Wednesday, May 14, 2008

How Do You Best Design for a Tornado?

Photo Art by Chuck Carlton

With the 1 year anniversary of Greensburg, Kansas being leveled by a EF5 tornado and the recent string of deaths and destruction due to tornadoes, I though I would ask the question "How do you best design for a tornado"? Better yet, how do you rebuild a community in the best sustainable way to survive a tornado and not have to rebuild, thus saving materials and energy.

In the Midwest plains, one of the main reasons that tornadoes form and continue a path of destruction, is the lack of natural features dispersing the wind forces. One idea that was raised in a recent Discovery channel program "Blown Away : Greensburg, Kansas" was to plant a forest ring of trees around the town. This was shown in models to mitigate the overall power of the tornado and as long as the correct trees and spacing are used this seems to be a good plan, however, how long until the groves of trees are tornado-protecting size?

To rebuild the homes that are more tornado resistant, what design techniques and materials should be employed? Let's start from the bottom; the foundation is an important aspect of many homes and unfortunately the trailer homes and others that rest on blocks or a concrete slab are the most at risk of destruction. In the Midwest, many new homes don't have basements anymore and storm cellars are becoming rare. So why not build a strong foundation that will act as an secure anchoring system for the house and give it's inhabitant's safe shelter? The reasons I found were flooding due to high water table, stresses and stains due to freezing in the winter, and a solid bedrock under site surface. Fair enough, if these factors are the case, then one must think about building a strong lower level that attaches firmly to the ground slab.

The latest material I am beginning to see in pre-fab construction and in the rebuilding efforts of the city of Greensburg is ECO-Block. ECO-Block is simpler and easier to install than wood-frame methods — and, even other ICF systems. It's faster, taking less time with fewer people. And it’s more sustainable, because ECO-Block buildings are vastly more energy-efficient. Stronger. Quieter. Resistant to fire, moisture, mold and the elements. Debris driven by high winds presents the greatest hazard to homeowners and their property during tornadoes and hurricanes. Recent laboratory testing at Texas Tech University (see video below) compared the impact resistance of residential concrete wall construction to conventionally framed walls. Only concrete wall systems, such as ICFs and concrete masonry, successfully demonstrated the strength and mass to resist the impact of wind-driven debris. The wood-frame walls failed to stop the penetration of airborne hazards. These insulated concrete forms can be filled with recycled fly-ash concrete to make the wall even "greener". Another advantage to ICFs is that they can be cut into radius rounded walls for greater aerodynamics if wanted.

After the walls are up, impact-resistant window systems, when properly installed, have a much better chance of surviving a major windstorm. Alternatively, impact-resistant shutters that close over window openings prevent flying debris from breaking windowpanes.
Strengthen door frames, hinges, latches, and dead-bolt locks should be strong enough to resist high wind pressures. Double doors such as French doors need special attention because of the stress on the center latch. Door frames must be securely anchored to house frames. Another idea is to reduce the design of adornments on the exterior that could become damaging projectiles.

Hip roofs without gables and with minimal eaves have proven to be the best roof design to combat high winds in hurricane and tornadic areas especially if the slope of the roof from horizontal is 35 degrees or greater (to reduce/eliminate the airfoil effect of extreme high winds that blow over the roof). Also, steep angled roofs seem to handle winds better than a standard gabled roof. Most importantly, the roof must be securely tied into the wall structure with metal roof tie fastener systems and the soffit and eave area should have reinforced connections. A home has to be well-connected in order to distribute the forces caused by wind pressure, which reduces the chance it will come apart or collapse.

Last year a post was started on the Alternative Building Architecture Usenet Group as a result of the Greensburg disaster. They suggest an aerodynamic cylindrical structure, maybe the Monolithic Dome?
If you can't build your entire house to be tornado resistant you can always just build a tornado safe room or shelter.

Some other good links:
The Institute for Business & Home Safety - Disaster Safety - Tornado

I'd love some feedback on this topic if you have further information to add, please comment below.

1 comment:

Richard Boles said...

I think the first thing that any house owner/builder needs to consider are the walls 'coz they keep the whole structure stand. The roofing must be a little lower, but it still depends on the over-all architectural design. Better ask the company you chose to help you do the project. Monolithic Dome is the most ideal tornado proof house. I saw that when I was in Miami years ago.