Protection from Extreme Wind
Follow Prevention Tips from the Federal Emergency Management Agency (FEMA) for Tornado and Hurricane. FEMA publications relating to preparing your family and your home for disasters includes Surviving the Storm, Surviving the Storm for Hurricane, Disaster Plan for Families, and Tornado Fact Sheet. All can be downloaded from their web site; publications are listed alphabetically. FEMA has a new animation demonstrating how a house can be damaged during a tornado.
One of the best discussions of current advice on tornado safety can be found on the Storm Prediction Center's Tornado FAQs page compiled by Roger Edwards.
Overpasses Are Not Safe Places In A Tornado
Three people were killed in Oklahoma City during the May 3, 1999 tornado seeking shelter under overpasses. The following is an excerpt from a presentation by D. Miller, C. Doswell, H. Brooks, and E. Rasmussen, G. Stumpf of NOAA. These are reasons why overpasses are unsafe. (The entire presentation can be viewed on-line.)
ALL tornadoes have some amount of debris within their near-surface flow. In the case of a strong or violent tornado, much more debris would be present, traveling at much higher speeds, especially when debris from man-made structures is involved. In strong and violent tornadoes, typically harmless everyday items such as shingles, boards, cans, dishes (or pieces thereof) become dangerous missiles and are responsible for most tornado casualties.
Climbing up under an overpass exposes one to higher wind speeds and more flying debris.
The narrow passage underneath an overpass might cause an increase in the wind speed under the bridge. The extent to which this is true, and the circumstances under which it could happen are not known, but this is at least a possibility. Also, most overpasses don't have girders or support beams for handholds or small ledges into which to crawl.
If an overpass is directly in the path of a tornado, the wind will change direction nearly 180 degrees as the vortex passes. Thus, if one side of the overpass was protected from the highest wind speeds as the tornado approached, that same side of the bridge will be completely exposed to the wind and flying debris as the tornado moves away and vice-versa.
Seeking shelter under a highway overpass is to become a stationary target for flying debris, with a substantial risk of being blown out and carried by the tornado winds. Safety in such a location is merely an illusion.
The American Red Cross also has excellent preparedness tips for thunderstorms and lightning, tornado and hurricane and others. There is also a downloadable publication on "Preparing for Disaster for People with Disabilities and other Special Needs." Other useful publications are: "Before Disaster Strikes: How to make sure you're financially prepared to deal with a natural disaster," and "After Disaster Strikes: How to recover financially from a natural disaster." Wonder what to include in a disaster kit? The Family Disaster Supplies Kit from the Red Cross is designed to be used in event of multiple hazards. See the complete listing of family preparedness publications on the website.
For protection in tornadoes and hurricanes in non-storm surge areas, build an In-residence Shelter: a small, windowless room accessible from all parts of the house which has been reinforced to provide protection in severe wind. If a residence does not have a basement, we recommend this type of shelter over any type of shelter outside the house because of: 1) reduced time to get to the shelter and 2) reduced risk of injury from flying debris.
Lightning kills more people each year than any other storm phenomenon other than flood. Lightning safety tips and dramatic lightning photos are available from the National Weather Service.
Keep alert. Watch your local television stations or listen to a radio. What if severe weather strikes late at night? What if you are watching a video or logged on to the internet (as were some of the victims of the Oklahoma City tornado)? A NOAA weather radio broadcasts weather, watches and warnings 24 hours a day. It has an alarm to alert you in event of a severe weather warning. The new SAME radios are programmable so you will only be alerted in event of a warning in the county or counties you have programmed; this eliminates the alert sounding in the middle of the night for a distant county.
Mobile Home Residents: Remember mobile homes are not safe in extreme winds. In the Central Florida Tornadoes of 1998, 42 people died: 34 in mobile homes, 7 in RVs and 1 in a car.
At the conference on the Great Plains tornado of May 3, 1999 held in May 2000 in Oklahoma City, many engineers and meteorologists drew attention to the fact that there are many things the homeowner can do to make his/her home more wind-resistant. Even in the Oklahoma City tornado, only limited areas experienced the most severe winds. Damage to most houses in the tornado path could have been greatly minimized with better construction. It IS possible to make an existing home more wind-resistant from strong winds, hurricanes and even weaker tornadic winds, at reasonable cost. Look at some of the projects described below and make a plan to accomplish one of these each year or as your budget allows. Also look at the 'How-To' booklets from FEMA. Some excellent step-by-step videos are available from the American Red Cross, the Institute for Business and Home Safety and the City of Urbana, IL (see below).
"Avoiding Hurricane Damage: A Checklist for Homeowners" from the Federal Emergency Management Agency is a good starting place to assess your home's wind resistance. A good place to start for deciding what wind-resistant measures you want for your home and determining approximate additional cost is the Fortified Home website sponsored by the Insurance Institute for Business & Home Safety. There is a cost estimate for each mitigation measure.
Building a new house? This is the best time to make sure your home is wind-resistant. See A Solid Frame from the This Old House website on building beyond the code.
