Subject matter related to shear wall construction
Why Retrofit?
Failed Cripple Walls Many west coast homes built before 1950 do not survive earthquakes very well. Houses of this era were built over a crawlspace beneath the first floor framing. The main floor of the house is supported on short "cripple-walls" that typically are not strong enough to brace against the shaking forces generated by an earthquake. The most devastating earthquake damage occurs to wood-framed homes when the cripple walls collapse and the house falls several feet to the ground. The image at right shows a house that fell about four feet (note the stairs leading up to the porch, and how the top of the front door is about three feet above the porch).

Siding Failure The image at left shows a house where the floor framing sat atop concrete footings; in this case the framing was not connected to the footings adequately, and the entire house slid off of them.

Other common weaknesses are the connection between the house's foundation to the cripple wall or between the cripple wall and the main floor framing.

Further problems include the strength of the foundation itself and other factors that usually have to do with the floor plan or shape of the house.
General Principles of Earthquake Retrofits
A typical retrofit involves three basic tasks: Connecting the cripple wall to the footings; strengthening the cripple wall; and connecting the main floor framing to the top of the cripple wall.
Two Engineering Approaches for Retrofits
Streamlined, simple design:
The philosophy behind this approach is that doing something, even if is not an all-encompassing design, is better than doing nothing at all. A retrofit using this "bare-bones" approach may cost $8,000 or so for the design and construction, and it might reduce earthquake damage by 80 percent. A very detailed analysis of the home and design & construction costs to strengthen every structural component that may be undersized by today's standards could easily cost $100,000 and might reduce damage by 95 percent. Most people cannot afford to spend that much money, and would simply do nothing if given only the thorough retrofit option. Furthermore, the huge increase in cost for a thorough retrofit over the streamlined retrofit does not result in a huge reduction in expected damage. This makes spending the extra money for a thorough retrofit a relatively poor investment. Note that the preceding examples are estimates only, and no one can guarantee or predict any level of damage that might result from an earthquake.

Plywood Siding Failure Several San Francisco Bay Area cities are developing a "standard" plan for earthquake retrofits that presents simplified designs for houses that meet certain shape and size requirements. As of 2009, the cities of Berkeley and Oakland both offer tax rebates to new homeowners who proceed with earthquake retrofits. To qualify for the rebate, your home must either meet the requirements of the standard plan or the retrofit must be designed by an engineer or architect. The cities accept the streamlined approach as an acceptable design that is eligible for the tax rebates.

Thorough, "complete" analysis and design:
Some home owners want more than the minimum protection of a streamlined design. In this case, engineers can perform as detailed an analysis and design as the home owner can afford.

A site visit by the engineer can give home owners a better idea of where to spend their money if they want to go beyond the minimum design; perhaps bracing the walls that flank a garage door, or anchoring patio roof covers, or tying two wings of the house together. Engineers often find plywood siding on newer houses that was not nailed correctly, such as that shown in the failure at right.
Beware of Ineffective Retrofits
Angle Iron California has only recently adopted statewide standards for residential earthquake retrofits as The California Existing Building Code (Title 24, Part 10). This code section shows only a small fraction of the conditions that occur in the field; contractors or engineers must invent their own details for other framing situations. Sometimes the invented connections do not consider all of forces and movement that must be resisted by an effective retrofit. There are no special training or licensing requirements for contractors who engage in retrofit work. Many home owners spent thousands of dollars on retrofit work that will not protect their homes at all, or certainly not as well expected.

The length of angle-iron with pairs of bolts at top and bottom (shown at left) was intended to prevent earthquake damage. This sort of connection is untested and most structural engineers believe that it is very poor. The shiny silver connectors at the left of the image were installed to "retrofit the retrofit." These new connectors have been tested extensively and are accepted by almost all building departments; angle irons are not. There are many other sorts of connections that will not perform as hoped in an earthquake.
Retrofit Costs
The following are only rough estimates; each building is different, and may have other issues such as termite damage that require repairs before an earthquake retrofit can be effective.

Design Costs:
The approximate costs to design an under-floor retrofit vary from about $1200 upwards. This cost varies depending on the contractor's experience (contractors who specialize in earthquake retrofits can execute a design more effectively, therefore requiring less extensive details on the engineered drawings), number of stories, building outline (many jogs in and out complicate the design), slope of the building site, and other factors.

Construction Costs:
For a simple under-floor retrofit, construction costs are approximately two to three percent of the value of the building.
An Earthquake Retrofit Success Story
Just before I graduated from college, I bought a home in a planned unit development in Paso Robles. The house has a concrete tile roof. Tile roofs add a lot of weight to a house, and increase the earthquake forces on them. About 60 other units in my development are virtually identical; the garages have narrow walls on either side of the garage door.

Garage Failure Eventually I changed jobs and moved three hours away, but kept the house as a rental. One time between tenants I took some initial steps to strengthen the walls that flank the garage door. I got about half-way through the process before my 'vacation' was over and I had to return to work. Soon the place was rented out to a professional roofer—I had installed about $25 worth of hardware, but I never completed the retrofit.

On December 22, 2003, the San Simeon earthquake hit. I called my tenant, the roofer, just after the earthquake to see how things were. He marveled, "It's really weird—all the other houses on the block had the ridge tiles on their garage roofs thrown about three feet to one side, but not a single roof tile moved on this house!" And that was with an incomplete retrofit.

The image at right shows narrow walls that did not have the strength needed to prevent severe damage to a garage. Current building codes require much greater strength than the tall narrow walls shown. A fairly inexpensive retrofit could greatly reduce the need for extensive repairs after a quake.
Why Condos are Vulnerable to Earthquakes
Condo Front Wall Many condos were built in the 1960's and 70's. Rows of identical units provided economical housing. However, even though the buildings may have met the building codes in effect at the time, experience has shown that some common types of condominium buildings from that era do not survive earthquakes very well. Enormous advances in earthquake-resistant construction methods have occurred in the last 20 years. Also a huge variety of construction hardware specifically intended to resist earthquakes is available today that did not exist 20 years ago.

The image at right shows a typical wood-framed condominium building. The front wall has many openings for garage and entry doors. The rear wall of the building is similar, with large openings for patio doors and windows. These openings leave little wall length to resist earthquake forces.

Weak Story Failure
The photo at left shows what used to be two 2-story buildings. The building on the left in the photo collapsed because the lower floor front wall did not have adequate strength to resist earthquake forces.
Related Articles
The following articles outline some of the issues related to condominiums and townhouses.

Seismic Retrofit Engineering Sequence(No longer available--e-mail thor at shearwalls dot com for info)
The basic steps an engineer would follow to provide an engineered earthquake retrofit design.