Tornado-Resistant Concrete Homes

What the 2011 Tornadoes Have Taught Us

The major tornadoes striking the central and southeastern regions of the USA have highlighted the interest in disaster-resistant construction. However, North American builders are unfamiliar with what needs to be done and the Guam experience is not being publicized or promoted on the mainland. So, repeatedly, unenlightened homebuilders are replacing the tornado-devastated houses with the same wood-framed technology that historically has been shown to have failed to survive tornadoes.

Such houses will disintegrate again when hit by a large tornado in the future. In effect, builders keep on making the same mistakes to the same inadequate building codes and, regretfully, innocent occupants will keep on losing everything, up to and including their lives.

It appears to be technically impossible to design and construct any single-family house using traditional North American “wood-frame” construction materials and methods that will withstand a direct hit from an EF4 or EF5 tornado without complete destruction. With careful attention to details, wood-roofed houses with reinforced concrete or masonry walls can be designed to resist earthquake or limited wind forces, but not forces imposed by an EF4 or EF5 tornado. There appears to be no documentation of any such wood-framed house ever having survived such a direct hit by a high intensity tornado wind anywhere in North America. Widely publicized events of 2011 graphically demonstrated this.

To really comprehend the intensity and frequency of tornado winds, one is directed to the referenced well-illustrated Wikipedia page listing F5 and EF5 tornadoes. Viewing the earlier mentioned videos would be time well spent by a prospective home buyer.

Is It Possible to Build Tornado-resistant Houses?

The answer is an emphatic YES, if built as reinforced concrete shells. Other systems may also provide tornado resistance, but a complete reinforced concrete shell is preferred because it provides other substantial benefits. Stay-in-place insulating forms called ICFs can be used during construction to provide the simultaneous added advantage of realistically conserving energy.

The technology for designing and building tornado-resistant houses has been around since 1963 — almost half a century. It was first developed in America, not in continental North America, but instead in the American protectorate territory of the Marianas islands, primarily on the island of Guam. It utilizes the principle of “box-rigid-frame,” a type of reinforced concrete shell design. This approach is described in an article titled “Disaster-Resistant Shell Houses” [PDF] in the American Concrete Institute’s Concrete International magazine, published in May 2008. The article discusses in detail the sterling performance of reinforced concrete houses over the past half century impacted with numerous strong typhoons and powerful earthquakes on the islands in the region.

In 1962, Typhoon Karen roared across the island of Guam with recorded wind gusts up to 207 mph, cleaning the island of most of the conventionally constructed houses. Following an appeal to the U. S. government by the Guam governor, President John F. Kennedy directed the development of typhoon-resistant houses. In this effort, the president enlisted the help of Henry J. Kaiser of Kaiser Hospital fame. At the time he also was CEO of Kaiser Permanente Cement Company as well as a homebuilder in Hawaii. Mr. Kaiser asked Dr. Alfred A. Yee, a world-respected structural engineer and author of a number of articles for various technical magazines, to develop the structural design for the first typhoon-resistant house. It was quickly done and thousands of these houses have been  subsequently built around the island including on the military reservations. And they are still being built.

The original houses used precast concrete walls which were fabricated at the building sites and tilted into position. The roofs utilized cast-in-place reinforced concrete. All of the elements of a house, floors, walls and roofs, were intimately connected with steel reinforcing bars in order to create a box, or shell. Instead of building storm shelters inside a house as is usually done in North America, the entire house becomes a storm shelter, a practice that was not used until the Guam typhoon-resistant reinforced concrete shell house concept was developed.

The concrete shells on Guam have performed without damage for fifty years. The only vulnerable features of such structures are the wall openings. Steel doors and window storm shutters are usually available from local building supply stores such as the Home Depot or Lowes. Special structural attention must be paid to the door mountings. As of the time of this writing [June 2012], foolproof protection of these openings, especially windows, does not yet appear to have completely matured. It is assumed that conventional metal garage doors will not survive a big tornado.

In 1993, Guam, also a frequent target of earthquakes caused by movement of ocean floor plates in the Pacific Ocean (often referred to as the “Ring of Fire”), was visited by a Richter 8.1 earthquake. There was no reported structural damage to a single reinforced concrete shell house.

In 1997, typhoon Paka hit Guam bringing wind gust velocities to over 240 mph, equivalent to an EF5 tornado. Again, there was no record of structural damage to the concrete shell homes. To see amateur videos of these incredible winds, look at Paka on the Internet. Some excellent amateur video of typhoon winds, even though not as severe as Paka, is shown on the Internet for Typhoon Pongsana.

So, reinforced concrete three-dimensional shells have repeatedly proven by actual performance to be reliable for houses exposed to nature’s most serious wind violence.

Storm Shelters

For homes that already exist in tornado alley, “storm shelters” are a very good investment. A large tornado will completely destroy a conventional wood-framed house. The objective is to save the occupants, fully knowing that the home will be decimated by a big tornado. The map on page 1 shows the regions of North America most likely to be visited by a tornado.

For homes already built, there are two options for building a secure safe room. One is to construct it inside the existing house. This can be expensive, messy and quite an inconvenience to those who live there. If you have a basement, it could be built there. The Internet has a great deal of information on safe rooms.

Regardless of where it is located inside an existing house, it should be made with reinforced concrete, both walls and ceiling. It should be designed by a licensed structural engineer and should be sufficiently strong to bear the weight of a collapsing house as well as flying debris, some including motor vehicles. The expense of utilizing a licensed structural engineer is insignificant with respect to the prospect of needing shelter and not having it. There likely may be savings in annual insurance costs if a shelter is installed.

Another option would be to build an underground shelter outside the house but close enough to the door to be quickly accessible. It should not be much more difficult than building a swimming pool, except that it is covered with a reinforced concrete roof. The Internet is full of details as to how to go about this, as well as references to contractors who would be interested. Again, it is important to have the structure designed by a licensed structural engineer. Post World War II bomb shelters would be ideal. There is extensive coverage of this subject on the Internet.

If you haven’t built your house yet, there are at least two options. One is to build a storm shelter inside a new wood-framed house. The other is to build the entire house as a storm shelter. Building the whole house as a shelter is one of the missions of this seminar since the topic is new and not well known in North America.

The principle difference between the two approaches is what is still standing on the lot after the passage of a tornado. With the small shelter, human life will likely be saved and injuries prevented, but the rest of the house will be gone.

With the concept of the “Whole House Shelter,” not only will the family be protected, but the house itself will be intact and livable after the disaster passes.  As a result, the house can continue to be lived in while repairs are being made, especially to the windows and doors.

If the prospective home buyer is willing to invest the extra money for a small storm shelter, instead why not spend the little, if any, extra required to make the entire house safe for those who live there?  It is quite possible that the savings in long term house hazard insurance will more than cover the cost difference and will save energy costs if built with insulating concrete forms (ICFs). If it is all concrete, it will be fireproof and termite-proof also, a potential saving in insurance premiums over the life of the house.

The U. S. Federal Emergency Management Agency (FEMA) provides on the Internet an extensive list of references for the design and construction of Safe Rooms and Storm Shelters. A publication referring to Safe Room details, FEMA 320, is available.   Wind test–http://www.youtube.com/watch?v=Jo8OuQUcpGo