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On Dec. 26, 2015, 12 confirmed tornadoes struck the North Texas area, killing 13 people. Property damage resulting from the tornadoes is currently estimated at $1.2 billion.
According to the National Oceanic and Atmospheric Administration National Weather Service, tornadoes are “nature’s most violent storms” and can be up to one mile wide, produce wind speeds in excess of 200 mph, stay on the ground for over 50 miles and cause devastating damage.
The devastation the Dec. 26, 2015, tornadoes left behind not only reflects the significant upward trend in the costs of great natural catastrophes worldwide, but also illustrates that building codes and construction practices do not adequately protect buildings against the forces generated by tornadoes. From an engineering perspective, the structural damage resulting from the Dec. 26 tornado outbreak was not unexpected, with the vast majority of damage occurring to single-family residences — structures that are designed for wind load magnitudes just a quarter to one-third as strong as common tornadoes.
Despite falling within “tornado alley” — property insurance policies issued to North Texas building owners typically cover damage caused by windstorm. However, this coverage comes at a cost as premiums are highest in the states most at risk for hurricanes and tornadoes — Florida, Louisiana, Mississippi, Oklahoma and Texas.
Even with the high premiums, if insurance carriers continue to pay claims year after year for property damage to buildings inadequately designed to sustain the strong winds associated with predictable tornadoes, carriers should consider strategies for managing the risk to mitigate the financial (and political) consequences associated with these devastating twisters.
The Perfect Storm
Weather experts report that the key atmospheric players responsible for the Dec. 26, 2015, tornados began with a strong Bermuda high pressure system parked over the Atlantic Ocean, circulating warm, moist, unstable air northward, feeding into the warm sector of a surface low pressure system. Fast winds subsequently flowed over a pronounced dip in the jet stream, while subfreezing air filtered southward into North Texas. The cold air overlapped warm air at low levels in the cyclone’s warm sector, creating an explosively unstable atmosphere. The central fast winds developed both directional turning and an increase in speed with wind shears, an ideal configuration for supercell thunderstorms that spawn nearly all violent tornados.
This ideal configuration did, in fact, spawn 12 reported tornados ranging from EF-0, such as the Sulphur Springs tornado with 70-mph winds and a path length of half a mile, a width of 25 yards and a duration of one minute, to the catastrophic Sunnyvale/Garland/Rowlett EF-4 tornado with 170-180 mph winds, a path length of 13 miles, a width of 550 yards and a duration of 17 minutes.
What Happens to a Building During a Tornado?
According to the Federal Emergency Management Agency, a tornado is defined as a violently rotating column of air with wind speeds that can be significantly higher than design wind speeds in modern building codes.
During a tornado, the winds blowing over a home exert an inward pressure against the windward wall, outward pressure against the sidewalls and leeward wall and upward pressure against the roof.
When wind enters a building through a broken window, door or roof section, that wind acts on the inside of a building much like air acts when forced into a balloon; it pushes on the walls and roof of the building from the inside. These forces within the building, added to the wind forces acting on the outside of the building, often result in building failure. Buildings are generally not designed to resist forces acting on both the inside and the outside of the building.
“The continued use of toe-nail connections to fasten the roof to the exterior walls makes the roof highly susceptible to blowing off during a tornado. The lack of lateral support for the walls means there is little capacity to resist the wind loads once the roof has been removed. When walls collapse, the residents are exposed to even more danger with the falling objects and wind-borne debris, increasing the risk of injury and even death.”
The performance of buildings during a tornado is not only dependent upon the location of the building in relation to the tornado’s path, but the age of the building, construction quality and roof type also significantly affect how a building performs during a tornado.
Although mitigation cannot eliminate damage resulting from natural catastrophes, such as tornadoes, it can help make property owners and communities more resilient to damage and less susceptible to losses resulting from natural disasters. Mitigation begins by identifying and understanding the risks in a current area and then taking individual or communitywide steps to reduce those risks.
Although the insurance industry encourages mitigation, individual building owners are not likely to take the steps necessary to make buildings more resilient to natural hazards because they underestimate the risk and are concerned about the costs associated with mitigation. Moreover, mitigation measures often require up-front expenditures yielding only possible benefits over the length of the life of the property.
Because most property owners do not consider the long-term investment in mitigation measures and/or underestimate the risk of loss, building codes can significantly reduce damage sustained during a tornado outbreak. It is possible to significantly increase use of mitigation measures by enforcing building codes, developing economic incentive programs such as tax rebates and adopting zoning ordinances. But collaboration between the public and private sectors is critical in this regard.
Texas has adopted the International Building Code (IBC) for commercial construction and International Residential Building Code (IRC) for residential construction, but individual municipalities determine whether to adopt the code as written, or adopt certain amendments to the IBC and/or IRC. Enforcement of building codes is also delegated to individual municipalities.
For example, the city of Garland has currently adopted the 2009 IRC and IBC, which requires resistance to 90-mph wind loads at a three-second burst, 33 feet above ground. In the 2015 edition of the IRC and IBC the building code, designed wind speed is 115-mph and 115-180-mph for hurricane-prone areas.
