Hurricanes are among the most destructive and costliest of all natural disasters. Since 1995, the United States has entered a cycle of more intense hurricane activity. Between 2003 and 2007 Florida was hit by nine hurricanes, resulting in 2.9 million claims and $31.3 billion in insured losses. The possibility of a major hurricane striking a large population center in Florida during the next 20 to 30 years of high activity emerges as a near certainty. As populations continue to increase in Florida coastal regions where the threat is highest, so does the possibility for even greater destruction. This risk can be devastating for entire communities, for the insurance industry, which ultimately must pay for a significant portion of the incurred losses, for local and state government who have to struggle with the impact on their populations and economies, and for the nation as a whole. For these reasons, it is essential to reduce the destruction expected from future storms.
How Wind Forces Affect Homes
Wind forces are complex. The effect of wind on a building depends on the interaction of many factors. Natural factors such wind speed, wind height, ground surface features, and the properties of the air affect possible windstorm damage. Building characteristics, including the shape, location, and physical properties of structures, also determine a structure’s overall resistance to wind damage. Together, these variables create differences in pressure that push and pull on the exterior surfaces of buildings during a hurricane.
Uplift occurs as wind flows over a roof. Similar to the effect on airplane wings, wind flow under a roof pushes upward while wind flow over a roof pulls upward.
Tilting or sliding occurs when horizontal wind pressures create a shearing action along the foundation.
Overturning, or rotating off the foundation, can also result from shearing action when a structure is otherwise unable to tilt or slide off the foundation.
A safe home is designed to resist these three effects of wind. The exterior surfaces of a home interact to function as the building envelope. Think of this envelope as a protective shield from the outdoor elements such as heat, humidity, and stormy weather. A stronger shield makes for a safer home and more comfortable occupants.
The structural components of a building envelope are the foundation, walls, and roof. A safe envelope has a continuous load path. This load path connects all the structural parts of a building envelope much like how a human skeleton supports and connects parts of our body.
The non-structural components of a building envelope include windows, doors, garage doors, and other openings in the structural components. These parts protect the inside of a building much like how human skin protects our internal organs.
In addition, the location of the structure has an impact on its ability to resist high winds. Winds are subject to “friction” – objects on the ground can significantly retard wind speed, while taller buildings may bear the brunt of much higher winds. If the structure is in an unobstructed location or within 1,500 feet of open water, it is more susceptible to damages caused by high winds. Proper landscaping may help to shield the home and divert winds around the building.
The weakest link in the building envelope is the point most likely to fail in a windstorm. When a hurricane or tornado strikes, a home is only as strong as the weakest link. Typically, when one link fails, the rest of the structure is subjected to greater stress and probability of failure. For example, high winds will exert uplift on a home’s roof. Normally, that force is exerted externally, by the passage of the wind over the roof and under the eaves. If, however, a window is blown out, or a gable wall is forced in by the shearing effect of the wind, the uplift force is multiplied: the wind coming in through the breach exerts an additional interior force lifting the roof. Likewise, small cracks in the building’s exterior may allow wind-borne water into the structure. Over a period of time, that water may saturate insulation, weighing down the ceiling or weakening other structural components. The collapse of an interior ceiling may damage the integrity of the load path, weakening walls or connections with the roof, and eventually lead to catastrophic damage.
Sometimes a high wind speed alone can break the building's load path or punch a hole in the building envelope -- the actual force of high winds can cause a door or window to break open or a gable end to shear. Other times nearby debris can be picked up in the wind and projected against the building envelope. Roof shingles from a neighbor’s home, branches from fallen trees, or unsecured yard furniture are examples of potentially dangerous wind-borne debris that can break windows or doors.
Once wind forces create an opening in the building envelope, the dangers of structural failure greatly increase. Obviously, wind damage will result if the building envelope fails, but water intrusion can be just as damaging – if not more so.
Once the wind has opened up a structure, wind-driven rain enters, causing additional damage. Direct wind damage to structures built to high wind standards has been reduced, but it has become clear that just improving the structural integrity of homes is not enough. Wind driven water intrusion can cause catastrophic damage to walls, ceilings, and interiors of homes, which leads to major disruption of households. Water intrusion can be of particular importance to the homeowner because some insurance policies do not cover water intrusion unless it originates from damage to the roof, walls, windows or doors of the home. When wind speeds get above 60 mph, rainwater is driven against the exterior of the home with great force. Any time water builds up on the exterior wall surfaces and there is lower pressure on the inside of the house, the water can penetrate in large quantities through cracks, holes and gaps in the siding and around windows and doors. When this happens for hours at a time the resulting damage and mold can be as devastating as wind damage. After a storm passes, mold can be a particularly insidious problem if no electricity is available to dry out damaged homes. Some studies have found that water damage can increase the cost of an insurance claim up to eight times as much as simple wind damage alone.
Hurricanes also bring the possibility of storm surges along coastal areas caused by wind-whipped tidal forces as well as localized inland flooding from the tremendous amounts of rain that fall during the storm. Windstorm policies will protect a homeowner from damages arising from high winds (and wind-borne water damage), but will not cover damage caused by storm surges, flooding and standing water. Flood insurance, available through the National Flood Insurance program is available to cover those perils. This program focuses only on windstorm coverage.
Chapter 6 Contents
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