Residential Seismic Reinforcement

Many of Rohnert Park's residential structures were built under what the building code refers to as Conventional Light-Frame Construction requirements. These are typically the older homes within our City. Conventional Light-Frame Construction or Prescriptive Construction generally means that as long as the building is not of "unusual shape" the building designer can use code-prescribed structural provisions without the use of an engineering analysis. Newer residential structures in Rohnert Park, because of their configuration, required engineered designs. By law, both prescriptive as well as engineered designs met the code requirements that were applicable at the time of permit application.

California's first building code was published in 1927. Many fundamental code requirements that are still appropriate were contained in that early document. Seismic resistant construction was not one of those requirements, however. Modern construction technology was in its infancy and the effects of earthquake forces on manmade structures were little understood at the time. Consequently, the early code writers gave much consideration to the vertical (gravity) loads that are imposed on a building's structure but ignored horizontal (lateral) loading. Thus, adequacy of framing was derived from considering roof & floor weights, for example, but not wind and seismic forces.

The importance of horizontal bracing became very apparent in the years following that first building code when prominent metropolitan areas in California suffered severe damage to their building stock from major earthquakes. Those building failures, along with increasingly sophisticated methods of engineering analysis and testing, began to produce important lateral load resisting construction techniques and products that apply to both prescriptive and engineered designs. The development and subsequent code required use of Near Source Factors, mentioned in the "Note to the design engineer" section, is the latest example of the ever-increasing understanding of seismic energy and mitigation efforts through new construction techniques.

The following information is relevant to any residential structure, whether it is of prescriptive or engineered design, and will provide all property owners with a number of practical retrofit measures. It is important to understand, however, that the installation of seismic resistant hardware, or employing any of the techniques described here, may not render your building entirely earthquake- proof. No one can provide that assurance to you. However, should you employ any of the techniques described you will, at the least, be better prepared.

For practical reasons, including legal reasons, the Building Department does not perform earthquake analyses of private property. To gain a thorough understanding of the earthquake resistant qualities of your home, as well as a specific design for addressing any deficiencies, you must engage a California Registered Engineer whose field of practice involves this kind of work. The Building Department highly recommends an engineering analysis to anyone who questions their home's vulnerability to seismic energy.

Foundation

One of the most important aspects of assuring seismic structural stability is the anchoring of the building to its foundation. Though it is unlikely that there are any homes in Rohnert Park that are not anchored to their foundations, it would be prudent for the homeowner to verify that their residence is attached to the foundation.

Homeowners wishing to verify foundation anchorage should know that the current building code requires a 5/8" bolt, which must be imbedded at least 7" into the concrete. These bolts shall be no further apart than 6 feet. Bolts shall be at a maximum distance of 12" and a minimum distance of 7 bolt diameters from each end. Foundation bolting can be verified by entering the crawl space and observing the wooden foundation plates. In the case of homes which do not have a crawl space (slab on grade), bolting can be verified by cutting back sheetrock at the interior of the home or by reviewing the bolt layout at the garage foundation plates, where sheetrock may be absent.

For those wishing to provide additional foundation to house connections (an excellent idea) there are a number of recently developed products that greatly increase wood frame structure/foundation stability. The most notable among these products are square metal bearing plates that are meant to replace the cut washers at the foundation bolt/sill plate interface. The metal bearing plates, along with a number of metal-to-wood to foundation connecting devices, can be installed as a retrofit modification. Should homeowners or building contractors wish further information on the topic of metal-to-wood connectors please contact the City Building Department at (707) 588-2239.

Building Lateral Bracing

Adequate lateral bracing is a prime factor in ensuring structural stability. However, the determination of adequate lateral bracing is produced by an engineering analysis. Many factors are involved in the analysis, including height and size of building, structural materials, building configuration and the area's known seismic potential.

Primary elements of lateral bracing in wood-framed construction are called diaphragms, shear walls & load path connections. Shear walls typically consist of plywood for which the location, thickness, nail size and nailing pattern are carefully specified. Because modern residences often are irregularly shaped, have large openings at perimeter walls (windows), are multi-storied and often contain voluminous interior rooms, shear walls become critical to structural stability. By using formulas prescribed in the building code design engineers develop the amount and placement of shear resisting elements as well as associated load path connecting devices.