In the hurricane context, the adoption of building codes with increased wind-resistance standards has resulted in a significant reduction in property damage from hurricanes. For example, after 1992’s Hurricane Andrew, building codes across the state were tightened. After Hurricane Charley in 2004, the codes were increased even more, requiring roofs in new construction engineered to withstand 150-mph winds, a mechanically-stronger truss system and impact-resistant windows or hurricane shutters. During Charley, homes built under the post-Andrew standards had a claim frequency of 60 percent less than homes built pre-1996 and claims for older damaged homes resulted in an average of $24 per square foot, compared to $14 per square foot for those constructed between 1996 and 2004.
Notably, there are no code-related 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. The minimum code requirements for wind loading in current building codes do not consider the inconsistent performance of different building components (walls versus roof, structural system versus envelope structural system versus power, gas and/or water systems) when subjected to tornado hazards.
According to structural engineers, to improve a building’s resistance to tornadoes, better building materials and techniques are required. Most agree that a continuous load path (one which connects the roof to the wall and the wall to the foundation) could significantly reduce the amount of damage a building sustains during a tornado — which could mean the difference between a structure that is repairable versus a total loss. For example, the Insurance Institute of Business and Home Safety, which identifies mitigation measures that strengthen buildings against natural catastrophes, recommends mitigating against high winds by using 8d ring shank nails instead of 6d common nails and staples to provide stronger support for a roof. Other high-wind resistant designs call for metal connectors and threaded rods tying the roof, walls and floors to the concrete slab and fortified walls and door openings.
In 2014, the city of Moore, Oklahoma, was the first city in the nation to adopt building codes that focus on the tornadic impact on homes. Moore’s new residential building codes include requiring roof sheathing, hurricane clips or framing anchors, continuous plywood bracing and wind-resistant garage doors. The homes would be built to withstand winds up to 135 miles per hour rather than the accepted standard building requirements of 90 miles per hour.
Another mitigation option currently available to Texas residents is to construct (or purchase prefabricated) safe rooms. Although there are more than 10,000 people on the waitlist, Texas residents living or developing in one of the North Central Texas Council of Governments region have an opportunity to qualify for up to half the cost of the construction or installation of an individual safe room, up to a reimbursement cap of $3,000.
Although it is unlikely that engineers can design a building to survive the winds associated with an EF-4 or EF-5 tornado, it is possible to design buildings to perform in other significant wind events, including small tornadoes. Given the frequency of tornadic activity in Texas, mitigation through strengthening building code requirements would provide a reduction in tornado and windstorm damage each year, which in turn will both save lives and reduce insurance losses. Such an increase in building code requirements combined with financial incentives for property owners to build tornado-resistant structures and funding for construction research are all necessary to improve the mitigation of catastrophic storm damage. In the context of a tornado, failure to mitigate could, very simply stated, mean the difference between life and death.
The opinions expressed are those of the author(s) and do not necessarily reflect the views of the firm, its clients, or Portfolio Media Inc., or any of its or their respective affiliates.
 The NOAA NWC meteorology glossary defines a tornado as, “a violently rotating column of air, pendant from a cumuliform cloud, and often (but not always) visible as a funnel cloud.”
 See NOAA NWC.
 Managing Large-Scale Risks in a New Era of Catastrophes, Insuring, Mitigating and Financing Recovery from Natural Disasters in the United States, Wharton Risk Management and Decision Processes Center.
 Hurricane and Tornado-Resistant Concrete Houses.
 David O. Prevatt, The 2015 Christmas Tornado Outbreak, Wind Hazard Damage Team, Dec. 29, 2015.
 Typically defined as “something more than an ordinary gust of wind, no matter how prolonged, and although whirling features which usually accompany tornadoes and cyclones need not be present, it must assume the aspect of a storm.” Fireman’s Insurance Co. v. Weatherman, 193 S.W.2d 247 (Tex. Civ. App.—Eastland 1946 (ref’d n.r.e.).
 Report Providing an Assessment of the Current State of the Market for Natural Catastrophe Insurance in the United States, Federal Insurance Office, U.S. Department of the Treasury, Sept. 2015, page 3.
 Jeff Halverson, The Meteorology that Led to the Deadly Late-December Tornado Outbreaks, The Washington Post, Dec. 29, 2015.
 Since 2006, tornadoes have been classified by the Enhanced Fujita Scale using EF0 through EF5. The EF scale provides a number of damage indicators for different types of buildings.
 See Taking Shelter from the Storm: Building a Safe Roof for your Home or Business, FEMA.
 Id. at page 9.
 Report Providing an Assessment of the Current State of the Market for Natural Catastrophe Insurance in the United States, Federal Insurance Office, U.S. Department of the Treasury, Sept. 2015, page 49.
 Managing Large-Scale Risks in a New Era of Catastrophes, Insuring, Mitigating and Financing Recovery from Natural Disasters in the United States, Wharton Risk Management and Decision Processes Center, page 10.
 NIST NCSTAR 3, Joplin Tornado Investigation, pages 362-364.
 Id. at page 51.
 James Pratt, Predictable Destruction: Building in Tornado Alley, Builder Magazine (July 24, 2013).
 http://www.cityofmoore.com/node/2111 (last visited Feb. 15, 2016).
 NCTCOG is centered around the two urban centers of Dallas and Fort Worth.