Introduction

Big Big Winds

Recent big tornadoes have dramatically demonstrated to the conventional home building industry in North America that the technology for building houses according to contemporary codes and construction practices does not adequately protect such houses against the forces generated by tornadoes.

The lessons learned — particularly at Tuscaloosa, Alabama; Joplin, Missouri; Moore, Oklahoma; and Illinois — are clearly delineated by the fact that not one single wood-framed house has survived in the direct path of EF4 and EF5 tornadoes. They have all indiscriminately been turned into kindling.

To get a feeling for the immense power of a tornado, the reader should watch two videos. One, a movie called Twister (1996), while oriented to a fictitious love story, is exciting and does a good job of portraying the emotional consequences of big tornadoes. The other is a contemporary documentary produced by NOVA and shown on PBS television called Deadliest Tornadoes (2012). Anyone contemplating building a new house or rebuilding in tornado-prone areas would be well served to view both of these videos. They are available for purchase on the Internet.

Tornadoes are far and away the biggest disasters of all ever to challenge home owners in North America. They can be four times more destructive than hurricanes or earthquakes. And yet the uncomplicated engineering and architecture required to build houses that are safe is essentially ignored.

It is not at all technically difficult to design and construct homes that will stand up to these monster winds. It has been effectively done for half a century, but not in North America. And they can be built for approximately the same cost as wood-framed houses.

Wikipedia provides an excellent discussion, excerpted here, of the various F and EF categories of tornadoes illustrated with photos of probable damage for each tornado classification. It is accessible at Wikipedia's Fujita scale page. Photographs used in the EF scale table below show damage done by EF4 and EF5 tornadoes.

Fujita Scale

The Fujita scale (F-Scale), or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation. The official Fujita scale category is determined by meteorologists and engineers after a ground or aerial damage survey, or both; and depending on the circumstances, ground-swirl patterns (cycloidal marks), radar tracking, eyewitness testimonies, media reports and damage imagery, as well as photogrammetry or videogrammetry if motion picture recording is available. The F-Scale was replaced with the Enhanced Fujita Scale (EF-Scale) in the United States in February 2007.

The scale was introduced in 1971 by Tetsuya Fujita of the University of Chicago, in collaboration with Allen Pearson, head of the National Severe Storms Forecast Center (currently the Storm Prediction Center). The scale was updated in 1973, taking into account path length and width. In the United States, starting in 1973, tornadoes were rated soon after occurrence. The Fujita scale was applied retroactively to tornadoes reported between 1950 and 1972 in the National Oceanic and Atmospheric Administration (NOAA) National Tornado Database. Fujita rated tornadoes from 1916-1992 and Tom Grazulis of The Tornado Project retroactively rated all known significant tornadoes (F2-F5 or causing a fatality) in the U.S. back to 1880.

In 2007, the Fujita scale was updated, and the Enhanced Fujita Scale was introduced in the United States. The new scale more accurately matches wind speeds to the severity of damage caused by the tornado.

Though each damage level is associated with a wind speed, the Fujita scale is effectively a damage scale, and the wind speeds associated with the damage listed aren't rigorously verified. The Enhanced Fujita Scale was formulated due to research which suggested that the wind speeds required to inflict damage by intense tornadoes on the Fujita scale are greatly overestimated. A process of expert elicitation with top engineers and meteorologists resulted in the EF scale wind speeds, however, these are biased to United States construction practices. The EF scale also improved damage parameter descriptions.

At the time Fujita derived the scale, little information was available on damage caused by wind, so the original scale presented little more than educated guesses at wind speed ranges for specific tiers of damage. Fujita intended that only F0-F5 be used in practice, as this covered all possible levels of damage to frame homes as well as the expected estimated bounds of wind speeds. He did, however, add a description for F6, which he phrased as "inconceivable tornado", to allow for wind speeds exceeding F5 and for possible future advancements in damage analysis which might show it.

Furthermore, the original wind speed numbers have since been found to be higher than the actual wind speeds required to incur the damage described at each category. The error manifests itself to an increasing degree as the category increases, especially in the range of F3 through F5. NOAA notes that "… precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar-looking damage from place to place—even from building to building. Without a thorough engineering analysis of tornado damage in any event, the actual wind speeds needed to cause that damage are unknown." Since then, the Enhanced Fujita Scale has been created using better wind estimates by engineers and meteorologists.

The five categories are listed here, in order of increasing intensity.