Follow the guidelines in the publications listed below for more specifics on what can be done. When choosing a contractor, mention that you want these features added to your house. They can add extra cost to your house but could save you a lot of money in the long run and, most importantly, could save the life of someone in your family. This Old House also has some guidelines for choosing a contractor.
- A publication is available from the Insurance Institute for Business & Home Safety, titled "Is Your Home Protected from Hurricane Disaster: A Homeowner's Guide to Retrofit."
- Another excellent video, Safe & Sound: Hurricane Resistant Construction, shows features of a wind-resistant house and different types of storm shutters. Despite the title, other than the section on storm shutters, most of the book and and the first half of the video apply to general wind-resistance and are not specific to hurricanes. There is no charge for the video but remaining quantities are limited; only one copy is available per household; contact IBHS to request a copy. IBHS has other publications of interest including Is Your Home Protected from Hail Damage: A Homeowner's Guide to Roofing and Hail.
- A new FEMA Building Performance Assessment Team (BPAT) report on the Midwest Tornadoes of May 3, 1999 has a "Recommendations" section which describes and illustrates alternatives for increasing the wind-resistance of your home (most of these apply to new construction). In particular, look at the drawings on connections. You can order a copy from FEMA Publications 1-800-480-2520 or download the appropriate chapter from the FEMA website. For recommendations for wind-resistance construction for a residential structure see Chapter 8. For recommendations on improving the wind-resistance of manufactured housing, see Chapter 7, section 7.1.2.
- Online tips for retrofitting your home for wind-resistance are available from the Florida Department of Community Affairs (FDCA). Residents of Florida can also learn about incentives for homeowners offered through programs of FDCA.
- More tips are available from the South Carolina Sea Grant Consortium, "Guidelines for New Home Construction in Hurricane Prone Areas".
- FEMA has a number of "How-to" booklets available on protecting your home from wildfires, flood, earthquake and wind. The booklets on wind relate to Garage Doors, Shutters, Brace Gable End Roof Framing, Double Entry Doors, and protecting your house from Trees and Potential Windborne Missiles.
- The American Red Cross has published a number of disaster preparedness materials for kids, families and communities. There is a booklet-Against the Wind: Protecting your home from hurricane wind damage,and video of the same name- Stock No. A5023V, 17min 58sec. Again, most of this applies to general wind-resistant construction, not just hurricane. This is a visual "how-to" video for retrofitting an existing house to make it more wind resistant. Available for $5 (there may be some additional cost for shipping & handling) but must be ordered from your local chapter of the American Red Cross. This video shows the viewer how to brace your roof, your garage door and double entry doors and how to install a variety of storm shutters.
- The City of Urbana, IL has produced a video entitled Inland Wind Resistant Construction: Upgrading the Wood Frame Home which provides a general introduction to the concept of upgrading a home's wind resistance. It is intended to be shown at home shows, to civic groups, etc. To request a copy of the videos, contact Carolyn Baxley, Baxley Media Group, 510 W. Main, Urbana, IL 61801, email@example.com, 888-808-8021. The videos will be distributed free of charge while supplies last.
- Clemson University has performed extensive research on retrofitting existing homes for wind-resistance. Information is available on their web site. This has detailed information for the home owner or home buyer. Read these pages before buying, building or remodeling a home.
- See the Red Pages for hazards information on tornado, storms, hurricane, flood and fire.
- The State Farm virtual house tour shows a house built with a number of safety features including features for wind-resistance.
- Wind Safe: 20 Things to Consider Before You Build, Buy or Remodel a Home," is available from WDM Associates, 1621 Northcrest Dr., Plano, TX, 75025, phone: (972)509-2151; Cost: $7.95 plus $1.50 for postage.
- Another new publication is Windstorm Mitigation Manual for Light Frame Construction, 1997. Available on-line or from Illinois Emergency Management Agency, Mitigation Section, 110 East Adams, Springfield, IL 62701; (firstname.lastname@example.org).
Please note the following sites are currently being revised: Wind-resistant construction products are often difficult to find. This site provides a listing of impact-resistant products tested and approved by the Dade County Building Code Compliance Office including impact-resistant skylights, shutters, windows, exterior doors, and overhead doors and approved wind-resistant roofing products (composition and wood shingles are about 1/3 of the way down the list). These lists are geared at contractors but, if you are a homeowner, the list will provide the name of the company and product ID #; you can contact those companies for further information. For information on hail resistance of various roofing products, see the IBHS (Institute for Building and Home Safety) page. The Disclaimer notice will explain the meanings of the 1-4 rating each product is given.
A free independent study course is available from FEMA: IS 394-Mitigation for Homeowners. The course provides non-technical mitigation techniques for a home or small business-both predisaster (preventive) and postdisaster (corrective). Of particular interest is Chapter 3: Reducing Risks from Wind. This is in pdf format.
For information on recommended fastening for roofs see "Roof Sheathing Fastening Schedules for Wind Uplift" from APA-The Engineered Wood Association. An excellent general description of how to improve the uplift resistance of your roof is given in "Retrofitting a Roof for High Wind Uplift". Tips for the do-it-yourselfer are included. You will need Adobe Acrobat Reader to view these documents. For more tips go to the APA website and click on Publications.
