Residential Building Codes for Wind

The most severe and least understood destructive forces imposed on houses are generated by tornadoes. Most of the funded research activity has been directed to hurricane amelioration in North America, more specifically oriented to experience along the east coast of the United States and to the Caribbean regions.

A psychological attitude seems to have developed that we should work to improve the design and construction of houses for small and intermediate wind forces and assume that little can be done about saving houses that will be ravaged by the more severe tornadoes and hurricanes. For communities already devastated by a major disastrous event, the attitude seems to be that it can not happen here again. "Let us hope the next bad one happens in a rural area instead" — where not so many people will be killed and not so many houses will be converted to kindling.

Following a particularly bad tornado in one state in 2012, the governor of that state was actually quoted as saying that the state was not interested in improving the building codes. Apparently, that has not been the attitude on the island of Guam where tornado and hurricane (typhoon) proof houses have been constructed for about fifty years.

The process of developing building codes in the United States is very methodical, (slow moving), and basically is controlled by Building Official members of the International Code Council (ICC) and one Professional Engineers association, the American Society of Civil Engineers, (ASCE). The ICC is the product of confederating three earlier existing regional code-writing organizations: International Conference of Building Officials (ICBO), Building Officials and Code Administrators International, Inc. (BOCA), and Southern Building Code Congress International (SBCCI).

Fifty states and the District of Columbia have adopted the I-Codes at the state or jurisdictional level. Federal agencies including the Architect of the Capitol, General Services Administration, National Park Service, Department of State, U.S. Forest Service, and the Veterans Administration also enforce the I-Codes. The Department of Defense references the International Building Code for constructing military facilities, including those that house U.S. troops around the world and at home. Amtrak uses the International Green Construction Code for new and extensively renovated sites and structures. Puerto Rico and the U.S. Virgin Islands enforce one or more of the I-Codes.

Since the early 1900s, the system of building regulations in the United States was based on model building codes developed by three regional model code groups. The codes developed by the Building Officials Code Administrators International (BOCA) were used on the East Coast and throughout the Midwest of the United States, while the codes from the Southern Building Code Congress International (SBCCI) were used in the Southeast, and the codes published by the International Conference of Building Officials (ICBO) covered the West Coast and across to most of the Midwest. Although regional code development has been effective and responsive to the regulatory needs of the local jurisdictions, by the early 1990s it became obvious that the country needed a single coordinated set of national model building codes. The nation's three model code groups decided to combine their efforts, and in 1994 formed the International Code Council (ICC) to develop codes that would have no regional limitations.

ICC Building codes

For the typical person involved in the daily business of building houses, this seems to be somewhat of a code smorgasbord. It would seem logical that the International Residential Code for One- and Two-Family Dwelling (IRC) would apply to all matters related to designing and building houses. However, while it covers most issues regarding home building, it is completely silent about provisions for tornado-resistant houses. It includes comprehensive provisions for hurricanes, termites, floods, seismic, decay, and storm shelters. But not one word about tornado protection. So, in effect, there is no tornado code protection for dwellings and their occupants in the IRC. It is possible that code writers consider hurricane winds and tornadoes as one topic. They are completely different, with maximum tornadoes being four times as destructive as a maximum hurricane.

To help bring some clarity to a confusing issue, the following nomograph and associated notes have been assembled to provide an overview of the perplexing issues to be considered regarding high wind protection.

Big wind category comparison - chart

Big Wind Category Comparison

NOTES — Big Wind Category Comparison

  1. Extended Fujita Wind Scale in current use since 2007. The Enhanced Fujita Scale (EF scale) rates the strength of tornadoes in the United States and Canada based on the damage they cause.

  2. Original Fujita Wind Scale in use from 1971 until February 2007. The F-Scale was replaced with the Enhanced Fujita Scale (EF-Scale) in the United States in February 2007.

  3. Power-point program [PDF] by Structural Engineer Kenneth A. Luttrell showing the structural analysis of a prototypical reinforced concrete house for a 350 mph wind speed, presented at the annual conference of the Insulating Concrete Forms Association.

  4. The Saffir-Simpson hurricane wind scale (SSHWS), (or the Saffir-Simpson hurricane scale (SSHS)) classifies hurricanes — Western Hemispheretropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds.

  5. Saffir-Simpson category 5 hurricane wind velocity at landfall

  6. International Residential Code for One- and Two-Family Dwellings
    This code provides criteria, in substantial detail, for earthquakes, insects, and winds, but only winds related to hurricanes. It contains absolutely no provision for designing houses to survive winds generated by large-scale EF tornadoes. (See tornado damage photos below from Wikipedia)

Enhanced Fujita scale: Damage indicators and degrees of damage for EF4 and EF5 tornadoes.

EF4 damage example — Single family residence with interior walls and floor above basement missing

EF4 Damage — Single Family Residence
Interior walls and floor above basement missing (Wikipedia)

EF5 damage example — House with not much more than the foundation left

EF5 Damage Example — House
Not much more than the foundation left

International Residential Code for One- and Two-family Dwellings (IRC)

http://publicecodes.cyberregs.com/icod/irc/2012/

Shown below is a wind design map from the IRC. It is noted that it shows only the hurricane zones. It is blank for tornado winds in the Plains states and southeast, the North American regions most affected by tornadoes.

R301.2(4)B REGIONS WHERE DESIGN FOR HIGH WINDS IS REQUIRED (by the IRC)

Map of United States showing where design for high winds is required

Noaa Tornado Activity Map

[Storm Prediction Center Statistics from NOAA (National Oceanic and Atmospheric Administration).] The map below from the National Oceanic and Atmospheric Administration (NOAA) shows the regions of the USA that are targets of multiple tornadoes on an average of around 1000 per season.

Note again that there is no reference in the IRC Wind Design map to tornado wind design provisions in the regions shown below.

Below is the NOAA Tornado activity map as shown previously. The IRC map is blank for these regions.

Tornado activity in the United States

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