1. Where the relative frequency of tornadoes is mentioned, it is the relative frequency in the United States, for which the best data exists. Strong tornadoes (F2 or greater) occur less often elsewhere in the world. The ratio seems to be similar except for some areas where it's less such as the United Kingdom, but because of few overall number of tornadoes there are few strong tornadoes. Areas of eastern India, and possibly a few other areas do have frequent severe tornadoes; however, data are scarce and statistics in these countries have not been studied thoroughly.

2. The rating of any given tornado is of the most severe damage to any well-built frame home or comparable level of damage from engineering analysis of other damage.

^*Fujita's initial wind speed estimates have since been found to be highly inaccurate. See Enhanced Fujita Scale. For detailed information, download the detailed Enhanced F-Scale document [PDF, 95 pp.].

Since the Fujita scale is based on the severity of damage resulting from high winds, an F6 tornado is a purely theoretical construct. Property damage cannot exceed total destruction, which constitutes an F5. (A tornado with wind speeds greater than 319 miles per hour is theoretically possible, and the 1999 Oklahoma City tornado may have been such an event. However, no such wind speed has ever been recorded and that measurement was not near ground level.)

In the Belly of the Beast

Would you ever want to witness the awesome power of big winds from a safe place inside the middle of the storm?

There are number of amateur videos taken inside incredible windstorms. Several of the most dramatic videos were recorded from the safety of DISASTER-PROOF concrete houses during major typhoons as they passed over the island of Guam. Here are links to a few of these videos from some of the more famous typhoons (equivalent to Atlantic hurricanes) that have impacted the Marianas islands in the Pacific Ocean:

• Karen — (1962) Where concrete disaster-resistant shell technology was born: Wind gusts to 207 mph
• Pongsana — Occupant comment "We are in a concrete house - reinforced concrete"
• Paka — Winds >240mph (Higher wind velocities of the wind gusts were not measured because weather installations blew away after 240 mph)
• Omar — Wood frame roofs being removed

To provide an idea about how often Guam is hit with major typhoons, the table below lists eight major wind events that have occurred there since 1963 and shows a comparison of some of these events to tornado categories. The term "typhoon" is used for the similar type of storm in the Pacific basin as for Atlantic "hurricanes" and Indian Ocean "cyclones."

It should be noted that most of the above Guam typhoons experienced winds in excess of Saffir-Simpson Hurricane Category 5, in a number of cases substantially exceeding the most severe Atlantic hurricanes experienced in the United States. The Saffir-Simpson hurricane scale, used in the United States, is shown below for comparison.

Saffir-Simpson hurricane wind scale
Category	Wind speeds
Five	≥70 m/s, ≥137 knots ≥157mph, ≥252km/h
Four	58-70m/s, 113-136knots 130-156mph, 209-251km/h
Three	50-58m/s, 96-112knots 111-129mph, 178-208km/h
Two	43-49m/s, 83-95knots 96-110mph, 154-177km/h
One	33-42m/s, 64-82knots 74-95mph, 119-153km/h
Additional classifications
Tropical storm	18-32m/s, 35-63knots 39-73mph, 63-118km/h
Tropical depression	<17m/s, <34knots <38mph, <62km/h

Most of the Pacific typhooons are well documented on the Internet with many detailed references for those persons interested in learning more.

Damage

Wind events cited in the Big Big Winds section above were classified as EF4 and EF5 tornadoes. To provide the reader with reality in this regard, a highly recommended quick visit to Wikipedia's Enhanced Fujita Scale page graphically displays tornado classifications both with well-written understandable text and actual damage photo examples. Classifications address damage inflicted.

Both EF4 and EF5 tornadoes leave wood-framed houses as piles of rubble. A tornado as small as an EF3 usually renders a conventional wood-framed house unsalvageable as shown in the Wikipedia damage photo for that category. Even an EF2 will do costly damage, perhaps making retrofitting of the house impractical.

If any reader is able to document the intact survival of a wood-framed house after a direct hit by EF3-5 tornadoes, the authors would deeply appreciate receiving any reference and documentation of such an event. No documentation has been reported to the authors since this website has been active.

Studies have revealed that big tornadoes, though not as widespread in their footprint as earthquakes and hurricanes, can be almost four to five times more destructive to the structures they directly impact than a Maximum Credible Earthquake or a Category 5 hurricane. So the corollary might be that if one designs a house to be tornado-resistant, one covers all of the other natural disasters at the same time as well.

