Tornado: Construction and Life Safety Issues
May 2013 – IBHS Experts Weigh in Building in Tornado-Prone Areas
After a tornado can be an emotionally charged time as entire communities face lengthy recoveries and must cope with extensive loss of life. As attention turns toward recovery and rebuilding, IBHS is committed to providing science-based information to address many issues that will arise.
The following Q&A is drawn from several IBHS staff with expertise in the fields of wind-related building performance for residential and commercial properties, the behavior of tornadoes and their destructive patterns, and building codes.
Question: What role can building codes play in protecting properties from tornado damage?
Answer: Building codes are minimum life safety standards. A strong, well-enforced building code provides uniformity and protection from a variety of hazards, but properties built to the codes used outside hurricane-prone regions, where design wind speeds are less than 110 mph, will offer little protection against tornadoes. The best option for building stronger structures that will have a fighting chance of surviving many weak or moderate tornadoes is to upgrade to construction standards that provide for the robust continuous load paths used in coastal hurricane-prone areas.
Question: How does a tornado destroy a house or business?
Answer: EF-2 and EF-3 tornadoes with 111 mph to 165 mph winds can destroy a property in four seconds. The biggest initial risk to properties is flying debris, which can shatter windows and burst open doors to create large holes in exterior walls. Once an opening the size of a window or door is created on the side facing the wind (where the debris is also hitting), air pressure builds up inside the building like inflating a balloon. This internal pressure adds to external wind forces acting to lift the roof off the building and pull side and back walls away from the building. The net effect is that forces trying to rip the home apart can be doubled. If the connections between the roof and walls are weak, these wind forces will drive the roof and walls to give way. Once the roof blows off the entire structure can collapse within seconds. This was illustrated through a series of wind demonstrations at the IBHS Research Center in 2010 [watch the video]. The demonstrations clearly reinforced the fact that conventionally built structures do not have much of a safety margin against structural damage and showed the brittleness of conventional connections. Widespread use of effective strapping, which might increase the cost of a wood frame house by two percent, would create houses that are significantly more resistant to all kinds of severe wind events. However, they will still experience significant structural damage when exposed to a direct hit from EF-3, 4 or 5 tornadoes. Read more about how a tornado destroys a house in Dr. Reinhold’s interview with Popular Mechanics.
Question: What does IBHS recommend for construction choices in tornado-prone areas?
Answer: The options will be based on personal choices and financial abilities. In the wake of tornado-related fatalities in several states, many home and business owners will make life safety their first priority. However, options for improving structural performance do exist and can produce effective results, particularly when combined with the use of a safe room. Houses and businesses are not typically designed to withstand the impact of a severe tornado. It is important to note that 77 percent of tornadoes have wind speeds less than 110 mph, and the recent more powerful tornadoes, such as those affecting Moore, Oklahoma; Tuscaloosa, Alabama and Joplin, Missouri, account for only one to two percent of tornadoes affecting the U.S. each year, according to the National Climatic Data Center (NCDC). Watch IBHS President and CEO Julie Rochman talk with NBC Nightly News about engineering buildings for tornadoes.
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Life Safety:
The best option for addressing life safety is the installation of a safe room. Guidelines for building a new or retrofitting an existing space using a safe room kit are offered by the Federal Emergency Management Agency (FEMA 320), which also provides grant funding to offset construction costs, and the International Code Council (ICC 500). FEMA guidelines require safe rooms to be designed to withstand the impact of a 15-pound 2″x4″ at 100 mph and to withstand wind loads from 250 mph winds. Construction costs for less expensive options typically range from $5,000 to $7,500. FEMA recognizes safe room construction as qualifying for community mitigation grants for which individuals can apply.
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Structural Safety:
Building to the IBHS FORTIFIED standards, which include options for new and existing residential buildings, as well as new commercial construction, offer some advantages, but, expectations need to be clearly established. The FORTIFIED for Safer Living® and FORTIFIED for Safer Business™ criteria will offer enhanced protection for about 90 percent of the areas affected by tornadoes in any given year, but will not provide the kind of personal life safety protection needed to withstand an EF-4 or EF-5 tornado, and possibly not in an EF-3 tornado. In those intense tornadoes, storm shelters designed to the level of protection established by ICC 500 or FEMA 320 (residential shelters) and FEMA 361 (community shelters) provide the most credible personal protection. The FORTIFIED Home Hurricane™ Program for new and existing construction’s gold designation will provide a significant increase in strength over conventional construction, but does not provide the strength increase of the FORTIFIED for Safer Living® program.
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Protecting Lives and Properties:
The ideal combination is installing a safe room and then strengthening the structural integrity of the rest of the house or business, by fully sheathing the walls and reinforcing the connections between the roof and walls and the walls and foundation. This combination addresses both life safety concerns and increases the likelihood that at least part of the structure will be left standing after a tornado to provide some shelter.
