The technical material is provided under the "Note to the design engineer" section while general information and suggested mitigation measures follow in the "Note to property owner" segment.

Wind

Note to the design engineer:
Per California Building Standards Code (CBSC), Rohnert Park's Minimum Basic Wind Speed designation is 80 MPH. While most of the City is considered Exposure B, new construction at perimeter areas of the City are designated as Exposure C. Design engineers are encouraged to contact the Building Department when there is a question regarding the appropriate Wind Exposure category for a specific project.

Note to property owners:
Most residents would be surprised to know that Rohnert Park's Wind Speed Designation is 80 MPH. For property owners the important understanding of these wind forces is that they typically affect appurtenant elements of structures rather than the structure itself. Examples of these appurtenant elements are fences, awnings, antennas, carports and patio covers.

Fences and residential canvas awnings are exempted from building code requirements. Therefore, the Building Department does not issue permits for their construction or inspect the work. The Department requires permits for commercial canvas awnings, large antennas, carports and patio covers, however. A review of adequate wind resistance is a part of the permit issuance process for these structural appurtenants.

Wind resistant measures for the non-permitted elements fall into the category of common sense maintenance: Fences that have rotted posts or that are weakened in other ways should be repaired. Awnings for residential structures are best purchased from a supplier who can verify the appropriate wind resistance. They should then be attached with appropriately sized lag bolts that are imbedded into structural framing.

As described earlier, large antennas, carports and patio covers are required to be approved and permitted by the Building Department and a review of adequate wind resistance is a part of that process. During the review the Building Department verifies that support elements (posts) are appropriately sized and properly imbedded in the earth or connected to a foundation element. Other wind resisting items that are considered in the review are beams, posts/beam connections and the adequacy of lateral bracing.

Seismic

Note to the design engineer:
Based on the California Building Standards Code, Rohnert Park is within Seismic Zone 4. Furthermore, the City is situated near the Rogers Creek Fault. The Maps of Known Active Fault Near-Source Zones in California and Adjacent Portions of Nevada, prepared by the California Department of Conservation, Division of Mines & Geology, classifies the Rogers Creek Fault as a Source Type A, with a Slip Rate of 9 and having the potential of a Maximum Magnitude of 7.

Because of the City's close proximity to the Rogers Creek Fault Near Source Factors must be calculated as part of the structural design formula for any building within the City limits. For determining Na or Nv, a Near Source Map specific to Rohnert Park, consult Map D-15 of the publication described above. Design Engineers are encouraged to contact the Building Department should there be a question regarding the applicability of either Na or Nv factors to a specific site located within the City.

Soils

Note to the design engineer:
Most soils in Rohnert Park are clays with low permeability, and are classified as Clear Lake clays. Though the underlying geology of the City consists of Holocene basin and Holocene fan deposits, liquefaction potential is expected to be low. However, the clayey soils have a high shrink-swell potential.

The Rohnert Park Building Department requires site-specific current soils investigations for all new structures and any addition greater than 500 sq. ft. The one exception to the site-specific requirement policy involves residential sub-divisions where a soils report, based on borings representative of the sub-division area, may be sufficient for individual buildings.

A current soils report means one which has been conducted within one year of the date of permit application.

It is recommended that design engineers consult the City of Rohnert Park's web site to review local code amendments relative to foundations and slab on grade construction. Within Chapter 15.04 of the City Municipal Ordinances sections 1506.3, 1815.1, 1815.3, 1900.4.4, & 1900 of the California Building Standards code have been amended to specifically address the expansive soil conditions within the City.

Note to property owners:
It is important for Rohnert Park property owners to know that these soil conditions, known as expansive, heaving or swelling soils, are not entirely unique to Rohnert Park. Both Petaluma and Santa Rosa have similar conditions within portions of their cities. In fact, expansive soils are found throughout the United States.

Expansive soils are clay soils and typically expand when wet and shrink when dry. Expanding soils can exert many tons, per square foot, of swelling pressure. The phenomena of expansive soils turns out to be a very complex subject and, until recently, not well understood:

What Causes Expansive Soils to Shrink or Swell?

As clay particles are formed, there are usually several points in the particle arrangement where there is an electrical imbalance; the electrical imbalance is increased whenever a "string" of clay particles is broken apart. Thus, the result is that a clay particle typically has a negative net electrical charge on its surface. Since nature likes all things to be balanced, whenever water molecule drifts close enough to the surface of a clay particle, the negatively charged surface of the clay particle causes the positive end of the water molecule to turn toward the particle and, if close enough to the particle, the water molecule is attracted to the clay particle surface sufficiently strongly that the water molecule becomes trapped. Also, unattached or "free" positively-charged particles, called cations, tend to acquire a spherical-shaped arrangement of water molecules which have their negative ends directed toward the positively-charged cation (and their positive ends directed away form the cation). When the free cation and its "captured" water molecules approach a clay particle, the attraction between the negatively charged clay particle surface and the positively charged outside of the cation's sphere of water molecules causes the cation to be "captured" by the clay particle. Thus increasing the amount of water associated with the clay particle.1

So now you know more about expansive soils then what you had ever cared to know. One more fact you should have in your store of knowledge on the subject, however: "The effect of expansive soil damage on a local, regional or national scale is considerable."1 In 1980 two researchers estimated the annual cost of expansive soil in the US to be $7.0 billion.2

Early attempts at dealing with expansive soil conditions included standard foundation walls with footings, multiple leveling jacks under the structure, non-engineered monolithic slabs and small, shallow & widely spaced piers. Engineering research, much of which has been based on experiencing what works and what doesn't, has gradually contributed to foundation designs that today resist the many tons of pressure exerted by expansive soil.

For this reason, Rohnert Park residential structures using perimeter foundations currently do not use footings but rather include piers that penetrate the soil, usually to a depth of nine feet. These piers are typically 10" - 12" in diameter and are placed approximately every six feet along the foundation stem wall (grade beam). Thus, these types of foundations are known as pier and grade beam.

Another residential foundation design that has been developed to counter-act the effects of expansive soil is called post-tensioned slab on grade. Post-tensioned slab construction is gaining in popularity, and they have been used successfully in Rohnert Park and adjacent areas. However, this type of foundation requires more attention to the moisture content in the areas surrounding and immediately adjacent to the building.

Maintenance Considerations Where Expansive Soils Exist

An important consideration for minimizing damage resulting from expansive soil movement is to maintain relatively constant soil water content around the building. Also, roof and surface rainwater should be directed away from the house and kept from pooling or standing at this area.

Gutters around the entire roof perimeter, along with downspouts that terminate at splash blocks, are mandatory building features in directing rainwater from the roof and away from the building. Drain swales that have been incorporated in surface areas, as required by the Rohnert Park City Engineer's Office, provide the means for the movement of surface water away from the foundation and to the street. Disruption of the drain swales by such things as sidewalks, fences & patios is a common reason for surface water to pond or flow in areas other than where intended.

Continuous and positive water flow away from the foundation is fundamental, but the careful watering of landscape features is also important. Avoid "flooding" planting beds adjacent to foundations. Maintain constant soil moisture around your foundation by providing the appropriate amount of irrigation to your plants & lawns on a regular basis.

1 So Your Home is Built on Expansive Soils, edited by Warren K Wray, PhD, American Society of Civil Engineers, 1995.

2 Krohn, J.P. & Slosson, J.E. (1980) "Assessment of Expansive Soils in the United States", 4th International Conference on Expansive Soils, Denver, Co.