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When the strong winds of a hurricane strike a house, usually it is the roof that is first to be blown away. Then, without the joists that hold the tops of the walls together, the walls blow down. Modern construction techniques try to solve this problem in a brute force way by using special metal attachment devices to more securely connect the roof structure to the top of the walls to keep the roof from slipping off the top of the house. The idea is that the horizontal pressure of the wind could push the roof sideways off the substructure unless it was securely fastened to that substructure.
In pictures taken while a hurricane was in the process of hitting one of the Hawaiian islands, it became obvious that roofs were not slipped sideways, but rather were lifted upwards from the walls. In effect the two slopes of a roof formed an airfoil, which, while not as efficient in shape as that of an airplane wing, was still sufficient to lift the necessary tonnage needed to rip the roof and its attachments from the substructure. Indeed Clemsen University determined that during a category five hurricane the lift could be up to 100 pounds per square foot. This, for a normal house, calculates to 100 tons, and that is in all likelihood more than the house AND its foundation weigh.
Aerial videos had been taken of this area both prior to (←) and after (→) the hurricane's passage. What was seen in the aftermath was that all of the steep roofed houses still had their roofs, while those that were much less steep were missing their roofs and usually their walls also. Flat roofed buildings usually showed collapse, probably due to their inability to drain the rainwater away fast enough and the weight of the water caused the collapse. But the other observations supported the notion of lift. If the roof was steep, the "wing" stalled, but if it was shallow, then it was more nearly like an airplane's airfoil and provided sufficient lift to destroy the house, and those houses were indeed decapitated or worse - the supporting walls collapsed, too.
For a moment let us turn to airplanes. The velocities of highly damaging hurricane winds are approximately those of the airspeed of a departing or landing 747 jetliner which weighs many tons. In other words, that airspeed on those airfoils is sufficient to lift many tons. This concurs with what was determined previously by Clemsen University.
The roofs of even modest homes approach or exceed the area of most 747 wings. And even with less than optimal aerodynamics, is it any wonder that roofs are lifted from houses? Indeed, even were the roofs perfectly attached to the substructure, it is not unbelievable that the lift during a hurricane could lift the whole house - perhaps concrete slab and all! You might think that a house is hugely massive, but consider how many trucks were used to deliver all the lumber, shingles, siding, etc. Those trucks could easily ride in a 747 cargo plane. This means that no amount of reinforcement of the structure would prevent the maximum damage. As medical doctors would say: "Attack the cause and not the symptoms." Thus the way to attack the 'cause' is to "foil the airfoil." This can be either done by proper design of a new house (e.g.: steeper roof), or retrofitting an existing home in some way. (Do not misunderstand that the reinforcing methods are not necessary. By no means! They will protect the structure under less than maximum conditions. But for minimizing roof damage, nothing would be simpler and less costly than finding a way to defeat the lifting power of the wind on the roof's airfoil. And that method is described next.)
shapes of roofs:
Initially, we shall concern ourselves with the simple roof both in terms of new construction and with retrofitting. Later we shall take up the other two simple roof styles - the barn and round. Of course, gambrel, hip, shed and dome roofs are composites of these simple plans, and they will not be considered in this writing.
In order to test the lift of wind on a common pitched roof, the setup to the right was constructed of a fan, an electronic scale on a table, and a roof of variable pitch attached to the pan of the electronic scale. The scale itself substituted for the body of the substructure of a house. The Lift was directly measured as the fan blew at different speeds horizontally across the roof from the side.
RESULTS: It is seen that very steep roofs have little lift, but that nearly flat ones have a great amount of lift. This is unfortunate since less steep roofs are easier to build requiring less material. Thus construction costs and wind stabilty run counter to each other. One final aspect of new construction would be the orientation of the new roof to the winds. Since many homes are built on mountainous terrain such as in the Caribbean, and Japan, the lessons learned here would be to situate the house so that the winds hit from the sides and not from the ends. Of course, this can only be done when one can predict the direction of storm winds such as if the home were to be built in a valley where crosswinds would never be strong.
In addition to increasing the steepness of the roof, it would be worthwhile adding as new construction the retrofitting device described in the next section. This would further diminish the lift on the proposed roof by a factor of 40%.
A second very common style of roof often seen on homes is the Dutch colonial, and of course on farmers' barns. These rustic barns should not be taken lightly as within them is often the farm family's livelihood - expensive equipment, livestocks and hay and grains. Often the barn's contents are more valuable than those in the farmhouse itself. Shown to the right are the lift curves of a typically shaped barn roof when the wind approaches from the side.
While a round or cylindrical roof is rarely seen on homes, it is often seen on skating rinks, supermarkets and older sports arenas and gymnasiums. As this study has moved from simple roofs to barn roofs and now to round ones, we see that we are approaching the curve and smoothness of design of the airplane wing. Shown to the right is with the wind striking the roof from the sides and going over the top. The first thing noticed is that with the plain roof, the lift dramatically increases as the curve flattens. This would be expected as the roof approaches the curvature of an airplane wing's airfoil.
An especially good reference
A person connected to the Natural Hazards Research and Applications (Information Center), of the Institute of Behavioral Science at the University of Colorado wrote an extensive paper on hurricane damage to homes - covering all the damages ranging from wind, to rain, to storm surge, etc.
Learn how to use this huge resource - the United States Patent and Trademark Office ("uspto") - as you investigate how one man made a major breakthrough along the lines talked about above - not by strengthening the roof, but rather by weakening the wind by foiling the airfoil. Literally, Mr. Ponder thought outside of the box. This is an extremely important concept for young scientists - looking at a problem from a totally different perspective. While everyone else was inventing metal clips to hold the rafters and roofing boards more strongly together to make a stronger roof, Mr. Ponder thought it'd be easier and more effective to weaken the wind.
See if you can find this invention by starting at the USPTO's home page and NOT use either the patent number or the inventor's name. Now you will have to do some word searching. You will undoubtedly find many examples of making wind resistant shingles, roofing tiles, flashings, etc. (Those inventors were NOT thinking outside of the box!)
Once you have mastered this USPTO resource, you will have a powerful tool at your disposal to be creative AND a first step towards perhaps making money from your inventions to support you financially as you make more creative discoveries - your own person "institute!"
Test your USPTO skills by finding the patents for the airplane or the wire paper clip or the telephone.
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