Tornadoes, cyclones, and other strong winds damage or destroy many buildings. However, with proper design and construction, the damage to buildings by these forces can be greatly reduced. Over time, a variety of methods have been studied and tested (both formally and incidentally by actual storms) that can help a building survive strong winds and storm surge. Local building departments may mandate their use in high velocity hurricane zones, or areas where buildings are likely to have to withstand a hurricane in their lifetime.
Storm surge considerationsModifica
A common problem for buildings during hurricanes is storm surge. Flooding occurs frequently in coastal areas and waves contain a tremendous amount of energy which can literally batter a building to pieces. Beach front buildings should be able to withstand the ocean rising 20 or more feet, with large waves on top of that. They should preferably be built on high ground where possible, in order to avoid waves knocking the building down.
If waves can reach the building site, the building should be elevated on steel, concrete, or wooden pilings and/or anchored to solid rock. Whether it is intended or not, walls on the first floor are often built with sheetrock, which can completely deteriorate when wet and/or exposed to lateral forces, leaving structural members in place, and allowing water (or high winds) to pass through. This "gutting" occurs frequently in storm surge areas. Designing so as to "sacrifice" the walls of the first floor in this way can save the rest of the building from destruction, although it is not an ideal solution. Building contents left on that level will be lost, and considerable damage to the building could still result in costly repairs such as mold, rot, and termite problems.
Wind loading considerationsModifica
Wind acting on the roof surfaces of a building can cause negative pressures that tend to create a lifting force. This is one of the most common ways a building can be destroyed during a storm. Gravity alone may not be sufficient to prevent the roof from lifting, or "peeling," off the rest of the building. Once this occurs, the building is weakened considerably and the rest of the building will likely fail as well. To minimize this, the upper structure should be securely anchored through the walls to the foundation.
Several methods can be used to securely anchor the roof. Traditionally, roof trusses were simply "toenailed" into the top of the walls. These nails provide little to no actual structural advantage; they're mainly used to hold the trusses in place while the rest of the roof is being built. Gravity and friction then ensure the roof stays put. Various products have been developed that can actually anchor the roof to the walls, which should then be anchored to a solid foundation. Metal straps that nail into the wall and wrap over the trusses are one method. Other methods, including temporary straps made of a special low-elongation material, have successfully been used and have an advantage, in that a building built before 1993 which may not be constructed to withstand wind loading can be quickly and temporarily strapped to the foundation to ensure structural stability. Hurricane harness strapping can easily be applied and removed after the storm to ensure the highest level of protection in extreme high wind storms.
Mobile home tie down to the foundationModifica
In the bulk of instances when interlocking metal pan roof systems installed on mobile homes are exposed to extreme high winds, such as hurricanes and the outer band winds of a tornado, irreparable damage occurs to the overall building structure once the fasteners attaching the metal roof panels to the structural frame begin to tear or rip through the aluminum metal pan base, under the pressure differential (lift) created by the high velocity winds passing over the surface plane of the roof. This event becomes compounded by the high velocity of wind entering the carport or other building add-on, which causes a mode of wind capture: transfer of the kinetic energy of the wind lifts the underside of the roof panels, resulting in complete devastation of the roof system and the roof line/siding section.
To mitigate this pressure differential, pre-installed aluminum tabular channels can permanently be fastened perpendicularly across the top of the interlocking ribs of the metal roof system without disturbing the flow of rainwater at the eaves, mid-span, and ridge locations of the building. Variable lengths of an extremely strong, low elongation, hurricane harness strap are cut to length, placed over the channels and fastened into ratchets which are attached to a variety of anchoring methods on opposite sides of the building. This engineered design provides an uninterrupted continuous load path between anchors. The ratchets apply a uniform counteractive load throughout the channel systems and throughout the roof assembly. The structure literally becomes sandwiched within the strapping and the anchors with addition to providing a positive active dead load to the outer wall systems and column supports, increasing the resistance to the lateral wind force being applied to the main structure during a storm event. In addition, this secondary measure of protection will visually alleviate any unforeseen building deficiencies within the structural confines of the building, or rusted or inadequate earth support ties to the lower frame chassis of the structure.
Earth sheltered construction is generally more resistant to strong winds and tornadoes than standard construction. It is for this reason that cellars, and other earth sheltered components of other buildings, can provide safe refuge during tornadoes.
The physical geometry of a building affects its aerodynamic properties and how well it can withstand a storm. Geodesic dome roofs or buildings made from wood, steel, or concrete have low drag coefficients and can withstand higher wind forces than a square building of the same area.
Even stronger buildings result from monolithic dome construction.
Garages, windows, doors, and other openingsModifica
These are generally the weak points susceptible to breakdown by wind pressure and blowing debris. Once failure occurs, wind pressure builds up inside the building and in seconds, may lift the roof off a building. Hurricane shutters can also provide effective protection.
An inward opening door can be blown into the house by wind causing potential structural failure. Various companies offer new doors that comply with the local building codes. Some companies offer retro-fit devices that can be professionally installed. These kits are often just as expensive as a new door. A good source for products include the Miami-Dade building code website . It shows how the products are to be installed to withstand the most punishing of winds. Some of the companies are local and many products were in use prior to Hurricane Andrew.
It is usually a requirement to install 150 miles per hour tested windows in hurricane prone areas. These windows should have plastic panes, shatter-proof glass or glass with protective membranes (Impact Glass). The panes have to be more firmly attached than normal window panes (possibly even using screws or bolts through the edges of larger panes). See hurricane shutters.
Windows protected by steel or heavy aluminum shutters may be best in some hurricane prone areas.
The choice of building materials can affect the ability of a building to withstand high winds. Although it is not always possible to use different materials, if the area is extremely susceptible to high winds, it is good practice to use the most resistant materials available.
Wood is the most common building material as it is readily available, relatively inexpensive, and has a degree of flexibility which can be beneficial in certain high stress situations. However, termite and dry rot are frequent problems in timber buildings located in areas susceptible to hurricanes, particularly in warm, humid climates. Weakened buildings cannot withstand wind loads as well as intact buildings can. To combat this, certain building codes require the use of pressure-treated wood for all structural elements of the building, which is designed to prevent rot and deterioration.
Also, wood and paper backed sheetrock provide food for black mold which can grow if the inside of the building gets wet during a storm. The mold can then be costly to remove and must be considered as a factor when deciding which building materials to use.
A building constructed with wood can effectively be built to withstand fairly high wind loads. However, flying debris - furniture, trees, parts of other buildings which are common in such a storm - can still damage or destroy a well-designed wood building even if the wind isn't sufficient to do so itself.
Reinforced concrete is a strong, dense material that, if used in a building that is designed properly, can withstand the destructive power of very high winds, pounding waves, and even high-speed debris. Concrete used in home construction must be reinforced with steel (commonly known as "rebar"). While the rebar can rust in wet or humid environments, there are various effective means to retard or prevent rebar corrosion due to moisture.
Examples of Cyclonic Construction Methods Modifica
- Hurricane preparedness
- HurriQuake nail (for resisting hurricanes and earthquakes)
- Structural engineering
- Structural design
- Earth sheltering
- Geodesic dome
- Monolithic dome
- Windstorm inspection