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Design Wind Speed

An important part of assessing the hurricane risks a home faces is to find out the design wind speed for the area where the house is located. Two different scales have been used over the years to define design wind speeds. The latest codes all use 3 second gust wind speeds and all wind speeds referred to in this Manual are gust speeds. Building codes used in Florida before 2002 used Fastest-Mile Wind Speed. Some design standards are written using Fastest Mile Wind Speeds. To allow their use, it is necessary to convert from 3 second gust speeds to Fastest Mile Speeds. To convert from a 3-second gust speed to a fastest mile speed, subtract 20 MPH from the 3 second gust wind speed. (See Table 1609.301 from the 2004 FBC below.)

TABLE 1609.3.1

EQUIVALENT BASIC WIND SPEEDS

V(3S) 85 90 100 105 110 120 125 130 140 145 150 160 170

V(fm) 70 75 80 85 90 100 105 110 120 125 130 140 150

3 Second Gust Wind Speeds for the continental United States.

These design wind speeds are determined based on balancing the risk of high winds in every area of the state. In general terms, the design wind speed for a given location is the wind speed that has approximately a 1% chance of being met or exceeded in any given year. Keep in mind that this does not mean that a home will not experience wind speeds greater than the design speed published in the code, it merely has less of a chance of experiencing that wind speed than areas with higher design wind speeds. The following map shows the design wind speeds and Windborne Debris Region (Effective 1-25-2007) published in the Florida Building Code.

Wind speed and windborne debris map for Florida (March 8, 2007)

Wind speed map for the Gulf Coast region—Texas to Florida

If the design wind speed in the area is:

120 mph and above = high risk for hurricane wind damage and water intrusion.

Between 100 and 120 mph = moderate risk for hurricane wind damage and high risk for water intrusion.

Between 90 and 100 mph = low risk for hurricane wind damage, but moderate risk for water intrusion (high risk of water intrusion in slow moving hurricane).

It is important to note the differences between the three methods of measuring wind speeds as displayed in the table below. The Saffir-Simpson Hurricane Scale uses Sustained Wind Speed measurements where as the Florida (and International) Building Code uses 3-Second Peak Gust Wind Speed measurement. These wind speeds represent different ways of looking at the same wind event. The difference is the averaging time used to describe the winds. At a sustained (1 minute) wind speed of 74 MPH, it takes 49 seconds for one mile of wind to pass a location so the 1-minute and fastest mile speeds are about the same value. However, over that 49- second interval, a 3-second gust of 94 MPH can be expected. When the designers factor in the Exposure Category, Enclosure Category adjustments (discussed later in this manual) and safety factors, we end up with stronger buildings than one might think just looking at the wind speeds in the various columns below. Consequently, if a home or retrofit solution is developed for a 130 MPH 3-second gust design wind speed, it doesn’t mean that failure will occur if the wind speeds exceed 130 MPH. On average it will take a 3-second wind speed of 150 to 180 MPH or more before failure becomes likely.

Saffir-Simpson Hurricane Scale