Windswept_ The Story of Wind and Weather - Marq de Villiers [82]
II
The critical thing to remember about wind is that the force it exerts doesn't just double when the wind speed doubles—if that were so, hurricanes would seldom cause any real damage, and on the other hand, windmills would only work in major gales. In fact, the inertial force of wind, the direct push of wind against an object, or the force exerted when something stops or changes the direction of moving air, is proportional to the wind speed squared.
A wind of 20 miles an hour will exert one pound of inertial force on a flat object 1 foot square. But this doesn't mean that the pressure will double if the wind doubles—that a 40-mile-an-hour wind will exert only two pounds of pressure instead of one. Instead, every doubling of the wind speed quadruples the inertial force, and a wind three times as fast exerts a force nine times as severe. You can see how rapidly this escalates if you take, say, the wall of a small shed some 400 feet square. It will face 400 pounds of pressure in a 20-mile-an-hour wind, 1600 pounds in a 40-mile-an-hour wind, 6,400 pounds in an 80-mile-an-hour wind and an astounding 14,400 pounds, more than 7 tons, in a 120-mile-an-hour hurricane. A building will have to be twice as strong to survive a 120-mile-an-hour wind as it would an 80-mile-an-hour wind. (See Appendix 11 for a wind force table.)
The effect is exaggerated by the so-called blast effect of a gust. In the case of a building, if a window or door suddenly breaks open in a gust, wind will explode into the building and destroy it from the inside. "No realistic amount of structural engineering can safeguard a building against the blast effect. It makes much more sense to take every possible precaution to ensure that windows and doors will not break or fly open during high winds."12 But sealing the house tight against the wind may not be the right approach either. It may make more sense to enable some kind of controlled air flow through a building during a major storm. One homeowner whose house survived a major hurricane in Florida had closed his doors and windows tight and even sealed two turbine vents in the attic. The house was doing well until an errant tile smashed a window, and the wind immediately started to inflate the house like a balloon. He was saved only because the plugs in the turbine vents blew out before the walls did, and the pressure dropped at once.
Aerodynamics have shown that winds will exert an upward lift on any roof pitched less than 30 degrees, an outward force on walls parallel to the windstream, and a drag force on walls and gable slopes facing downwind. "In any major windstorm, then, some parts of a building will be pushed inward while other parts are being pulled outward. This combination of pushing in one place while pulling in another is particularly abusive and leads directly to many structural failures. Moreover, as wind direction shifts, and it usually does during any major storm, many of the forces reverse direction. Often a building will be weakened during the first half of a hurricane, then will collapse when the winds reverse their direction after the eye passes through."13
Interestingly, the pagoda style of roof design generates what wind-tunnel experts call a negative lift where they overhang, helping to keep the roof in place in typhoons. Which may explain why so many pagodas have survived for so long in typhoon-prone parts of the world.
A survey of Dade County, Florida, homeowners after Hurricane Andrew had passed by found that many of them had taken refuge in a bathroom or closet in the middle of a house. Such spaces, however, provided little more than emotional cover in a storm that was known to have driven two-by-fours through concrete walls. Much of Florida was considering adopting a code that would require new houses to have at least one room thought to be projectile-proof. Under consideration was an eight-foot-square room sheathed in two-by-four studs covered with four inches of plywood.14
Because hurricane winds