By Evan Gurley
Every year, natural disasters involving extreme wind events take aim at residential areas. Tornados, hurricanes and tropical storms strike with lethal force. For North America, some years are far worse than others.
2011 was an unusually active and deadly year for tornados across the United States, with a total of 1,691 of the angry funnel clouds reported across the country – the second most in history! Two years ago, an EF-5 tornado hit the city of Joplin, Mo., leaving 157 people dead. The Joplin tornado was the deadliest single tornado since modern recordkeeping began in 1950 (1).
Are tornados growing in strength?
When we look at the storms that made headlines in 2012, the tornado that hit Henryville, Ind., on March 2 stands out. Packing winds of 175 mph, it struck the small town of 2,000 residents, killing 11 and nearly wiping out all the buildings and aboveground structures.
The National Weather Service (NWS) office in Norman, Okla., announced that a tornado slammed into El Reno, Okla., on May 31, 2013, and was responsible for killing 20 people including three storm chasers. It was the widest tornado ever recorded in the United States at 2.6 miles wide.
NWS confirmed that the El Reno tornado, which was originally classified as an EF-3 tornado (See Figure 1), was reclassified as an EF-5 tornado. It generated wind speeds of 296 mph and left a 16.2-mile-long swath of destruction, making it one of the strongest tornados ever recorded.
Some communities, homes and businesses are equipped with shelters, basements or rooms designed to withstand the forces and high-velocity projectiles associated with extreme wind events. However, many communities and individuals do not have these resources available and remain at risk. Precast concrete, as a building component, can help provide protection and peace of mind for families and communities when these dangerous storms approach.
A home or business “built to code” is no panacea
Based on 60 years of tornado history and more than 150 years of hurricane history, the United States has been divided into four Wind Zones that geographically reflect the number and strength of extreme wind events. Figure 2 shows these four zones, where Zone IV experiences the highest frequency and strength of high-wind events.
Your home or place of business is most likely built in accordance with local building codes that require buildings to withstand a “design” wind event. In most tornado-prone regions, the building code design wind event is a storm with 90 mph winds. For hurricane-prone regions, the code for design wind events ranges from 90 mph to 150 mph.
A home or building designed and constructed to code does not mean that your home or office building can withstand wind from every event. An intense tornado or extreme hurricane can pack winds much greater than local code requirements.
“Near-absolute protection” requires safe rooms
Extensive testing by Texas Tech University and other wind engineering research facilities shows that walls, ceilings and doors commonly used in building construction to meet minimum building code requirements for standard building construction cannot withstand the impact of projectiles carried by extreme winds (2). Most homes, even new ones constructed according to current building codes, do not provide adequate protection for occupants seeking safety from tornados. Only specially designed and constructed safe rooms, which are built above the minimum code requirements as defined by the Federal Emergency Management Agency (FEMA) offer life-safety occupant protection during a tornado or strong hurricane.
Having a safe room in your home, business or community can help provide “near-absolute protection” for you, your family, employees and visitors from injury or death caused by extreme winds (3).
FEMA safe room guidelines are the Gold Standard
To ensure that safe rooms are structurally sound and will provide near-absolute protection from devastating storms, FEMA has developed design, construction and operation criteria (FEMA 320 and FEMA 361) for architects, engineers, building officials, local officials, emergency managers and prospective safe room owner/operators. FEMA 320 outlines the design criteria for the development of residential safe rooms (16 persons or less) located in basements, garages, interior rooms and standalone structures (buried or exposed). FEMA 361 contains the design criteria for construction and operation for both community (greater than 16-person capacity) and residential safe rooms.
Using the FEMA guidelines as a pre-standard, design and construction professionals, led by the International Code Council (ICC) and the National Storm Shelter Association (NSSA), have joined forces to produce the first ICC/NSSA Standard for the Design and Construction of Storm Shelters (ICC-500). Precasters who manufacture safe rooms meeting this standard are able to assure prospective owners and occupants that precast concrete safe rooms will provide life-safety protection (see Figure 3). While fully supporting this effort, FEMA has continued to promote its 320 and 361 guidelines to communities and individuals seeking further guidance on best practices.
Typical residential buildings are no match for windborne missiles
When a tornado or hurricane starts ripping apart structures and buildings, the wind field associated with that storm becomes filled with flying debris. Once airborne, these objects become dangerous projectiles, or “windborne missiles.” When wind speeds are extreme, debris missiles are propelled with enough force to penetrate or perforate windows, walls or roofs. For example, a standard 2×4 weighing 15 lbs, when powered by a 250-mph wind, can have a horizontal speed of 100 mph! At this speed, it can penetrate most common residential building materials (See Figures 4 and 5).
Numerous instances show that during extreme winds, building failures occur even on the far peripheries of a storm’s path. These failures can generate projectiles of various sizes, perhaps whole roofs, that could damage other structures. This means that just one weak building can produce windborne missiles, which could damage other buildings that otherwise would have suffered little or no damage.
When an urban area is stuck by an extreme wind event, the projectile load carried in the debris cloud represents a substantially increased hazard compared with an equivalent tornado in an open, rural setting. In addition to increasing a tornado’s damage potential, it is well known that being hit by a projectile is among the three major causes of causalities in tornados (the other fatalities occur when people become airborne or are crushed within collapsing structures).
The precast advantage: university projectile impact tests
FEMA has long recognized that precast concrete safe rooms provide effective protection against the most dangerous storms. Precast concrete inherently possesses the vital characteristics, such as strength, penetration resistance and durability, required to withstand the hazards of violent weather. Precast concrete’s superior structural characteristics make it one of the most preferred construction materials for safe rooms.
Texas Tech University, a leader in wind engineering research, conducted debris impact tests on various nonproprietary materials to determine how these different building products are affected by high-speed debris impact, simulating the conditions in an extreme wind event.
The test setup included a pneumatic cannon that can accelerate a timber plank to a speed of 150 mph (See Figure 6). The pneumatic cannon can also simulate other types of debris, such as lumber, fence posts, bricks, PVC pipe and steel conduit. A 15-lb 2×4 traveling at 100 mph is equivalent to the weight and speed of debris in a 250-mph tornado – the maximum wind speed of 99% of tornados (4).
Building materials that were tested included concrete masonry unit (CMU) wall sections, reinforced concrete, plywood-and-metal combinations and doors. Tests of reinforced concrete slabs, compared with the other building materials, clearly showed that concrete excelled and incurred no damage in nearly every scenario. Even when a 4-in.-thick concrete test specimen (reinforced with No. 3 rebar, 6 in. OC) was hit with a 2×4 plank traveling at 140 mph, no damage was reported; in fact, the missile was reduced to splinters. Under the same conditions, other building elements did not fare nearly as well. In addition to the Texas Tech University tests, numerous industry tests confirm that precast concrete’s strength and durability offer unmatched resistance to windborne projectiles during tornados and hurricanes.
Precast concrete safe rooms can be manufactured to exacting specifications by most precast concrete producers. To find out more, visit precast.org.
Evan Gurley is a technical services engineer with NPCA.
- The May 21, 2001, tornado is ranked as the 7th deadliest in U.S. history.
- A Summary Report on “Debris Impact Testing at Texas Tech University,” prepared by the Wind Science and Engineering Research Center, Texas Tech University, Lubbock, Texas. June 2003 : (www.depts.ttu.edu/nwi/research/DebrisImpact/index.php)
- “Near-absolute protection” as defined by FEMA means that, based on current knowledge of tornados and hurricanes, the occupants of a safe room built according to FEMA guidelines will have a very high probability of being protected from injury or death.
- 90% of tornados have wind speeds ≤ 150 mph.