Tornado Hazard Mapping for Tornado-Resistant Design of Buildings and Infrastructure

Tornadoes typically affect much smaller geographic areas compared with other natural hazards like earthquakes and hurricanes, but occur at a much higher frequency and cause more deaths than those two hazards combined.  During the period from 1950 through 2011, U.S. tornadoes caused about 5,600 fatalities. This number well exceeds the toll for U.S. hurricanes and earthquakes over the same period (3,102 and 459, respectively). U.S. average annual property losses due to tornadoes are approximately the same as losses due to hurricanes.  Inflation adjusted insured catastrophe losses for the two decades between 1995 and 2014 were $155B for tornadoes and tornadic storms, compared to $161B for hurricanes and tropical storms (in 2014 dollars).

The National Institute of Standards and Technology (NIST) conducted a detailed investigation of the deadliest tornado since official record-keeping began in 1950, the May 22, 2011 EF-5 Joplin tornado, which caused 161 fatalities and more than 1,000 injuries.  NIST found that the majority of the fatalities, 135 of 161 (more than 84%), occurred inside buildings.  This, despite an official tornado warning time of about 17 minutes, greater than the National Weather Service (NWS) national average warning time of approximately 14 minutes.  This particular finding gives rise to an important question:  can building safety in tornado be enhanced through tornado-resistant design procedure and practice?

The NIST investigation report also outlined 16 specific recommendations, among which was recommendation calling for the development of nationally accepted Performance-Based Design (PBD) standards for tornado-resistant design of buildings and infrastructure.  The conventional wisdom is that designing for tornadoes is not warranted since the probability of a tornado strike on any particular building is considered too low.  However, research underway towards implementation of the NIST tornado recommendations appears to indicate that tornado strike probabilities may be an order of magnitude greater than previously assumed. Additionally, considering the lopsided death tolls caused by tornadoes and earthquakes, one can justifiably argue that tornado resistant design should have the level of importance as seismic design.  This is not the case currently, as tornadoes remain the only major natural hazard not considered in the design of buildings and other structures in modern building codes and standards. 

Designing for tornadoes requires a set of tornado hazard maps that accurately reflect the tornado risks for different regions of the U.S. and can be used to derive appropriate tornado design criteria for structural design purposes.  Such maps need to be developed using up-to-date tornado database, tornado parameters, and science-based tornado risk assessment methodology, with appropriate consideration of tornado climatology.  This paper describes a major, on-going research and development effort to create these new tornado hazard maps.  This paper will also provide context for several companion papers, also presented at this conference, which describe in detail various aspects of the data, analyses and modeling   underpinning this tornado hazard mapping effort.

Contact Author: Long Phan/Group Leader/Structures Group/NIST/Email: long.phan@nist.gov