Lessons learned from recent major seismic events resulted in new requirements in the edition of the building code following the event. This was particularly true in the case of the Loma Prieta (1989) & Northridge (1994) events. Consequently, both prescriptive and engineered buildings constructed under the 1995 & 1998 editions of the California Building Standards Code (CBSC) contain far more lateral (earthquake) load resisting elements than structures designed and constructed under earlier code editions. One of the most prominent changes imposes restrictions on the use of narrow exterior walls, which act as lateral bracing elements, especially adjacent to large openings.

A major reason for the restriction on narrow lateral bracing walls was the heavy damage that adds to single family residential structures at the garage opening area. This was particularly true during the Northridge Earthquake in structures where the garage supported a second story. Lateral bracing on both sides of garage door openings is of great importance where the garage supports a second level. Addressing missing shear resisting elements at this location should be a priority for all homeowners, especially those in a seismically active area. Fortunately, this is one area that many homeowners may be able to inspect and retrofit themselves, if it is necessary.

The investigation process is simple if sheetrock is absent at the interior of this portion of the garage: Is there plywood at this interior location, or is there not? If sheetrock is present drive a nail in an inconspicuous location, and where there would not be a framing member. If the nail easily penetrates the sheetrock, shear ply is absent.

A superior method of addressing this issue, should it be determined that shear resisting panels are indeed absent, would be to then engage an engineer to calculate the load resisting requirements and provide a retrofit design. However, if that is not possible, it should not be beyond the means of most homeowners, in terms of carpentry skill or cost, to provide some measure of shear resistive protection here.

Cut washers at the foundation bolts should be replaced with the square 2" x 2" x 1/4" metal plates. Five-ply plywood, at least 1/2" in thickness, would be best used for the shear panel. The object is to have a single piece of plywood span the complete width of the wall, as well as the total wall height, and continue on to the beam that supports the garage door opening. The result is an inverted "L"- shaped piece of plywood. The plywood should be nailed to each wall and the beam with No. 8 common nails. The nails should be spaced no greater than 6" apart at all edges and no further apart than 12" in the field. Homeowners are encouraged to contact the Building Department for further clarification, should it be necessary.

Water Heater Bracing

Lack of seismic bracing of water heaters has been identified as a major contributor to earthquake damage to buildings. In some cases structural fires, which have occurred during an earthquake, have been attributed to a broken gas line at a water heater. Moreover, broken water supply lines at water heaters have contributed to significant water damage to buildings that were otherwise unaffected during a seismic event.

The issue of unbraced water heaters in California is of such importance that State Senate Bill 304, passed in 1998, requires all new and replacement water heaters sold in California to be braced, anchored or strapped to resist falling or horizontal displacement due to earthquake motion. Additionally, this bill also requires water heaters to be seismically braced prior to the sale of any real property which contains a water heater.

Local hardware & building supply stores carry manufactured water heater seismic braces. Two are required for each water heater. One brace is installed at the upper 1/3 of the unit and the other brace is installed at the lower 1/3. Be sure to follow all of the manufacturer's instructions.

Masonry Chimneys

A very common category of earthquake damage to residential structures is the cracking or toppling of masonry chimneys. It has been found that even those residential masonry chimneys that have been built to the latest building codes have been subject to failure. Consequently, some cities, such as the City of Los Angeles, will not allow the utilization of masonry chimneys in new residential construction. Based on this information, the Rohnert Park Building Department suggests that homeowners with existing masonry chimneys, at the very least, inspect them for damage on a yearly basis.

Interior Hazards

All citizens should be aware of the danger of falling objects within the interior of a building during an earthquake. Freestanding furniture items such as hutches & bookcases, should they topple, can easily injure an adult or kill a small child. They should be attached with metal brackets to wall framing. A careful inventory of the interior of each room will reveal other items which either should be restrained or, in the case of smaller objects, moved to a lower location. For further information on interior hazards, check out the "Putting Down Roots in Earthquake Country" web site referenced below.

Other Information Resources

Those interested in learning more about seismic energy, and how it relates to structural stability, can find a wealth of information on the Internet. We have provided below a few of best of those resources. Please note: The City does not endorse specific products or manufacturers by the listing of associated web sites.