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Name: Sara
Status: Student
Grade: 6-8
Location: NJ
Country: United States
Date: April 2006


Question:
What shape and material of a building can better with-stand high winds?



Replies:
Simply put, structural engineering is transferring the loads, or forces on the structure into the ground. For wind, primarily the force is horizontal, although some roof shapes result in uplift.

As you know a building is usually composed of several elements. The outer wall transfers the wind force to columns, or inner wall. So, you see it depends on the materials. Wood is weaker in bending than comparable steel thickness. No easy answers, but consider an all concrete building, or an all steel building. If everything was the same, dimensions, height etc. the answer would be the relative strength of the materials.

Now, consider the shape. A tube can be stronger than a box, but a triangle can be stronger than a tube. Again, no simple answer. The new proposed high rise in Chicago is a twisted triangle, or tube., but to achieve the height, it cannot be too wide. Is it economical?

Unfortunately, the related aspects are too numerous for a simple answer. Look around, towers are strong, usually some form of braced steel triangles, but they do not have any skin, or outer surface, they let the wind pass trough, but they are not buildings. The design has to fit the function.

Hope this helps.
James Przewoznik


Hi, Sara. I grew up in Oklahoma City, where they have a lot of tornadoes in the spring, and I moved to Florida in July 2004, just in time to experience several hurricanes, so I feel uniquely qualified to answer this question. There are a couple of shape variations that lend wind resistance to a building, and the material of choice tends to be concrete.

Tornadoes are small and brief compared to hurricanes, but can have much higher winds (200-300 MPH). Hurricanes, though, can last an entire day (like Frances in 2004), so even at lower wind speeds, they can just "wear down" a structure from continual load and vibration.

In Oklahoma, with it is relatively warm climate, most houses still use traditionally framed 2 X 4 wood stud walls and roofs with gable ends. Brick is a popular external covering material. The brick gives effective protection from flying debris, but there is still a lot of roof damage in storms. In really bad storms (like the F5 that hit Moore several years ago), homes were completely demolished except for the concrete safe rooms that several had. There is not much in the way of traditional wood stud framing that could withstand that kind of storm.

In Florida, you notice hip style roofs and a lot of stucco used o the outside of houses. The external walls are usually cinder block ad concrete. The roof rafters are tied down with steel straps. The stucco covering is vulnerable to impact and moisture damage, but the cinder block construction resists flying debris as well as brick, and is more resistant to sustained high winds. Around windows and doors, the blocks are filled with concrete and rebar. Hip style roofs are used because that style is more resistant to high winds. The hip roof shape has no large flat areas. A gable end roof does, and that exposes a lot of surface area when it is hit by a perpendicular wind gust, resulting in high loading.

Florida building codes are updated after most major storms. They continually try to incorporate any 'lessons learned' after a storm. This has resulted in structures that are better able to withstand that sort of weather, resulting in less disruption to people's lives and a faster recovery after storms. Oklahoma has never done this to my knowledge. They would do well to learn the lesson. After the Moore F5, there was a boom in installing concrete safe rooms, but the general construction techniques still did not change.

There are building techniques that are very resistant to wind (and can be very efficient as well), but are not widely used. They are still considered 'alternative' construction, although they have been around many years. Here are a couple of these techniques:

Insulated concrete forms. These are foam forms that interlock to build your external walls (like big building blocks). Rebar is placed in gaps between the foam, then concrete is poured to fill the gaps. The foam is left in place after the pour, and serves as insulation. Any traditional exterior covering (stucco, siding, brick, etc.) may be used. The result is a solid external wall made of steel reinforced concrete with insulation built onto it already. Bolts are embedded into the tops of the walls to attach the roof structure. The house can look like any other house or anything else the owner wants when done. The solid construction is very resistant to high winds.

Monolithic concrete domes: A membrane is inflated to the dome shape, and lightweight concrete is sprayed into position, along with reinforcing wire mesh and/or rebar. This is strong, solid construction with no flat walls or corners that resists high wind and impact very well, but it obviously results in a dome shape, which some people may object to. Domes can also be constructed from spray foam insulation in the same manner, then covered using a traditional siding or stucco, or they can be assembled from geodesic panels that use traditional wood framing. The shape gives these dome homes strength and wind resistance, but to withstand impact from flying debris, concrete is the material of choice.

Earth-Sheltered Homes: These homes are built at least partially below grade or have earth berms around them, usually to help with energy efficiency, since the ground tends to remain at a steady temperature all year round. The gentle slope of the berms reduces the flat areas and corners, which helps reduce wind loading, and the earth sheltering helps to spread out loads. These homes also tend to use much more concrete in their construction, which gives them additional strength.

Underground homes: Storm shelters have for years been constructed of steel or concrete and buried underground. They are completely out of the wind except for the doors. This leads to other problems, though, especially moisture.

Hope this helps you out. An internet search on "insulated concrete form" or "monolithic concrete dome" or "earth sheltered home" will yield a lot more information on these alternative building methods.

David Brandt, P.E.



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