The South Carolina Sea Grant Consortium has guidelines on choosing a roofing contractor. When a strong wind hits a house, the first damage is usually loss of roof shingles. SCSGC has tips for "Improving the Wind Resistance of Roof Systems: Asphalt Shingle Roofs". A longer version on this topic also from SCSGC is entitled, "Holding on to Your Roof: A Guide to Retrofitting Your Roof Sheathing Using Adhesives". It can be downloaded or ordered free of charge (scroll down to the title).
Also see the IBHS publication, "Wind Resistance of Roofing Coverings".
Hurricane shutters protect against wind and windborne debris. For more information, see the NOAA's Hurricane Research Division Hurricane Shutter page. Shutters add to the value of your home even if you never have to use them.
South Carolina Sea Grant Consortium has listed tips for consumers on Selecting Storm Shutters.
Remember mobile homes are not safe if a tornado or hurricane threatens. As mentioned above the FEMA BPAT report for the Midwest Tornadoes of 1999 includes a section on mobile homes with an easy-to-understand graphic showing correct tie-downs. Click on Chapter 7, section 7.1.2. The GoPBI site has some good suggestions for mobile homes including nice illustrations of tie downs.
IBHS (Institute for Business and Home Safety) has produced a Disaster Planning Tookit for the Small Business Owner (in pdf format). This can be ordered from IBHS or downloaded from their web site.
FEMA has a how-to series available on protecting businesses from disasters.
The American Red Cross also has guidance for business and industry.
Get a weather radio for your business. The SAME radios are programmable to avoid warnings from distant counties.
The Disaster Resource Guide 2000 provides a directory of resources for business continuity. It can be viewed on-line or request a free copy from Disaster Resource Guide, P.O. Box 15243, Santa Ana, CA, 92735, fax (714) 558-8901.
The South Carolina Sea Grant Consortium has tips on "Preparing your Business for a Hurricane".
The Coastal Services Center of NOAA has developed a method whereby local and state governments can determine and set priorities for coping with the coastal hazards to which their localities are prone. This has been issued as a tutorial on CD ROM and is available free of charge. Ask for NOAA/CSC/99044-CD Community Vulnerability Assessment Tool. It is also available on-line.
NOAA has developed a program known as StormReady to help prepare communities for tornadoes, hurricanes and other major storms.
NOAA, FEMA and the NWS cooperated to make the course Community Hurricane Preparedness available on the internet.
The Wind Science and Engineering Research Center worked with FEMA to develop guidelines for architects and engineers in designing school and other community shelters See FEMA 361: Design and Construction Guidance for Community Shelters
Have a NOAA weather radio. The SAME radios are programmable to avoid warnings from distant counties.
Do houses explode in a tornado?
Buildings appear to "explode" when hit by a tornado. However, the cause of the outward and upward movement of walls and roof is wind-related, rather than induced by atmospheric pressure change. When an opening appears in a windward* wall, such as when a piece of debris breaks a window, winds enter the building and exert an outward pressure on the walls causing the walls to be pushed outwards. This gives the impression of an explosion.
Why is it no longer recommended to open a window when a tornado is imminent?
While it is true that pressure on the roof can be relieved by opening a window on the leeward* side of the house, the direction of an attacking wind will be unknown. Even when the tornado approach direction is known the winds of the tornado are spinning; so depending on which part of the tornado passes over the house, the winds on the wall could be from any direction. Opening a window on the "wrong" side could be detrimental to the building and waste valuable time needed to reach a place of safety. Additionally, most residences and commercial buildings have enough natural venting to relieve any change in atmospheric pressure which may occur. (One square foot of opening per 1000 cubic feet of volume will "vent" the atmospheric pressure change effects of a severe tornado.) Finally, it is noted that severe winds act on a building before the largest portion of the atmospheric pressure change can become effective; hence, window failures or failures of other components are likely to "open" the structure to a greater degree than an open window would. Therefore, when a tornado is approaching, the best action is to take shelter immediately.
Is it true that tornadoes sometimes skip over a house?
Tornadoes are very complex systems and we do not fully understand the nature of their winds. It is for this reason that researchers still "chase" tornadoes with the goal of better understanding their characteristics. It does appear that there are occasionally limited areas of stronger winds. However, it cannot be assumed that because a building with minimal damage is observed next to a building with significant damage that the first building was "skipped over". The performance of buildings in windstorms is directly related to their design and construction. Engineered (professionally-designed) buildings follow building codes. Minimal damage in an engineered building more likely indicates that the building was well-designed and constructed. Houses are not designed by engineers but can also be constructed with wind-resistant features, particularly the connections to the foundation and between elements of the house (e.g., walls and roof). Some of these features are described in the booklets listed above under "Protecting your House".
*windward: towards the direction from which the wind is blowing; leeward: towards the direction towards which the wind is blowing
This information was extracted from a National Severe Storms Lab Technical Memorandum, TM ERL NSSL-82, "The Tornado - An Engineering Oriented Perspective," by Dr. James R. McDonald.