Floyd, Fran and Bertha -- they’re meaningful names to people who live along North Carolina’s coast and face the potential for devastating damage to their homes and businesses every hurricane season.
Nothing can prevent a storm from hitting, but a team of North Carolina State University researchers is testing new designs for "breakaway walls" that could reduce damage to homes and buildings should a hurricane make landfall.
Their findings are included in the most recent edition of the Federal Emergency Management Agency’s (FEMA) Coastal Construction Manual.
Breakaway walls are designed for use on the ground floor of buildings in coastal flood zones. To minimize damage from storm surges, the National Flood Insurance Program suggests that these homes and businesses be built on pilings, or "stilts," and that the ground floor be used only for access, parking or storage. Property owners who choose to enclose this space are urged to use walls that will break away from the rest of the house when pressure exerted on them by a storm surge reaches a predetermined stress load -- usually between 10 and 20 pounds per square foot. Stronger walls would absorb the force of the surging water, jeopardizing the integrity of the entire foundation.
To determine what materials and designs will work best for breakaway walls, a trio of NC State researchers tested eight experimental wall prototypes. The researchers are Dr. C.C. Tung, professor emeritus of civil engineering; Spencer M. Rogers Jr., senior coastal engineering specialist with North Carolina Sea Grant and an adjunct civil engineering faculty member; and Dr. Bohumil Kasal, associate professor of wood and paper science.
Each 8 x 8-foot wood-wall prototype was tested in simulated hurricane storm surge conditions at a wave tank testing facility at Oregon State University in Corvallis, Ore. The researchers placed the prototypes into the tank, which measures 342 feet long, 12 feet wide and 15 feet deep, and directed increasingly strong waves and rising water levels at them until they broke apart. Due to the size and depth of the tank, the researchers were able to test when and how the walls would fail in hurricane-force breaking waves, which exert an exceedingly high-pressure burst against walls as they crest.
Based on their findings, the researchers developed practical guidelines for builders to follow, such as using exterior siding no thicker than ½-inch plywood or equivalent material; using studs no bigger than 2x4s for breakaway walls; and placing the studs at least 24 inches or more apart. A FEMA technical bulletin containing the full results from the NC State-Oregon State research study on breakaway walls is on the Web at http://www.fema.gov/MIT/job15.pdf.
The research was funded by the National Science Foundation and FEMA.
Next, the NC State trio would like to test unreinforced hollow-cell masonry walls -- also known as cinder block walls. Because they sink, hollow cinder blocks have the advantage of not becoming large, floating debris after a hurricane, making cleanup easier and reducing potential damage to surrounding buildings. However, testing cinder block walls is more difficult than testing wood walls, because of long cure times for the mortar and the potential of damage to the wave tank itself.
FEMA’s Coastal Construction Manual is available on a CD-ROM with interactive links for calculations, cross-references and other useful Web sites. The CD-ROM and a printed version of the manual -- which fills three binders -- are available from the FEMA Publications Service Center at (800) 480-2520. There is no charge for single copies.
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