Each year in the U.S., more people are killed by tornadoes than by earthquakes and hurricanes combined. But the chances of one particular structure being hit by a tornado are so remote that no national codes address the often-deadly storms. Of late, the fickle twisters are shifting northward, invading—and often leveling—more densely populated areas. In response, communities, engineers, researchers and others are rallying defenses as never before to minimize twister-triggered death and destruction.
One of those communities is Moore, Okla. Last year on May 22, the city was hit by an Enhanced Fujita Scale 5 (EF-5) tornado—the strongest storm rating. The twister slammed into two schools, killing 10 children.
The shock of the event spurred Moore officials to take defensive and landmark action against future storms. On March 17, the city's lawmakers unanimously passed 12 changes to Moore's residential building code. The law went into effect immediately.
In Moore, new single-family houses must be built to withstand wind speeds of 135 mph—rather than the model International Building Code (IBC) 2009 requirement of 90 mph—and have roof sheathing, hurricane clips or framing anchors; continuous plywood bracing; and garage doors all able to withstand 135-mph winds. The code does not address retrofits.
Moore is the first city in the U.S. to have a residential code that addresses tornadoes. As such, it has become the focal point of a tornado-defense discussion.
This is in part because Moore has incurred great losses from twisters—and not only last year. At 302 mph, the strongest recorded winds in world history hit Moore during a 1999 EF-5 tornado. The 1999 and 2013 tornadoes combined bring Moore's property damages to an estimated $3 billion and death toll to 60, according to the Oklahoma Dept. of Emergency Management. Moore's population is only 58,000.
Moore's losses pale alongside the nation's. In 2011 alone, tornadoes caused $10 billion in property damage and 553 deaths, according to the National Oceanic and Atmospheric Administration (NOAA) Storm Prediction Center statistics. The strongest twisters ravage the midsection of the nation—from Springfield to Baton Rouge and from Cincinnati to Amarillo. At least 800,000 square miles of U.S. territory falls in the EF-5 zone.
Moore's code foreshadows new design guidelines for commercial and residential construction in addition to updated model code shelter requirements coming out this month. The 2015 edition of the model International Building Code, published by the International Code Council, has provisions for schools up to 12th grade with six or more students, and emergency call centers. The IBC code, if adopted by jurisdictions, would require those occupancies, located in tornado areas with wind speed possibility of 250 mph, to have tornado shelters to meet the ICC 500 Standard for the Design and Construction of Storm Shelters.
This is the first time a model code addresses safe rooms. But the tornadic-design push goes beyond shelters. The Applied Technology Council (ATC) is producing first-ever tornadic design guidelines for residential and commercial construction.
The guide is based on an upcoming expanded commentary in ASCE 7 on tornadic design, developed by the Wind Load Subcommittee of the American Society of Civil Engineers. Changes to the commentary are informed by recent university research, the Miami-Dade County hurricane code and forensic investigations of structural damage in tornado-ravaged cities.