Risk Category and the Florida Building Code

Our product approvals are largely focused on Risk Category II as per the Florida Building Code. You can read the details of each risk category here:
https://www.floridabuilding.org/fbc/thecode/2017_Code_Development/Wind_Maps/table_1604_5.pdf

 

Exposure Categories

1609.4.2 Surface Roughness Categories
A ground surface roughness within each 45-degree (0.79 rad) sector shall be determined for a distance upwind of the site as defined in Section 1609.4.3 from the following categories, for the purpose of assigning an exposure category as defined in Section 1609.4.3.
Surface Roughness B. Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.
Surface Roughness C. Open terrain with scattered obstructions having heights generally less than 30 feet (9144 mm). This category includes flat open country, and grasslands.
Surface Roughness D. Flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats and unbroken ice.

1609.4.3 Exposure Categories
An exposure category shall be determined in accordance with the following:
Exposure B. For buildings with a mean roof height of less than or equal to 30 feet (9144 mm), Exposure B shall apply where the ground surface roughness, as defined by Surface Roughness B, prevails in the upwind direction for a distance of not less than 1,500 feet (457 m). For buildings with a mean roof height greater than 30 feet (9144 mm), Exposure B shall apply where Surface Roughness B prevails in the upwind direction for a distance of not less than 2,600 feet (792 m) or 20 times the height of the building, whichever is greater.
Exposure C. Exposure C shall apply for all cases where Exposure B or D does not apply.
Exposure D. Exposure D shall apply where the ground surface roughness, as defined by Surface Roughness D, prevails in the upwind direction for a distance of not less than 5,000 feet (1524 m) or 20 times the height of the building, whichever is greater. Exposure D shall apply where the ground surface roughness immediately upwind of the site is B or C, and the site is within a distance of 600 feet (183 m) or 20 times the building height, whichever is greater, from an Exposure D condition as defined in the previous sentence.

Wind Speed
Basic wind speed data is calculated by evaluating the statistical analysis of the region’s climate over a period of 50 years. The highest wind occurrence in that period will then become the established design wind load, with an annual probability of the occurrence of “0.02″.
The basic wind speed for the majority of the United States is 90 miles per hour (mph). Coastal regions have much higher wind speeds due to high winds generated by hurricanes; design wind loads on the east coast range from 100 mph to 190 mph. There are also special wind regions to account for inland areas that have higher wind loads. For example, the Front Range of Colorado sits in a “special wind region” and the predetermined wind loads for building design can vary from 90 miles per hour (mph) to 180 mph.

Exposure Category
Exposure category is based upon ground surface roughness, which is determined from the topography, vegetation, and existing structures. ASCE 7 defines three exposure categories: B, C and D. Exposure B is defined as “urban and suburban areas, wooded areas, or other terrain with numerous, closely spaced obstructions having the size of single-family dwellings or larger”. Exposure C is defined as “open terrain with scattered obstructions having heights less than 30 feet. This category includes flat open country and grasslands”. Exposure D is defined as “flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats, and unbroken ice”.