado that struck Joplin, Mo. The final report includes clarifications and supplemental text suggested by organizations and individuals from across the nation in response to the request for comments on the draft Joplin report, released November 21, 2013.
The NIST Joplin tornado study was the first to scientifically study a tornado in terms of four key aspects: storm characteristics, building performance, human behavior and emergency communication—and then assess the impact of each on preventing injury or death. It also is the first to recommend that standards and model codes be developed and adopted for designing buildings to better resist tornadoes.
As part of its report, NIST recommended that nationally accepted performance-based standards for the tornado-resistant design of buildings and infrastructure be developed and adopted in model codes and local regulations to enhance the resiliency of communities to tornado hazards. “The standards should encompass tornado hazard characterization, performance objectives, and evaluation tools. The standards shall require that critical buildings and infrastructure such as hospitals and emergency operations centers be designed to remain operational in the event of a tornado.”
The study points out that currently there are no standards for the tornado-resistant design of ordinary buildings and infrastructure, except for safety-related structures in nuclear power plants and storm shelters or safe rooms. Even they, the report notes, have inconsistencies in the way tornado hazards are characterized, as reflected in the different tornado regionalization and associated tornado design wind speeds for the contiguous United States.
According to the report, “performance-based standards for tornado-resistant design of ordinary buildings–including critical facilities, commercial and residential buildings–will result in more tornado-resilient communities … by explicitly considering tornado hazards, which will be characterized by the most up-to-date tornado data and risk-consistent science-based methodologies, as a structural design condition.”
In its report, NIST also recommends the development of “risk-balanced, performance-based tornado design methodologies such that all building components and systems meet or exceed the same performance objectives when subjected to tornado hazards.”
As NIST points out, there is currently no methodology for building design that specifically considers the design hazards associated with tornadoes. “The minimum code requirements for wind loading in current building codes do not take into account the inconsistent performance of different building components … when subjected to tornado hazards.” The report also cites the performance of building envelopes: “failure of building envelopes, despite the robust structural system that could withstand the tornado without structural collapse, often resulted in extensive damage to building interiors.”
As an example, the report noted the “failure of building envelopes at St. John’s Regional Medical Center (SJRMC)… was the primary cause for the complete loss of functionality of this critical facility, which occurred despite the robust structural system that withstood the tornado without structural collapse.” The report said that while the building envelope’s failure caused devastating destruction, the “majority of the impact-resistant windows on the fifth floor (Behavioral Health Unit) of the West Tower of SJRMC remained intact, whereas most regular dual-pane insulated windows at SJRMC were broken when exposed to the same tornado hazards.”
According to the report, NIST will now work with the appropriate code development organizations to use the study’s recommendations to improve model building codes and lay the foundation for nationally accepted standards. NIST plans to work with organizations representing state and local governments—including building officials—to encourage them to consider implementing its recommendations.