Devastation

The tragedy in North America is that builders and building code bodies ignore relatively simple construction methods available that are definitely capable of resisting EF-5 tornadoes. Most homebuilders tend to rebuild the destroyed houses with the same vulnerable technology with which they were built originally. The International Residential Code for One- and Two-Family Dwellings does not provide for tornado-resistant home design and construction.

After such disasters have occurred, a number of expert sources have publicly offered subjective explanations for failures and have proposed "disaster-resistant" solutions that have not been documented as being capable of resisting severe tornado winds. Most of them are based on trying to upgrade wood-framed technology that has consistently failed in major tornadoes and hurricanes over the years.

Promoters have proclaimed "tornado proof" solutions without showing documentation that a single house constructed according to their concepts has ever survived a direct hit by an EF3 (136-165 mph winds) to an EF5 (Greater than 200 mph winds) tornado. Upon inquiry to those making such claims, technical solutions, verified by conventional structural analysis, usually have not been forthcoming. In addition, promoters of houses with reinforced concrete walls and wood-framed roofs are also tending to imply tornado-resistant assertions. The conventional wood-framed roofs are consistently destroyed even when the concrete walls are still intact.

Some of these experts continue to pursue ways to build a "survivable" wood-framed house even though it has been amply demonstrated to be outside the capability of the materials and procedures used for assembly. It is simply not possible to cost effectively accomplish within the parameters of the home building assembly techniques being used in North America. Most of the market-oriented claims that new houses being built are tornado-resistant, earthquake resistant, and hurricane-resistant are mainly based upon undocumented opinion and not on actual field experience, rigorous structural analysis, or research laboratory verifications.

According to a representative of the Insurance Institute for Building and Home Safety (IBHS) Research Center, they have been testing house performance under high velocity wind conditions at their facility in South Carolina. To date, they have imposed wind velocities of up to 110 mph to full scale wood framed houses in their wind tunnel tests. These wind velocities fall into the range of a Category 3 hurricane (111-129 mph) and a Category EF1 (86-110 mph tornado). As of this writing, they have not been tested for maximum strength hurricanes nor tornadoes. A category 5 hurricane is currently rated as wind velocities "157 mph or higher" and an EF5 tornado is rated as "greater than 200 mph."

Fire Danger

Tornadoes usually occur in conjunction with large thunderstorms. As a result, debris left in the wake of the storm is likely quite rain-soaked and not conducive to generating a firestorm.

However, during the passage of the tornado, natural gas lines and electric power lines servicing houses are ruptured, thus creating an important opportunity for fire. The damaged service lines are usually shut down quickly by emergency crews after the passage of the storm, reducing the fire danger. Road crews quickly clear the roads to provide access for emergency vehicles.

Water Damage

Water damage to the interior of a conventional home after a tornado has taken the roof off and blown it away can be extensive. When a moderate tornado removes only the roof, but leaves the walls standing, rain damage to the interior of the house can be most severe and very costly to salvage and restore. There is also a concern that people living in the house will be sucked out into the vortex when the roof is gone.

Contents

• In a Nutshell
• Introduction
  • Big Big Winds
  • In the Belly of the Beast
  • Damage
  • Devastation
  • Fire Danger
  • Water Damage
• What Recent Tornadoes Have Taught Us
  • Is It Possible to Build Tornado-resistant Houses?
  • Storm Shelters
• Residential Building Codes for Wind
  • International Residential Code for One- and Two-family Dwellings (IRC)
• Disaster-resistant Shells
  • Three-dimensional Concrete Shells
  • Disaster-resistant Concrete Shell Variations
  • Sample Structural Analysis for a Concrete Shell
  • Architectural Appearance
• Walls
  • Tornado-resistant Houses With Insulated Concrete Walls
• Roofs
  • Wood Framed Roofs
  • Insulated Reinforced Concrete Roof Decks
  • Ridge And Valley Beams
  • Constructing the Concrete Roof Deck
  • Reinforced Concrete Roof
  • What Can Be Done If the Roof Leaks?
  • Roof Appearance
• Putting It All Together
  • Roofing
• Energy Conservation Considerations
  • Energy Code/R-Value Map — Canada
  • Energy Code/R-Value Map — United States
  • The Role of Foamboard Insulation
• Indoor Environment Systems
  • Indoor Air Quality
  • Ductless Heat Pumps
• Electrical and Mechanical
• Design Considerations
• Custom Versus Production Houses
• Conclusions
• World of Concrete Seminar
• About the Authors
  • Kenneth A. Luttrell
  • Joseph C. E. Warnes
• References
• Disclaimers
• Contact Us

— formerly Tornado-Proof Houses
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