B-29 2020-Current: Residential Wind Design Criteria

Lateral load design for wind is required by the California Building and Residential Code. The following are the design requirements for residential structures in Sonoma County. Wind speed and exposure category are the first parameters needed to determine the forces a structure should be designed to withstand.

Version: 04/01/2020

Wind Speed

For structures designed by an engineer the basic wind speed is 95 mph.

For structures designed per the prescriptive bracing methods of the California Residential Code the Ultimate Design Wind Speed is 110 mph.

Exposure Category

The exposure category is a measure of the openness of the terrain and the potential for large wind forces to develop.

Inland sites within Sonoma County shall be designed for Exposure C, unless determined to be B or D during site review. Ocean frontage sites are Exposure D and some protected or less rural sites might be Exposure B. 

A letter from the licensed design professional stating that they have personally been on the site and determined it to be exposure B will generally be accepted.  Another option is for the design professional to provide a topographic map and aerial photographs that demonstrate that the site and surrounding terrain conform to the criteria for exposure B.

Glossary

Basic Wind Speed:

is the nominal design 3-second gust wind speed in miles per hour at 33 feet above the ground for Exposure C category.

Exposure B:

shall apply where the ground surface roughness condition, as defined by Surface Roughness B, prevails in the upwind direction for a distance of at least 2,600 feet or 20 times the height of the building, whichever is greater.

Exposure C:

shall apply for all cases where Exposures B or D do not apply.

Exposure D:

shall apply where the ground surface roughness, as defined by Surface Roughness D, prevails in the upwind direction for a distance of at least 5,000 feet or 20 times the height of the building, whichever is greater. Exposure D shall extend inland from the shoreline for a distance of 600 feet or 20 times the height of the building, whichever is greater.

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. This category includes flat open country, and grasslands.

Surface Roughness D:

Flat, unobstructed areas and water surfaces. This category includes smooth mud and salt flats.

References

• 2019 California Building Code
• 2019 California Residential Code
• ASCE7-16  Minimum Design Loads and Associated Criteria for Buildings and Other Structures