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When a hurricane makes landfall, the shear force of hurricane strength winds can destroy buildings, topple trees, bring down powerlines, and blow vehicles off roads. When flying debris, such as signs, roofing material, building siding, and small items left outside, is added to the mix, the potential for building damage is even greater. Threats to human safety from hurricane force winds are equally severe. Many people have been killed or seriously injured by falling trees and flying debris. This is one reason why it is important for residents and businesses to assess their property before hurricane season to ensure that landscaping and trees do not become a wind hazard. When a hurricane warning is issued, all lawn furniture and other outside objects that could become projectiles in high winds should be secured or brought inside. In hurricane prone regions, trees should be pruned each spring to remove dead and weak branches. Hurricane winds impact homes and other buildings in two different ways: 1) differential pressures act on the building envelope, which includes roofs and walls (and their associated components), and 2) wind-borne debris may strike a building, with windows and doors the most susceptible to impact. Additionally, downed trees and power lines, being blown-down onto a building, is another way hurricanes can cause damage to structures. Rainwater damage to a building’s interior will normally result after damage to the building envelope has occurred. Differential pressure is defined as the difference in pressure between two points of a system, such as the opposite sides of a house or building. Excessive differential pressures caused by wind flow on two, opposite sides of any building component (roofs, walls, structural beams, etc.) could deform or dislodge these materials when the differential pressures, the more severe of which tend to be outward forcing, exceed the resistance capacities of the components and/or their connections to other parts of the building. For example, roof shingles or roof tiles can be broken or lifted off in this manner and be carried away by the wind, only to become wind-borne debris. Wall sidings, windows, skylights, and doors etc. can be damaged in a similar fashion. Even plywood roof panels or the entire roof structure can be uplifted and transported, though this normally only occurs at higher wind speeds. With a roof gone, the walls of a building are much easier to be blown down by a hurricane's winds.
Wind-borne debris generated from loose-laid ground materials and/or from dislodged building components can fly an unbelievable distance, up to several hundred feet, and impact neighboring buildings. Almost all building envelope materials, particularly windows, doors and skylights, as well as most wall sidings and roof coverings used for homes, are at risk for impact from such high-energy projectiles. Breaches to the building envelope by such projectiles will allow rainwater intrusion and subsequently damage the building interior. It may also pressurize the interior of the building, increasing the uplift on the roof and the outward forces on the walls, making them more likely to fail. In high category hurricanes (e.g. category 3 or higher), wind-borne debris damage occurs essentially in parallel with the damage due to differential pressures. Wind speed tends to decrease significantly within 12 hours after landfall (for more detail see Interaction between a Hurricane and Land). Nonetheless, winds can stay above hurricane strength well inland. For example, Hurricane Hugo (1989) battered Charlotte, NC, (175 miles inland) with gusts to nearly 161 kmph (100 mph); these winds were strong enough to topple trees and power lines across roads and houses, leaving many without power and closing schools for as long as two weeks.
An often-misunderstood aspect of hurricane winds is the potential for increased damage as wind speeds increase. The forces against structures do not increase linearly, they increase exponentially (power of 3), and as wind speed increases. A 241 kph (150 mph) wind is 20% stronger than a 201 kph (125 mph) wind. However, the destructive power of a 241 kph (150 mph) wind compared to a 201 kph (125 mph) wind is actually 73% greater. For example, Hurricane Andrew’s sustained winds of 165 mph were 160% more powerful than Hurricane Katrina’s sustained winds of 120 mph at coastal Mississippi. Measuring the Severity of Hurricane Winds In the Atlantic and East Pacific regions, hurricane winds are categorized on a scale of 1 to 5 by the Saffir-Simpson Hurricane Wind Scale. This scale provides examples of the type of damage and impacts (in the United States) associated with winds of the indicated intensity. In general, damages rise by about a factor of four for each category increase. Typhoons in the North Pacific Ocean (e.g. those impacting Taiwan, Japan, the Philippines, and Mainland China) are also classified according to the maximum wind speed at their center.
The Saffir-Simpson Hurricane Wind Scale does not address the potential for such other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes. It should also be noted that the general wind-caused damage descriptions are to some degree dependent upon the local building codes and how well and how long they have been enforced. Hurricane wind damage is also dependent on other factors such as the duration of high winds, change of wind direction, amount of accompanying rainfall, and condition of affected structures. Depending on circumstances, less intense storms may still be strong enough to produce significant damage, particularly in areas that have not sufficiently prepared in advance of the storm. For more information about the Saffir-Simpson Hurricane Wind scale, as well as the different stages of hurricane development, please see the section on a Hurricane's Life Cycle.
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