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Wind Design for Buildings
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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Update: June 2012
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